Clarification and Defence

Australasian Journal of Philosophy

Vol. 78, No. 2, pp. 248-254; June 2000

THE UNITY OF CONSCIOUSNESS: CLARIFICATION AND DEFENCE Tim Bayne

I. Introduction In 'The Disunity of Consciousness' [9], Gerard O'Brien and Jon Opie argue that human consciousness is not synchronically unified. They suggest that the orthodox conception of the unity of consciousness admits of two readings, neither of which they find persuasive. According to them, 'a conscious individual does not have a single consciousness, but several distinct phenomenal consciousnesses, at least one for each of the senses, running in parallel' [9, p. 387]. They call this conception of consciousness the multi-track account. I make three points in reply: (1) O'Brien and Opie's characterisation of the orthodox conception of the unity of consciousness is problematic; (2) their arguments in support of the multi-track account are unpersuasive; and (3) the phenomenon of inter-sensory integration suggests that O'Brien and Opie are wrong to claim that 'the only sense in which it is con'ect to talk of a 'unified' consciousness...is that in which the representational contents of the various components coincide' [9, p. 387].

II. The Orthodox Conception O'Brien and Opie reject what they claim is the common sense, and philosophically orthodox, assumption of the unity of consciousness. Unfortunately, they are not as clear as they might be about what exactly they mean to deny when they say that consciousness is not unified. They say that the unity of consciousness is the claim that at any one time a subject's phenomenal consciousness is a single thing [9, p. 378][ This could mean several things. First, it could mean that all of a subject's conscious experiences are part of a single phenomenal field: they are part of a single global experience. This, it seems to me, is what is normally meant by the claim that consciousness is unified [see e.g. 4, 6], but it does not seem to be what O'Brien and Opie mean by it. O'Brien and Opie seem to take the claim that a subject's consciousness is a single thing to mean that various contents of a single person's phenomenal field are made conscious by a single consciousness-making mechanism [9, pp. 379, 388, 390]. To reject the claim that consciousness is single-track is, they claim, to reject the claim that there is a single consciousness-making mechanism or process underlying all of a subject's conscious states. Thus, the multi-track model of consciousness which O'Brien and Opie endorse is the claim that the various contents of a single person's phenomenal field depend on ' . . . t h e combined output of myriad consciousness-making mechanisms distributed right across the cortex... On this concept i o n , . , a conscious individual does not have a "single consciousness", but several distinct phenomenal consciousnesses, at least one for each of the senses, rttnning in parallel' [9, p. 387]. O'Brien and Opie are, of course, at liberty to use terms in whatever manner 248

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they please, but this use of 'the unity of consciousness' does seem to me to be in danger of engendering unnecessary confusion. Having said that, I will follow what I take to be their usage. In the first part of their paper, O'Brien and Opie argue that single-track models can take two distinct forms: 'as a serial stream containing only one contentful element at a time, and as a single experience embracing multiple contents' [9, p. 379]. O'Brien and Opie dub this first form the monophonic model, and liken it to a 'solo performance, in which a single voice is all we hear' [9, p. 379]. They dub the second form the polyphonic model, and liken it to polyphonic choral music, in which there are two or more simultaneously active voices. According to the polyphonic model, 'consciousness incorporates a number of distinct contentful elements (e.g. from a number of distinct modalities) fused into a single experience...the brain binds together a collection of informational elements into a single conscious experience' [9, p. 379]. O'Brien and Opie also attach a coherence condition to the polyphonic model of consciousness. This condition involves the claim that all the representational contents of a single consciousness at a time must be compatible or coherent [9, p. 392]. There are two problems with this account of the orthodox conception of the unity of consciousness. First, the distinction between the monophonic and the polyphonic models is ill-defined. Second, one can hold the single-track model of consciousness without endorsing the coherence constraint. I take these two issues in turn. In order for the monophonic model to be distinct from the polyphonic model we have to be able to individuate contents. The problem here is not (primarily) epistemological, but metaphysical. What is the difference between having a single experience with a single content, and having a single experience with multiple contents? How should we count contents? O'Brien and Opie give us no advice at this point. Their argument against the monophonic model takes for granted the thesis that if an experience is multi-modal, then its contents are multiple. Unfortunately, O'Brien and Opie provide no argument for this thesis, and it is not at all obvious. In fact, nothing about counting contents is obvious. Is seeing that a brown cat is lying in the sun an experience with one content or two? Is being proprioceptively aware of one's entire body a single experience with a single content (feeling one's whole body), or is it a single experience with multiple contents (feelings one's right leg, feelings one's left leg, and so on)? I am not claiming that these questions are in principle unanswerable, but they are very forbidding. Consider now the coherence constraint on single-track accounts of consciousness. O'Brien and Opie argue that pathologies of the visual system and the phenomenon of sypthesthesia suggest that both intra and inter-model incoherence is possible, but they need not have looked so far for examples of incoherence with a single consciousness at a time. Consider the case of the straight stick that looks bent in water but feels straight. This seems to be a straightforward example of intermodal representational inconsistency. In addition, one can entertain (and probably even believe) an incoherent thought. As O'Brien and Opie point out [9, p. 385], certain psychologists have suggested that one of the functions of consciousness is to ensure coherence between the contents of simultaneous conscious states. But it is surely unfair to paint all single-track accounts of consciousness with the colours of a few. This is not a merely terminological point. O'Brien and Opie claim that cases of incoherent consciousness 'create a problem for the single-track polyphonic model of consciousness' [9, p. 393]. This claim does not go through unless we

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build the coherence condition into the definition of the single-track model. Once the coherence condition is decoupled from the polyphonic model, the problem O'Brien and Opie raise is dissolved. III. O'Brien and Opie's Arguments for the Multi-Track Model O'Brien and Opie give two arguments in support of the multi-track model. The first argument is phenomenological; the second draws on neuroscientific evidence. I begin with the argument from phenomenology. Close attention to our instantaneous experience reveals it to be a complex aggregate of many elements--a sum of relatively independent parts. Right now, for example, as you concentrate on these sentences, your phenomenal experience is a very complex affair: visual experiences (the shapes, textures and colours of these sentences, together with other objects in the room), auditory experiences (noises from outside the room), tactile experiences (the chair pressing against your body), proprioceptive experiences (the position of your limbs), and understanding experiences (what these words and sentences mean), to name a few, together comprise your instantaneous phenomenal field. These parts are independent because they are distinguishable in experience, and because any one o f them can be removed or lost without affecting the others (try closing your eyes for a moment) [9, p. 387; emphasis in last sentence mine]. In fact, there are two arguments here. The first argtunent is that consciousness must be multi-track because one can distinguish different elements within one's phenomenal field: Call this the distinguishability argument. O'Brien and Opie are surely correct to claim that one can distinguish various features within one's phenomenal field, but I do not think that this provides support for the multi-track theory. Call those material things (e.g. neuronal events) that are the bearers of conscious contents the vehicles of consciousness. It is vital to distinguish between the unity of consciousness at phenomenal or introspective level and the unity of consciousness at a vehicular level [see 4 and 7]. Consider two conscious contents, C1 and C2, that are united in one phenomenal field. It is one thing to suppose that C1 and C2 are distinguishable in experience, that they are phenomenally separable. It is another thing to claim that C1 and C2 are produced by distinct consciousness-making mechanisms. There may be some deep connection between the phenomenal distinguishability of conscious contents and their neurophysiological independence, but such a connection would have to be argued for. There does not seem to be any quick and easy route from phenomenology to the neuronal structure of consciousness. For all we know, contents that are indistinguishable in experience may be produced by different consciousness-making mechanisms, while a single consciousness-making mechanism might produce contents that are phenomenally distinguishable. I call the second phenomenal argument that O'Brien and Opie give for the multi-track theory the subtraetability argument. The idea is that a conscious content is produced by a distinct consciousness-making mechanism if one can subtract the conscious content in question without affecting the rest of one's phenomenal field. Thus, they claim that one's visual contents involve an independent consciousness because closing one's eyes does not affect the rest of one's conscious states. The neurophysiological argument that O'Brien

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and Opie give builds on the subtractability argument. As they point out, there are a number of neuropathologies involving deficits that are phenomenally localised. For instance, it seems to possible to lose the visual perception of motion without this deficit affecting other aspects of one's phenomenal field. The subtractability argument is no more persuasive than the distinguishability argument. As O'Brien and Opie themselves point out, 'a single--track theorist could argue that the deficits in consciousness experienced as a result of localised cortical ablations [or from closing one's eyes!] are due to the relevant contents never being passed on to the executive system' [9, p. 391]. Indeed, they could. O'Brien and Opie find such an explanation unsatisfactory: 'the multi-track reading does seem more parsimonious, given that it doesn't require an additional executive consciousness-making system over and above the distributed information-processing modules that we know to exist in the brain' [9, p. 391]. But in fact it is the multi-track model that is unparsimonious. The multi-track model needs more than mere information-processing modules, it needs a mechanism/process that bestows consciousness upon some of that information. In fact, unlike the single-track model, the multi-track model posits multiple consciousness-making mechanisms or processes--that's what makes it multi-track! Finally, it is worth noting that if the distinguishability and subtractability arguments were sound, they would seem to entail that a normal human being has an indefinite number of consciousnesses, and certainly more than one per sense modality. Consider the contents of a typical tactile field. Right now I can feel the carpet beneath both feet, I can feel the pressure of the chair on my behind, and I can feel the computer keys under the fingers of both hands. Each of these experiences is phenomenally distinguishable. In addition, it seems that each of these experiences can be lost without affecting the others. Losing the tactile sensation in my right foot no more affects tactile sensation in my right hand than losing my eyesight does. According to O'Brien and Opie's phenomenal criteria for individuating consciousnesses particular parts of, say, one's tactile field would count as individual consciousnesses in their own right. Similar points could be made regarding proper parts of other sensory modalities. What is there to be said for the claim that 'the consciousness' of a normal human being is multi-track, that a single phenomenal field may be subserved by a number of distinct consciousness-making mechanisms? First, note that the multi-track model is not supported by the fact, if it is a fact, that there is no consciousness module, no one point in the brain through which information must pass in order to become conscious. Such a consciousness 'Grand Central Station' is one form that a single-track model might take, but it is not the only such form. In recent years, a number of theorists have suggested that consciousness involves distributed time-locked neuronal oscillations [see e.g. 2, 3, 12]. Are such theorists multi-track theorists? Not necessarily. As far as I can tell, most of these authors are silent on the question of whether all the elements of a single phenomenal field are made conscious by being time-locked in a single global activation pattern. Insofar as we understand the neural basis of consciousness, it would seem that a deep contrast between inter-modal integration and intra-modal integration is rather difficult to draw. There is no reason to suppose that the mechanisms of binding the output of different sensory modalities into a unitary experience of multi-modal objects, are, in essence, any different from the mechanisms that bind the output of various discriminations within a modality. Just as the brain binds information derived from different modalities into (usually) coherent

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multi-modal representations of objects, so too it binds colour, spatial, figure-ground, texture and other discriminations into a (usually) coherent visual perception of an object. Intra-modal binding can break down, so too can inter-modal binding. If one holds that the senses are distinct consciousnesses because they involve distributed mechanisms that produce conscious representations that need to be bound together, then one should conclude that the neural mechanisms that makes, say, figure-ground judgments also supports a distinct consciousness. However, Jon Opie points out (personal communication) that neuroscientists Zeki and Bartels argue that there are distinct consciousness-making mechanisms in a normal subject [see 13, 14]. Examining the justification for this fascinating claim would take an entire paper in its own right, and I must restrict myself to one brief observation here. Suppose that Zeki and Bartels are correct when they claim that visually-perceived motion and colour each have their own conscious correlate: once conscious, motion and colour must be bound together in some way in order to be part of a single phenomenal field [13, p. 1584]. What happens when this mechanism of binding breaks down? Is there a conscious experience of visual motion and colour that is not part of a larger phenomenal field? Such a case would be even stranger than visual agnosia, in which the patient perceives the various features of an object but cannot bind them into a unitary conscious percept. In visual agnosia, both vision and motion are parts of the subject's larger phenomenal field-the subject is aware of them both (albeit not as part of a single visual object). In the scenario that I am suggesting, although there is colour consciousness and motion consciousness in the subject's brain, there is no larger experience of which colour consciousness and motion consciousness are parts. This is not intended as a knock-down drag-out objection to the Zeki-Bartel's proposal, but it does reinforce just how radical a proposal it is. IV. Intersensory Integration Finally, I turn to O'Brien and Opie's claim that [t]he sense in which it is correct to talk of a 'unified' consciousness, one incorporating elements from the different modalities, is that in which the representational contents of the various components coincide: we see our bodily parts in positions w e feel them; we hear sounds emanating from objects in the direction we see them; we taste the food that we can f e e l in our mouths; and so on. [9, p. 387] I am far from convinced that one can legitimately argue from the mere coincidence of representational contents to the fact that they have been produced by distinct consciousness-making mechanisms. As far as I can see, representational coincidence does not support the multi-track model, and representational integration does not support the single-track model. Be that as it may, it seems to me that the phenomenon of inter-sensory effects indicates that the contents of our various sensory modalities do not merely coincide. We do not have to look far for examples of such effects. As anyone who has recently had a cold knows, one's olfactory experiences intimately affect one's gustatory experiences. Consider also ventriloquism. The visual experience of seeing the ventriloquist influences the direction from which his/her voice is heard. Shutting one's eyes

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actually changes the content of one's auditory experience. The McGurk effect is another illustration of the fact that what we hear and see does not merely coincide. In their seminal study, McGurk and MacDonald [7] dubbed the sound of someone saying [ba] onto the lip movements for [ga]. Normal adults reported hearing [da]. Note that this is what subjects reported hearing. Again, visual information is involved in forming an auditory percept. 'By merely closing the eyes, a previously heard [da] becomes [ba] only to revert to [da] when the eyes are open again' [7, p. 747]. The McGurk effect is robust under both repeated exposure to the stimulus and to knowledge of the illusion involved. More recently, Jordan and Bevan [5] discovered that the visual perception of facial orientation can affect the accuracy of auditory speech report with incongruent audiovisual speech stimuli. Such inter:sensory effects are not merely restricted to visual and auditory experiences. What we see influences what we feel, and vice versa. Patients who have had limbs amputated often experience a phantom limb. Sometimes this limb feels paralysed, a feeling which can cause severe discomfort. In an effort to relieve this discomfort, V. S. Ramachandran created a visual illusion that the arm had come back [10, 11]. The patient puts his normal arm, say, the right one, in front of a mirror and 'puts' the phantom arm on the other side of the mirror. He then moves his right arm until it is superimposed on where the phantom limb is felt to be. In seeing his right arm move, the patient feels his phantom arm move. Information provided by vision informs the content of the patient's somatosensory experience. (For other examples of inter-sensory effects see [1] and [4].) One might claim that the subjects of inter-sensory effects are misreporting their phenomenology, l Perhaps subjects in the McGurk study actually do hear [ba] even though they report hearing [da]. But it seems to me that one would only find this position attractive if one was already committed to the claim that the outputs of various modalities merely coincide. V. Conclusion I have argued for three points. (1) O'Brien and Opie unfairly lumber single-track models of consciousness with a coherence condition. (2) Neither the distinguishability nor the subtractability arguments support the multi-track model, and even if they did, they would lead to the highly counter-intuitive conclusion that the tactile awareness I have of my right big toe might be a distinct consciousness. (3) The widespread phenomenon of intersensory effects suggest that many of the contents of one's phenomenal field do more than 'merely coincide'. I haven't provided a knock-down, drag-out proof that consciousness is unified (in O'Brien and Opie's sense), but I have tried to show that O'Brien and Opie's attack on the unity of consciousness--although thought-provoking--is unsuccessful. 2

University of Arizona

Received: March 1999 Revised: October 1999

i I thank Chris Maloney and an anonymous reviewer for this point. 2 Thanks to David Chalmers, Josh Cowley, Chris Maloney, Dave Truncellito and especially Jon Opie and an anonymous reviewer for useful comments on previous drafts of this paper.

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The Unity of Consciousness: Clarification and DeJence REFERENCES

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