THE BEHAVIORAL AND BRAIN SCIENCES
(1980) 3, 373-415
Printed ln the United States ofAmerica
Against direct perception ...
28 downloads
559 Views
6MB Size
Report
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form
THE BEHAVIORAL AND BRAIN SCIENCES
(1980) 3, 373-415
Printed ln the United States ofAmerica
Against direct perception S. Ullman Artlffclstlntsi/IQfHico Laboratory, Massachusetts lnst/lute of Technoloay, Csmbridae, Msea. 02139
Abstract: Centr(d to contemporary cognitive science is the notion that mental processes involve comptltntlon� defined over internal representations. This vie w stands in sharp contrast to the "direct approach" to visual perception and cognition, whose most prominent proponent has been J.J. Gibson. In the direct theory, perception does no t involve computations of any sort; it Is the result of the direct pickup of available information. The publication of Gibson's recent book (Gibson 1979) offers an opportuni t y to e�Camine his approach, and, more generally, to contrast the theory of direct perception with the computational/representational vi ew. In t he Brst part of the present article
(Sections 2-3) the notion of "direct perception" is examined from a theoretical standpoint, and a number of objections are raised. Section 4 is a the prohlem of perceivi11g the three-dimensional shape of moving objects is examined. This problem, which has been extensively
"case study":
studied within the Immediate perception framework, serves to illustrate some of the inheren t shortcomings of t hat approach. Finally, in Section 5, an attempt is made to place the theory of direct perception in perspective by embedding it In a more comprehensive framework. Keywords: nrti6ciol intelligence, compu tational models; direct perception; ecological optics; Gibsoni an theory; Information pickup; visu al
repr esent ation
1. Introduction Gibson's recent book (Gibson 1979) is his third in thirty years devoted to the development and exposition of the theory of direct perception. The interest in Gibson's Infl u enti al theory has often transcended the interest in perception alone. One reaso n is that his approach to cognition in gene ral stands in sharp contrast to another prevailing approach, the com puta tionalfrepre.�ntationul one. According to the latter view (of which generative grammar, theories in cognitive psychology, and some of the work in artinci al intelligence are current examples), mental processes involve computations defined over internal representations [see Pylyshyn: "Computation and Cognition " BBS 3(1) 1980]. In the direct theory of perception, mediating constructs arc unnecessary, and in the early stages of his theory Gibson expres.�ed the hope t hat the direct approach, if successful, would extend to other areas of psychology as well: (The theo ry of di rect perception] "... if successful, will provide a basis for a stimulus-response psychology, which oth e rwise seems to be sinking in the swamp of i nte r vening variables" (Gibson 19C:i0, p. 70 1). In tht� pn.·-�m1t puper the concept of direct visual perception (henceforth uhhmviuted as DVP) will ue examined. The overall plan of the puper is us follows. First, a brief descri ption of the concept will be given.This is only intende d t o sta te the main points o f relevance t o the ensuing discussion, not to summarize Gibson's theory. For a comprehensive presentation of t he theory in different stages of its evolu tion, see Gibson (1950, lHfifi, 1979). These books describe differen t approaches to direct perception, not all of which (especially the 1950 formulation) are r etained in the current formulation of the theory. The notion of DVP is then examined pri m aril y from a theoretical standpoint (for discussions of empirical evidence against direct perception see Epstein & Park (1964}; Gyr (1972a,b); Epstein (1977). Section 2 examines what I t means for perception t o be di rect, and Section 3 raises g eneral arguments against the plausibility of direct perception. Section 4 is a "case study": the application of the theory to a particular pmblem, the perception of moving objects, Is discussed to highli�ht some of t he inherent shortcomings o f e 1980 Csmbridae
UniVersity Pross
0 140-52SX1801030373-43$04.00
the direct approach. Finally, Section 5 tries to put the DVP approach In the perspective o f a more comprehensive framework, and to Identify some of its missing ingre dien ts.
1.1 Dir ect visual perception Visual perception and its relation to the struct u re of the en vironment are viewed by the theory of di rect visual perception as a sequence of two direct and una mbig uous mappings: "stimulation is a fun ctio n of the environment, and perception is a function of stimulation" (Gibson 1959, p. 459). The first mapping is between various aspects of the environ ment and some spat lotem poral patterns of the visual array, sometimes called ''higher order stimuli" (the more recent formulations of the theory emphasize the transformation� ancl invariants in these patterns). The second mappin g is between stimuli and percepL�. When an observer moves in the environ· ment, some aspects of the light array that reaches his 1!-ye.� change , while others remain unchanged. The information in the.se trunsformation� and i nvariances specifies the environ ment: its layout , changes of layout, and the oceurrencrt•nt kind of relation. The term ''immedint
Journtll will bB
con/1/dttred f(1f pubf/ost/® In s lster IBBue ss
Continuing commentary on th/s ari/Die.
by o. J. I'Sraddlck
Exp•rlmenfalP•ycllology, Unlwrtllty ol C•mbrld�e. Cambrtdf/• CB2
O•�rtm�to/
sEa, Etlflland
Direct perception: an opponent and a precursor of computational theorlea The theory of "direct Visual perception" can be seen
as
a
metalheory;
lhat Is, a theory of what would conslilule an adequa1e theory of visual perception. In
my view, the arguments marshaled a gains t it in the se
terms by Ullman are quite compelling. A theory (however complete or accurate) as to which properties of the proximal stimulus are
to
yield our perceptions is
extracted
not good enough. It is not good enough
because something better promises to be available, accolJnt of such relationships plus an account
of
namely
an
the mechanisms
n. Furthermore, what is promised s i not which permit the extractio
just
perhaps not at all) a moleetAar accOlX1t of 1he mechanisms at the level of single-unit physiology, but a clst inctivcly psychological or computational account which would organize our unders1andng of them. Ullman's quotation "the retinal mosaic Is sensitiveto transformations as such" snows that even Gibson found It dlfflcult to eschEIIN an account thai 1eant on physiological mediation in this Instance, an (or
-
account that Hew in the face of retinal physiology even as It was known
In 1957.
However, this statement does raise an Issue lhat is important
and far from resolved for representatlonal 1heories, whether physiolog-
THE BEHAVIORAL ANOBAAIN SCENCES (1980). 3
381
Commentary/Ullman: Against direct perception ical or computational: That is. the information about transformations is
perceptual processes revealed by lhasa laboratory demonstrations
clearly present in the pattern of retinal activity: representational theo·
exist only to be activated in such rare and highly artificial circum
risls have to argue thai it is not explicit at that level, but that it may
stances.
become so in some higher-level representation that is derived from the
Actually, there is a continuity between the perception of the Ames
retinal representation by computational processes. The Idea or infor
demonstrations or the Mach illusion end that of Gibson's moving
mation explicit in a representation has not to my knowledge been
observer in a richly textured environment. The completeness of the
rlgourously defined. Certainly, it has not been established that informa
information in Gibson's optic array depends on what he cells "ecologi
t in any particular, tion about a stimulUs variable has to be explici
cal laws" -that is, certain constraints on what could eKist in the
obvious sense at some level of representation in order that this
envronmenl to produce the optic array (for example, that objects have
variable should be available to conscious perception, or that it should
continuous surfaces). In perceiving the Ames room, much m()(e
drive verbal or motor behaviour. Barlow ( 1972), for example, has
specific constraints are apparently implicitly accepted by the percep·
argued that perceptually relevant variables are represented by the
tual system (such as that artificial surfaces meet at right angles). Yet
activation of ever smaller and more specific populations of neurones at
these specific constraints do not act to prevent us seeing trapezoidal
higher levels ifl the visual pathway; but this proposal is frankly specula
enclosures correctly when we have richer stimulus information, as from
tive. It is dillicult to lind arguments against the opposite view, that even
motion parallax or binocUlar disparity. It seems that the perceptual
uniquely significant stimuli could be represented at the highest levels of
system will always adopt a wor1s with
speculative - by mathemaicians t which try to derive Gibsonian invar
the problem. The task at hand 101' Ullman is to demonstrate that the relation between stimAus and percept is not one-to-one. That is. he must show that with lhe stimulus (optic array) constant, changes
iants from the hierarchy of known, or hypothesized, neurophysiological processes in the brain. (See, for example. Blaivas 1975; Hoffman
1966.)
internal to perceivers have an elfecl on perception. Experimental or real life instances of this kind are actuaUy not hard to provide. For e)(ample. an article by Gyr, Wiley and Henry (1979) in this journal discussed numerous cases in which. with visual input constant. changes in internal feedbacks of the motor system produced changes in perception. In tact. based on reported experimental findings. a theory of perception was outnned in which motor processes are
by Frederick Haye.,.Roth nt.R•ndCorpor•tk>n,s.ttt•Motrlce, C•HI. 11040tl
Mediating the so-called Immediate processes of perception
cf�rectly involved in perception. More precisely, motor events - via
Ullman's reaction against direct perception
efferent copies. and the like (Evarts 1971) - produce internal e)(pecta tione about external visual afference befo1e movements (of the eye,
substantial understanding of computation and vision. I would readily
theories rests on a
ally myself with his concluslon that perception requires representations
and the like) producing such alterence are made. The theory argues
end mediating processes; hence. so do theoretical explanations of
that internally prodUced expectations are then matched with external
perception. However, t befieve Ullman has presented a weak argument
atference. and that it is the nature of this relation which determines the nature of the percept. In such a model It is clear that the connection between the external stimulus and the percept cannot be one-to-one. The logic of the above approach is, on the whole, not the logic
against the Gibson position that somewhat confuses the issues. In particular, he attacks the Idea of immediate perception by first assign
Ing only two possible meanings to the concept of "immediate" and then rejecting "immediate" perception as implausible.
employed by Ulman. Only in his brief review of the ilusions first
The weakness or his argument stems trom the fact that his two
discovered by Ernst Mach does Ullman explicitly and empirically
suggested meanings do not exhaust the space of plausible meanings,
demonstrate that the perception of structure and motion might be a
so one could simply dismiss his argument as Illogical. The first sense of
fu-1ction of two (presumably relatively Independent) variables: the
"Immediate" he considers would treat perception as immediate when
incoming image and what Ullman calls the current interpretation of the
aft preperceptual processes lie outside the perceptual system itself. He
observer. Here then is a critique ot direct visual percepUon based on
rightly rejects such an extreme interpretation. In the second case, he
the fact that the relation between external visual stimulus and percept
suggests that "immediate" should describe any system with nonde
is not one-to-one. Nowhere else in Ulman's paper, however, is this
composabte !unctions. Hence. by showing that several exemplary
particular logic elq)ficitty developed further.
perceptual functions reveal Intervening variables and subprocesses.
Instead, UUman's primary tactic in criicizing t direct perception seems to be to argue that the perceptual process is decomposable and that
he claims to have rejected the other plausible interpretation of "imme diate" perception.
the physiology or psychology underlying perception is probably quite
This approach to criticizing Gibson, while somewhat flawed, does
complex. To this approach he devotes a great many pages and
reveal weaknesses in Gibson's theory as well as strengths in UHman's
examples. However, it seems to this commentator that such an
conceptual framework. unman has attempted to define and operation
approach to the problem can only lead to wasted effort. For regardless
alize "immediate perception," because Gibson has failed to do so
of how complex the physiological or psychological mechanism may be.
himself. We should realize the difficulty this presents to a critic like
the fundamental question remains whether this mechanism adds
Ullman: he must first add enough straw to the phantom to have a straw
information to the perceptual process or is merely a highly sophisti
man worthy ot destruction. Ullman, on the other hand, has placed his
cated resonator to external and complete information residing In the
Interpretation ot direct perception (or "ecological optics") on the
top
optic array, which is Gibson's daim. Certainly Gibson himself recog
of a llvee.e .J vet theory that also includes information processing and
nized the physiology underlying perception as qui t e complex.
physiological levels of analysis.
In this connection, Ullman goes into an e)(ampie borrowed from the
A stronger attack on the theory of "Immediate" perception would
calculator. He suggests that if arithmetic were to be done with such a
rest on the necessity ot mediating processes. I have argued previously
mechanism via table-lookup, the process from the Input of an arith-
that perception requires specific kinds ot mediations, in particular,
886
TI-E BEHA'VIORAL ANO SAAI'I SCIENCES (1980),
3 ·-
Commentary/Ullman: Against direct perception memory or computational ''state." My own criticism of the immediate t lheory (Hayes-Roth 1977) was formulated in reaction to perc epion Turvey's version of the theory (Turvey 1977). Because both Gibson and Turvey adhere to similar positions, my previous comments bear repeating in this context. Immediate perception theories eschew the use of mediating concep tual structures and associated recognition and classification alogrithms. Computer scientists now commonly believe that the recog nition and classification of even simple graph structures necessitate the representational power of the ftrst-order predicate calculus, plus the computational power of serial pattern-matching procedures that successively test alternative Interpretations of stimuli by matching perceived objects to variables In the general pattern templates:. Thus, perception of even some simple line drawings requires just the sort ot mediations that n i formation-processing theories postulate. For a more detailed explanation, I refer the reader to my previous article (Hayes Roth 1977), but I will summarize the situation in the folowing paragraph. The need for mediation (memory, sequential algorithms) arises from the impossibaity of directly detecting whether an arbitrary paltern occurs in the visual field. To delect such a pattern (for example, a triangle or a regular graph of n vertices), the Visual system must consider alternative sets of vertices and lines to check whether aP the necessary, pattern-defining conditions hold. A priori, the environment may exhibit an arbitrary pattern in myriad different configurations. To guarantee immediate perception of these would require ettectlvely prewiring as many logic networ1<s as possible configurations. Because the number of possible configurations increases exponentially with tile number of lines in the pattern and in the visual field, no a priori bound on the complexity of such networks exists. Moreover, the number of networks required grows so rapidly that immediate perception appears infeasible: neither natural nor artiftciat systems can immediately reCOQ· nize patterns of arbitrary complexity. UUman cited the prohibitively large number of stimulus patterns in a similar argument to support the need for processes and rules of formation over direct coupling of input output pairs. As an alternative to more and more networking, we indirect realists postulate more and more information processing. Many problems that appear extremely complex in one system become tractable when memory, appropriate representations, and matching procedures enter the picture. As a simple example, most naflKal languages do not conform to the very restricted syntactic structure of regular grammars and, as a consequence, these languages cannot be recognized by devices without pushdown stacks (or equivalent memory apparatus). Thus, if we should demand arbilrarBy that theories of speech percep· lion exhibit "immediate" recognition, or even if we permitted state memory but disanowed register memory, we could not model �ech understanding. Gibson and his folowers have, I believe, adopted a similarly untenable position in the domain of visual perception. Since publishing my earlier critique, I have developed with the help of my colleagues at Rand a possible compromise between the two theoretical positions. Not all of the immediate perception theory can be salvaged, because it is untenable in the ways previously mentioned. The new theory retains its emphasis on immediate coupling between simulus t and Interpretation, but it postulates a model-based feedbacl< loop to mediate this process. Basically, an observer builds an internal model of his environment and perceives by fitting the model to the situation. Thus, from the point of reference of the viewer, the model simulates the features of the scene, which simply confirm the interpre· tation. From the environmental reference paint, the objects in the scene adjvst the parameters of the modal to make it conform to the observable features. For this overall scheme to work, a person in 8 new situation would require some time to construct the initial model. The model would embed possible alternative values of as-yet-unspecl· lied features (such as the sex of the observed person, the shape of his nose, and his exact orientation) as feasible parameter value ranges. Finally, the parameter tuning process would require a feedback loop that selected correct parameter values. (Similar analog-to-digital conversions typically accomplish this by successively testing the goodness-of-fit between the possible model parameters and incoming
signals.) The feedback loop could approach instantaneous reaction times in particular doma.ins with suitable machine architectures. In summary, tne Gibsonians have emphasized correctly the value of information that derives from spatial and temporal organization in the environment. They have erred by asserting dogmatically that this infom18�on could directly support perception without mediation. Ullman has proVided several empirical counterexamples to theirtheory. I have objections and have described briefly tried to elaborate his theorecal it an apportunity for theoretical synthesis that we are currenUy attempting to develop.
by Geoffrey E. Hinton
Pto(Jrlnt In �tlv• Scl•nce
C.()()(l. C•nler for Hu,..n lnfomr•lion PrtJCo.. l ng,
IJttWea/fY tJI C.IIOfnt•. S•n or�o, L• Jo/111. C•llf. f12C»3
Inferring the meaning ot direct perception
Everything Ullman says is very sensible, but It Is not entirely satisfying, because it tails to answer the most puzzling QUestion about J. J. Gibson's views: how could someone who says so many sensible things about perception maintain that perception Is direct and does not involVe computation? Either Gibson is baing very slay or there is a deep misunderstanding about wllat it means tor perception to be direct. I shall try to show that although Gibson expressed his views in a contusing way, their main content is not reaMy In conflict with the computational view that there is more to perception than meets the eye.
Ulman's e)(ample of doing simple arithmetic s i very helpful in elucidating one aspect of what Gibson means by direct perception. There is 8 level of description at which doing arithmetic involves a number of steps, but recalling that 3 plus 4 makes 7 is a single step. lhis level corresponds quite well with the "naiVe" or "everyday" psychology that we use to get tnrough the day. People can describe a sequence of steps that they went through in performing a complex addition, but they cannot generally giVe any decomposition of the step of recaling that 3 plus 4 makes 7. It just does. Similarly they cannot say l)ow they saw the digit 3. This is an unanatysable single step in everydaypsychology. One thing that Gibson means by saying percep tion is direct is that un�ke doing arithmetic or following an argument, it dOeS not n i volve a sequence of mental operations of the kind that we can introspect upon or instruct others to perform. This seems to be right, and it has important implications for any theory of perception. It suggests, for example, that Helmholtz's analogy between vision and conscious inference should not be taken too Ateralty. Digital computers have influenced theories about human perception and cognition in two rather different ways. By proVlding a medium for building wori
for higher-order stirr.�lus determinants ol perceplion (see wanaoh 1948; Btown 1931; R.ock & Ellenholtz 1959). Whether ooe lends lo agree or disagree with , no one studying perception can aford to iglore them; it t Gibson's formulaions
claim that specifying these stimulus factors, whatever they are, is a
one wishes to pul forth an ind�ect lheotY of a given phenomenon one lll\JSt fhl
higher-order stimulus attribute, such as a texture gradient. is uncov ered; and because of an isomorphism between the stimulus attribute (lor example. steepness of texture gradient) and percept (for example, slope of plane with respect to the line of sight) the correlation i s at least a plausible theory of the perception. Predictions can be made and
demonstrate why a lirecl lheory is inadequate to 8JCI)Ialn it. Ulman's target article
sulficient explanation of the percept. 8
and this open peer commentary on il should be thought of as a tribute to Gibson.
theory must successfully circumnavigate the
Whose recent dealh we ah mourn. He woUld suretyhave welcomed and thoroughly
Brief mention should be made ol certain other difficulties with direct theory. Such
8
problem of perceptual organization. The attempt to account for what
enjoyed the dobale.
goes with what. or what is figure and what Is ground, by implying that In
2. Ullman wriles as il lhe computatiOnal/represenlallonal theory were the llrst
the conditions of daily life - In contrast to the impoverished pictures
or only cognitive appoaeh lo cllaUenga or offer an alternatiVe to the direct theory
and fine drawings of the laboratory - this information is directly avail
t and does not repesent Whereas tt is in factrelallvolynew In lhe field of percepion
able simply wal not do. For example. It might be said that depth
experimental psychology. He !hereby iglotes enemalives that go back at lea.st to
information of various kinds suffices to eslabtish figure and ground because it tells us what is In front and what behind. But - to give just
Helmholtz, alternatives that stll have various cont9fl1)otery representatives within
one argument against this claim - the depth relations among objects that are very far away. so as to be beyond the range of binocular information or Information derived from observer motion. must be determined by pictorial information which. by deftnition. is no richer than that which can be given in pictures. II nonplctorlal depth informa
the fteld of psychology. 3. Since I am seeking lo analyze any kind ot drecl theory and nol only Gibson's, I do nol feel obliged to be confined to his particular way of relerring to the stimulUs, which in any event changed in lhe courae of his writings. Tnus. 1 wil use terms such as "stltrutus," "stimulus information," "retinal image," or
"Information pick up," more or less i'llerchangeably. 4. In
ion recognit
ol the lact lhal we ollen need to leam lo d'oscriminate
carv�ot directly specify what is figure and what is ground, then t ion certain internal organizational principles applied to the content of the
similar-appealing members of lhe same category or clah
scene's image must do so just as in the case of pictures and line
James Gibson, Eleanor Gibeon, and their associates tacitly acknOWledge 11\allhe
drawings. In fact, certain organizational principles- for example those i terposition - even determine what is in front of what under governing n
ty of stimulus inlorma6on does not always provide a aufllcient e,.Pana avallabii
such conditions. I suspect that the "experience error" is often respon sible for the belief that the phenomenal organization is directly given by the stimulus (Kohler 1947). A direct theory is unable to deal with the many cases in which one perception depends upon another. This point is relaled to the forego ing one in that often a particular perceptual outcome depends upon how the incoming stimulus array is organized. Thus, for example, there
is reason for believing that only if one region is ftgural will the deletion
and accretion of elements of a neighboring one signify its occlusion and disocclusion (see Kaplan 1969, but also Rock & Gilchrist 1975); only if an illusory contour becomes a phenomenal reality will certain other perceptions that depend upon such a contoured region emerge or disappear as the case may be, and so forth. The example UNman gives of the Mach book viewed by a moiling observer fo!lowing the
from one another.
llon ol perception (see E.J. Gibson 1969). It is argued thai learning here consisls In differenllellng among atlmukJs objects by extracting lhe relevant information !hal ls already given. II seems lo me 11\at the necessity ol sUCh perceptual learning raises serious dlfllcullios lor a direct theory even if lhe learning is one ol differentialion rather lhan of ervichmeol
by Sverker Runeaon Poylcofog/ab lnotkutlon�. Uni-Mtyoll}ppolll •• 6·76104 Upfnnlo, S-•n
There Is more to psychological meaningfulness than computation and representation Ullman's version of Indirect percepllon Is remarkably different from traditional ones in that It concedes two of the most important and controversial constiluents of the direct perception approach: that the
achievement of a perspective reversal iiiU8trates this same point. The
available information is rich and sufficient and that perception does not is consciously repre t involve any junctions at which the informaion
perception of the book as moving depends upon the perspective depth
t or meaningless form. (I will assume that Ullman's sented in a primiive
reversaL By way of conclusion, a few words about the problem of veridicality
"decomposing the relation between stimuli and percepts in psychologi cany meaningful terms" does not refer to conscious components.)
are in order. One of the appealing features of a direct theory is Its potential for dealing with this problem. After all. if we can somehow
Thus it seems thai In Ullman's view. as well as In Gibson's, what we perceive is the world and not images of it.
directly pick up the Information about what is "out there" because the
The major point of disagreement put forward in the target article
information is so much richer than earlier philosphers and psycholo
concerns the nature of the mechanisms or syslems that enable us to
gists appreciated, then veridicallty is no longer a problem. Conversely, theories that rely heavily on constructive events within the organism do have a problem accounting tor veridicality. The point 1 would like to emphasize, however. is that, for all theories, perception must do justice
perceive the world. Although I am not convinced that "direct versus indirect" are appropriate terms for it, the issue Is a substantial one
to or accqunt for the stimulus. There must be a good match between
among theories of perception. Ullman claims that perceptual function ing is (necessarily?) representational/computational whereas Gibson suggests that concepts like resonance and tuning provide better
lhe constructed percept and the stimulus. That Is what distinguishes
analogies. Unfortunately, Ullman shares the common misapprehension
perception from other kinds of cognition such as imagining or thinking.
that Gibson's terms "resonance" and "tuning" are meant to be psychologically ernpty and hence that perceptual mechanisms are of
Thus the percept must be grounded In the stimulus, and t�.is in Itself serves as a constraint that assures that perception will have some degree of correlation with what is producing the stimulus. But, in addition, the requirement has led to the evolution of rules and principles of preference that wnt generally guarantee veridicalily. One of the virtues of this way of looking at the problem fa that illusions then become explicable in terms of the misapplication of the same rules and principles.
interest only to, and could be studied only by neurophysiology. True enough, there isn't much computation implied by these notions, but there might be other Interesting things involVed. Thus, in technology, questions about what makes a device resonate, or how it can be made tunable, reach far beyond simple checking out of the deta�s of the hardware; they extend into advanced theory. And the principles behind a device that selectively picks up something delicate are certainly
THE BEHAVlORAL AND BfWi.l SCIENCES (1980), 3
399
Commentary/Ullman: Against direct perception worth stu
THE BEHAVIORAL ANO BRAIN SCIENCES ( 1980),
3
409
Respon.se/Ullman: Against direct perception perception. Perception plays a role in determining the behav ior of animals, and animals' movements and locomotion affect the visual stimulus that reaches their eyes (Jones & Pick, Mace, Prindle et al., Reed, Shaw & Todd). 7. Cognftlve problem-solving tn perception. Johansson et al. sketch what they consider to be the accepted distinction between direct and indirect theories of perception. This characteri7.ation contrasts the "direct recording of informa tion" with cognitive problem solving, or reasoning-like activi ty, that draws on past eltperience to supplement the raw material in the visual stimulus. (See Epstein, characterization 4 and Rock for similar definitions.) Additional characterizations of the DVP controversy that have been offered in the past are listed in note 1 of the target article. They include the role of past experience in percep tion, the degree to which the environment s i speciBed by a dynamic visual array (see also the commentary by Pradzny), and the difference between continuous optical flow and discrete sampling of the visual array. ln light of these widely different characterizations, is it possible to determine what the direct perception controvery is "really" about? The above characteri:zatlons were supported primarily by citations from Gibson's writings. Using this kind of support alone is not guaranteed to yield >1 unique definition of direct visual perception, since different characterizations may be proposed, corresponding to different aspects of the Gibsonian approach. To keep the discussion coherent. the characteri:zations that correspond to distinct aspects of the theory should not be confounded. In my paper, I drew a distinction between two separate aspects of the Gibsonlan approach: ecological optics and the direct pickup of information. A number of the commentaries objected to the argument against DVP by supporting the Gibsonian approach to ecological optics. A fundamentaI difficulty with these arguments Is that ecologi cal optics and DVP are not identical, and the former does not entail the latter (see section 2 above). The fact that the visual array is rich in information does not imply that constructs internal to the perceiver have no place i n the theory of perception. The various proposed characterizations can also be exam ined In light of their relevance to the theory of direct perception vis a vis alternative theories. The various charac teriutions are not all of equal relevance in this respect. If direct perception is defined, for example. as "unaided by instruments," then one can make a strong case for directness in perception. This distinction would not be essential, however, from the point of view of contrasting the Gibsonian approach with alternative theories of perception. The conclu sion is that in putting forward a characterization of DVP it is important to evaluate its "discriminative value," that is, whether it captures an essential distinction between direct perception and alternative theories. One way o£ assesis ng the discriminative value of a given distinction between DVP and alternative theories is to consider the following "modification test." Suppose that a proposed characterization is based on a proposition P which is held true in the indirect view but not in the DVP formulation. The question the(l is whether the DVP theory can be modi ned to admit the correctness of P without losing its essential i not an essential characterl:zation character. If it can, then P s of the DVP controversy. The discriminative value criterion reveals a weakness in Cyr's emphasis on motor feedback. In Gibson's formulation motor feedback s i indeed superfluous. But its role can be acknowledged with only a marginal change in the directness aspect of the theory. For example, one could replace the visual array by a "sensory array" that Includes some nonvisual Information about the posture and motion of the perceiver. It can then be argued that the sensory array is only a slight
410
Tf£ BEHAVIORAL ANO BRAIN SCIENCES (1980), 3
modification of the visual array, and that perception is still nothing beyond the "direct pickup of the information In the sensory array." (See, for example, the afferent-efferent loops in Reed's formulation of direct perception. They could, in principle, include motor feedback without drastically affed ing the directness of the theory.) . It is also unclear whether motor feedback plays an essential role in all, or most, perceptual tasks. A variety of visual tasks, say stereoscopic vision, or the recognition of various objects, can be achieved under brief presentation, without the use of eye movements or other motor activities. rf motor feedback is not essential in these tasks, then it does not provide an adequate characterization of indirectness in perception. It should be noted that Gyr's argument is not invalid: there is good evidence that motor feedback plays at least some role n i perception, and this fact does run contrary to the Gibsonian formulation. It is, however, a relatively weak characteriza tion, which bears more on the information content problem than on the directness controversy. For similar reasons, Pradzny's description of DVP as the study of dynamic perception is also a weak characterization of DVP. Dynamic transformations of the visual array ar� certainly relevant to ecological optics and the information content problem, and Gibson was a pioneer in stressing their importance. The use of dynamic patterns is not excluded, however. from the indirect theories, such as the computa tional/representational approach. It fails, therefore, to provide an essential characterization of the DVP controversy. As to Weimer's definition, I find it difficult to agree with his characterization of cognition as a ''fallible device to deal with the unexpected," or the speculation that "when evolu tion s i completed" there would be "no need for cognition beyond perception." His main argument is aimed at estab lishing the necessity of conscious processes in cognition. It does not seem to me, however, that the necessity of conscious cognition lies at the heart of the DVP controversy. First, at present this problem appears to lie outside the psychological theories of perception, direct or indirect. Second, the direct and indirect theories also disagree in their account of tacit, unconscious perceptual processes, and consequently Weim er's characterization is lacking in discriminative value. Mackworth's characterization stresses the role of processes "in between" the initial registration of the visual input and the final percept. This emphasis does not seem to capture the essence of ''directness" in perception. Tt is conceivable, for example, that a theory could explain the internal proces.�es involved in perception, without describing perception as the "final product" of these processes. The relation between interal processes and perception may be more complex, and consequently the "in between" relation is not an indispensa ble aspect of the indirect theories. In Mackworth's view the distinction between a one-stage and multistage process in perception is primarily an empirical question. This view is not entirely correct, since the decompo sition of a process into substages depends on our description of the process, and thus of the chosen level of description. The main objection to viewing the perceptual process as a single stage Is not that it is empirically wrong, but that it does not provide an adequate theory. Finally, it should be noted that perception is not necessarily viewed as a single-stage process in the DVP formulation, since it involves a continuous inter play between the perceiver and his environment. It seems, therefore, that the main distinction is not the "one-staged ness" of perception, but the adequacy of a theory that does not elaborate the decomposition of the internal perceptual process. The same fundamental distinction applies when the theory of perception is extended to include perception-action Inter actions. The interrelations between perception and motor activities do not Imply that the processes involved are direct.
Response/Ullman: Against direct perception The role of perception in controlling behavior in fact suggests a possible advantage of explicit internal representa tions of the environment. An action to be taken in response to visual input often depends on a configuration of the environ ment, say, that object A s i in front of object B. The in-front-of relation may be determined by different kinds of visual information, such as stereo, motion. or interposition. It might be advantageous if "A in front of B" was represented by the same brain event, regardless of its visual source. More gener ally, the "convergence argument" for internal representations of the environment runs roughly as follows: the conditions that lead to an action can be "summarized" in a fixed intemal form that is then used in executing the action. The conditions that determine a motor activity often depend on the configu ration and properties of the external enviTOnment. Conse quently, an explicit representation of some aspects of the environment and its properties may be an efficient "internal form." The characterization of indirect perception described by Johansson et al. has a number of different components: (i) the information in the visual stimulus is insufficient, (ii) past experience plays a major role In perception, and (iii) the perceptual process is a logicul reasoninglike activity that involves making assumptions and drawing inferences. These various components are in many respects independent. As one example: past experience can in principle act by modifying the structure of the system rather than adding to a bank of experiences. In this manner it can conceivably play a signifi cant role in perception without requiring an explicit reasoninglike activity. Using the conjunction of the different components to characterize the indirect approaches fails, it seems to me, to characterize adequately the possible alternn tivL "li to the "direct recording of information." A theory cnn reject the "direct recording" without necessarily relying only on past experience and cognitive problem solving. The more fundamental distinction (albeit not necessurily the generally accepted one) is that the direct recording of information cun be replaced by a systemlltlc theory that uses constructs internal to the perceiver. Such theories mt�y agree with pk\rts of Gibson's ecological optics, but not with the directnes.� of the information pickup. The objection to DVP that I huve discussed, in terms of the meaningful decomposition of the perceptual process, seems to rrw to provide a strong characterization of the direct percep tion controversy. Such a decomposition s i an essential nspec.:t of the alternative theories, and the DVP theory cannot be modified lo meet this objection without losing its essential churat:ter. If it is acknowledged that the rnuln concern of the theory of perception is to explnin the perceptual ..Information pickup" in terms of constructs internal to the perceiver (such as internal processes, computnt ions, representations, and the like), then the DVP formulation no longer holds.
4. Meaningful decomposition, computation, and internal representations. In the indirect theories, constructs internal to the perceiver are used in the explanation of the perceptual process. This brings such concepts as computations, processes, and internal representations into the realm of the (indirect) theory of perception. The use of these concepts in the theory of perception has been examined in a number of the commentaries (Grossberg, Hinton, Johansson et a!., Runeson, Weimer). On the most general level, the very use of computational terms i n psycho logical theories has been questioned. Weimer's objection to the standard notion of computation is based primarily on the argument that rule-governed symbol manipulation cannot account for conscious experience. A second objection to the computational approach (for example, Prindle et al.'s) is that algorithms cannot provide proper explanations. My own view regarding Weimer's objection was stated
briefly in section 5 of the target article and in note 13. I see no strong reason to believe that the computational view in its present form is the ultimate approach to the problem of perception in all its aspects. But this is only marginally relevant to the direct perception controversy. The merit of the indirect approach is not that it provides the ultimate theory of perception, but that, in the current state of knowl edge. it provides a more adequate theory than the direct theory (cf. von Fieandt and Welker). As to the problem of an algorithm as an explanation. it appears to confuse algorithm-as-a-theory with the theory of a computation. An algorithm as the theory may indeed lnck explanatory adequacy, even if it correctly describes the behavior of the system it purports to explain (cf. Chomsky 1965). But for a system that performs computation, there can be a theory of the computations it performs, and this theory should not be confused with an nlgorithm-as-an-explanation. A number of the commentaries examined the form of the computations used in perception - in terms of whether they should be regarded as coordinated sequential steps, as distrib uted parallel processing (Hinton), or as dynamic patterns (Grossberg). Although this is not an immaterial problem, I would like to deemphasize its pertinence to the DVP controversy. For the argument against direct perception, the form of the internal computations is of secondary importance. The tP.rm "compu tation" was not intended to menn computerlike operations (Prindle et al.). I have in fact, examinecl elsewhere (Ullman 1979b, ch. 3; 1979d) the possible use of distributed, uncoordi nated computations, similar to those described by Hinton. Grossberg prefers the language of differential equations and dynamic patterns, and therefore Bnds the term "resonance" appealing (see also Ruoeson for a similar argument). But his enterprise, if I understand it correctly, is aimed ut deriving a theory of the computations underlying perception (as well liS other activities), and in this crucial respect it s i in accord with other computational theories. 5. Explanatory adequacy and truth value. The objections I have raised against direct perception were based primarily on the explanatory inadequacy of the DVP formulation (cf. Kocnderink). A number of the commentaries sought to strengthen the case by attempting to disprove direct percep tion on either logical or empirical grounds. It seems to me, howevm, that no such strong case has ·been made. The additional arguments do not disprove DVP, but challenge it again on the basis of its explanatory inadequacy, relying primarily on what 1 have described as the psychologically meaningful decomposition of the perceptual process. llnyes-Roth argues that the tasks performed by the visual system require a certain "representational and computational power." These requirements imply that the perceptual system must incorpor:�te mediating processes and representa tions; hence direct perception is logically impossible. There are a number of problems in npplying this semifor mal argument to the DVP controversy. I shall mention two
here.
First, according to the DVP formulation, it would be wrong to consider the patterns of light stimulation on the retina as the input to the visual process. It Is claimed that the visual system can "resonate" directly to lnvariances in the patterns of light, in which case the recognition of triangles (considered in Hayes-Roth's commentary) could be performed directly. The crucial Issue here is whether abstract properties and invarlances can, In general, be picked up directly by the visual system. (See section 3.2 of the target article for a discussion of this issue.) A second shortcoming of the "computational power" argu ment is that a system may employ some rule for associating inputs and outputs without having a meaningful decomposlHE BEHAV,()RAL AND BFW-1 SC1ENCES (1980), 3
411
.l
l
·l
I j 1
l ·! :�
·'
:l
:r l
·'
,;
·I i .
·
l i
!
I I
Rej�rences /
Ullman: Against direct perception
lion. To stay with the semiformal line of argttment, consider a Turing machine that instantiates, by virtue of its internal states and transition table, some mapping rule between inputs and outputs. [t is possible, in principle, that the only descrip tion of this mapping rule would be to give essentially the complete ··blueprint" of the machine. The same problem may conceivably arise in a complex networklilce system, where the network may be entangled in a manner that prohibits meaningful decomposition. lf this were the case for the perceptual system, there would be no significant middle ground between ecological optics and a detailed description at the mechanism level. The fundamental objection to direct perception therefore rests, not in the computational power argument, but in the arguments for a meaningful decomposi tion and adequate level of description. Empirical evidence against direct perception is described by Loftus & Loftus. They argue that in metacontrast experi ments, for example, the perception of a stimulus A is not completely determined by A alone, but depends also on a second stimulus B. that is presented at a different location or a different time (see also a similar argument by Rock). The acceptance of such empirical results as evidence against direct perception depends crucially on the argument support ing tbe explanatory adequacy of decomposi11g the perceptual process. Otherwise, one could, in principle, describe A and B together as a single event C, and argue that C alone deter mines perception "directly.'· An extreme example along this line would be the use of a ·'history function," similar to the one described by Shaw & Todd. In discussing the Mach phenomenon, I have argued that perception can sometimes be naturally described as a function of two ··arguments": the visual array and the cunent interpretation of the observer. Shaw & Todd argue that this is not the only possible formula tion. The "current state" of the observer may itself be a function of earlier stimuli and earlier internal states. By a repetition of this argument, perception could be described without resorting to changing internal states. Perception at any given time t could be described as a function of a fixed initial state (say, the state at birth), and the stimulus history from the i.nitlal slate up to time t. The main objection to the "history function" formulation is not that it is descriptively Incorrect, but that it is unsatisfactory as a psychological theory of perception. The argument centers, again, around the notions of meaningful decomposition, internal constructs, and explanatory adequacy, rather than truth va.lue. A fundamental distinction between the direct and indirect theories, closely related to the use of ··history functions," is the reliance of the direct approach on external constructs. £t seeks to explain perception in terms of such constructs as "looking llround, getting around, and looking at things," and overt movements of the body, head, eye, pupil, and lens (Mace, Reed). When perception is explained in these terms alone, concepts akin to the "history function" become unavoidable. At the heart of the disagreement here lies the problem of psychologically meaningful constructs. From a vantage point that denies the legitimacy of internal constructs in the theory of perception, DVP s i implied. As n i all the arguments considered above, the controversy centers primarlly around what constitutes an adequate decomposi tion of the perceptual process in psychologically meaningful terms.
References Attneave, F. (1972) Repre.�entatlon of physical space. In: Codlngprocmu tn h..man memory, ed. A. W. Melton & E. Martin, Washington, D.C.: V.Ji. Winston & Snns. [SUI A!lneave, F & Frost, R. ( 19601 The determination of p<m:eived tridimensional
412
THE BEHAVIOOAL AND BRAIN SCIENCES (1980), 3
orientation by minimum crlteriL Perceptll>n and P&ychbphystc� 6(6 B):391-96. [SUJ llnlow, fl. 8. (1972)Single units ond sensation: A neuron doctrine for perc:e(> tual �ychology. Perception 1:371-94. [OJB] Barrow. H. C. & Tenenbaum, J. M. (1978) Recovering intrinsic scene charac terblics from Images. In: Comptsfer t>Uion 1!/flems, eel. A. R. H•ruon & E. M. Riseman. New York: Academic Press. [CEH) Bentley, 0., 6 Hoy, l\. 1\. (1974) The neurobiology of cricket song. Sct�ttfic Arnerk:on 231:3-t-4... Aug. [SU) Betnlleln. N. (1967) Theco-ordination and regt.diJtion of mooement&. New York: Pergamon Preos. [WMM) Blalvas, A. S. (1975) Visual Analysis: theory uf lie group representations. Math· et114tlc41 Btosclcnus 28:4S-67. UWG] Bloclo, B. (1948) A lei of pc»tulat� for phonemic nnn)ysis. lAnguage 24:346. (SJK] (1950) Studies In colloquial Japanese IV: phonemics. lAnguage 26:86125. [SJKJ Brauruteln, M. L. (1962) Oepth perception In rotating dot patterns: effects of numerosity and perspective. journal of E:rpenmontal Psychology 64(4):415-70. [SUJ Breltmeyer, B. C. & Can�. L. (1976) Impllcallons of sustained and transient chAnnels for theories of visual pattern masking, saccadic suppression and Information processing. PaOJChologtcal Reolew 83:1-36. (GULl Btldgtomo.n. 8.: Hendry, 0.; & Stork, L. (1975) Failure to detect d!Jplacernenl o£ the visual world during saccadic eye movements. Vtsino Rychology of knoWing. ed. J. n. Royce & W. W. l'lozeboom. New York, Pazis, London: Cordon & nreoch.
[SUI
Gyr, j.; Willey, R.: & Henry, /\. (1970) Motor-sensory feodback and geometry of vlsuol spoco: o replication. Belrautoral nnd Ilrnln ScU!nces 2:59-
64. [JWC. SU) . Hoy, C. J. (1966) Optical motions and space perception - and extension of Gib son's anolyslr. !'tychologlcnlllevlew 73:5150-65. [SUI Hayek, 1'. 1\. (1969)Tho primary of the abstract. In: &yond reductionism, ed.
A. Koestler & ). II. Srnythles, pp. 309-33. New York: Macmillan. (WBW] Haye&-lloth, F. (1977) Critique of Turvey's "Contrasting orientations to the theory of visual Information prOCC$l$ ng." Psycltologteol Reolew 53135. [f'HRJ Hecht, S. (1934) VJ.osion lJ, the nature of the photoreceptor process. In: A hand book of gerrero/upertmental psychologv. eel. C. MurchinsorL Woroester: Clark Unlverslt'y PrOSI. [CRL] Hclmholb:. H. von, (1963) Trtalue on phflllo/oglcal optla, ed. j. P. C South :.U. New York: Dover. (80, WE, SWZ) Henle, 1\4. (1974) 011 naive realism. In: Pen:eplfon:e#Dy•ln honor of)amu j. Ctblon, ed. n. 0. Macleod & H. L. Piclt, Jr. Ithaca and London: Coroell University Preu. [SU) Hill. A. L. (11772) Direction constancy. Perception aru/ PzvchopluJ.U:S ll:l7578. [IRJ Hinton, G. ( 1979) SmJMO dcmonstrotlon of the effects uf stnreturol descriptions io 111ental lmagery. Cognllloc Science 3:231-50. [SUI Hochberg, j. (1974) lil&her-ord�r stimuli and Inter-response coupling in the perccptlol> of the visWll world. In: Perception: t:Mays In Jronor ofjamu j. Glbfon, cd. R. D. Mocled & H.L. Pick, Jr. Ithaca and London: Cornell \Jntv.,tslty Pre$8. [SU] Hochberg, ]. & McAlister,/\. (19�) A quantltatlvcappronch to figural "gl){)(! ness." }ollrnol ofE%1�mcmtal P1vchology 46:361-64. SUJ [ Hnflmno. W. C. (1900) The lic-algcbra of vblUII perception. jouroalof Matlr· emoltca/ Psvchology 3:65-91!. [JWGJ (1977) An Informal, ltlslorlcol d�riptlnn (with bibliography) of the "L.T.G./N.I'." Cailters de Pt1JOI1ologlc 20:135-74. (KvF] )atiSSon, C. & johons.�on, C. (1973) VIsual perception of bending motion. l'er an ecological psvcliologv. ed. R. Shaw & J. Brnus£ord. Hillsdale, N. ).: Erlbaum. (ESR) Mack, lt.. ( 1970) An investigAtion of the relationship betweeneye and retinal Image mcwement n i the perception ol movement. l'erceptton and Ptvcho Phlllfcs 8:291-98. [88) (1979) Nonvisual detennlnnnts of perttption. &lutuirt1l end Bratn Sclcnce6 2(1):75. (SUI Madcworth. A. K. (1976) Model-driven interpretation b1 intelligent vision •Y'· terns. Perceptwn 5:349-7n [AKM) Marcel. A. (In press) Con,cious and unconscious perception: vlstllll m11sklng, word recugnitiori and an approach to consciousness. Cognltluc Psuchnlo· gy. (Gl\L] Marmolln, R. (1973) Visually perceived n••tion in depth resuhlng from proxl· mal changes. Ptrcepllon and PlfiC}IopiiJIIIcl 14(1):133-48. (S'UJ Marr, D. (1976) Early proeess!ll& of vlsual lnf<m•tion. Phi/Oipirlcal Tramoc· lions of the Royal Sot:Utv of L.tmdon 275(942)483--S34. (SU. SWZ) ( 1977) Artilklal intelligence' - e personal view. Artt/ickll lntelltgttnce 9:3748. (SUI Mnrr, D. & Ntshlharn. K. (1978) Representation nnd recognition of the spatial organizntlon of three-dimensional shape£. Proceedings of the RD11al $()(:1. ety - LondDn B 200:269-9-1. (SUI Marr. D.; Palm, C.; & l'oggio, T. (1978) Analysls of a cooperativestereo algo rithm. Btolog1C4/ Cybernellcr 28:223-39. UFDJ Marr 0. & Pogglo T. (1976) Cooperative computation or j�ereodisperfty. Science 194:283-8'7. UFD, CE.H) (1977) From understanding comput•tlon to unders!Jlnding neural circuitry. Neuroscience Rt•earch P10gsam Bulfelln 15(3):470-8& (SU) ( 1979) A computational theory of human stereo vision. Proceedlngt of the lloyal Socfcly-London B 204:301-28. (SU] Mast�rton, R. B. & Rerkley, M. A. (1974) Brain function: changing Ideas on the
414
THE BEHAVIORAL ANO 81W'l sae-ICES (1980), 3
roleof sensory, motor And 11ssoc:lation cortex In behavior. Annual Reol�w ofPIIJCiaoiDil'J 25:277-312. (E$1\J Metzger, W. (1930) Optiscbr untersuchungen lm Canzfeld TI. Pkvcltolo,Uche FortChung. 13:6-29. (RKJ)
(1972)Critical remarks to j. J. Gibson's conception of "direct"visual percep tion. I.e.. or revived prephysinloglcal realism. In: The ,nychology ofkn01.0· lng, ed. J. R. Roy� & W. W. R07.eboom. New York. Parb, London: Gor don & Breach. (SU] Meyer, D. E. & Schvaneveldt, R. W. (1971) Focllltalion In recognizing palu of words: evidence of a dependc.onco between retrieval operations. ]oumdl of £xperlmental P1vchology 90:227-34. [GRL) Michaels, C. F. & Carella, C. (in press) The theory ofdirect perception. New York: Prentice-Hall. (SSP] Minsky, M. (1967) Comp��tclln:finII• and ln/infle machines. Englewood Clift's. N.).: Prentice-Hall. (RS) (1968) Matter. mind and models. In: Sert14nllc Information procettlng. ('.ambridge, Mass: MIT Press. (AS) (1975) A framework £,or representing knowledge. In: The p.sycho/ogy Df compulltf ol.t1011, ed. P. H. Winston. New York: McGraw-Hill. (WE, KP) Mochoue, A. (1963) The p [GEHJ
Slomon, A. (1978) The computer raoofut1on In plrl/011op/Jy: plrtlosop!.uscience
and moclels ofmtr1d. Suss
Smith, N. W. (1971) Aristotle'' dynamic approach to .sensing and some current implications. )oumnl of the History of tire 8ehaotorol Sciences 7:37577.
[SSP)
(1974) The ancient background to Greek psychology and some implications
for today. P•vcho(ogtca( Record 24:309-24. [SSP) Snellen, j. W. (197!l) Set point and exercbe. In: Essays on fcmperatureregufa. liOn, ed. j. Bligh & R. E. Moore. Amsterdam: Natth·Holland Publish· ing. [SSP)
Sutherland, N. S. (1979) The representation of three-dimensional objects. Nn· tur" 278:395-98. [SUI Taylor, J. G. (1962) The behavioral ba.rts ofperception. New Haven: Yale Unl· versity Press. [SU) Thatcher, 1\. W. & john, E. R. ( 1977) Fou111falfona ofcognftloe prote3su. Hillsdale, N.j.: Erlbaum. [SU) Titchener, E. B. (1926)J\ textbook of'f)$!/Chologv. Now York: Macmillan. [IRI Tondeur, P. (196.5) fntroducfton fo the lie groupstmd trarttformatton groups.
Heidelberg: Springer.
[KvFJ
Turvey, M. T. (1976) On j>Criphcrnl and central proccesos ln vision: inferene
ull. Psvclwlogfcallteolem 80:l-52. [GRL) (1977) Contrasting orientations to the theory of visual Information process ing. P•ychologlcalllaofew 84(1):67-88.
[WE, FHA, SU)
(1979) The thesisofefference-mediotionof vl51on can.not be rotionaliz.ed. Behaoforal tJnd Brain Sciences 2(1):81-83. [SU) Turvey, M. T. & Show, R. {1979) The primacy of perceiving: An ecological re· formulation for understanding memory. Tn: Per�ecllvt$ on memory rtt· search: etsa!J$ tn honor of Upr»afa Un!oersttv'• /SOOth anniversary, ed. L.· G. Nlllson. Hillsdale, N.J.: Erlboum. [SSP, RSI Turvey, M,; Shaw, R.; & Maoc, W. (1978) l!sues ln the theory of action: de-
grees of freedom, coordlnative structures �tnd coalitions. In: Attention and
performance VII, t:d. J. Roquin. Hillsdale, N.j.: Erlbaum.
(WMM, SSP, RS) Ullman, S. (1978a) Artificial lntelltgonce systenu and human cognition: the missing �nk:. Behtlvfor�Jf and Brain Sclenca 1(1):1 17-19. [SU) (1978b) Menial representations and mental experiences. Behat>loral and Brain Sc!errce• 1:605-6. [SU) (1979:a) The Interpretation of >lructure from motion. Procudtng• offhe RoiJtll Socfetv- London B 203:41l5-26. [SUI (1979b) The lnfel"f)retotton ofotsiUll mouon. Cambridge und London: MIT Press. (RS. SU) {1979c) Reln�•Uon and eonstrllined optimi7.1ltlon by loco] processes. Com· pUlergtap/11�and fmaiJ• proceutng 9(6): lliS-25.
[SUJ
von Fieandt, K. (1966) Tha worftl of perception. Chicago: Dorsey
Press. [T(vF]
Nor diU: P•ylco/ogl '.tT. [ICvF] von Fieondt, K. & Cib.son, J. j. (1959) The sensitivity of the eye to twn.kindsof continuous transformations of a shadow-pattern. journal of Experimental Psychology 57:344-•17. [lerfmentaf P•ucholo&lJ 38:210-24.
[Ill)
(1976) On perception. New York: Quadrangle.
[GBZJ {In press) The perception of a stable environment. Sctenttfic Amerl·
can. [GI3Z) Wallach, H. & O'Connell, 0. N. ( 1953) The kinetic depth effect. )o umol of Erperlmentcl Psychafll8M 45(4):205-17. [SU) Wallach, H.; O'C'..onnell, D. N.; & Nelsser, U. (1M3) Tho memory effect of vi· sua! perception of throc-