THE CRIPPLING HOMUNCULUS Inefficient executive control and the persistence of intrusive memories
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THE CRIPPLING HOMUNCULUS Inefficient executive control and the persistence of intrusive memories
ISBN: 9789036737258 Author: Johan Rokus Leendert Verwoerd Cover illustration: Image of an artistic representation of trauma (www.mentalhealth.about.com/library/mh/anx/blrelive.htm) Printed by: Offsetdrukkerij Ridderprint B.V., Ridderkerk © 2009, J.R.L. Verwoerd
RIJKSUNIVERSITEIT GRONINGEN
THE CRIPPLING HOMUNCULUS Inefficient executive control and the persistence of intrusive memories
Proefschrift ter verkrijging van het doctoraat in de Gedrags- en Maatschappijwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus, dr. F. Zwarts, in het openbaar te verdedigen op donderdag 26 februari 2009 om 14:45 uur door Johan Rokus Leendert Verwoerd geboren op 5 april 1975 te Woerden
Promotor: Prof. dr. P. J. de Jong Copromotor: Dr. J. P. Wessel Beoordelingscommissie: Prof. dr. A. Arntz Prof. dr. R. de Jong Prof. dr. R. de Raedt
Contents Chapter 1 General Introduction ............................................................................................................7
Chapter 2 Distractibility and individual differences in the experience of intrusive memories ..........27
Chapter 3 Individual Differences in Experiencing Intrusive Memories: The Role of the Ability to Resist Proactive Interference .............................................................................................39
Chapter 4 Low Attentional Control and the Development of Intrusive Memories Following a Laboratory Stressor............................................................................................................61
Chapter 5 Pre-Stressor Interference Control and Intrusive Memories ...............................................75
Chapter 6 Preferential Processing of Visual Trauma-Film Reminders Predicts Subsequent Intrusive Memories ...........................................................................................................................95
Chapter 7 Summary and discussion..................................................................................................111 References ...................................................................................................................................123 Samenvatting ..............................................................................................................................135 Dankwoord .................................................................................................................................147 Curriculum Vitae .......................................................................................................................151
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Chapter 1 General introduction
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Chapter 1
‘about two years ago, I was dully walking along a Sydney street, worrying about nothing in particular, when – boom! I was small. I was turning the corner past the tankstand and putting my hand out to push open the back door of the Ocean Grove house. It was made of diagonal latticed slats painted dark green and rough to the touch. I nearly keeled over with the vividness of that door. It came from nowhere. It hadn’t entered my mind for over forty years.’ - Helen Garner An old black and white photo, stuck at one corner to the bottom of the drawer. I ease it out carefully, not to tear it. Its shiny surface is covered in tiny cracks, but the picture’s still quite clear. As I gaze at it, my heart starts beating painfully. All those faces, once so intimately, so intensely known. - Phil Salmon
Memory can sometimes present itself like a great mystery. Returning to childhood within a fraction of a second, sitting on the couch with your first love, lost nostalgic moments affected by the passage of time suddenly return with great clarity and physical reliving. This class of involuntary memories possesses a kind of intrinsic beauty for the rememberer which inspired the great novelist and memory-explorer Marcel Proust to formulate his famous Petite Madeleine cookie example. Not surprisingly, these memories are also known as aesthetic memories (Berntsen, 2007). A completely different class of involuntary remembering involves the stressful intrusions after the experience of a traumatic event. According to the most recent version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV; APA, 1994), a traumatic event is defined as an experience where a person “experienced, witnessed, or was confronted with an event or events that involved actual or threatened death or serious injury, or a threat to the physical integrity of self or others” with a subjective response of “intense fear, helplessness or horror” (APA, 1994, pp. 427-428). These horrific experiences will often result in stressful memories more likely to be described as poison instead of joy. For people who develop a posttraumatic stress disorder (PTSD) after trauma, the repeated experiences of intrusive memories are felt as a heavy burden which could deregulate everyday functioning (e.g., work, family) for decades. Prominent examples are cases of holocaust survivors,
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Vietnam veterans and people confronted with sexual abuse in their childhood who remain stuck in the past and become prisoners of their own persistent memories (cf. Schacter, 2001). The psychological symptoms of PTSD comprise: (i) recurrent intrusive distressing recollections of the traumatic event, (ii) avoidance of trauma-related stimuli and/or a general numbing of emotional responsiveness; and (iii) increased levels of arousal (Steel & Holmes, 2007; APA, 1994). Ample evidence has shown that traumatic events appear to be relatively common experiences. For example, the National Comorbidity Survey (NCS) under more than 5800 American citizens aged 15-54 showed prevalence rates of 60.7% of male and 51.2% of female respondents having experienced at least one DSM-III-R traumatic event in their lifetime (Kessler et al., 1995). Another study using the more liberal DSM-IV stressor definition uncovered rates of trauma exposure of 89.6% (Breslau et al., 1998, cited in Duke & Vasterling, 2005). Finally, in a study among 900 US college students, the prevalence rate of having experienced a potentially traumatic event was found to be around 67 % (Bernat, Ronfeldt, Calhoun, & Arias, 1998). In spite of this high prevalence of exposure to traumatic stressors, relatively few people exposed to trauma will subsequently develop PTSD. For example, among adults living in metropolitan Detroit, 89.6% reported exposure to DSM traumatic stressors, yet only 13% of the women and 6.2% of the men had developed PTSD (Breslau, Davis, Andreski & Peterson, 1991, cited in McNally, Bryant & Ehlers, 2003). This indicates that most trauma survivors are well able to reduce the occurrence of stressful intrusive memories in the first months after trauma exposure. Cognitive models of PTSD have based their ideas regarding dysfunctional processing in response to a traumatic event on theories in the experimental memory and attention literature. The underlying idea is that the wealth of knowledge about basic memory and attention processes that has been acquired in the past decades may help explaining the nature of trauma-related intrusions after the experience of a traumatic event. Prevailing models (e.g., Ehlers and Clark, 2000; Brewin, Power & Dalgleish, 1996) have explained persistent intrusions in such a way that dysfunctional information processing during trauma would result in a weakly integrated representation of the traumatic event in the autobiographical knowledge base (cf. Conway & Pleydell-Pearce, 2000). That is, Conway and colleagues (Conway & Pleydell-Pearce, 2000; Conway, Singer, & Tagini, 2004) suggested that individual autobiographical memories are embedded in higher level structures of life themes (e.g., childhood, work, marriage) and general events ( e.g., holiday in Spain, conference in
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Chapter 1
Finland) which together form an integrated autobiographical knowledge base. Weakened integration of the trauma memory in this knowledge base may facilitate the triggering of intrusive memories after trauma and possibly results in the inability to obtain control over these stressful experiences. Although these models are informative in describing what may go wrong when people are confronted with traumatic events, it remains an open question why some individuals show a strong tendency towards dysfunctional information processing in response to trauma but others do not. A large body of work on risk factors has not yet resolved this more specific issue (see Brewin, Andrews, & Valentine, 2000; Ozer, Best, Lipsey, & Weiss, 2003 for reviews). This crucial why-question points to the possible role of innate or early acquired individual differences in basic memory and attention processes. In the experimental memory literature, the importance of individual differences in explaining human cognition and behavior has now been widely acknowledged (e.g., Unsworth & Engle, 2007). Theories of working memory have included a homunculus-like supervisory component that may be responsible for differential performance on a wide range of cognitive tasks and behaviors. A collection of theoretical models has given different headers to this component and its functions, such as central executive (Baddeley, 1996), supervisory attentional control (Atkinson & Shiffrin, 1968), attentional control (Unsworth & Engle, 2007) and executive control (Friedman et al., 2008). The central premise of these theories is that people are born with or grow up in early life with differential resources in executive/attentional ability (Friedman & Miyake, 2008; Engle, 1995). This raises the questions about the implications of individual differences in this basic ability for functioning in everyday life, especially, when one is being confronted with intrusive trauma-reminders. According to working memory models, a well-developed executive ability supports active maintenance of goal-directed information and resistance or inhibition of irrelevant information (Unsworth & Engle, 2007). Thus, executive ability supports flexible control over the contents of consciousness. This would be crucial at times when the risk of being overwhelmed with emotional information during and after a traumatic experience grows. Following this line of reasoning, it can be speculated that a deficit in this basic ability would set people at risk for the persistence of intrusive memories after trauma. A stepwise investigation of this putative prospective relationship between pre morbid executive ability and intrusive memories will be the major theme of the present thesis.
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Because ethical considerations prohibit the exposure of people to real traumatic events, the main questions related to pre morbid cognitive vulnerability and intrusive memories are examined with the methodology of presenting non-selected participants with trauma films as an experimental analogue of real-life trauma. The central idea behind this methodology is that analogue intrusions elicited by trauma-films and intrusive memories after trauma represent qualitatively similar kinds of memory that only differ in intensity. That is, psychopathological intrusions can be viewed as an extension of a continuum from our common, everyday intrusions (see Holmes & Bourne, 2008). This general introduction is build op as follows. Before introducing the main research questions presented in the empirical studies, I will first discuss the concepts that are critical for the present research project. Following a short definition of the contents and qualities of intrusive memories in the clinical literature, I will discuss how two prominent information-processing accounts of PTSD explain the development and maintenance of intrusive memories: the cognitive model of Ehlers and Clark (2000) and dual representation theory (Brewin, Power & Dalgleish, 1996). Next, relevant models of executive control and working memory are reviewed. In the experimental literature, there is some consensus that executive control may not be conceptualized as a unitary construct, but that it consists of different sub-functions, each with their own involvement in regulating particular aspects of goal-directed behavior. This will result in a specification of the earlier suggested relationship between pre morbid executive control and intrusive memories. Following this review, I will introduce a model depicturing underlying mechanisms for persistent intrusive memories after trauma. Based on this model, the general outline of this thesis is presented with a short overview of the main research questions addressed in the subsequent empirical chapters.
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Chapter 1
1.1. How do cognitive models of PTSD explain image-based intrusive memories? Intrusive memories can be defined as vivid sensory-perceptual memories of earlier stressful or traumatic events. For example, Ehlers et al. (2004) describe the case of a patient who kept seeing headlights coming towards him (as he had seen them shortly before his head-on car crash) (see also Ehlers & Steil, 1995; Ehlers et al., 2002). These unwanted memories enter consciousness involuntary and are often accompanied with a strong sense of reliving of the original emotions. Intrusive memories can be experienced in different modalities, but visual intrusions are most common (Ehlers et al., 2004). Intrusive memories can be differentiated from intrusive thoughts which are more likely to be experienced as verbal rumination (e.g., Ehlers et al., 2004). Although both intrusive memories and thoughts might be explained by similar underlying mechanisms, the focus of the present thesis will be on image-based memories. Several cognitive models have been put forward to address the core phenomena of PTSD. These models vary widely in their level of abstraction and number of representational elements or components that describe these phenomena (see Dalgleish, 2004 for a review). For example, higher level schema-based models try to deal with the integration of the subjective meaning of traumatic events into pre-existing knowledge structures (i.e., schemas) of the self and the world. In contrast, other models are more trauma-centered and try to explain the (pathological) nature of the trauma memory and its relationship with the persistence of trauma-related symptoms such as intrusive memories. Important to mention here is the clinical observation of a compelling difference between intentional/generative retrieval and direct/associative retrieval of the trauma memory. Intentional retrieval consists of a strategic and effortful search process for the recall of knowledge in long term memory (e.g., Conway & Pleydell-Pearce, 2000). For example, when you meet a person you recognize as someone you met regularly in college, it will often require an effortful strategic search process to come up with sensory-perceptual details of the specific moments you shared together. In contrast, direct retrieval is an associative and more automatic retrieval process in which there is direct match between a retrieval cue and the content of a specific representation in long term memory. In the context of trauma, intentional recall of the trauma memory is often fragmented and poorly organized with an additional difficulty of recalling the exact temporal order of events. On the other hand, direct retrieval of the trauma memory
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results in intrusive reliving experiences of very specific moments during trauma that involuntary enter awareness in a very vivid and emotional way (Ehlers & Clark, 2000). In the following, I will discuss two prevailing trauma-centric cognitive models that have specifically focused on the imbalance between the degree of intentional and direct retrieval of the trauma memory in explaining persistent intrusive memories and PTSD. I will start with Ehlers and Clark’s (2000) cognitive model for persistent PTSD. This model, primarily intended to provide a theoretical context for the development of a new cognitive behavioral treatment for PTSD (Ehlers et al., 2003; Dalgleish, 2004), gives a detailed account of how dysfunctional encoding of the trauma memory may lead to a processing bias for cues that resemble the earlier traumatic event. This processing bias may then – among other things – be responsible for the unbidden occurrence (i.e., direct retrieval) of stressful intrusive memories after trauma. The second model is dual representation theory (DRT) of PTSD as developed by Brewin and colleagues (Brewin et al., 1996; Brewin, 2001). DRT has its main focus on the representation of the traumatic event in long term memory. The model accounts for the imbalance between the degree of intentional and direct retrieval after trauma by arguing that direct retrieval of image-based intrusive memories and intentional retrieval of autobiographical knowledge are supported by different memory systems in long term memory. Persistent intrusive memories are the result of a relatively large amount of information stored in the memory system supporting direct retrieval of the trauma memory.
Ehlers & Clark’s (2000) Cognitive Model of PTSD Central to Ehlers and Clark’s (2000) cognitive model for persistent PTSD is a sense of serious current threat. That is, in contrast with people who recover from trauma within a relatively short period of time, for people with persistent PTSD the world remains a dangerous place in which normal functioning is impossible. This appraisal of current threat is intimately linked with reliving experiences in the form of intrusive memories, hyperarousal symptoms and strong emotions that apparently come out of the blue. Intrusive memories are interpreted as a sign of an incomplete integration of the trauma memory into the general structure of autobiographical memory which supports direct retrieval of sensory-perceptual details. The model proposes that this lack of integration is the result of dysfunctional processing during the encoding of the traumatic event. In this respect, conceptually-driven
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Chapter 1
(meaning-based) processing is differentiated from data-driven processing (i.e., processing sensory impressions). Conceptual processing is thought to support intentional retrieval of the trauma due to integration of trauma-related information with other autobiographical information whereas data-driven processing supports re-experiencing symptoms. The idea is that people with a tendency to encode the trauma in a sensory/perceptual way will show enhanced perceptual priming for stimuli with a temporal association with the traumatic event. Enhanced perceptual priming can be conceptualized as increased activation of sensory fragments of the trauma in long-term memory. This perceptual priming will support a biased processing of trauma-reminders in the external environment and the subsequent emotional reliving (via direct retrieval) in the form of distressing intrusive memories.
Brewin, Power & Dalgleish’ (1996) Dual representation theory Brewin et al.’s (1996; see Brewin, 2001) dual representation theory shares assumptions about encoding with the Ehlers and Clark (2000) model, but makes the additional claim that the representation of the trauma in long-term memory is distributed over two different memory systems. First, intentional retrieval of the traumatic event is supported by a Verbally Accessible Memory system (VAM). VAM memories have in common with normal autobiographical memories that they are accompanied with autonoetic awareness (a feeling of remembering; Tulving, 2002) and can be communicated. Second, involuntary intrusive memories are supported by a different Situational Accessible Memory system (SAM). SAM is a perceptually-based memory system that can be accessed by situational cues with a temporal association with the traumatic event. This memory system echoes earlier proposals for a perceptual representation system (PRS, Tulving & Schacter) in the literature on implicit memory. SAM-based intrusions lack contextual integration in autobiographical memory and are often accompanied with a strong sense of stressful reliving. Furthermore, this memory system is not accessible for voluntary retrieval/introspection. Brewin et al. (1996) argue that individual differences in the tendency to encode the traumatic event in a sensory/perceptual way will predict the amount of information stored in the SAM (for direct retrieval) and VAM systems (for intentional retrieval). Considering this, a relatively large amount of information stored in SAM would result in a relatively high frequency of image-based intrusive memories. The crucial point here is that recovery after trauma heavily depends on the ability to transport information stored in the SAM system into the VAM system. A more complete
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VAM system will result in an integrated trauma memory which will ease intentional retrieval and significantly reduce the probability of intrusive re-experiencing. Both Ehlers & Clark’s (2000) cognitive model and DRT (Brewin et al., 1996) highlight aspects of dysfunctional information processing (i.e., encoding, storage) in patients with persistent PTSD. However, these models are less able of explaining individual differences in dysfunctional/healthy processing during the first weeks after a traumatic event. For example, it remains unclear which underlying mechanisms might help trauma survivors to transport information from the SAM to the VAM system during natural recovery. Similarly, it is unclear which underlying mechanisms are responsible for a gradual reduction in perceptual priming, intrusive memories and eventually, the sense of current threat in the aftermath of a traumatic event. In the present dissertation I propose a model that describes a role for individual differences in pre-trauma executive control in explaining the difference between healthy processing towards natural recovery or persistent pathology as described by both clinical models. Before presenting a more detailed account of this model, it is important to introduce some key concepts in the experimental memory literature and summarize relevant earlier findings in the neuropsychological literature.
1.2. Theories of working memory and executive control Working memory as a dynamic system instead of a passive store. According to recent conceptualizations, working memory is described as a set of temporary active LTM representations under supervision of an executive attentional controller. This attentional controller manipulates working memory contents in line with currently activated goals (Kane et al., 2007). This dynamic conception of active memory has important consequences for explaining individual differences. Important in this respect is the construct of working memory capacity (WMC). WMC has been defined as the ability to actively maintain or recover access to goal-relevant information in a wide range of contexts that also trigger irrelevant interfering responses/memories/behaviors. This use of the term capacity strongly differs from the view of primary memory as a passive store with a limited number of items or chucks (7 + 2; Miller, 1956, cited in Kane et al., 2007). WMC as ability depends on the amount of executive resources available for goal-directed behavior. Thus, WMC is just as important in the retention of a single representation of a goal as it is in determining how
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Chapter 1
many representations can be maintained. WMC does not refer to memory, but to how executive attention is used to maintain or suppress information. WMC is usually measured with complex span tasks (e.g., Operation Span; Conway et al., 2005) that have been found to predict performance on a wide range of intelligent behaviors. These WM-span tasks present participants with the traditional memory span demand to immediately recall short lists of unrelated stimuli. Additionally and critically, these tasks also consist of a secondary processing component (to prevent rehearsal) through which the maintenance of memoranda is challenged. Operation span (e.g., Conway et al., 2005) for example requires subjects to read aloud and verify arithmetic equations in combination with the instruction to memorize unrelated words following the equations (e.g., IS 6 + 5 = 11? Yes/NO Ball). Empirical work has shown relationships between WM-span tasks and standard measures of intelligence (Kane et al., 2004), active goal maintenance during Stroop performance (“name the color, ignore the word”; Kane & Engle, 2003) and flexible target responding in an attentional orienting task (Unsworth & Engle, 2004). More importantly, Brewin and Smart (2005) showed a negative relationship between performance on a measure of WMC (i.e., Operation span) and the frequency of personally relevant intrusive thoughts using a thought suppression paradigm (Wenzlaff & Wegner, 2000). In general, these studies on WMC provide accumulating evidence for the prominent role of executive control in explaining individual differences on a wide range of every day behaviors. However, the literature on WMC does not speak about the possible existence of specialized executive abilities that would be independently involved in different aspects of cognition and behavior.
Executive control as a multi-component construct A separate literature (e.g., Friedman & Miyake, 2004) has used latent variable analyses to address the question of the unity and diversity of executive functions. The general idea behind these studies is that executive control can be defined as a group of semi-independent abilities, each responsible for a different aspect of behavior. For example, Miyake et al. (2000) divided executive functioning into three correlated but separable abilities: Shifting between task sets, Updating of working memory contents and Inhibition of prepotent response tendencies. Furthermore, these abilities were found to be separately related to performance on neuropsychological tasks of frontal functioning such as the Wisconsin Card Sorting test (WCST) and the Tower of Hanoi. Another division of executive functioning
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which may be of particular importance for the present purposes is the distinction between inhibition at the response level and inhibition at the cognitive level. Friedman and Miyake (2004) examined the relationships between executive measures of inhibition of prepotent responses (Response Inhibition; e.g., the Stroop, stop-signal and antisaccade tasks; see also Miyake et al., 2000) and executive measures of resistance to proactive interference or cognitive inhibition (resistance to PI; indexed by list-learning paradigms). Response inhibition may be involved in preventing the natural but socially inappropriate tendency to say something embarrassing about a colleague’s new purple dress. As an example of resistance to PI, consider the first weeks after purchasing a new mobile phone. During this period, it would be difficult to access the new number in long term memory because the extensively used old number will persistently intrude into conscious awareness. Flexible access to the memory representation of the new number would require a well-developed ability to resist unwanted interference from the old number. This executive ability of resistance to PI seems to play an important role in explaining individual differences on complex measures of WMC (e.g., Kane & Engle, 2000; Rosen & Engle,1997, 1998). This could mean that part of the variance of well-known indices of WMC may be attributed to the specific ability to resist or inhibit interference from working memory. Considering this, in the study of Friedman and Miyake (2004) resistance to PI (but not response inhibition), showed a relationship with everyday self-reported intrusive thoughts. As trauma-related intrusive memories may be seen as a profound example of experiencing unwanted interference in real life, a deficit in the general ability to resist PI in working memory may set people at risk for persistent intrusive re-experiencing and PTSD. A large body of work in the neuropsychological literature has compared PTSD patients and trauma-exposed controls on a wide range of cognitive measures. Several studies which focused specifically on indices of proactive and retroactive interference, found recall deficits in PTSD patients when initial learning was taken into account (Uddo, 1993; Vasterling et al., 1998, 2001; Yehuda et al., 1995; see also Vasterling & Brailey, 2005). Moreover, Vasterling et al. (1998) found that the tendency to make intrusion errors on cognitive interference tasks was positively correlated with re-experiencing symptoms. These clinical results could be informative about underlying pathogenic mechanisms such as resistance to PI. However, they leave undecided whether neuropsychological abnormalities are a consequence of PTSD or represent a pre morbid vulnerability factor (e.g., Vasterling &
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Chapter 1
Brailey, 2005). The studies in the present thesis were intended as a first step to solve this critical issue.
Inhibition theory Until now, I have discussed the role of executive control in the service of the ongoing processing of active goals in working memory. It is unclear however what would be the consequences of exerting control over material deemed as unwanted or irrelevant. When irrelevant/unwanted material such intrusive memories no longer interfere with the goals in working memory has it become less accessible, or less available? Inhibition theory of executive control deals with this important question by taking the activation metaphor (i.e., the idea that memories can become activated or de-activated) one step further. Inhibition theory as formulated by Anderson and coworkers (Anderson & Spellman, 1995; Levy & Anderson, 2002; 2008) proposes that a healthy functioning memory system needs an active ability to forget as well as the well-known ability to remember. At this point, it is important to note that there are two conceptualizations of the construct of cognitive inhibition. First, in its weaker sense, cognitive inhibition is described as an executive control function or ability to prevent irrelevant information from entering working memory (e.g., resistance to PI) (see Hasher et al., 1999; Lustig et al., 2001). Second, in a stronger sense, cognitive inhibition is described as the consequence of an activation reducing suppression mechanism. In the latter description, inhibition reflects a reduced activation level of the memory representations of earlier interfering material (see Anderson & Spellman, 1995). The central premise behind this idea is that a target memory can be made (temporarily) less available by suppression of its representation below a certain baseline state of activity in LTM. In the literature, two case examples of memory situations are described in which LTM (below baseline) inhibition would be involved: The need for selection during retrieval and the need to stop retrieval itself. As an example of selection during retrieval, imagine a situation in which a person’s goal is to retrieve the name of an old classmate after seeing an old high school picture. Although this retrieval cue seems quite specific, it will often not be specific enough to prevent the activation of several memory traces with potential candidate names. Inhibition theory states that in this situation, inhibitory mechanisms (i.e., executive abilities) are recruited to inhibit competing traces, resulting in the selection of the appropriate name
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into consciousness awareness. In contrast, the inhibited competing traces are temporarily less available due to a below baseline activation level. This inhibition process during selective retrieval is widely known as “retrieval induced forgetting” (Anderson & Spellman, 1995). The need to stop retrieval arises when someone is confronted with a cue or reminders and does not want an associated memory come to mind. In case of trauma-reminders in the external environment, this would mean that repeated attempts to stop retrieval of the overwhelming intrusions would result in a gradual decrease in activation of the representation of the trauma memory (see Levy & Anderson, 2008 for a suggested link between inhibition in its stronger sense and trauma-related intrusive memories). In the past decade, empirical studies have tried to test the ideas behind the need for selection and the need to stop retrieval. Standard methodologies such as the retrieval practice paradigm for selective retrieval (Anderson, Bjork, & Bjork, 1994; Anderson & Spellman, 1995) and the think-no-think task (Anderson & Green, 2001) for stopping retrieval have been developed to test the crucial claim of below baseline activation of LTM representations. Although the ideas presented by inhibition theory may sound intriguing from the perspective of a memory theorist, they are in apparent contrast with some basic assumptions in the clinical literature. For example, in the clinical psychology literature, not talking about and trying to forget a traumatic experience is a first step towards development of psychopathology such as PTSD (i.e., avoidance; APA, 1994). One solution to these inconsistent views may be found in the proposal that motivated attempts at avoidance after a traumatic event are the expression of an inability to exert executive (inhibitory) control (see Levy & Anderson, 2008). Although this sounds plausible, it may be quite challenging to conduct an empirical test of this hypothesis in the context of real life intrusive memories. More importantly, before addressing questions about the role of inhibition in a stronger sense, it is important to establish whether there is, indeed, a link between executive control and image-based intrusive memories. Hence the main focus of the present thesis will be on the prospective relationship between pre-morbid executive control (i.e., resistance to PI) and the frequency of intrusive memories after a stressful or traumatic event.
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Chapter 1
1.3. Direct retrieval of intrusive memories after trauma: underlying mechanisms and individual differences Studies investigating intrusive memories will almost by definition use subjective indices such as questionnaires, structured interviews and diaries. This may give information about several aspects of intrusive remembering such as the frequency over time, experienced modalities (e.g., visual, tactile, auditory) and unhealthy appraisals (“I am going mad”, “the world is no longer safe” [e.g., Ehlers et al., 2004]). However, a relatively unexplored aspect of intrusive re-experiencing is the retrieval process itself. What is the exact process by which retrieval cues are (automatically) selected and how does this cue interact with the memory contents of the intrusions? Increased knowledge of the mechanisms supporting the retrieval process of intrusive memories may lead to a better understanding how risk factors such as executive control may contribute to the persistence of these memories. Earlier in this introduction, I briefly introduced a model on the role of executive control in explaining natural recovery/persistent symptoms after a traumatic event. A more detailed account of this model – which heavily relies on an integration of different ideas presented in cognitive theories of PTSD - will be given below. Central to the retrieval of intrusive memories is the direct mapping of the cue and the memory contents. Simple cues in the external environment may be powerful enough to directly trigger stressful contents without the cue elaboration process needed for the intentional recall of everyday autobiographical memories (Conway & Pleydell-Pearce, 2000). In an attempt to explain why seemingly unimportant cues can obtain such a powerful intrinsic meaning, cognitive models of PTSD have used insights from the literature on implicit memory (priming; e.g., Tulving & Schacter, 1990), episodic memory (encoding specificity, transfer-appropriate processing; e.g., Tulving, 2002) and autobiographical memory (direct retrieval, weakly integrated representations; e.g., Conway & Pleydell-Pearce, 2000). Considering this, it has been argued that the expression of an implicit perceptual representation system (PRS) in long-term memory (i.e., perceptual priming [Tulving & Schacter, 1990]) might explain the maintenance of intrusive memories (Ehlers et al., 2006; Ehlers & Clark, 2000). This enhanced perceptual priming hypothesis (e.g., Ehlers et al., 2006; Michael & Ehlers, 2007) states that a primary focus on sensory (i.e., visual) details during trauma encoding, would give rise to the preferential processing (due to enhanced activation in
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PRS) of those environmental stimuli that bear a strong perceptual resemblance to the actual traumatic situation. This preferential processing of perceptual trauma-reminders would then increase the probability that the trauma memory would intrude in conscious awareness due to direct retrieval (Conway & Pleydell-Pearce, 2000) of fragments of the sensory-perceptual representation. Taken together, the foregoing discussion indicates that sensory reliving of a traumatic situation triggered involuntary by cues reminiscent of those present during the experience is mediated by a different (i.e., more perceptual) memory system (i.e., SAM; Brewin et al., 1996) than the retrieval of normal verbally accessible memories (i.e., VAM; Brewin et al., 1996). In terms of the model depicted in figure 1, well-developed executive control (i.e., resistance to PI) existing prior to the exposure of a traumatic event allows gradual control over the processing/attentional bias for perceptual trauma reminders (i.e., retrieval cues) in the external environment (i.e., Ehlers & Clark, 2000). This gradual control may counter the direct retrieval of stressful intrusive memories in SAM and support the formation of more healthy associations with the trauma reminders in VAM (i.e., Brewin et al., 1996; see Brewin 2001 for a slightly different account of the effects of exposure therapy). In contrast, inefficient executive control may set people at risk for a prolonged engagement with perceptual reminders in the post-trauma environment. This inability to disengage attention from the stressful aspects of the trauma reminders (e.g., “danger”) may maintain imagery (SAM) reliving of the trauma and prevent the formation of more healthy VAM associations to the trauma cue.
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Chapter 1
Figure 1. Model explaining the gradual decrease of intrusive memories
Pre trauma
Post trauma
• Sight of a car Trauma reminder
Executive control
• Feeling of… • Smell of… • Sound of..
Resistance to PI
+
VAM SAM
Note. SAM = situational accesible memory; VAM = verbally accesible memory; thick black arrow = strongly associated; thin dashed arrow = weakly associated; dark grey oval (SAM) = highly activated; = inhibitory connection; + = supports the formation of new associations.
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1.4. Aim and outline of the present dissertation The central aim of the present dissertation is to explore the role of pre-trauma executive control on the persistence of image-based intrusive memories after trauma. Based on the foregoing discussion of the experimental memory literature, it is proposed that this relationship might be restricted to the specific executive ability of resistance to proactive interference. A second aim of the present thesis is to explore the mechanisms by which executive control may counter the retrieval process of intrusive memories after trauma (see the model presented in figure 1). The idea is that individual differences in the (in)ability to disengage attention from perceptual reminders after trauma predicts the frequency of
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intrusive re-experiencing. This ability to disengage attention is thought to depend on the amount of executive resources existing prior to experiencing a traumatic event. Based on this model, the empirical chapters of the present thesis focus on three target questions: 1. Is there a prospective relationship between pre-“trauma” executive control and the frequency of subsequent “trauma”-related intrusive memories? 2. If so, is this relationship based on a relative (in)ability to resist PI from working memory? 3. Is there a relationship between the ability to disengage attention of visual traumareminders and the frequency of subsequent image-based intrusive memories?
The first two chapters explore the relationship between executive control and intrusive memories related to a stressful life event in a retrospective design. As our main hypothesis of a specific relationship between resistance to PI and intrusive memories has not been tested before, we used these cross-sectional examinations as stepping stone for testing the prospective model depicted in figure 1. The main goal of the study described in chapter 2 was to test the relationship between self-reported distractibility (as an index of executive control) and the frequency of recent intrusive memories in a large sample of undergraduates (N= 400). In chapter 3, we tested the idea that persistent intrusive memories can be best explained by inefficient executive control at the cognitive level. Non-selected participants filled in a standard index of intrusive memories (i.e., IES; Horowitz et al., 1979) and completed objective measures of executive control at the cognitive level (termed “resistance to proactive interference”; i.e., paired associates list-learning) and objective measures of executive control at the response level (termed “response inhibition”; i.e., Stroop, Random Number Generation). It was hypothesized that intrusive memories would be primarily related to individual differences in the ability to resist PI. Yet, no relationship was expected with indices of response inhibition. Based on the ideas presented by inhibition theory, it was also examined whether individual differences in the ability to reduce the accessibility/availability of irrelevant material (i.e., an effect of well-developed ability to resist PI) would be related to the frequency of intrusive remembering.
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Chapter 1
The last three chapters of this dissertation use a prospective design with a trauma film as an analogue stressor for eliciting intrusive memories. Before discussing details of the studies presented in these chapters, I will present an overview of the ins and outs of the trauma film paradigm (e.g., Holmes & Bourne, 2008).
The use of trauma films in analogue settings An important conclusion from early studies using analogue stressors to induce intrusions was that the tendency to experience intrusive memories was common in the population at large and expected to occur following mild as well as severe stress events (see Holmes & Bourne, 2008 for a historical review).This earlier work has given input to more recent developments of the trauma-film paradigm (e.g., Holmes et al., 2004; Holmes & Bourne, 2008) in which participants are presented with a short film (8-12 min) depicting traumatic events (e.g., scenes of a traffic accident; Holmes et al., 2004; Holmes & Steel, 2004; Stuart, Holmes & Brewin, 2006 ) or horrific movie scenes (e.g., Kindt & van den Hout, 2003). During the film, participants could either be asked to view the film as they would naturally do or to adopt a specific processing task or instruction manipulation. After film viewing, participants complete some post-film control indices (e.g., film-related distress) and are subsequently asked to record any spontaneously occurring intrusive memories of the trauma film in a oneweek diary. After one week, participants return to the lab to submit their diaries and to complete some follow up measures. As noted earlier, the trauma-film paradigm supports experimental control over trauma which has the opportunity to study the role of potential pre morbid risk factors such as executive control on persistent intrusive memories in an analogue setting. The remaining chapters were designed as follows. The prospective studies in chapter 4 and 5 addressed the first two target questions more specifically. These questions were based on the model’s underlying assumption of a link between pre-trauma resistance to PI and SAM-based intrusions. In chapter 4, we explored the relationship between pre-film self-report executive/attentional control and the frequency of intrusive memories in the four days after the presentation of a stressful film. Furthermore, we also tested whether the diary method would be preferable above retrospective ratings of intrusive memories via a questionnaire. The main goal of chapter 5 was to examine the
- 24 -
relationship between a performance-based measure of resistance to PI (as a specific executive ability) and image-based intrusive memories related to an earlier presented stressful film. These memories were recorded in a one-week diary and via a questionnaire (i.e., a modified IES) at a follow-up session after 7 days. Finally, chapter 6 was devoted to the second theme and third question in this dissertation: control over the involuntary retrieval of intrusive memories after a stressful event. In terms of the model: we examined the association between the perceptual cue and SAM-based intrusive memories. It was tested whether an inability to disengage attention from visual reminders of an earlier presented stressful film would predict subsequent intrusive memories recorded in a one-week diary. Biased processing of visual film reminders was explored in a single target rapid serial visual presentation paradigm (RSVP; Most et al., 2005). The degree of interference by these film-reminders during the detection of neutral targets was used as an index of the (in)ability to disengage attention. To further support the role of attentional disengagement in the prediction of image-based intrusive memories, we also included a self-report measure of attentional control that was presented before the presentation of the stressful film. This made it possible to undertake a preliminary investigation of the model’s link between pre-trauma executive control and biased processing of perceptual trauma reminders.
- 25 -
Chapter 1
- 26 -
Chapter 2 Distractibility and Individual Differences in the Experience of Involuntary Memories J. Verwoerd, I. Wessel. Personality and Individual Differences (2007: 42, 325-334) The present study explored the idea that the tendency to experience intrusive memories might be associated with relatively weak cognitive control in general as indexed by the general propensity to become distracted by irrelevant information. A sample of undergraduate students (N = 413) filled in self-report measures of involuntary memories, distractibility, depression and repressive coping. The results showed a significant relation between involuntary memories and distractibility, independent of both trait depression and repressive coping, indicating a general vulnerability factor. As cognitive control may be sensitive to circadian variation, time of day effects in the experience of intrusions were also explored. No significant relation emerged. A proposed relationship between deficient inhibitory mechanisms of working memory and vulnerability for developing and maintaining intrusive memories after experiencing a stressful event is also discussed.
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Chapter 2
1. Introduction Intrusive memories are involuntary recollections of specific events in the past that are hard to control (American Psychiatric Association, 1994). These memories are the hallmark symptom of posttraumatic stress disorder (PTSD), but they are also found together with other pathologies like depression (Brewin, 1998; Kuyken & Brewin, 1994). Apart from memories of trauma, involuntary memories of past events occur also in the normal population (e.g., Berntsen & Hall, 2004; Brewin, Christodoulides & Hutchinson, 1996). Intrusive traumatic memories and involuntary memories in the normal population have in common that they are primarily visual and are mainly triggered by specific cues in the external physical environment, but also by internal cues (present only in thoughts). Although involuntary memories are a normal everyday phenomenon, it appears that people differ in the frequency of experiencing these memories (Horowitz, Wilner & Alvarez, 1979). There is some evidence that differences in working memory capacity may help to explain inter-individual differences in experiencing involuntary memories. For example, Klein and Boals (2001) found that the frequency of experiencing involuntary memories and attempts to avoid such memories are relatively high in individuals who are relatively low in working memory capacity (WMC). WMC is the ability to keep information quickly retrievable and usable under conditions in which there is interference from information that is strongly elicited by task context but that nevertheless would lead to inappropriate responding on a current task (Engle, 2002). The assumption is that it depends on the ability to keep irrelevant information out of working memory (Rosen & Engle, 1998). In line with this, it has been shown (Brewin & Beaton, 2002; Brewin & Smart, 2005) that individuals who are relatively high in working memory capacity are better able to suppress unwanted thoughts. Perhaps, a general inability to keep distracting information out of awareness could be seen as a personality trait which is also responsible for inter-individual differences in involuntary memories. The main purpose of the present study is to investigate this possibility using self-report measures of involuntary memories and cognitive failures. The degree to which people report cognitive failures has been assumed to be an index of the inability to keep irrelevant information out of awareness (Friedman & Miyake, 2004). Earlier research showed that depression is associated with both increased distractibility and the occurrence of involuntary
- 28 -
intrusions (Ellis, 1990; 1991). Therefore, in this study we also measured trait depression (Spielberger, 1995) to see if any relation between distractibility and involuntary memories would not be an artifact of variations in the tendency to react with high levels of negative feelings in a wide variety of contexts. In addition, the present study explored a number of variables that may shed light on the nature of the association between weak cognitive control and the occurrence of involuntary memories. The first variable was repressive coping (Weinberger, 1990). People with a repressive coping style appear to be particularly adept at using protective strategies to avoid self-threatening material from entering awareness (Myers & Derakshan, 2004). Thus, it is possible that having a repressive coping style lowers individuals’ distractibility, thereby moderating the relation between distractibility and involuntary memories. Second, the role of valence was explored. Since distractibility was hypothesized to be a general and stable vulnerability trait, it was expected that valence of reported memories would not play a moderating role in the relationship between distractibility and involuntary memories. Third, earlier research (Hasher, Zacks & May, 1999; Yoon, May, & Hasher, 2000) found a relation between times of day of preferring habitual and intellectual activities and peak moments of circadian variation. Following this, Meccaci, Righi & Rocchetti (2004) found that self-reported cognitive failures were most noticeable at off-peak times of circadian rhythms. In this study, we explored whether reports of involuntary memories followed a similar pattern. In sum, in this study cognitive failures were predicted to be associated with involuntary memories. This relation was expected to be independent of trait depression, coping style and valence. It was also predicted that off-peak times of circadian rhythms would go together with reporting the highest frequency of intrusions. Finally, it was assumed that a general inability to keep distracting information out of awareness would be responsible for interindividual differences in involuntary memories
2. Method 2.1. Participants A sample of 488 undergraduates completed several questionnaires as part of a larger test battery at the beginning of their first year. Because of a strict time schedule, not all of the participants completed all the questionnaires. Therefore, in this study only participants who
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Chapter 2
completed the five most important questionnaires (CFQ, IES, STDS_State/Trait, IMQ, and MEQ) were used for further analyses, leaving a total of 413 valid cases. Additionally, the subsidiary analysis with repressive coping was done over the 358 participants who completed both the anxiety and defensiveness measures used for constructing this scale. Mean age of the participants (95 men, 285 women and 33 with unreported gender) was 20.4 years (range: 1851).
2.2. Materials The Cognitive Failures Questionnaire (CFQ; Broadbent, Cooper, FitzGerald & Parkes, 1982) is a 25 item questionnaire, related to everyday slips and errors in the different cognitive domains of perception, memory and motor functions, experienced over the last six months. Total scores on the CFQ range from 0 to 100 and the response format uses a 5-point Likert scale (0 = never, 4 = always). The CFQ had good internal consistency (Cronbach’s α = .86). The Impact of Events Scale (IES; Horowitz et al, 1979) is a 15-item self-report measure of involuntary memories of some specific emotionally evoking event (range: 0-75). Items are scored on a four-point scale (0 = not at all, 5 = often) with item-loadings of 0, 1, 3, 5. The internal consistency of the IES was high (Cronbach’s α = .93). The Involuntary Memory Questionnaire (IMQ) was constructed for the present study and measures characteristics of the most prominent involuntary memory in the past week. Following Brewin, Christodoulides & Hutchinson (1996), involuntary memories were defined as: “suddenly appearing memories about specific events where you were personally involved. These memories can be positive or negative, they are difficult to control and they can hinder you by your daily tasks.” The questionnaire consisted of the following items: First, participants were asked to describe briefly the content of their most prominent involuntary memory in the past week. Next, they rated the frequency of their memory, together with how positive, how negative, and how intense they experienced their memory on 100 mm Visual Analog Scales (VAS) (range: 0 = not at all, 100 = extreme). Finally participants were asked during which period(s) of the day the intrusions were most noticeable to them. On a timescale with 30-minute intervals (range: 0h – 24h) they reported the length (e.g. 5:00 – 8:00 a.m.) of every single period they had experienced involuntary memories. They were also asked to describe their activities in that period(s).
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The Morningness-Eveningness Questionnaire (MEQ; Horne & Östberg, 1976) consists of 19 items related to time of the day preferences for practicing physical and mental activities and subjective alertness after rising and before going to bed. People can get scores in the range from extreme-eveningness (16) to extreme-morningness (86). The MEQ has good internal consistency (Cronbach’s α = .83). In order to assess repressive coping, scores on the Trait scale of the State-Trait Anxiety Inventory (STAI; Spielberger & Vagg, 1984) and the Social Desirability Scale (SDS; Crowne & Marlowe, 1960) were combined. The STAI consists of 20 items scored on 4-point scales, range 20-80 as a measure of anxiety. The SDS consists of 33 dichotomous items (1 = true, 0 = not true) as a measure of defensiveness. Following Mendiola (2002), a repressor score was calculated by subtracting the SDS from the STAI. Next, the difference score was again subtracted from the highest conceivable positive score. This resulted in an ascending repressor score (range: 0-36). The State-Trait Depression Scales (STDS; Spielberger, 1995) were used as indicators of State (STDS-S) and Trait (STDS-T) depression in the normal population. Each subscale consists of 10 items scored on 4-point scales, range: 10-40. The state items are used to measure participants’ reports on how they feel “at the moment” by rating the intensity of their feelings. The trait items measured how participants “generally” feel (Spielberger, 1995).
2.3. Statistical analyses The data were analyzed using Pearson’s product-moment correlations and hierarchical multiple regression (HMR) techniques. The first purpose of the regression analyses was to investigate whether an association between cognitive failures and involuntary memories was moderated by trait and state depression or by repressive coping. In these analyses the IES was used as the dependent variable. The second purpose was to investigate whether characteristics such as valence would moderate the association between cognitive failures and the frequency of the most prominent intrusive memory in the past week. These analyses used the IMQ frequency as the dependent variable. A measure of valence was calculated by subtracting the two valence ratings of the IMQ (positive minus negative). Following this, 100 points were added to the difference score, resulting in a valence score (range: 0 = extremely negative memory - 200 = extremely positive memory). Prior to conducting the analyses, interaction terms were computed as follows. First, for the IES-analyses the repressor score, the CFQ total
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Chapter 2
score and the STDS subscale scores were transformed into z-scores. Next, the transformed repressor score and STDS subscale scores were multiplied with the CFQ total score, resulting in three different interaction terms. Second, for the IMQ-analyses a similar multiplication was conducted with the CFQ total scores and both valence and time since event. The regression analyses were conducted in a similar fashion. The first step included the main predictors, whereas the second step entered the interaction terms. The first two analyses used the IES total scores as the dependent variable. The first analysis looked at the possible moderating effects of state and trait depression. Therefore, the two scales of the STDS entered the model in the first step together with the CFQ total scores. The interaction terms between the CFQ and the STDS subscales entered the model in the second step. Second, a similar analysis with the IES looked at repressive coping as a moderating variable. The repressor score and the CFQ total scores were included in the first step and the interaction term between the repressor score and the CFQ total score entered the model in the second step. As for the analysis with the IMQ frequency scores as the dependent variable, the analysis with valence was conducted in a similar way.
3. Results
3.1. Descriptive statistics and contents of reported involuntary memories Mean, standard deviation and range of the scales and questionnaires used in this study are shown in Table 1. Table 2 shows that from a total of 413 participants who filled in the IMQ, only 200 gave a description of their most prominent involuntary memory in the past week. It appeared that valence ratings of those participants differed slightly from participants who did not describe a specific involuntary memory, t(412) = 2.02, p< .05. That is, the first group reported slightly more positive involuntary memories. Frequency of reported memories was also higher for the memory group in comparison with the group who reported no memory, t(412) = 3.56, p < .01. Intensity ratings did not differ between the two groups, t(412) = -0.64, p = .52. The contents of the reported memories were compared with items used to measure life stress on the Life Experience Scale (LES; Johnson, Sarason, & Siegel, 1979). It appeared that 20 % of the memories resembled LES categories of negative events (e.g., death of a close family member, serious illness of a close family member, death of a spouse, divorce of parent, broken relationship and trouble with study/ employer) and 11 % to
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LES positive events (e.g. reconciliation with boyfriend/girlfriend, major change in social activities, major change in closeness of family members). However, 69 % of the reported memories could not be classified with the LES (e.g., trouble with a car, dating, going on holiday). Pearson product-moment correlations between all measures are given in Table 3. Because of the large number of individual correlations, a Bonferroni correction was applied, resulting in an alpha of .001. As predicted, the results showed a significant correlation between the CFQ and IES, r(412) = .36, p < .001. The correlation between the two measures of involuntary memories (IES and IMQ-freq) appeared to be rather low, r (412) = .23., p< .001. Table 1. Mean, Standard Deviation (SD) and Range of the scales used in this study N
Mean
SD
Range
MEQ
413
47.23
8.77
22-77
CFQ
413
41.61
11.75
10-77
IES
413
20.42
17.16
0-66
STDS-S
413
22.09
4.01
10-30
STDS-T
413
24.23
3.91
10-32
IMQ frequency
413
50.00
25.36
0-100
IMQ Valence
413
103.63
66.03
0-200
Repressor score
358
20.27
6.48
0-36
Note. MEQ = Morningness-Eveningness Questionnaire; CFQ = Cognitive Failures Questionnaire; IES = Impact of Events Scale; STDS-S = State-Trait Depression Scales: State; STDS-T = State-Trait Depression Scales: Trait; IMQ frequency = frequency ratings Intrusive Memory Questionnaire; IMQ Valence = valence ratings Intrusive Memory Questionnaire.
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Chapter 2
Table 2. Number of participants who described and did not describe an involuntary memory plus means and standard deviations (SD) of Valence, Intensity and Frequency of those memories (IMQ) Memory description Number (%) Valence Intensity Frequency (SD) (SD) (SD) Yes 200 108.71* 60.00 54.49* (48.5) (67.45) (23.21) (24.63) No 213 95.32* 58.53 45.67* (51.5) (63.71) (22.78) (25.61) * p < 0.05 3.2. The effect of depression on the relation between the CFQ and IES The results of the HMR analysis with the IES as dependent variable and STDS-T , STDS-S and the CFQ total scores as predictors in the first step accounted for a significant 19% of variance in reported involuntary memories, F(3, 409) = 32.31, p < .001. Independent of each other, both the Trait depression scale and CFQ scores were positively related with the IES: CFQ, β = 0.29, t(412) = 6.31, p < .001; STDS-T, β = 0.18, t(412) = 2.76, p < .01. State depression appeared to be only marginally related to the IES, STDS-S, β = 0.10, t(412) = 2.20, p = .06. The two interaction terms between cognitive failures and the depression scales entered the model in the second step and added no extra variance, ∆R² = .008, F-change (2, 407) = 2.12, p = .28.
3.3. The influence of repressive coping on involuntary remembering (IES) Including the repressor scores and CFQ total scores in the first step of the regression model resulted in a significant amount of explained variance, R² = .19, F (2, 355) = 42.68, p < .001. There was a negative association between repressive coping and involuntary remembering, β= -.21, t(357) = -4.25, p < .001. The CFQ total scores were positively related to involuntary remembering, β = .35, t(357) = 7.11, p < .001. Finally, the interaction between the CFQ total score and the repressor score did not reach significance, ∆R² = .00, F-change (1,354) = 0.19, p = .89. Repressive coping did not moderate the relationship between distractibility and involuntary memories.
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3.4. Involuntary memories of one particular event The HMR analysis with IMQ-freq as the dependent variable and valence and CFQ total scores as predictors in the first step resulted in 4% of explained variance in the prediction of IMQ-freq, F (1, 410) = 8.75, p < .001. There was a positive relationship between CFQ total scores and IMQ-freq, β = 0.17, t(412) = 3.30, p < .01, and a negative relation between valence and IMQ-freq, β = -0.13, t(412) = -2.50, p < .01. Including the interaction terms between valence and cognitive failures in the second step did not add any extra explained variance in IMQ-freq, ∆R² = .00, F-change (1,409) = .04, p = .85. Table 3 P-M correlations between the scales and questionnaires used in this study Scales 1 2 3 4 5 6 7 _ 1. CFQ .36* .21* .28* -.20* .15 0.15 2. IES _ .26* .30* -.28* .10 0.23* 3. STDS-State _ .57* -.35* .14 0.03 4. STDS-Trait _ -.49* .19 0.06 5. Repressive _ -.17 -.09 score 6. Valence _ -.10 7. IMQ-frequency _ Note. CFQ = Cognitive Failures Questionnaire; IES = Impact of Events Scale; STDS-State = State-Trait Depression Scales: State; STDS-Trait = State-Trait Depression Scales: Trait; Valence = Valence ratings Intrusive Memory Questionnaire; IMQ-frequency = frequency of memory on Intrusive Memory Questionnaire. *p errors neutral).
3.3. Prospective relationship between preferential processing of perceptual film-reminders and subsequent intrusive memories As shown in table 2, the first step of the HMR analysis with the transformed IMSscores as the dependent variable resulted in a significant 18% of explained variance for the two control variables. Inspection of the individual predictors showed a significant contribution of neuroticism, semi-partial r = .41, p < .05. In line with predictions, the inclusion of the RSVP difference score in the second step resulted in an additional 22% of explained variance, semi-partial r = .52, p < .01. The HMR analysis with the (transformed) number of diary intrusions as the dependent variable showed a slightly different pattern. In this analysis, the three control variables did not explain significant variance in diary intrusions. However, when the RSVP difference score was included in the second step, an additional 27% of explained variance was added to the model, semi-partial r = .55, p < .05. 3.4. The role of pre-film attentional control on intrusive memories and RSVP performance As shown in Table 2, the HMR analysis with the IMS as dependent variable showed a non-significant trend of attentional shifting/disengagement in the second step, semi-partial r = -.28, p = .08 The inclusion of the RSVP difference score in the third step resulted in an additional 15% of explained variance (first analysis: 22%), semi-partial r = .45, p < .01. An almost similar pattern of results was found with the number of diary intrusions as dependent variable: this time the second step showed a significant effect of shifting/disengagement, semi-partial r = -.38, p < .05. In the third step, the RSVP difference score added another 16%
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(first analysis: 27%) of explained variance to the model, semi-partial r = .48, p < .01. Note that overall, the addition of self-reported ACS resulted in a substantial reduction in explained variance (7 % and 11% for IMS and diary, respectively) compared to the initial analyses. Finally, in the HMR analysis with the RSVP difference score as dependent variable, the first step with neuroticism and prior film exposure failed to reach significance, R² = .13, p = .10 (see Table 2). However, the addition of the shifting/disengagement scale in the second step showed a significant relationship with the strength of film-related interference on the RSVP, semi-partial r = -.37, p < .05. Participants relatively low in self-reported attentional control showed relatively strong film-related interference on the RSVP.
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Table 2. Summary of Hierarchical Regression Analyses of IMS scores (with and without ACS_shifting), Diary intrusions (with and without ACS_shifting) and the RSVP difference score (N = 36) Dependent Predictors (∆)R DF F-change B SE Beta ² IMS scores Step 1 Prior film exposure Neuroticism
.18
Step 2 Lag 2 score
.22
2, 33
1,32
3.55* -0.57 0.06
0.52 0.02
-0.17 0.40*
0.72
0.21
0.50**
-0.06 0.01
0.33 0.02
-0.03 0.16
0.47
0.13
0.55**
-0.57 0.06
0.52 0.02
-0.17 0.40*
-0.12
0.07
-0.28
11.64**
difference
Diary intrusions Step 1 Prior film exposure Neuroticism
.02
Step 2
.27
Lag 2 score
2, 33
1,32
0.41
12.01**
difference
IMS scores Step 1 Prior film exposure Neuroticism Step 2 ACS_Shifting 106
.18
.08
2, 33
1,32
3.55*
3.12
Step 3 Lag 2 score
.16
1,31
8.20**
difference
0.66
0.23
0.46**
-0.06 0.01
0.33 0.02
-0.03 0.16
-0.10
0.04
-0.40*
0.39
0.14
0.46**
Diary intrusions Step 1 .02 Prior film exposure Neuroticism Step 2 .15 ACS_Shifting Step 3 .16 Lag 2 difference score Lag 2 score
2, 33
1,32 1,31
0.41
5.72* 7.50**
difference
Step 1 .13 2, 33 2.47 Prior film exposure -0.82 0.38 -0.36* Neuroticism 0.01 0.01 0.10 Step 2 .12 1,32 5.19* ACS_Shifting -0.11 0.05 -0.36* Note. ACS = Attentional Control Scale; IMS = Impact of Movie Scale; Lag 2 difference score = number of errors in target detection for RSVP film trials minus number of errors in target detection for RSVP neutral trials. *p < .05 **p < .01.
107
4. Discussion The main finding of the current study showed that preferential processing of perceptual reminders of an earlier presented trauma-film predicted subsequent intrusive memories. In addition, this distracting property of the film-reminders was associated with habitual self-reported attentional control. This pattern of findings is consistent with an earlier trauma-film-study showing that low self-reported attentional control has predictive validity for the occurrence of subsequent diary intrusions (Verwoerd, de Jong & Wessel, in press). The present findings raise the question of how attentional control might contribute to the reduction of real-life trauma memories. One explanation might be that lack of attentional control would result in a prolonged engagement with trauma-reminders, resulting in an increased probability of persistent triggering of intrusive memories. This would be in accordance with earlier clinical findings that PTSD patients have specific problems with attentional disengagement (Pineles et al., 2007). Individual differences in pre-trauma cognitive (i.e., attentional) control might further help explaining individual differences in this increased engagement with perceptual trauma-reminders and thereby in the (non)persistence of intrusive memories. That is, a general, pre-morbid deficiency in executive attention might set people at risk for developing persistent pathology (i.e., attentional bias, intrusive memories) in the aftermath of an actual traumatic event. Accordingly, it has been shown that pre-trauma cognitive control as measured by an AB-AC list-learning paradigm was associated with a relatively frequent occurrence of subsequent intrusive cognitions related to a stressful film (Wessel, Overwijk, Verwoerd & de Vrieze, 2008). The current associations of pre-film self-reported attentional control with biased processing of film reminders and subsequent intrusive memories further supports the role of individual differences in premorbid cognitive control in the prediction of post-trauma symptomatology (see also Verwoerd, de Jong & Wessel, in press). However, future studies should decide whether the relationship between pre-trauma executive ability and the frequency of subsequent intrusive memories is indeed mediated by the (in)ability to disengage attention from perceptual “trauma” reminders. The current results on attentional control and intrusive memories are in line with the notion that perceptual priming explains the triggering of visual intrusions (Micheal & Ehlers, 2007; Ehlers et al., 2006). The theoretical framework behind the perceptual priming hypothesis emphasizes a strong link between the encoding and retrieval context of the trauma memory. That is, a primary focus on sensory (i.e., visual) details during trauma encoding would result in enhanced priming (biased processing) for retrieval cues with a direct 108
perceptual resemblance to the earlier trauma context. In this light, testing hypotheses about preferential processing in the post-trauma environment would be best accomplished with the use of perceptual stimuli. Note however, that earlier studies measuring attentional bias in clinical populations mainly used words as trauma-related stimuli (see Buckley, Blanchard, & Neill, 2000 and Williams, Mathews & MacLeod, 1996 for reviews). Following this, the present design has a clear advantage over other methodologies testing biased processing of trauma-related material. First, the selection of visual film-distracters with direct reference to the earlier presented stressful film-fragment would more closely resemble visual reminders in the external post-trauma environment. Furthermore, the use of a single target RSVP (cf. Most et al., 2005) as a measure of the (in)ability to disengage attention from “trauma” reminders demonstrated that preferential attention towards emotional stimuli could disrupt goal-directed behavior of detecting neutral targets. This process is difficult to capture with a response-time measure such as the modified Stroop-task (e.g., Williams et al., 1996) in which the target and distracter share the same dimension (cf. Most et al., 2005). A first limitation of the present study was the use of an analogue design. Studies using analogue stressors and populations are open to the objection that the studied processes are dissimilar to those observed in the aftermath of an actual trauma. Therefore, these laboratory findings surely have to be extended to other methodologies with survivors of actual traumatic events. Some support for the validity of the current findings can be found in earlier traumafilm studies which have shown to be a valid tool for studying peri- and posttraumatic processing (see Holmes and Bourne, 2008 for a review). In line with this, if only quantitative differences exist between analogue and traumatic symptoms, the present effects could as likely be more (rather than less) enhanced for real-life trauma relative to the current analogue trauma. Secondly, the error rate on the RSVP was quite low which could indicate the existence of floor effects on this measure. Possibly, rotated buildings were generally rather easy to detect in a stream of mainly landscape scenes, irrespective of the presence of a distracter stimulus. Thus, the generally low detection threshold for the rotated target buildings may have attenuated the disruptive effect of the presence of a distracter stimulus. Future studies may increase task difficulty by including targets that would “pop up” less easily from the background (landscape) scenes which may enhance the sensitivity of the task as an index of attentional control.
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Chapter 6
Third, although the present findings provide a case for the role of deficient attentional control in explaining the present relationship between RSVP performance and intrusive memories, it should be acknowledged that the correlational nature of the current study does not rule out alternative explanations. Therefore, future experimental research should manipulate attentional control to further test the validity of “trauma”-related disengagement problems in explaining persistent intrusive memories Finally, in contrast with earlier studies that used distracters with intrinsic negativity (e.g., mutilated faces, Most et al., 2005; Smith et al., 2006), the present study did not find a main effect of visual distracters related to an earlier stressful film. Although this null finding might seem puzzling at first sight, it has to be noted that we selected stills from highly specific scenes of a stressful film fragment. This will make it almost by definition very difficult to find neutral control stimuli that would match the film stills on a wide range of dimensions (e.g., a systematic difference in perceptual quality was still possible). Therefore, we only used the neutral distracter stimuli as a more general baseline for the construction of our difference score. That is, the lag 2 difference score measured the relative difficulty in the detection of targets after film distracters. In our study, no specific predictions were made about the absolute difference in target misses between film and neutral distracters. In conclusion, the present study provided evidence to suggest that an inability to disengage attention from perceptual trauma-reminders contributes to the frequency of intrusive memories. Future work might examine how pre-trauma cognitive control may influence the degree of preferential processing (i.e., prolonged engagement with) perceptual trauma-reminders. Flexible control over trauma-reminders in the external environment may support goal-directed behavior thereby reducing the probability of re-experiencing unwanted intrusive
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memories.
Chapter 7
Discussion
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The central aim of this dissertation was to examine the prospective relationship between inefficient executive control and persistent intrusive memories. The current research questions are based on the clinical observation that during natural recovery after trauma, most people experience some form of reliving of the earlier traumatic event, but only a minority of them will show persistent intrusive memories over time (e.g., Rothbaum et al., 1992). This observation raises the question of what mechanisms might underlie individual differences in vulnerability/resilience. Considering this, the empirical chapters described in this thesis were devoted to answering this crucial question by examining the predisposing role of executive control. More specifically, it was explored whether persistent intrusive memories may be the result of innate or early acquired individual differences in the specific executive ability of resisting PI in working memory (e.g., cognitive inhibition). The choice for the potential involvement of this specific executive ability was based on recent developments in the experimental memory literature. That is, it has been argued that executive control may not be conceptualized as a unitary construct, but that it consists of a set of related but separable executive functions, each devoted to a specific aspect of goal-directed behavior (e.g., Hedden & Yoon, 2006; Friedman & Miyake, 2004; Friedman et al., 2008). The executive ability of resistance to PI was most likely to be involved in gradually decreasing interference of stressful intrusive images in the aftermath of a traumatic event. The analogue methodology of the trauma-film paradigm (e.g., Holmes & Bourne, 2008) presented in chapter 4, 5 and 6 made it possible to investigate the potential involvement of pre-stressor executive control in a fully prospective design. In earlier clinical studies, this idea of pre-trauma vulnerability could only be tested indirectly, by comparing PTSD patients and trauma-exposed controls (e.g., Vasterling & Brailey, 2005) or by retrospectively collecting archival data about more general ability measures such as IQ (Macklin et al., 1998). Thus, positive outcomes of such studies do not address the question whether specific neuropsychological abnormalities are a consequence of PTSD or represent a risk/resilience factor for PTSD (cf. Vasterling & Brailey, 2005). Considering this, the prospective studies in the current thesis were intended to collect preliminary evidence to resolve the issue about the direction of the relationship for the specific symptom of intrusive memories. A second theme of this dissertation was to examine - from a cognitive perspective factors contributing to the direct retrieval of intrusive memories. As noted in the introduction,
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increased knowledge of the mechanisms supporting involuntary retrieval of intrusive memories may lead to a better understanding of how risk factors such as executive control may contribute to the persistence of these memories. In earlier studies, it was suggested that enhanced perceptual priming (e.g., Michael & Ehlers, 2007; see also Ehlers & Clark, 2000) for stimuli reminiscent of the earlier traumatic event may be responsible for the (early) preferential processing of trauma reminders and subsequent retrieval of intrusive memories. Generally in line with this earlier work, our model as presented in chapter 1 also emphasizes the perceptual nature of trauma reminders as potential triggers of intrusive memories. Moreover, we proposed an additional role for the involvement of attentional control. That is, it was hypothesized that an inability to disengage attention from perceptual trauma-reminders might be crucially involved in the development of persistent intrusive memories after trauma. In this final chapter, I will provide a summary of the empirical studies presented in chapter 2 to 6, and discuss the results in terms of the model presented in chapter 1. Furthermore, reflections on future work addressing untested aspects of the model are provided. Subsequently, some methodological issues are discussed. I will conclude with clinical implications.
7.1. Summary of empirical findings Prior research has shown that image-based intrusive memories usually found in response to trauma also occur on a regular basis in the normal non-traumatic population (Bywaters et al., 2004). These earlier findings were confirmed in the first analogue study presented in chapter 2. The results of this cross-sectional examination showed a positive association between selfreported distractibility and the frequency of intrusive and avoidance symptoms related to a stressful life event. Furthermore, it was shown that the relationship between distractibility and intrusive memories was still present after controlling for differences in depressive feelings, indicating unique explanatory properties for cognitive ability. Interestingly, we also found a main effect (but no significant interaction with distractibility) for repressive coping. That is, people with a repressive coping style reported fewer recent intrusive memories. This indicates that both a well-developed cognitive ability as well as a motivational tendency to avoid negative affect may protect people from experiencing persistent intrusive memories related to stressful life events.
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The cross-sectional study presented in chapter 3 used objective performance-based measures of resistance to PI and response inhibition to explore their relationship with intrusive memories related to a stressful life event. The results were in line with our predictions. That is, after correcting for possible confounding effects of depression and age of the reported stressful life event, a unique relationship between intrusive memories and the ability to resist PI appeared. Moreover, this relationship was especially pronounced in the group of female participants. By contrast, this was not the case for the executive ability of response inhibition. These results are in line with the idea that separate executive abilities may underlie different aspects of (abnormal) behavior (e.g., Nigg, 2000). For example, it has been suggested that a lack of response inhibition might be involved in dysfunctional impulsive behavior (e.g., ADHD; Nigg, 2000; Pennington & Ozonof, 1996). In chapter 3, we also proposed that resistance or inhibition of unwanted material from working memory might have consequences for the accessibility of that unwanted material at a later point in time. It was hypothesized that an inefficient ability to resist PI would result in a relatively increased accessibility for earlier interfering material at a later point in time. More importantly, it was hypothesized that this increased accessibility would be related to a relatively high frequency of intrusive memories of an earlier experienced stressful life event. Inspection of the results showed that this last hypothesis could not be confirmed. No relationship between an index of memory accessibility (i.e., reaction times) and intrusive memories emerged. The study reported in chapter 4 was intended as a first step in exploring the prospective relationship between self-report executive control and intrusive memories. In this study, a trauma film was used as an analogue stressor for eliciting intrusive memories. We further explored whether the instruction to keep an intrusion diary would trigger unforeseen recording strategies. These potential strategies may interfere with the natural course of experiencing intrusive memories. Results showed that for participants who were asked to fill in a diary, a negative relationship between self-reported executive control and intrusive memories at a follow-up session emerged. By contrast, in the group of control participants, no evidence was found for a prospective relationship between executive control and intrusive memories. These results provide preliminary evidence for the idea of inefficient executive control as a pre-trauma vulnerability factor for persistent intrusive memories. Furthermore, the results of our diary manipulation suggest that the instruction to fill in a diary can be a valid method for recording spontaneous intrusive memories related to a stressful film.
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However, the small sample size of this study (n= 17 in the diary group) calls for careful interpretation of the results. The design of the prospective study presented in chapter 5 integrated several ideas tested in the foregoing chapters. The main goal was to examine whether individual differences in pre-stressor resistance to PI would predict the frequency of image-based intrusive memories one week after the experience of a stressful event (a film fragment). The results were in line with these expectations. That is, low performance on the index of resistance to PI was related to a high frequency of diary intrusions and intrusive memories at a one-week follow-up session. Moreover, results showed that resistance to PI was still able to predict intrusive memories over and above prior trauma exposure, depression and impact of the stressful event (film). These findings provide some evidence for the idea that the specific executive ability of resistance to PI may function as an independent pre-morbid risk factor for persistent intrusive memories after trauma. Important to note, inspection of the modalities of the diary intrusions showed that the majority of the recorded memories were experienced as visual intrusions. This is in line with earlier clinical observation regarding the modality of trauma-related intrusive memories (e.g., Ehlers et al., 2004), thereby providing evidence for the external validity of the diary method. The main goal of the study presented in chapter 6 was to explore cognitive mechanisms underlying the retrieval of intrusive memories after a stressful or traumatic event. Based on earlier work on the role of enhanced perceptual priming in explaining intrusive re-experiencing (e.g., Michael & Ehlers, 2007), we proposed an additional role for attentional control in predicting persistent intrusive memories. It was hypothesized that a relative inability to disengage attention from perceptual “trauma”-reminders in an RSVP paradigm would be associated with a relatively high frequency of subsequent intrusive memories. Results showed a pattern of findings in line with these expectations. That is, performance on the RSVP task significantly predicted the frequency of subsequent intrusions. To further explore whether this relationship would indeed reflect genuine differences in the (in)ability to disengage attention, several other variables were included in the analyses. First, self-report indices of film-related distress and neuroticism were included to control for any effects of increased levels of anxiety (state and trait) on the processing of film-related distracters on the RSVP (see Barnard et al. 2005). Furthermore, a pre-film self-report index of attentional control was included (see chapter 4) to more directly test the involvement of
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attentional control in RSVP performance. Inspection of the data showed that the relationship between film-related interference on the RSVP and the frequency of diary intrusions remained significant after controlling for effects of neuroticism and film-related distress. Moreover, a significant association between pre-film self-reported attentional control and RSVP target detection emerged, suggesting that individual differences in the disengagement component of attention played a role in the prevention of processing film-related distracter stimuli.
7.2. Discussion of the results in terms of the intrusion model In this paragraph I will return to the model introduced at the end of chapter 1. Based on an integration of ideas presented in the experimental memory literature and the clinical literature on PTSD, the model provides a schematic overview of underlying mechanisms/processes involved in persistent intrusive memories after trauma. More specifically, it was intended to explain how pre-trauma resistance to PI as a specific executive function may contribute to persistent intrusive memories or natural recovery after trauma. The empirical chapters of this thesis were devoted to test the tenability of several components of this model. First, the results of the studies presented in chapter 2 , 3 (retrospective) and 4 (prospective) were used as steppingstone for the design of the study presented in chapter 5. This study was used to test the supposed prospective link between resistance to PI and SAMbased intrusive memories. The results supported the model’s assumptions and were in line with an earlier prospective investigation of Wessel et al. (2008). That is, a relatively low pre“trauma” ability to resist PI in working memory was associated with a relatively high frequency of SAM-based intrusions (i.e., large dark oval in the model p.15). These analogue findings present important information about the potential role of low resistance to PI as a cause (instead of a consequence) of persistent intrusive memories after an actual traumatic event and may therefore help explaining unresolved issues in earlier clinical studies. As mentioned in the introduction, Vasterling et al. (1998) found a positive relationship between performance on indices of interference control (e.g., commission errors) and re-experiencing symptoms in a group of PTSD patients (see also Vasterling & Brailey, 2005). The study of Vasterling and coworkers (1998) presents evidence for the idea that deficient executive control plays a role in the persistence of intrusive memories. However, it remains undecided whether this deficit was already present prior to the traumatic situation.
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Second, further inspection of the model shows a strong link (thick black arrow) between perceptual reminders of earlier trauma and the frequency of SAM-based intrusive memories. This link emphasizes the potential importance of biased processing of perceptual trauma reminders in the prediction of image-based intrusive memories. The idea is that prolonged engagement with these reminders would be associated with a high frequency of intrusive memories (large black oval). The results of the analogue RSVP study presented in chapter 6 supported this part of the model. That is, the degree of film-related interference on the RSVP task was related to the frequency of intrusive memories. Furthermore, the majority of these memories were experienced in the visual modality, supporting the idea that intrusive memories are indeed mainly image-based. Together, these findings support the view that an inability to disengage attention from perceptual trauma reminders may set people at risk for the persistent experience of SAM-based intrusive memories after real trauma. Considering the foregoing discussion of results in terms of the intrusion model, it will be clear that several questions remain unanswered. For example, it would be an interesting avenue for future research to further explore whether individual differences in the (prolonged) engagement with perceptual trauma reminders would depend on pre-trauma resistance to PI. One of the most important proposals of the model is that the established association between pre-trauma resistance to PI and SAM-based intrusive memories (see chapter 5) will be (partly) mediated by an individual’s ability to disengage attention from perceptual trauma reminders. This proposal is depicted in the model by an inhibitory connection between resistance to PI and the association between the perceptual cue and SAM-based memories. Future analogue studies may explore this more complex relationship by testing whether film-related interference on the RSVP task would mediate the relationship between performance-based indices of pre-film resistance to PI and SAM-based intrusive memories. The model further shows that pre-trauma resistance to PI may also support the formation of newly acquired VAM association to the trauma cue. This idea was based on Dual Representation Theory (DRT; Brewin et al., 1996; Brewin, 2001) of PTSD which stated that the formation of verbally accessible association to trauma cues would be crucial for recovery after trauma. This ability to form new VAM-based associations after trauma exposure can be expressed by the degree of coherence and or fragmentation of the trauma narrative (cf. Brewin et al., 1996). Thus, a first step to investigate this part of the model
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would be to relate performance on indices of (pre-trauma) resistance to PI with subjective ratings of the coherence and fragmentation of the trauma narrative. This will probably require the use of clinical groups because the degree of memory fragmentation after viewing a stressful film might be very low and not useful for exploring individual differences. Followup studies may then further explore whether a more detailed and coherent trauma narrative may be predicted by an individual’s ability to disengage attention from perceptual trauma reminders.
7.3. Methodological issues 7.3.1. Use of analogue groups and trauma films The empirical studies presented in chapter 2 to 6 used student samples to investigate the relationship between inefficient executive control and the (sub-)clinical phenomenon of intrusive memories. Studying the natural occurrence of intrusive memories related to stressful life events or trauma films in a sample of healthy undergraduates may result in floor effects and a restriction of range on self-report measures of individual differences. Similarly, the use of student samples with above average scores on measures of intelligence may result in ceiling effects and restriction of range on indices of executive ability. Although the mean frequency of intrusive memories was generally low in the studies using the trauma-film paradigm, the range of these intrusions, together with the intrusions reported in two retrospective studies presented in chapter 2 and 3 was substantial enough to detect the expected correlations with executive ability. These findings concur with earlier observations that image-based intrusions also occur on a regular basis in the normal nontraumatic population (Bywaters et al., 2004). Regarding individual differences in executive control, the modified versions of the interference tasks used in this dissertation (i.e., AB-ACAB list-learning, CVLT) were difficult enough to create a considerable amount of variance on these measures. Taken together, after inspection of the relevant distributions, there is no reason to believe that methodological artifacts such as restriction of range could have reduced the sensitivity to find significant correlations between cognitive ability and intrusive memories. However, the generally low frequency of elicited diary intrusions in the present studies (chapter 5: m = 1.56; chapter 6: m = 2.00) might cause problems when designing future experimental studies. That is, follow-up studies may want to explore protective effects of certain personality traits. Such studies are designed to show a reduction in the frequency of
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intrusive memories in a group of people selected on the personality trait in comparison with a non-selected control group. The presently used trauma-film may not be sufficiently intense to elicit a considerable amount of intrusions, needed for investigating difference between groups in future experimental work. A more general point of concern for working with first-year undergraduates is that these students are often obliged to participate in a large number of experimental studies. This may potentially result in a reduction of their motivation to successfully complete every study to the best of their ability. This may be especially problematic in less structured assignments as recording naturally occurring intrusions in a one-week diary. One way to circumvent this problem might be to ask participants to fill in a confidential questionnaire at follow-up with questions about motivation during the whole procedure. Of course, it must be clear to them that their responses would have no consequences for payment or study credits. Participants with low motivation during one or more parts of the experiment could then be excluded from further analyses. A better, but more time-consuming and expensive procedure would be using community samples in future studies. These samples are often better motivated to participate in experimental psychology studies and do often better represent the general population than a sample of university students.
7.3.2. RSVP task In chapter six, we found a significant relationship between film-related interference on the RSVP-task and subsequent (diary) intrusions. Based on earlier RSVP studies (e.g., Most et al., 2005; Smith et al., 2006) and on the significant correlation with an index of self-report attentional control we proposed that good performance on the RSVP task would depend on an individual’s ability to disengage attention from visual film reminders. Although this proposal seems plausible, it has to be noted that the correlational nature of the study presented in chapter six did not definitely rule out alternative explanations. For example, because it was very difficult to match our film distracters with appropriate neutral control stimuli, it is possible that the significant relationship between interference for film distracters and intrusive memories was unrelated to the earlier presentation of a stressful film. That is, the significant correlation with intrusive memories would not depend on film distracters as genuine reminders with acquired emotionality, but on particular aspects (e.g. brightness, color) of these distracters. Without a control condition in which half of the participants would
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complete the RSVP task before the stressful film, it is impossible to make strong conclusions about film-related interference and its relationship with intrusive memories. Furthermore, the involvement of attentional control on RSVP performance was not tested experimentally which makes alternative explanations possible for the correlation between film-related interference and intrusive memories. Future studies should manipulate attentional control by comparing RSVP performance in an experimental dual-task condition (e.g., counting backwards in threes during target detection) with performance in a neutral condition without a processing manipulation. If the ability to disengage attention from negative distracters plays a crucial role in the detection of target stimuli on the RSVP, performance in the dual task condition should be worse in comparison with the neutral condition. Another interesting research avenue for future studies is to focus more specifically on the time course of distracter-target interference on the RSVP. Earlier studies compared lags of 200 ms (lag 2) and 800 ms (lag 8) and found interference effects on the shorter lag 2 (errors in target detection after negative distracters > errors in target detection after neutral distracters) and no effect of the presence of a distracter on the longer lag 8. With the selection of better matched neutral distracters in future studies (which is difficult but not impossible), it would be interesting to compare different time windows (e.g. lags of 100, 200, 400 and 600 ms) for measuring film-related interference on the RSVP. In line with earlier findings in studies that used paradigms measuring spatial attention (e.g., dot probe; Bar-Haim et al., 2007), interference effects would also be expected at larger time windows of 400 and 600 ms. Of course, the interest in exploring different time windows on the RSVP also holds for the correlation with intrusive memories, which may also depend on a specific time window between distracter and target.
7.4. Future directions Based on the predictions made by our model, future studies should test whether a premorbid deficit in the ability to resist interference in working memory is also involved in the prediction of intrusive memories after real trauma. For example, a prospective study of soldiers before deployment to a war zone would provide a fruitful research direction for further examining resistance to PI as a vulnerability factor for persistent intrusive memories.
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In an ideal study, soldiers are tested shortly before deployment and than followed and tested on their symptoms for at least one year after returning from their mission. Furthermore, a commonly held view in the clinical literature is that in the immediate wake of trauma, repeated exposure to the stressful trauma memory may support emotional processing and eventually, recovery. Based on this assumption, early intervention programs have been developed. The aim of these psychological debriefing programs is to focus on the traumatic event and to educate people about post-traumatic stress. Although talking about the traumatic experience might seem helpful at first sight, outcome studies have come up with mixed and, in some respect even disappointing results. That is, some studies show no differences between debriefing programs and no intervention at all, while other studies even suggest that such programs could work counterproductive because they would intensify the very symptoms they try to prevent (Mayou, Ehlers, & Hobbs, 2000). Thus, it is possible that early exposure to trauma memories via debriefing procedures interferes with the normal cognitive processing leading to recovery. In line with this, in a recent monograph, McNally, Bryant, and Ehlers (2003) suggested that massive debriefing programs may underestimate the psychological resilience of trauma survivors and that it would be more helpful to focus attention on individuals whose symptoms have failed to abate within the first weeks posttrauma. These persons may be at the highest risk for developing psychopathology such as PTSD. In the current dissertation, I propose that people with a well-developed executive ability would not need an intervention to support the formation of healthy associations to trauma reminders. Executive control and more specifically resistance to PI may help traumaexposed people to disengage attention from the threatening aspects/associations of trauma reminders which may help them with the formation of integrated verbally accessible associations (see the model presented in chapter 1). Building on these last conclusions, a fruitful direction for future research would be to investigate the possibility to train executive control in people with an increased risk of persistent intrusive memories after trauma. Although this may seem in contrast with our earlier proposal of the genetic origins of executive control, high heritability does not mean that environmental factors cannot and do not affect executive functions (cf. Friedman et al., 2008). According to Friedman et al. (2008): “…heritability is an estimate of the genetic influence on individual differences around a population mean (i.e., the population variance), rather than an estimate of the influences on the mean itself. Thus, environmental factors can
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influence a population’s average at the same time that genetic factors influence its variance (Scarr, 1992). The effects of training on executive functions may be analogous.” (Friedman et al., 2008, p. 218). In line with this reasoning, accumulating evidence in the experimental and neuro-imaging literature has shown that executive processes and their underlying brain regions are plastic and adaptive and can be modified by training (e.g., Erickson et al., 2007; Kramer, Larish & Strayer, 1995). For example, some studies have investigated rehabilitation of executive functions in clinical (e.g., ADHD; White & Shaw, 2006) and aging populations (Kramer et al., 1995). Whether these observed training induced changes in behavior and brain activity would transfer to traumatized people with inefficient executive control is an important question that deserves further research.
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Samenvatting
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Samenvatting
Een natuurlijke reactie in de nasleep van een traumatische gebeurtenis is het ongewild ervaren van sensorische beelden, ook wel intrusieve herinneringen genoemd. Slachtoffers van traumatische gebeurtenissen zoals oorlogsgeweld, seksueel misbruik of een ernstig verkeersongeval worden als het ware teruggeworpen in de tijd om delen van het trauma, inclusief de oorspronkelijke intense emoties opnieuw te beleven. Als voorbeeld van zo´n ervaring geeft de Londense psycholoog en trauma specialist Anke Ehlers een verkeersslachtoffer die bij het zien van stralen zonlicht plotseling een angstig beeld voor zich zag van naderende koplampen. Die lampen had hij ook gezien tijdens het verloop van het verkeersongeval. Dit soort zeer stressvolle, over het algemeen visuele intrusies, kunnen het dagelijks leven danig beperken en lokken in veel gevallen ook sterke vermijdingsreacties uit van situaties en zaken die sterk aan het trauma doen denken. Ondanks de heftige reacties die een traumatische gebeurtenis teweeg kan brengen, vertoont het merendeel van de overlevenden een opmerkelijk veerkracht. Voor de meeste mensen geldt dat ze binnen enkele weken van de steeds terugkerende intrusieve herinneringen/herbelevingen verlost zullen zijn. Slechts bij een relatief klein aantal personen zullen de symptomen voortduren en uitmonden in een klinische stoornis, waarvan de Posttraumatische Stress Stoornis (PTSS) de bekendste is. Of mensen nu wel of niet een PTSS ontwikkelen kan niet goed worden voorspeld op basis van de ernst van het trauma. Deze klinische observatie suggereert dat er individuele verschillen in het spel zijn die mede uitmaken of iemand wel of niet last blijft houden van ongewenste intrusieve herinneringen na een trauma. Maar wat voor individuele verschillen zijn dat dan? Wat voor factoren zorgen ervoor dat sommige mensen wel last blijven houden van intrusieve herinneringen en anderen niet? Deze vraag is de leidraad voor dit dissertatieonderzoek. Om de vragen die in dit proefschrift naar voren te komen beter te kunnen begrijpen, volgt nu eerst een introductie van enkele essentiële concepten uit de aandacht- en geheugen literatuur. In hedendaagse theorieën over het werkgeheugen speelt het begrip controle een belangrijke rol. Het werkgeheugen zou omschreven kunnen worden als de centrale processor van de menselijke geest, verantwoordelijk voor het selecteren, actief houden en manipuleren van informatie ten behoeve van specifieke doelen. Het idee is dat er aangeboren verschillen bestaan in de mate waarin iemand in staat is zijn werkgeheugen te controleren. In de
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literatuur wordt dit aangeboren verschil in resources/capaciteit uitgedrukt met de homunculus-achtige term “executieve controle”. Het filosofische begrip homunculus voert terug op bekende problemen m.b.t. het wetenschappelijk verklaren van bewustzijn, de vrije wil en de verhouding tussen lichaam en geest. Dat wil zeggen: de notie van executieve controle veronderstelt iemand die deze controle uitvoert – een denkbeeldig mannetje in ons hoofd, ook wel homunculus genoemd. Bij gebrek aan voldoende conceptueel gereedschap zal op deze plaats niet verder worden ingegaan op deze filosofische kwestie/queeste en zal het homunculus-probleem zich in een nieuw jasje als executieve controle blijven voordoen. Uit onderzoek blijkt dat executieve controle een grote rol speelt bij het reguleren van verschillende aspecten van doelgericht gedrag. Kerndoel van de huidige thesis is het stapsgewijs onderzoeken van de voorspellende rol van executieve controle bij het in stand houden van intrusieve herinneringen. De hypothese is dat de mate van executieve controle over het werkgeheugen voorafgaand aan een traumatische ervaring de uitkomst kan voorspellen tussen natuurlijk herstel of het in stand houden van intrusieve herinneringen. Dit idee is meer specifiek en gedetailleerd weergegeven in het model dat aan het aan het einde van het eerste hoofdstuk wordt gepresenteerd. Dit intrusiemodel was het uitgangspunt voor de empirische studies die in deze thesis zijn beschreven. Behalve executieve controle en intrusieve herinneringen spelen in dit model ook herinneringscues in de externe omgeving een belangrijke rol. Uit klinische observaties bij patiënten met PTSS blijkt dat intrusieve herinneringen voornamelijk worden opgeroepen door perceptuele omgevingsstimuli die een directe relatie hebben met stimuli die tijdens het trauma aanwezig waren. Een voorbeeld van zo’n perceptuele herinneringscue is het zonlicht dat voor een angstige herbeleving aan een verkeersongeluk zorgt. In het eerder beschreven voorbeeld van een visuele intrusie heeft het zonlicht een sterke perceptuele relatie met de koplampen die het verkeersslachtoffer tijdens het ongeluk op hem af zag komen. Doordat de herinnering aan het trauma sterk in het geheugen geactiveerd is (dit wordt ook wel priming genoemd) zal het zonlicht ogenschijnlijk automatisch voor de activatie van het angstige beeld van naderende koplampen zorgen. De vraag die in deze dissertatie wordt onderzocht is of natuurlijk herstel na een trauma (bij de meerheid van de slachtoffers) afhangt van de vaardigheid om de aandacht tijdig van perceptuele herinneringscues af te wenden. Controle over herinneringscues zoals het zonlicht kan personen in staat stellen om nieuwe, meer gezonde geheugenassociaties te vormen, zoals zonlicht ---> warmte. Dit betekent dat cues als
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Samenvatting
zonlicht of koplampen langzaam aan geen angst of gevoelens van dreiging meer oproepen (intrusies), maar hun normale neutrale (of positieve) betekenis weer terug krijgen. Zoals vermeldt in het model vergt dit proces van natuurlijk herstel na een traumatische gebeurtenis een zekere mate van executieve controle. Personen die van nature een gebrekkige executieve controle bezitten zullen in de nasleep van een traumatische gebeurtenis veel moeite hebben met het afwenden van de aandacht van herinneringscues die stressvolle intrusies oproepen. Dit betekent dat deze cues hun sterke associatie met de stressvolle herinneringen behouden, waardoor het erg moeilijk is om nieuwe, meer gezonde associaties te vormen. In het huidige proefschrift worden verschillende componenten van het model onderling en in samenhang met elkaar onderzocht. De nadruk ligt hierbij op het stapsgewijs onderzoeken van de relatie tussen executieve controle en intrusieve herinneringen. Het laatste empirische hoofdstuk richt zich op het additionele belang van perceptuele herinneringscues bij het in stand houden van het frequent ervaren van intrusieve herinneringen. In de verschillende studies wordt gebruik gemaakt van ongeselecteerde groepen psychologiestudenten. Het werken met “gezonde” proefpersonen heeft als voordeel dat het cognitief functioneren bij deze groep voorafgaand aan de ervaring van een stressvolle of traumatische gebeurtenis bepaald kan worden. Hierdoor is het relatief eenvoudig is om de unieke bijdrage van executieve controle bij het in stand houden van symptomen zoals intrusieve herinneringen te bepalen. Bij een groep met PTSS patiënten is dit veel complexer omdat de traumatische ervaring, samen met veel voorkomende comorbide stoornissen zoals depressie en angst (e.g., paniek, fobieën) invloed kan hebben op het verminderd cognitief functioneren na een trauma. Hierdoor is het erg lastig om oorzaak en gevolg m.b.t. gebrekkige executieve controle en traumagerelateerde symptomen te achterhalen.
Studie 1 (retrospectief): Intrusieve herinneringen en afleidbaarheid in het dagelijks leven In de studie die beschreven wordt in hoofdstuk 2 vulde een grote groep van ongeveer 400 studenten een aantal vragenlijsten in om de relatie tussen executieve controle en intrusieve herinneringen te onderzoeken. Een eerste vragenlijst vroeg naar de mate van afleidbaarheid in het dagelijks leven in de vorm van het maken van cognitieve fouten. Deze vragenlijst werd gebruikt als maat voor executieve controle met het idee dat een groot aantal cognitieve fouten
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vergeleken kan worden met een verminderde executieve controle. Voorbeelden van items op deze cognitieve fouten vragenlijst zijn: “afspraken vergeten” (nooit – zeer vaak), “Beginnen met iets en er halverwege mee ophouden omdat u afgeleid wordt door iets anders” (nooit – zeer vaak), en “In een gesprek iets eruit flappen dat wel eens beledigend voor anderen zou kunnen zijn” (nooit – zeer vaak). Daarnaast werd proefpersonen gevraagd om terug te denken aan een zeer stressvolle/traumatische gebeurtenis uit het verleden en hierover, indien aanwezig, het aantal recente (afgelopen 7 dagen) intrusies en vermijdingsreacties te rapporteren. Voorbeelden van items behorend bij de intrusievragenlijst zijn: “beelden ervan kwamen plotseling boven” (helemaal niet- vaak), “sterke gevoelens overspoelden me” (helemaal niet- vaak) en “ik probeerde er niet over te praten” (helemaal niet- vaak). De verwachting was dat personen met een relatief groot aantal cognitieve fouten het meeste last zouden hebben van recente intrusies en vermijdingsreacties n.a.v. de stressvolle levensgebeurtenis. In overeenstemming met de hypothese was er een significant verband te zien tussen het aantal cognitieve fouten in het dagelijks leven en het aantal intrusieve herinneringen. Eerder onderzoek heeft laten zien dat een negatieve stemming zowel samenhangt met een groot aantal cognitieve fouten als met een hoge frequentie van het aantal intrusieve herinneringen. Het was dus belangrijk om te kijken of de relatie tussen cognitieve fouten en intrusieve herinneringen niet gewoon een bijeffect was van een algemeen negatieve stemming. Dit bleek niet het geval; de relatie bleek onafhankleijk van verschillen in depressie. Behalve voor depressie werd er ook succesvol gecontroleerd voor de leeftijd van de stressvolle gebeurtenis (i.e., hoe lang geleden vond de gebeurtenis plaats?) en voor de impact die de gebeurtenis in het verleden had. Dit patroon van resultaten is in overeenstemming met het idee dat individuele verschillen in executieve controle een unieke bijdrage kunnen leveren aan de verklaring van individuele verschillen in intrusieve herinneringen n.a.v. stressvolle en traumatische gebeurtenissen.
Studie 2 (retrospectief): De rol van specifieke executieve functies bij het verklaren van intrusieve herinneringen De eerste studie maakte gebruik van een zelfrapportage maat voor executieve controle. Het valt echter niet uit te sluiten dat een dergelijke zelfrapportage geen accuraat beeld verschaft van de “echte” executieve controle functies. Om die reden maakt studie 2
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Samenvatting
gebruik van een computer taak om individuele verschillen in executieve controle in kaart te brengen. Daarnaast is het belangrijk vast te stellen dat executieve controle geen ondeelbaar construct is. In de experimentele geheugen literatuur wordt uitgegaan van het idee dat executieve controle verschillende onafhankelijke functies dient. Zo kan er controle plaatsvinden op het niveau van motorische acties (het remmen van een automatische respons) of op het niveau van het werkgeheugen (het tegengaan/remmen van ongewenste informatie). In de studie in hoofdstuk drie werd de executieve functie die betrokken is bij het weren van ongewenste informatie uit het werkgeheugen vergeleken met de executieve functie om een automatische respons tegen te gaan. Ter illustratie: een goede vriend geeft u zijn nieuwe telefoonnummer door. Bij de meeste mensen zal het de eerste weken zeer moeilijk zijn om dit nieuwe nummer direct op te roepen wanneer ze de vriend willen bellen, omdat het veel sterker geactiveerde oude nummer zich steeds opnieuw zal opdringen. Een flexibele toegang tot het nieuwe nummer vergt een goed ontwikkelde (executieve) vaardigheid om interferentie van het oude nummer uit het werkgeheugen te weren. Deze vaardigheid zorgt ervoor dat het oude nummer langzaam minder actief wordt in het geheugen om hiermee het nieuwe nummer voorrang te geven. Een mooi voorbeeld van de vaardigheid om een automatische respons tegen te gaan wordt gegeven door de Amerikaanse geheugen psycholoog Michael Anderson. Hij beschrijft een situatie waarin hij op een warme dag het raam open wil zetten om wat frisse lucht op te vangen. Op het moment dat hij dit doet valt er een cactus naar beneden die in het raamkozijn stond. In een eerste reactie wil hij de plant opvangen om te voorkomen dat de pot op de grond valt en breekt, maar realiseert zich in een fractie van een seconde dat dit misschien niet zo’n goed idee is. Net op tijd kan hij de al in gang gezette actie tegengaan en een zeer pijnlijke ervaring voorkomen. Uit het bovenstaande moge duidelijk zijn dat zowel de vaardigheid om ongewenste informatie uit het werkgeheugen te weren, als de vaardigheid om een automatische response te remmen van essentieel belang zijn voor de controle over specifieke componenten van ons gedrag. Voor het weren van ongewenste herinneringen lijkt het tegengaan van responsen minder relevant dan het tegengaan van irrelevante informatie in het werkgeheugen. Om te onderzoeken of inderdaad voornamelijk een gebrekkige vaardigheid om irrelevante informatie uit het werkgeheugen te weren samenhangt met het frequent ervaren van intrusieve herinneringen, zijn proefpersonen in studie 3 onderworpen aan een aantal computertaken. Dit om beide executieve functies in kaart te brengen. Als maat voor de
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vaardigheid om irrelevante informatie uit het werkgeheugen te weren werd een taak gebruikt waarbij proefpersonen verschillende lijsten met woordparen moesten leren. Deze lijsten met woordparen waren zo samengesteld dat het leren van de tweede lijst sterk interfereerde met de woorden die eerder geleerd waren (e.g., bakker - brood [lijst 1]; bakker - meel [lijst 2]; slager - vlees [lijst 1]; slager – touw [lijst 2]). De snelheid waarmee proefpersonen de tweede lijst tot een bepaald criterium konden leren werd gebruikt als maat voor de vaardigheid om interferentie tegen te gaan en gerelateerd aan het aantal intrusieve herinneringen. Het aantal intrusies werd vervolgens gerelateerd aan de taakprestatie op een tweetal computertaken welke in eerder onderzoek model stonden voor de vaardigheid om een automatische respons te onderdrukken: de kleurwoorden Strooptaak en de random number generation taak. Bij deze laatste taak wordt van proefpersonen gevraagd om cijfers tussen de 1 en de 10 in een willekeurige volgorde op te noemen, 1 getal per seconde. Het is bij deze taak de kunst om een automatisch opkomende neiging om te gaan tellen (1, 2, 3…) tegen te gaan. Voortbouwend op de resultaten van studie 1 bleek een gebrekkige vaardigheid om interferentie uit het werkgeheugen te weren gerelateerd aan een groot aantal recente intrusies aan een stressvolle gebeurtenis uit het verleden. In overeenstemming met de voorspelling dat vooral de controle over het werkgeheugen van belang is voor de verklaring van intrusieve herinneringen was er geen verband tussen de frequentie van het aantal intrusies en de prestatie op de taken die model stonden voor de vaardigheid om een automatische response te onderdrukken. Wederom bleek de relatie tussen executieve controle en intrusieve herinneringen onafhankelijk van depressie, geslacht en het aantal maanden geleden dat de stressvolle gebeurtenis plaatsvond. Deze resultaten leveren aldus additionele ondersteuning voor de hypothese dat natuurlijk herstel of persistentie van intrusieve herinneringen na een stressvolle
levensgebeurtenis
(deels)
afhangt
van
een
specifieke
executieve
functie/vaardigheid. Dit is de vaardigheid om ongewenste informatie uit het werkgeheugen te weren.
Studie 3 (prospectief): Zelfgerapporteerde executieve controle als voorspeller van het aantal intrusieve herinneringen aan een stressvolle film De studies in hoofdstuk 2 en 3 onderzochten de relatie tussen executieve controle en intrusieve herinneringen op een moment in de tijd. Het valt op basis van die gegevens niet uit te sluiten dat juist vanwege de intrusies de executieve functies achteruit zijn gegaan i.p.v. dat
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Samenvatting
gebrekkige executieve functies hebben geleid tot meer intrusies. Een belangrijke volgende stap is daarom om te toetsen of verschillen in executieve controle voorafgaand aan het trauma ook daadwerkelijk voorspellend zijn voor het al dan niet optreden van intrusieve herinneringen. Een belangrijke voorwaarde voor een dergelijke studie is dat individuele verschillen niet kunnen worden herleid tot verschillen in de traumatische ervaring. Een relevante ingang voor prospectief onderzoek naar de rol van executieve controle is het nabootsen van een traumatische gebeurtenis in een gecontroleerde analoge situatie. Uit eerder onderzoek blijkt dat het aanbieden van een stressvolle film als een analoog trauma een valide methode is om individuele verschillen in de frequentie van het aantal intrusieve herinneringen te onderzoeken (zie Holmes & Bourne, 2008 voor een overzicht). Het idee hierachter is dat de ervaring van een stressvolle film en de ervaring van een trauma qua impact en intensiteit sterk van elkaar verschillen, maar kwalitatief geen ander soort ervaringen zijn. Studie 3 was bedoeld als eerste stap om de invloed van pre “trauma” executieve controle op het in stand houden van intrusieve herinneringen te onderzoeken. In deze studie werd een stressvol filmfragment gebruikt als analoog trauma. Voorafgaand aan de presentatie van de stressvolle film vulden proefpersonen een vragenlijst in waarin gevraagd werd naar de vaardigheid om de aandacht te controleren. Voorbeelden: “Als ik me concentreer, kan ik mijn aandacht zo richten dat ik niet merk wat er om me heen gebeurt” (bijna nooit – altijd), “Ik kan snel van de ene naar de andere taak overschakelen” (bijna nooit – altijd), en “Als er een afleidende gedachte in me opkomt, kan ik mijn aandacht hier gemakkelijk van wegleiden” (bijna nooit – altijd). Na afloop van de film vulde de helft van de proefpersonen een dagboekje in om gedurende drie dagen plotseling opkomende herinneringen aan de film te noteren. Na afloop van de drie dagen kwamen ze nog een keer terug voor een tweede bijeenkomst, waar ze het dagboekje inleverden en een vragenlijst invulden die nogmaals vroeg naar het aantal intrusieve herinneringen aan de film. De andere helft van de proefpersonen vulden na het zien van de film geen dagboekje in en kwamen zonder verdere informatie na drie dagen terug om de intrusievragenlijst in te vullen. In lijn met de hypothese was er voor de dagboekgroep een verband te zien tussen de mate van aandachtscontrole voorafgaand aan de film en het aantal intrusieve herinneringen tijdens de tweede bijeenkomst. Dit verband trad echter niet op bij de controle groep die alleen een vragenlijst achteraf invulde. Dit resultaat suggereert het bestaan van een prospectieve relatie tussen executieve controle en intrusieve herinneringen na een stressvolle gebeurtenis.
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Verder lijkt het gebruik van een dagboekje een sensitievere methode om natuurlijk en spontaan opkomende intrusies te registreren dan enkel een vragenlijst achteraf.
Studie 4 (prospectief): De executieve vaardigheid om ongewenste informatie uit het werkgeheugen te weren voorspelt het aantal intrusies n.a.v. een stressvolle film De resultaten van studie 3 waren in overeenstemming met het idee dat cognitieve controle prognostische waarde heeft voor het ontwikkelen van intrusieve herinneringen. Echter, de kleine groep proefpersonen en het feit dat executieve controle werd gemeten met een vragenlijst vraagt om replicatie van de resultaten in een grotere groep waarbij gebruik gemaakt wordt van een computertaak i.p.v. zelfrapportage. Deze huidige prospectieve studie richtte zich specifiek op de rol van individuele verschillen in de executieve functie/vaardigheid om ongewenste informatie uit het werkgeheugen te weren. De vraag was of een gebrekkige controle over het werkgeheugen voorafgaand aan de presentatie van een stressvolle film samenhangt met een relatief groot aantal dagboekintrusies in de 7 dagen na het bekijken van de film. Om individuele verschillen op deze executieve functie in kaart te brengen werd een variant gebruikt op de computertaak waarbij proefpersonen interfererende lijsten met woordparen moesten leren (zie studie 2/hoofdstuk 3). De prestatie op deze taak werd gerelateerd aan het aantal dagboekintrusies 7 dagen na de vertoning van de stressvolle film. De resultaten lieten een duidelijk verband zien dat in lijn was met de assumpties uit het intrusiemodel: executieve controle over het werkgeheugen voorafgaand aan een stressvolle gebeurtenis was een goede voorspeller van het aantal intrusieve herinneringen aan deze gebeurtenis. Dit gevonden effect bleek niet afhankelijk te zijn van additionele factoren die een bewezen invloed hebben op het in stand houden van intrusieve herinneringen na een traumatische situatie. Na statistische controle voor de invloed van depressie, geslacht, de impact van de stressvolle gebeurtenis (de film) en het wel of niet hebben ervaren van traumatische gebeurtenissen in het verleden, bleef executieve controle een goede voorspeller van het aantal intrusieve herinneringen.
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Samenvatting
Studie 5 (prospectief): De rol van perceptuele herinneringscues bij het voorspellen van intrusieve herinneringen aan een stressvolle film In de studie die beschreven wordt in hoofdstuk 6 werd de focus verlegd naar een nog niet onderzochte component van het intrusiemodel. Er werd gekeken naar de invloed die herinneringscues in de externe omgeving kunnen hebben op het in stand houden van intrusieve herinneringen. De vraag was of het niet kunnen afwenden van de aandacht van perceptuele herinneringscues aan een stressvolle film voorspellend is voor het aantal filmgerelateerde intrusieve herinneringen in de week na het bekijken van deze film. De keus voor het selecteren van perceptuele cues was gebaseerd op de klinische observatie dat de meerderheid van de herinneringscues na een traumatische gebeurtenis een perceptueel karakter bezitten (zie het voorbeeld over het zonlicht). In de huidige studie werd gebruik gemaakt van een aandachtstaak waar een groep proefpersonen een snelle sequentiële reeks landschapsfoto’s op het computerscherm te zien kreeg (100 ms per foto). Tussen deze landschapsfoto’s zat 1 foto van een gebouw; deze was 90 graden geroteerd en fungeerde als doelfoto. Proefpersonen werd gevraagd om na iedere trial van 17 opeenvolgende foto’s de richting van de rotatie van het gebouw aan te geven (naar links of naar rechts). Dit konden ze doen met de pijltjes toetsten. De afhankelijke variabele bij deze taak was het aantal fouten in rotatiedetectie van de doelfoto’s. Dit aantal fouten werd vergeleken tussen twee condities. In de eerste conditie kwam er vlak voor het verschijnen van de doelfoto een afleidende foto in beeld. Deze afleidende foto bestond uit een neutraal gezicht of dier en week duidelijk af van de landschapsfoto’s die als achtergrond dienden. In de tweede conditie was er ook een afleidende foto te zien, maar nu bestond deze uit personen en objecten uit de eerder vertoonde stressvolle film. Het verschil in het aantal fouten tussen de trials waar film afleiders voorkwamen en de trials waar neutrale afleiders voorkwamen werd gebuikt als maat om problemen met het afwenden van de aandacht te bepalen. Het idee hierachter was dat de neiging om relatief veel fouten te maken op trials waar film afleiders voorkwamen (t.o.v. neutrale afleiders) model staat voor problemen met het loskoppelen van de aandacht van herinneringscues aan een eerdere stressvolle gebeurtenis. Deze verschilscore werd vervolgens gebruikt om het aantal intrusieve herinneringen aan de film te voorspellen. De resultaten waren in lijn met de eerdere verwachtingen: De neiging om relatief veel fouten in rotatiedetectie te maken na het verschijnen van een herinneringscue aan de stressvolle film hing sterk samen met het aantal dagboek herinneringen in de week na het bekijken van deze
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film. Dit betekent dat het ongewild vasthouden van de aandacht op de stressvolle aspecten van herinneringscues voor een relatief hoge frequentie van het aantal intrusieve herinneringen kan zorgen.
Conclusie Dit proefschrift heeft zich beziggehouden met de vraag of een aangeboren verschil in controle over het werkgeheugen (i.e., mentale capaciteit) kan voorspellen of iemand na een traumatische gebeurtenis nog jarenlang geplaagd zal worden door steeds terugkerende intrusieve herinneringen. In lijn met het intrusiemodel uit het eerste hoofdstuk lieten de resultaten van de verschillende empirische studies een consistent patroon zien: Een gebrekkige executieve controle voorgaand aan een stressvolle gebeurtenis blijkt voorspellend voor het aantal intrusieve herinneringen aan deze gebeurtenis. De resultaten van deze analoge studies bieden interessante aanknopingspunten om de rol van premorbide executieve controle als kwetsbaarheidfactor verder te onderzoeken. Toekomstig onderzoek zou zich kunnen richten op de vraag of pretrauma executieve controle een goede voorspeller is van het cruciale verschil tussen natuurlijk herstel dan wel het ontwikkelen van PTSS na een traumatische gebeurtenis. Hiervoor zou een groep personen met een verhoogd risico om een trauma mee te maken (e.g., soldaten voorafgaand en na afloop van een uitzending) gedurende een bepaalde periode gevolgd kunnen worden.
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Samenvatting
146
Dankwoord
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Dankwoord
Dan is het nu tijd voor het laatste en ongetwijfeld meest gelezen gedeelte van dit proefschrift: Het dankwoord. Een aantal personen hebben de afgelopen vier jaar - direct dan wel indirect bijgedragen aan het schrijven van mijn proefschrift. Hen wil ik graag persoonlijk bedanken. Allereerst mijn copromotor en begeleider Ineke Wessel. Beste Ineke, na een wat stroeve start hebben we de juiste toon gevonden. Ik zie onze samenwerking als zeer vruchtbaar en wil je bedanken voor de dingen die ik de afgelopen jaren van je heb geleerd. Hoe moeilijk was het voor mij om kort en bondig te zijn tijdens het schrijven….. Dankzij jouw kritische blik en grondig commentaar is mijn schrijfvaardigheid met sprongen vooruit gegaan, wat heeft geleid tot een aantal mooie publicaties. Ik zie onze toekomstige samenwerking en discussies tijdens mijn post doc in Groningen met plezier tegemoet. Vervolgens mijn promotor Peter de Jong. Beste Peter, dank voor je enthousiasmerende en inspirerende houding, het grote aantal creatieve ideeën dat me steeds weer vooruitgeholpen heeft en het waanzinnig snelle commentaar op mijn stukken. Het is toch de droom van elke AIO om een ’s middags opgestuurd stuk de volgende ochtend al weer met uitgebreide suggesties in je mailbox terug te hebben. Heel veel dank hiervoor, ook voor je constante betrokkenheid gedurende de ups en downs van mijn promotie traject. Uiteraard wil ik ook een aantal woorden richten tot mijn directe collega’s, te beginnen met mijn kamergenoten. Esther en Sippie, bedankt voor de gezellige tijd en de goede gesprekken tijdens de eerste helft van mijn AIO-tijd. Complimenten voor het feit dat jullie het zo lang uithielden met een warhoofd en chaoot als ondergetekende…; ik kijk er met plezier op terug. Vervolgens een woord van dank voor de (voormalige) bezetting van kamer 315 – bij mijn weten de enige kamer met een officieel kameruitje. Charmaine (please, learn the Dutch language quickly, so that you can read my acknowledgements), Els, Klaske en Koen (Heineken rulezz!!), het was een bijzonder fijne tijd om met jullie een kamer te mogen delen. Heel veel dank hiervoor. Ik kijk nu al uit naar het volgende kameruitje. Klaske, bedankt voor de mentale steun gedurende de afrondende fase van het proefschrift. Uiteraard wil ik ook de rest van de (voormalig) AIO’s en collegae K & O bedanken voor de onmisbare support tijdens lunch, borrels, congressen etc etc. In alfabetische volgorde: Annemiek, Boukje, Corine, Daan, Esther S., Eva, Femke, Jessica, Jorg, Maartje, Madelon, Marieke, Martine, Rineke, Victorine en natuurlijk de vaste staf die hier niet bij name is genoemd. Een speciaal woord van dank wil ik reserveren voor Maurice Nieuwenhuis. Tijdens je studie en vooral gedurende het jaar dat je als vrijwilliger op onze afdeling werkte hebben we samen
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vele, vele uren in het lab doorgebracht (250 - 300 proefpersonen getest!). Dank hiervoor. Ik wens je heel veel succes toe tijdens je nieuwe functie als promovendus; ik kom zeker een keertje langs voor een bakje koffie. Laat ik het secretariaat niet vergeten. Fenna, Ingrid, Josje, Lucia en Mirjam, heel veel dank voor het feit dat ik altijd bij jullie terecht kon; voor allerlei logistieke problemen, of gewoon voor een praatje. Bedankt ook alle EPP-promovendi voor alle gezellige momenten tijdens en vooral na afloop van de leerzame symposia. Tenslotte wil ik Bert Hoekzema en Pieter Zandbergen bedanken voor de onmisbare technische support tijdens het opzetten van de experimentele studies. Vriendenclub/koffieclub Waarder en omstreken, jullie namen mogen op deze plaats zeker niet ontbreken. Harry, Jantiene en “lieve” Lydia, bedankt voor de open deur, de vele kopjes koffie en de gezellige avonden aan de speltafel. Laten we ze vooral in ere houden. Evert-Jan en Dijanne, Dick en Franka, Jan en Cora, Anton en Dikkie, Rien en Marijke, Wolter en Marja en Johan Bos, allen hartelijk dank voor de morele support bij het schrijven van mijn proefschrift. Rien, wat gaaf dat je als paranimf naast me wilt staan tijdens mijn verdediging! Bouqie. Een simpel woord met een rijke inhoud: Nikolai, Piotr en Vladimir (Matthijs, Martijn en Frederik), wij 4-en vieren dit jaar ons eerste lustrum. Bedankt voor het samen beleven van literatuur, goede muziek en de zuivere smaak van sherrie. Boven alles staat echter onze vriendschap. Ik hecht hier erg veel waarde aan en hoop dat deze nog jarenlang voort mag duren. Frederik, bedankt voor je deskundige advies bij het ontwerpen van de omslag. Matthijs, bedankt dat je ook als paranimf bij mijn verdediging aanwezig wilt zijn. Ik wil natuurlijk ook mijn familie bedanken, in het bijzonder mijn moeder, broer, schoonzusje en de kleine Thomas. Mark, je brede algemene ontwikkeling maakt dat ik veel van je geleerd heb over wetenschap, literatuur, filosofie, religie en de betere films. Ik hoop dat we onze lange gesprekken ook in de toekomst een plaats kunnen geven. Moeder en schoonzusje, bedankt dat jullie altijd voor mij klaar willen staan, zowel dichtbij als op afstand. Het belang hiervan kan niet genoeg onderstreept worden. Laat ik afsluiten met die personen wier namen niet in dit dankwoord voorkomen, maar die toch hebben bijgedragen aan mijn morele en intellectuele ontwikkeling als mens en wetenschapper. Misschien zijn we elkaar uit het oog verloren, misschien spraken we elkaar slechts kortstondig op een congres, op vakantie, tijdens mijn vorig leven in de sport, of waar dan ook ter wereld. Slechts een klein aantal van jullie zal deze woorden daadwerkelijk lezen,
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Dankwoord
wat niet wegneemt dat ik een ieder hartelijk wil bedanken voor het feit dat ik voor kortere of langere tijd lief en leed met jullie heb mogen delen. Zonder jullie had ik nooit die keuzes kunnen maken die uiteindelijk hebben geleid tot de totstandkoming van dit proefschrift.
Johan Groningen, januari 2009
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Curriculum Vitae
Johan Verwoerd was born in Woerden on April 5th, 1975. After being trained as an instructor in the field of sports and recreation, he started his studies of psychology in September 1999. In February 2004 he received his Master’s degree in cognitive psychology at Leiden University under supervision of Dr. Gezinus Wolters. His Master’s thesis described the role of perceptual and conceptual priming in prospective memory tasks. In September 2004 he moved to Groningen for a PhD at the department of Clinical and Developmental psychology under supervision of Dr. Ineke Wessel and Prof. dr. Peter de Jong. This resulted in the present dissertation. Since Oktober 2008, Johan has extended his work in Groningen as a post doctoral researcher.
Publications Verwoerd, J., Wessel., I., de Jong, P. J., & Nieuwenhuis, M. M. W. (in press). Preferential processing of visual trauma-film reminders predicts subsequent intrusive memories. Cognition and Emotion. Verwoerd, J., Wessel, I., & de Jong, P. J. (in press). Individual differences in experiencing intrusive memories: The role of the ability to resist proactive interference. Journal of Behavior Therapy and Experimental Psychiatry. Wessel, I., Overwijk, S., Verwoerd, J., & de Vrieze, N. (2008). Pre-stressor cognitive control is related to intrusive cognition of a stressful film. Behaviour Research and Therapy, 46, 496513. Verwoerd, J., de Jong, P. J., & Wessel, I. (2008). Low attentional control and the development of intrusive memories following a laboratory stressor. Journal of Psychopathology and Behavioural Assessment, 30, 291-297. Verwoerd, J. & Wessel, I. (2007). Distractibility and individual differences in the experience of intrusive memories. Personality and Individual Differences, 42, 325-334.
Verwoerd, J., Wessel., I., de Jong, P. J., & Nieuwenhuis, M. M. W. (2008). Pre-stressor interference control and intrusive memories. Manuscript submitted for publication.
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