The Strategic Eye: Kosslyn’s Theory of Imageryand PerceptionReview Exchange

MARK ROLLINSDepartment of Philosophy, Washington University, St. Louis, MO 63130, USA; E-mail:mark@twinearth.wustl.edu

Stephen Kosslyn,Image and Brain, Cambridge, MA: The MIT Press, 1994, vii+516pp., $60.00 (cloth), ISBN 0-521-42992-7.

On the face of it, the main goal of Stephen Kosslyn’s most recent major work,Image and Brainwould seem to be to deliver the fatal blow to ‘descriptionists’,who deny that iconic representations contribute anything significant to cognition.Acknowledging the notoriously indeterminate nature of behavioral evidence in thisregard, Kosslyn aims to show the truth of his ‘pictorialist’ theory by appealing toneuroscience. The result, he claims, is decisive: “(T)he ‘imagery debates’ are for allintents and purposes settled” (p. 377). However, Kosslyn’s book is as much aboutperception as imagery, and whether or not the imagery (or any) debates are everfinally settled, the evidence and arguments that Kosslyn offers for an intimate rela-tion between imagery and perception are of considerable interest, both to scienceand to philosophy.

That relation has three dimensions: (1) Imagery and perception depend on someof the same mechanisms. (2) Imagery plays an essential role in perception and (3)perception generates images. One important consequence of these interconnectionsis that perception is veryplastic, but not always as the direct result of the top downeffects of background knowledge or theories. To be sure, imagery and percep-tion are also, on Kosslyn’s account, cognitively penetrable in the familiar sense:“(O)ne’s knowledge and beliefs apparently can affect visual processing almostfrom the start” (p. 87). Nonetheless, spatial properties are sometimes representedby spatial properties of the brain, and some operations cannot be properly describedas inferential at all. Moreover, on Kosslyn’s account, imagery and perception de-pend heavily onattentionalandperceptual strategies, which are, in effect, variableways of controlling available resources.

This emphasis on what might be called ‘the strategic eye’ is philosophicallysignificant, because it sheds new light on the nature of perceptual plasticity and per-ceptual content. And Kosslyn’s massive compendium of evidence makes a centralrole for perceptual strategy very hard to deny. However, while strongly endorsinghis approach to perception and imagery, I shall argue that he does not deal fullywith two problems in his 1994 book.1 One problem arises from evidence thatimagery and perception arenot interrelated in the ways that Kosslyn describes.

Minds and Machines11: 267–286, 2001.© 2001Kluwer Academic Publishers. Printed in the Netherlands.

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Thus, his theory of the particulars of perceptual strategy might be false. I do notthink that the apparent counterevidence does, in fact, show his theory to be false, forreasons I shall discuss, but the other problem is, that if his theory is correct, then thevariability that is introduced by it poses a prima facie threat to explanation. I shalltry to show that Kosslyn’s attempt to deal with this methodological problem is onthe right track, but stops short of an essential step. That, in itself, cannot be taken asa criticism of work that lays the foundation for the step so impressively and pointsin the direction of it so clearly. Yet I also want to show how Kosslyn’s treatmentof more empirical problems suffers from the incompleteness of his response to themethodological challenge. My suggestion will be that we should view the putativeconflicting studies as not only compatible with Kosslyn’s theory, but as addingto the arsenal of techniques with which the methodological problems posed byvariations in strategy might be overcome.

1. Kosslyn’s Theory

At bottom, Kosslyn’s construal of perceptual strategies derives from the funda-mental principle ofImage and Brain; viz, that perceptual abilities have to be under-stood in terms of interactions among seven major subsystems: (1) a visual buffer,identified with topographically organized visual areas in the occiptal lobe; (2) anattention window that selects and sends information to (3) a ventral system in whichthe shape, color, and texture of objects are processed and to (4) a dorsal subsystemresponsive to spatial properties like location and size; (5) an associative memorywhere concepts are stored; (6) an information look-up subsystem used to retrieveadditional information when object identification is difficult; and (7) an attentionshifting capacity, which not only serves to relocate the attention window, but alsoprimes representations of sought properties, thus making them easier to encode.

This subsystems approach stands in sharp contrast to a strongly modular, stage-dependent account. Kosslyn’s subsystems are, in Simon’s (1969) terms, “nearlydecomposible”; i.e. independent at a coarse level of description, but interactive atfiner grains. Specifically, two subsystems can draw on the same resources (e.g.shape recognition as well as spatial trajectory from motion); hence they are mu-tually constrained, and “the state of one. . . can affect the workings of another,violating [Jerry] Fodor’s notion of strong modularity” (p. 30). Kosslyn now relieson a connectionist computational model in which there is concurrent processingamong subsystems: No central executive turns subsystems on and off in sequence,nor does one subsystem have to wait until another is finished in order to begin.Processing involves constantly mapping input, often degraded or partial, in a seriesof cascades and in parallel streams, and computation is cooperative. There is re-current feedback, with subsystems activated later in a sequence providing aid toearlier processes.

It is important to note that the subsystems theory is driven primarily by consid-erations ofefficiency: The main reason for positing a distinct subsytem is always

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that it would allow a given perceptual task to be performed with fewer errors and/ormore quickly than if the task were done by another subsystem that already had ajob to do. But what gives the system its power is the potential it has forcombiningsubsystems in a variety of ways, in order to effectively perform a given task. Thisamounts to giving a major role to perceptual strategy: “A strategy consists of a setof subsystems working together to accomplish a task” (p. 43) and any task can beperformed by many different types of combination. The visual system thus operateson a principle of opportunistic processing: In the face of impoverished stimuli,perceivers will deploy novel strategies, making use of groups of subsystems thatthey might not ordinarly use. The capacity to do this is essentially an emergentproperty of concurrent processing and cooperative computation, but it depends inparticuarly interesting ways on the interdependence of perception, imagery andattention in Kosslyn’s account.

Kosslyn uses the idea of a basic set of subsystems, employed to produce cer-tain perceptual abilities, as a principle of organization for his book. Thus after anintroduction in Chapter 1 (“Resolving the Imagery Debates”) and an overview inChapters 2 (“Carving a System at its Joints”) and 3 (“High Level Vision”), Kosslynsets about the business of explaining how his theory accounts for high level vision,in particular, object recognition. But in a way that is characteristic of his currentgeneral approach, he treats object recognition as dependent on five different abilit-ies that themselves depend on various combinations of subsystems. Each of theseis the subject of a chapter. The first ability (Chapter 4, “Identifying Objects inDifferent Locations”) illustrates especially well how the strands in Kosslyn’s richtapestry are woven together.

In that chapter, Kosslyn argues that considerations of efficiency imply a funda-mental division of labor of the visual system into two distinct ‘what’ and ‘where’processing streams; the ventral and dorsal subsystems, respectively. One importantconsequence of the split what–where system is that it underwrites a major role forattention, because some mechanism is required to both separate and coordinate thetwo kinds of information. This emphasis on attention is not wholly surprising. Thepoint that links it to Kosslyn’s earlier work is that, in addition to overt shifts ofattention due to head, body and eye movements, there arecovertattention shiftseven with a fixed gaze. (Focus on the palm of your hand at a certain distance, andwithout moving your eyes you can still shift your attention from the tip of onefinger to another.) The question is, in what does covert attention shifting consist?Kosslyn’s answer is that it involves moving the window of attention around regionsof activity in the visual buffer without any corollary eye movement. So construed,image scanning is dependent on the same mechanisms that drive covert attentionin perception.2

Furthermore, the causal interactions between perception and imagery can beunderstood partly in terms of attention. As Kosslyn explains in Chapter 5 (“Identi-fying Objects When Different Portions Are Visible”), the ventral (“what”) systemincludes intermediate processes that extractnonaccidentaland signal properties

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from the input array in the visual buffer. The nonaccidental properties are edge-based features of the sort exemplifed by the points of intersection described byHoffman and Richards (1985). Signal properties are surface-based characteristicmarks. Representations of these are matched to stored representations, in the formof distinctly coded visual memories. This activates a pattern code which is matchedto categories in associative memory. Driven by attention, the process can then causethe relevant visual memory (a ‘compressed image’ in Kosslyn’s terms) to produceactivity in the visual buffer. This constitutes the image proper. Thus do imageryand perception interact.

An essential role for attention is also natural corollary of processes in the dorsalsystem. In Chapter 6 (“Identifying Objects in Degraded Images”), Kosslyn posits aspatial coordinate subsystem, as well as a subsystem for representingcategoricalspatial relationsin propositional form, e.g. “above”, “inside”, etc. On Kosslyn’saccount, spatial categorical relations can often be computed by the operations ofvisual routines of the sort that Ullman has described (1984); and these (e.g. tracingcontours) essentially involve attention. These routines can take the place of ob-ject centered representations (Chapter 7, “Identifying Contorted Objects”). Whenvisual routines are used to help identify an object, the rate of scanning is understrategic control. Moreover, Kosslyn argues that the reference frames required forthe spatial coordinate subsystem can be described in many ways and vary fromtask-to-task. One sort of variability is simply due to the fact that where one fixatesattention defines the origin of a spatiotopic map (367; a spatiotopic map specifiesa location, size, and orientation in three dimensions for each part of a perceivedobject in relation to the perceiver’s body). As it happens, changes in referenceframes for images are a form of strategic plasticity that is especially important.Such changes appear to be blocked under certain conditions, and some researchersargue that this is inconsistent with Kosslyn’s view that imaging is like perceiving.This is the evidence to which I alluded earlier and which I discuss more fully below.As noted, I shall argue that the threat is more apparent than real. But the evidencealso points, in ways I will develop, to some interesting conclusions about what isneeded to explain performance on imaging tasks.

Strategic abilities also emerge as important for the generation of images ofcomplex shapes. (This subject is treated in Chapter 9, “Generating and Maintain-ing Visual Images”, following a summary and reprisal, in Chapter 8, “IdentifyingObjects: Normal and Damaged Brains”, of the theory of object recognition in lightof studies of brain damage). Kosslyn argues that there are two ways to generateimages: one in which categorical spatial representations are used to arrange parts,and one in which coordinate spatial representations are used to arrange them. Inaddition, on his account, there are two types of images: ventral system imagesbased on visual memories involving object properties; and dorsal system imagesbased on attention to purely spatial properties. The former type is familar; thelatter are produced by seeing patterns in, e.g., surface textural properties (like thoseexhibited by floor tiles). These four parameters (two ways of generating two types

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of imagery) result in four types of image generation. Although he notes that manycircumstances will not allow any processing options, Kosslyn’s argument is that thetypes of image facilitate different tasks differently, thus pointing to the possibilityof task-specific combinations of parameters.

2. The Methodological Problem of Perceptual Strategy

On the subsystems approach, then, there is no single way to perform a task. Rather,many different combinations of subsystems, i.e. different strategies, will do thetrick. Because a mechanism or structure is available for recruitment in variousstrategies, it can have multiple functions. This appears to create a problem foridentifying those mechanisms or structures in order to explain the performance ofa task. The standard mode of explanation in cognitive science involves specifyingthe nature of a macrolevel ability bydecomposingit into more basic functions.While abilities can certainly be decomposed on Kosslyn’s model, the danger isthat one will be led by the process to miss the very fact of multiple functionalityof the components that make the macrolevel ability up. The tendency will be tobreak different capacities down into different constituent processes. As a result,explanations that are given of the capabilities in question will rest on a false andartificial theory, one that is, in effect,engineeredto account for data but that is nota realistic model of human neuropsychology. What will be left out is precisely theflexibility of the human visual system.

Daniel Dennett has pointed out the difficulties posed for the identification offunctions by multiple functionality in this regard. What happens, he asks, if spe-cial functions change their identities in profound ways, so that they come even to“contribute to functions in which their special talents play no discernible role”?(Dennett, 1992, p. 271). Such cases of “multiple, superposed functionality”, heargues, are very difficult to understand from the perspective of “reverse engin-eering”, because they imply that the functions in question somehow both retainand give up their special information carrying roles. There is, I think, a possibleconfusion in Dennett’s worry. He points to both a metaphysical problem and anepistemological one. The first concerns how mental states mightbe (i.e. consistin) distinctive functional roles performed by physical structures, if the roles bothchange and remain the same in some sense. The second problem is how we canknowwhat those roles are; i.e. when to assign a particular function, or under whatdescription to include in it explanations.3 I return to this confusion briefly at the endof this essay. But focusing for now on the epistemological problem (which is theone addressed by Kosslyn), we can see the danger clearly: Standard decompositionis especially likely to mislead in the case of multiple functions because the structureor mechanism at the bottom of the line has a dual status. While it may be possibleto identify each of the functions it performs, in so far as they are independent, theyare likely to be attributed to distinct structures.

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Kosslyn sometimes speaks as if the problem of attributing functions to struc-tures can be handled by using a variety of empirical methods to converge on apreferred account, but he also addresses the problem of variable strategies; andwhat he says is clearly pertinent to the issue of multiple functions.4 As a way of re-sponding to the problem, he proposes certain techniques for defining and weightingprinciples of strategy and, by extension, principles of multiple functionality. As henotes, there is no point to trying to specify the optimal set of subsystems to be usedfor a task, because there is no guarantee that subjects would use it. Yet, he argues, itis still possible to restrict performance on a task to arangeof variation. This wouldallow generalizations to be made about the probable scope of multiple functions.For example, Kosslyn argues that it is possible to limit performance on a giventask to a specific class of strategies by making the task very difficult to performwithout the use of certain subsystems (p. 397). As an extension of this method, thesubsystems in question can be weighted by showing that they are likely to play agreater and greater role in affecting the relevant variable (e.g. speed or accuracy)as the task becomes more and more difficult. Finally, as a test of the weighting,performance over time can be studied to see if improvement due to practice in atask for which a certain process had been weighted leads to general improvementin other tasks for which that process was similarly weighted.

Nonetheless, the problem of variation would seem to threaten even the projectof defining a range of strategies that are likely to be deployed on a task. For, asKosslyn himself notes, there can be “differences in cognitive styles, which leadsubjects to select certain types of strategieseven if they are not particularly effectivefor a given task. . . . (P)eople develop preferred strategies for a number of reasons,not all of which are based on how useful the strategy is” (p. 400; emphasis added).Yet he expresses confidence that, in the future, we will find ways to “assess cog-nitive styles” (p. 404). Presumably this means that it will be possible to formulatehigher order generalizations about tendencies to use certain strategies: We maydiscover constraints on cognitive styles and conditions under which one strategy ismore likely to be selected than another on the basis of a variety of criteria. If thiscan be accomplished, Kosslyn suggests, the impact on cognitive science could beprofound.

I think he is right about that. However, he does not say exactly how this levelof understanding is to be achieved, nor does he explain the sorts of reason a personmight have for adopting a nonutilitarian strategy. More importantly, by focusing onthe notion of a cognitive style at thepersonallevel, the deeper possibility is missedthat thebrain’s visual systemmight exhibit a variety of subpersonal strategies,some of which are employed, even though they are not very useful.5 In principle,brain design could allow for nonutilitarian strategies in the same way that it isopen to nonoptimal ones; viz. evolution makes room for coincidences and mistakes.Moreover, unconsciously deploying subsystems in this way could be the resultof faulty training of the individual perceiver; somehow, ineffective strategies gotrewarded in the past. Still, it is not obvious why or when such strategies should

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continue to be employed. One answer is that other, more effective combinations ofsubsystems that might normally be available – or that might be learned quickly bytrial-and-error – are blocked by the nature of certain tasks. If that is so, we shouldbe able to learn a great deal about our strategic options and cognitive styles byanalyzing cases of that sort.

As it happens, I am going to argue, certain studies have already been donewhich have precisely that effect. However, the investigators do not interpret theirresearch in that way; and in his replies, Kosslyn does not emphasize the role ofstrategies in the tasks in question. This is where his response stops short. I shallclaim that, if he had developed the notion of perceptual strategies further, he couldhave rebutted their conclusions more effectively and defused the methodologicalworries expressed here. But a move in this direction, I shall argue, should also belinked to an important reconstrual of what it means for a mental image to be like apicture.

3. The Empirical Problem: Evidence AgainstPerceptual Similitude

In Chapter 10 (“Inspecting and Transforming Visual Images”), Kosslyn addressesa number of potential counterexamples to his claim that imagery and perceptiondraw on the same processes: cases in which perception is impaired but imagery isnot; cases of imagery in the blind; and cases in which it appears that the perceptionof a figure supports multiple interpretations, whereas imaging the same figure doesnot. I shall concentrate on the last of these.

The argument against perceptual similitude comes from Chambers and Reisberg(1985). Subjects were shown an ambiguous figure, like the duck–rabbit or theNecker cube, and then asked to visualize it. Chambers and Reisberg found that,in the visualization phase, subjects were unable to ‘reverse’ the figure, e.g. identifyit as a rabbit after having first labeled it as a duck. This led the experimenters toconclude that imaging is different from perceiving in an important respect: Unlikea perceived stimulus, images are sometimes not ambiguous.

In his 1994 reply, Kosslyn argues that there is strong competing evidence thatimages can be reinterpreted and that images “must specify geometric informationin the way that perceptual representations do” (p. 336). He claims that the failureto get this result in the Chambers and Reisberg experiment was due to two factors:(a) thecomplexityof the stimuli, which prevented subjects from maintaining theimages long enough to reorganize the figure in order to ‘see’ the alternative contentin it; and (b) the effect ofverbal coding, which is known to impair the ability toreorganize images.

However, Kosslyn’s appeal to the complexity of the stimuli to explain why im-ages sometimes cannot be reinterpreted is unconvincing. For one thing, complexityis a relative notion; one image is only more-or-less complex by comparison to oth-ers. Kosslyn simply asserts that the figures used by Chambers and Reisberg weresubstantially more complex than the ones used by him and other experimenters

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(e.g. ice cream cones and jump ropes). In particular, it is hard to accept the claimthat, in Chambers’ and Reisberg’s experiments, imaging tasks were “no mean featfor a pattern as complex as [a map of] Texas” (p. 338) in light of Kosslyn’s own mapscanning research. Further, Kosslyn himself argues that the imaging system hasresources for mitigating the effects of complexity (at least for ventral images); e.g.chunking (p. 323). Indeed, he notes, other research shows that “subjects can indeedreconstrue even relatively complex images if they understand how to reorganize theobject”. Thus the appeal to image complexity here begins to seem ad hoc.

The argument in regard to verbal coding is closer to the target; but it is at bestincomplete. The evidence that verbal coding interferes with the reinterpretation ofimages is that when such coding is inhibited, subjects are much better at reversingthe ambiguous figure (p. 339). Kosslyn’s explanation is that verbal coding leads tothe storage of specific parts and properties in associative memory, which are thenused to reactivate certain perceptual units during imagery in ways that preserve theinitial interpretation.6 However, the problem with this explanation is that verbalcoding does not always interfere with the reinterpretation of images (at least itdoes not always prevent it altogether). So the question of why it does so in thesecases remains unanswered. Further, if verbal coding is taken to imply conceptual-ization (e.g. bringing a figure under a ‘duck’ concept), it is not obvious how thatapplies to the reversible perspectives and spatial reorganization of Necker cubesand Schroeder staircases. For those, some other explanation is required. Finally,we still need to know why verbal coding does not interfere with reinterpretation inperception.

Nonetheless, Kosslyn is right to say that something about the nature of thetasks explains these results in a way that does not really undermine the perceptualsimilitude thesis. A clue is provided by the cases of intact imagery and impairedperception that he discusses, as another example of potential counterexamples tohis account. These cases are the converse of those in the Chambers and Reisbergexperiments: The brain damaged patients can do things with images they cannot doin perception; with Chambers and Reisberg’s subjects, it is the other way around.However, the two kinds of case are still similar in the sense that in both the “imagescontain ‘previously digested’ information; they are already organized into percep-tual units that have been previously interpreted” (p. 329). Thus they need not beaffected by the problems of perceptual organization that appear in perception forbrain damaged patients, nor can they always be reinterpreted from memory. Butwhat this shows, Kosslyn says, is only that the mechanisms that are in fact sharedby imagery and perception are not alwaysusedin the same way. The questionthen would be, what exactly is blocked in our ordinary deployment capabilitieswhen there are perceptual deficits, on the one hand, and when images cannot bereinterpreted, on the other? The answer is suggested by the fact that, according toKosslyn, there are important differences inattentional strategiesadopted by thepatients in the cases of brain damage reviewed by him. Could it be that, on theother side of the coin – where subjects cannot reinterpret images – the explanation

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is likewise in terms of constraints on attentional strategies imposed by the task?The answer, I believe, is yes.

First, as I note below, Reisberg has often emphasized the importance of per-ceptual organization as the central determinant of the meaning of an image, but intheir 1992 article, Chambers and Reisberg explained the nonambiguity of imagesas due to the fact that the initial interpretation causes attention to be drawn to asalient region of the display. As a result, they claimed, the information requiredto produce an alternative interpretation is irretrievably lost through fading. Forexample, a ‘duck’ interpretation makes the region of the beak salient and causesthe portion of the figure opposite that region – which would be the rabbit’s face –to disappear, leaving a gap in the figure. Consequently, a ‘rabbit’ interpretation isno longer possible. I shall shortly argue that the idea that attention is compelled inthese cases is right, but that Chambers and Reisberg draw the wrong conclusionfrom that fact.

Second, the evidence that Kosslyn thinks controverts Chambers’ and Reisberg’sview clearly shows that attention is a key factor in distinguishing their resultsfrom others that support the reinterpretability of images. For instance, as I havenoted, if subjects “understand how” to reorganize an object, they can reconstrueeven complex figures. Butunderstanding howis not simplyknowing thatthere isan alternative interpretation and possessing the requisite background knowledgeneeded in principle to make it. Instead,understanding howis a kind of know-howproduced by telling the subjects to regard the front of one animal as the back ofanother, or by using a demonstration figure; in short, by calling their attention to adifferent region of the figure. (Reisberg, 1994, has specifically said that the relevantknowledge is procedural knowledge.) One might say that they were unable to “hiton the [appropriate attentional] strategy” (Kosslyn, p. 338) required to reorganizethe figure. The question then is, why? What is it about the Chambers and Reisbergtasks that interfered with the use of relevant attentional strategies? To put the pointdifferently, the perceptual similitude issue now becomes one of attentional similit-ude; and the question is why attention is compelled so decisively in the Chambersand Reisberg experiments in a way that it seems not to be for other tasks.

Before proposing an answer, let me point out that, when it becomes clear thatthis is what the implication of Chambers’ and Reisberg’s research is – that inimaging we cannot always employ the same strategies available in perception, i.e.use common mechanisms in the same way – it hardly seems to represent a majorchallenge to Kosslyn’s basic claims about the link between imagery and perception.Nonetheless, the evidence regarding the apparent limits on attention calls for anexplanation.

The answer lies, I believe, in a closer look at the tasks. It should be noted firstthat the types of figures used in many of the Chambers and Reisberg experimentshave distinctive features; the multiple interpretations they admit are not merely theresult of the general indeterminacy of perceptual stimuli. For example, as Reis-berg points out, the alternation between interpretations may change figure-ground

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relations. This need not be true of ordinary cases of multiple interpretations ofperceptual stimuli, e.g. when a bird is seen as a duck or a goose. A similar pointcould be made about perspective reversals. To that extent, reinterpreting images ofsuch figures makes special demands on the image system. However, what failuresto reinterpet thus show is not that perception and imagery do not draw on the samemechanisms and structures; rather, they reveal important microproperties of theoperation of the subsystems they do share.7

Consider the cases in which subjects are unable to reinterpret figures after ro-tating them; for instance, when a reoriented nonsense figure should be seen as amap of Texas, but it isn’t. One possible explanation is that the categories usedfor recognition differ in type: ‘Nonsense-figure-with-a-certain-shape’ is not on apar with ‘map of Texas’. In light of that, reinterpretation may require more than asimple rotate-and-match operation. For example, in ordinary perception, subjectsmight employ the strategy of redefining the top of the figure in various ways, thenrunning visual routines in order to identify, e.g., left-right relations. It would thusnot be just the location of the top that determines how the image is construed.Instead, the construal would depend on the use of a more complex strategy, ofwhich defining a standard orientation for the figure is only a part. The problemin the Chambers and Reisberg visualization task, then, is that, because the top ispre-established, this avenue of reinterpretation is blocked. That would also explainwhy reinterpretations are sometimesnot blocked. For example, in experiments byFinke et al. (1989), subjects could rotate the letter D, place it on top of the letter J,and identify the result as an umbrella. Supplementary strategies would again haveto be brought into play because the entities to be matched before and after rotationare of fundamentally different types, i.e. letters and objects. But in this case, theuse of such strategies is not inhibited because the initial identification of letters ismade soautomatically(letter recognition being highly overlearned) that it placesno restrictions on attention, which is then free to discover alternative saliencies.

I am not concerned to show that this is actually what happens in these cases. Thepoint is that this alternative account shows how the evidence that is supposed to un-dercut the perceptual similitude thesis can also be explained in terms of constraintson available strategies.

The account of tasks requiring conceptualization (as with the duck-rabbit figure)also points to the general conclusion that reinterpretation fails because strategiesordinarily available in perception are not permitted, or are limited, by the natureof the task. It might be true, as Chambers and Reisberg suggest, that the initialapplication of a concept to a visual array biases the direction of attention in anysubsequent scrutiny of an image and thus interferes with reinterpretation. But threepoints must also be borne in mind:

(1) Even when this happens, the explanation cannot simply be that the prioruse of a concept limits attentional strategies, because concept application does notalways have that effect. The difference is most likely due to differences in thedetails of the task, and such details will need to figure as much in explanations of

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cases that show a strong top-down, concept-deployment effect, as in those that donot.

(2) It is not clear that reinterpretation is in fact prevented by a loss of informationfrom the visual array, in the way that Chambers and Reisberg (1992) describe. Thataccount gives attention a wholly subsidiary role to play: The imager simply cannotlocate what is no longer there. But if that were the case, then why could subjects not‘redraw’ the portion of the mental image from which information had been lost?The point is that a closer investigation seems necessary of how the actal mech-anisms for disengaging and shifting attention are affected by concept applicationunder certain circumstances. The result again may be that the explanation has asmuch to do with those mechanisms as it does with the nature of concept use.

(3) Sometimes it is a prior constraint on attention that drives concept applicationrather than the other way around.8 In that case, it is not ultimately the fact thatan image has been previously interpreted that prevents reinterpretation, but thepresence of factors in the task that affect attention. This difference is importantbecause it suggests that the Chamber and Reisberg results may show somethingabout the way information is registered perceptually, rather than – as they suggest– about how it is or is not made available in memory.9

For example, evidence from Ramachandran (1992) suggests that perspectivereversals and gestalt switches on ambiguous figures in perception depend on whathe calls the “capture” of certain information: Some of the details in a visual stim-ulus – e.g. the precise location of the spots on a moving leopard – are not actuallyregistered in perception, but are simply assumed by the visual system to correspondroughly to information of another sort (e.g. location of a surface with a contour).Yet the captured information remains, to some extent, usable for certain purposes(for instance, identifying the leopard as such); and it contributes to perceptual ex-perience. But if a strategy of partial-rather-than-detailed-representation is used inperception, it should have an impact on tasks that later require memory images.This impact, I want to suggest, could account for failures of reinterpretation. Thus,in the case of ambiguous figures perceived as bistable, the particular way inform-ation is encoded and captured can change back and forth over very brief periodsof time, but if the bistability is not immediately apparent to the perceiver, and thevisual stimulus is removed before the alternative content is seen, then we shouldexpect bistability to be lost from the memory image.10

Of course, that would not show that imaging is different from perceiving ordoes not draw on perceptual resources. To the contrary, it makes clear that imagingdepends on the same resources that perceiving does (except for the visual stimulusitself, which is no surprise), while theusesof these resources are constrained bythe nature of the mechanisms and the tasks in which they are employed. Thus, theanalysis in terms of perceptual strategies takes the force out of the Chambers andReisberg experiments as counterexamples to Kosslyn’s theory.

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4. Recent Developments

In articles describing his research since the appearance of his 1994 book, Kosslynmakes arguments that are similar in certain respects to some of those I have justset forth, and he provides some evidence for how different strategies might beavailable in perception and imagery. In general, his claim is that the results of otherresearchers that apparently conflict with his own can be explained in terms of thenature of the task with which the subjects were presented in each case. For example,Kosslyn says that that there are actually three types of images: spatial, figural, anddepictive. These are required by different tasks, and only the last involves the bufferin area V 1. Thus, he argues, certain recent brain imaging studies of (e.g. by Rolandet al., 1994; Mellet et al., 1998) have found no V1 activity, because what theirsubjects were required to do did not require the production of detailed depictiveimages.

However, that does not explain the results of Reisberg’s experiments, whichdid require depictive images. Reisberg has argued that a distinction can be madebetween interpretations of an image that are ‘compatible’ with a given perceptualreference frame and those that are not. (The reference frame specifies perceptual or-ganization, e.g. top-bottom, figure-ground relations, etc.) He claims that his resultsshow that only reinterpretations compatible with the original reference frame canbe made. In reply, Rouw et al. (1997 and in press [b]) argue, first, that Reisberg’sdistinction between compatible and incompatible changes can be expressed in asomewhat different way. Specifically, the properties that define perceptual organ-ization are said to be ‘high’-level, and things like T- and Y-junctions between parts(which depend on prior perceptual organization) are ‘low’. High level propertiesare “immediately visible, whereas the low level ones require perceptually breakingdown” the larger units (Rouw et al., 1997, p. 223). The first type is said to be“explicitly encoded”, the second only implicit. Rouw et al. then provide evidencethat new interpretations of an image can be given that depend on access to lowlevel properties, even if those properties are incompatible with a prior perceptualreference frame. Thus reinterpretation failures cannot simply be due to the level ofproperties that must be accessed or to reference frame incompatibility. Moreover,if, as Rouw et al. (1997) suggest, the stimulus “is organized and interpreted duringthe act of perception” (p. 210), then the continued accessibility of low level prop-erties suggests that the information that is necessary for reinterpretation remains inmemory, albeit implicitly (as I have argued).

This still leaves open the question of exactly why Reisberg’s subjects had suchtrouble in reinterpreting their images; but in general Kosslyn’s view is clear: Thereorganization of images is hard and consumes valuable resources through elabor-ate processing, more so than when a perceivable drawing is present. But that doesnot undercut the perceptual similitude thesis. In certain respects, imaging may notbe exactly like seeing a drawing, but that is not a deep fact about it. Divergencesbetween seeing and imaging have more to do with the demands that imaging makeson shared resources than with differences in the resources they share.

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To appreciate the larger significance of Kosslyn’s position in this regard, it willbe useful to consider the sense in which low level properties might be ‘impli-citly’ encoded. One obvious construal of the notion of an ‘implicit’ property isthat such properties are simply those that must be inferred from the presence ofexplicit ones. Strictly speaking, however, this implies that explicit properties arethose that are articulated in descriptions of perceptual organization; for it is onthe basis of descriptions than inferences are made. But Kosslyn does not think thatperceptual organization is always represented in a description (Rouw et al., in press[b]). Moreover, although inductive inference plays an important role in Kosslyn’saccount, it is clear that the extraction of implicit properties depends in importantways on noninferential processes as well. We can see the full spectrum of processesand subsystems engaged by ambiguous pictures (and partially blocked in memoryimages of those pictures) by looking at what is required for a recognition of objectsthat are depicted from noncanonical (atypical) perspectives.

In such a case (for example, when there is an extreme foreshortening of therepresented object), information about object and spatial properties is integratedin associative memory and then compared to stored amodal (descriptive) repres-entations. The best matching representation is then treated as a hypothesis, whichis tested by searching in the picture for distinctive characteristics of the objectit represents. Attention is shifted to the location of the expected chararacteristic,and that primes the representation of the characteristic in the ventral subsystem.This priming facilitates the encoding of the sought property. In imaging tasks,the priming is supposed to cause a reconstruction of the shape of the object inthe visual buffer; thus “one can identify patterns and properties that were onlyimplicit in a stored visual memory” (p. 322). The point then is that, while implicitproperties might be made explicit in images as part of an overall hypothesis testingprocess, they are discovered through something other than a logical derivation ofa conclusion from premises, viz, attention shifting and the priming of topographicdisplays. In effect, this shows that explanations of the operation of the ‘strategiceye’ depend on understanding thelogic of visual discovery, as well as the logic ofconfirmation. The discovery process will be constrained by the capacity limits andcontrol mechanisms, along with the abilities of the perceiver. These affect the easeor difficulty with which a percept or image is reconstructed. But differences in easeor difficulty between perceptual and imaging tasks do not show that perceiving andimaging do not employ the same sort of information processing operations.

Beyond that, however, I think that Reisberg’s studies and Kosslyn’s responsesto them help forestall an important possible confusion about the contribution ofspatial organization to perceptual content. Of course perceptual organization doesprovide one dimension of the content of perceptual experience. But how are we tounderstand it? A perceptual reference frame cannot actually determine the referentof an image or percept, because the same perceptual organization is compatiblewith the representation of several different individuals or types of object. Thus,the impact of a reference frame on the content of a mental representation can-

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not be understood simply in terms of a relation between reference and meaning,i.e. the semantic properties that belong to an image. Instead, Reisberg’s claim –that perceptual organization is somehow a central determinant of image content –means only that spatial organization somehow imposes limits on the subsequentprocessing of visual information.

Some current accounts of the effect of perceptual reference frames on percep-tion (especially on picture perception) suggest that reference frames control thecontent of perceptual experience to a significant extent by restricting the propertiesthat can be seen as belonging to an object. This could be the case, whatever elseis required to establish a particular object or type of object as the referent of theperceptual experience. That view would seem to be supported by Reisberg’s results.Roughly, the idea is that the content of a perceptual experience is determined partlyby a type of visual indexical reference; i.e. egocentric spatial properties that com-prise the location of objects and their organizational characteristics (top-bottom,right-left, etc.) in relation to the perceiver’s body and point-of-view. In particular,such a reference frame constrains the detection of what have here been called ‘im-plicit’ properties: Given a certain frame, some properties can be detected and otherscannot. The content of a perceived stimulus (scene or picture) is then analyzed interms of which properties can be detected after a search, which must be inferred,and which are excluded altogether. In effect, this sort of analysis identifies layers ofmeaning or content in a perceptual experience (see Peacocke, 1987; Lopes, 1993,1992).

This is a promising line of thought; but I will not develop further here. The pointI wish to make is this: Reisberg suggests that a certain primacy attaches to spatialorganization in the control of subsequent processing of visual stimuli or images,and that primacy is in some sense fundamental. Expanding this view in terms ofthe accounts I describe above, that would mean that the perceptual reference frameis a fundamental determinant of the meaning of images in general. But Kosslyn’swork shows that this sort of primacy is not fundamental. Which sort of propertiestend to regulate processing in this way can vary across tasks. My point then is thatthose regulatory effects are part and parcel of perceptual strategies, which shouldbe factored into explanations of perceptual experience. It is a mistake to treat per-ceptual reference frames as always playing a special role in theindividuation ofmental content. When they do play a special role (which is not in every case), itcomes from the fact that knowing the sort of organization that has been assigned(under certain conditions) is essential for attributing perceptual content and forexplaining task performance in those terms. But individuating content, on the onehand, and attributing it, on the other, are two different problems.

5. Mental Pictures and Perceptual Equivalence

The comparison between imaging and picture perception brings me to my finalpoint. There has been, in the literature on the subject, some confusion about exactly

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what issue separates Kosslyn and Reisberg. This confusion appears, to some extent,in Kosslyn’s 1994 book, where he argues for two points in response to the Reisbergexperiments: that images are nonpropositional and that imaging is like perceiving.Thus he says that the findings of Chambers and Reisberg “led them to suggestthat imagesare not spatial patterns, but ratherare interpretations. . . of the sortspecified in a propositional representation” (p. 336, emphasis added), and he citescontrary evidence which he takes to show that “visual mental images are depictiverepresentations” (p. 339), but the pictorial format issue can be distinguished inprinciple from the perceptual similitude question: Imaging and perceiving might ormight not share a processing pathway, whatever the format of imaging happens tobe; for it could be that perception itself requires only propositional representations.

Further, while it is true that Chambers and Reisberg have linked their work tothe format issue, they have also said that their primary point isnot that imageshave a descriptive or propositional format (although they may give more weight todescriptions than Kosslyn does; cf. Chambers and Reisberg, 1985, p. 324; Reisbergand Chambers, 1991, p. 347; Reisberg, 1994). Thus, Chambers’ and Reisberg’sclaim is not that images simplyare interpretations, as Kosslyn sometimes says,but rather that they require interpretations, with which the spatial patterns must beconjoined. The question then is why images are sometimes inextricably bound toone interpretation and what this tells us about the similarity between perceivingand imaging. (For a fuller discussion of this point and the Chambers and Reisbergexperiments, see Rollins, 1994.)

Nonetheless, I want to argue that there is a legitimate sense in which the issuesof format and perceptual equivalence do coincide. If that is the case, then the evid-ence for perceptual equivalence also supports Kosslyn’s pictorialist construal ofimages. However, the overlap of issues depends on a different theory of depictionthan the one that Kosslyn appears to endorse. Clearly, the question of whetherimages are picture-like or not can only be answered after one has said what it is forsomething to be a picture: The comparison of images to pictures requires a theoryof depiction. As I construe it, a full theory of depiction has two parts. It tells us, onthe one hand, what distinguishes pictures from other types of representation, and ittells us, on the other hand, how to distinguish one picture from another.11 I focusonly on the first part here.

It is tempting to think that images are like pictures in the sense that both repres-ent by virtue of some significant degree of similarity, if not outright resemblence,between the representation and what it represents. Kosslyn’s language, and hisemphasis on the functional topography of V1 in both imagery and perceiving,often suggest this view (e.g. 1994, p. 5). But the topographic organization of brainareas might be functional in some important sense, while their similarity to whatthey represent is still an ephiphenomenon. For instance, the organization might bedictated by a principle of efficient wiring or the nature of operations downstream,without it being the case that any of those operations are analogous torecognizinga similarity. It might be helpful to wire a circuit breaker box in a way that mirrors

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the actual layout of rooms in a house (i.e. it might be conducive to the efficientoperation of the electrical system). The organization of the box could then serveas a visual wiring diagram. But its value to the electrician or homeowner in thatregard is a byproduct of, and distinct from, its contribution to the flow of energy.In the case of topographic patterns in the brain, there need not be anything like arecognition of similarity by, or within, the visual system. It is possible, of course,that topographic organization does more than facilitate information processing. Thevery fact that information upstream is encoded in brain regions that are systematic-ally arranged near to, or far from, one another might be used as input to subsequentprocesses; but even if that is the case, such information aboutproximitiesamongregions of neural activity is not the same as information about similarities betweenthose patterns and the parts of an object they represent.

In any case, a great deal of criticism has been levied against the resemblencetheory of depiction. At a minimum, if to be a picture is to represent through re-semblence, it will be impossible to say exactly what a picture represents (becauseit will resemble objects under many descriptions) or to exclude things that areintuitively not pictures, yet resemble each other; e.g. twins. Moreover, an objectresembles its picture, as much as the other way around. Yet we do not say that theobject represents a picture of it.

However, I think that there is a more general principle at work in Kosslyn’sappeal to topography, and it is related to a different theory of depiction. One mightsay that the significance of topography is that it is simply an important mark ofthe areas of the brain that are dedicated to fundamental visual processes. Whilethe topographic organization is probably a nonaccidental property of those areas,it is only the fact that they are involved in imaging, rather than the topographythat is characteristic of them, that supports the claim that images are picture-like.As it happens, this analysis is consistent with a recent theory of depiction, theRecognition Theory, as it has been called (see Schier, 1986; Rollins, 1999). Ac-cording to that account, a picture is defined as an object that activates the sameperceptual processes as the item or scene which the picture represents, althoughthe picture is not itself that item or scene. This is, of course, only a necessaryand not a sufficient condition for something to be a picture; by itself, it does notrule out treating a twin as a picture. However, once depiction is defined in termsof perceptual equivalence, the door is open to narrowing the definition by addingfurther psychological constraints, e.g. on the operation of attention in detectingsurface properties on a picture that are not present on a twin or on the object whichthe picture represents. Indeed, these additions are essential if Recognition Theoryis to avoid the problems that beset an appeal to resemblence.12 Thus, the concept ofperceptual equivalence between pictures and their objects need not – indeed, shouldnot – be taken to imply that the perceptual strategies that are used to recognize anobject in a picture and in a real world scene are exactly the same.

If this Recognition Theory is applied to images as mental pictures, it leads tothe conclusion that an image is just activity in areas of the brain that are dedicated

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to fundamental visual processes; activity that is produced by information stored inmemory (or such information together with some perceived features that can beassociated with it). What it is to be a mental picture is thus specified in terms ofperceptual equivalence, so that the format issue collapses into the question of per-ceptual similitude. The shift to this theory of depiction is important: Once we adopta recognition theory, it then becomes clear that Kosslyn is right to say that both theformat issue and the perceptual similitude issue have been decisively resolved infavor of pictorialism. The evidence strongly suggests that imaging and perceivingproduce essentially the same sort of activities in the visual system, and, accordingto a Recognition Theory of depiction, that means that images are like pictures.

This brings us back to the methodological problem with which we began. Al-though, as I have suggested, the issue between Kosslyn and Reisberg is largelyperceptual equivalence, in so far as Reisberg and his co-workers do see their re-search as having implications for the format question, we might now understand itlike this: Pictorialism requires perceptual equivalence, and perceptual equivalencerequires strategic equivalence. In the absence of the latter, pictorialism must fall.But a picture need not engage all and only the perceptual strategies that would beengaged by the object it represents. Thus not all of the strategies available in theperception of an object have to be available for the extraction of information froma mental picture of it. Such a definition is not required by a Recognition Theory ofdepiction.

Indeed, rejecting that definition is helpful for removing the problems of explan-ation produced by variable strategies, problems to which Dennett has earlier givenvoice. I suggested that Dennett’s remarks tend to conflate metaphysical concernswith epistemic ones. But the appeal to strategies now eliminates the metaphysicalconcerns. As a metaphysical matter, the point is that an image, taken as a mentalpicture, consists in activity in areas of the visual system dedicated to certain basiccapacities. When there is such activity in the absence of a visual stimulus, we mightsay, the relevant brain regions function like a mental picture, but different strategicuses or deployments of those capacities need not be viewed as changingthat func-tion. Not every use of a representational function contributes to the identity of thatfunction. In particular, the control operations that are applied to it do not.

The epistemological challenge – how we know which functions are deployed toproduce an image withwhat content – remains. Yet a hard problem is not neces-sarily an impossible one. Kosslyn has outlined several converging techniques forrestricting the range of strategic options on a particular task. I have suggested thatstudies in which certain strategies are blocked provide even further insight intowhat Kosslyn calls “cognitive styles”. My point now is simply that we can gainsome important leverage for assigning content to images by knowing somethingabout the strategies that are likely to be used to generate them, as well as thestrategies which they, in turn, could support.

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6. Conclusion

Whether one agrees that Kosslyn’s defense of pictorialism is successful or not, onecannot fail to appreciate the way in which his work integrates so much currentresearch into a coherent theory. HisImage and Brainprovides both a guidingoutline of that research and many of the details of the operation of the strategiceye. It is a major contribution to cognitive neuroscience, one to which philosopherand scientist alike should pay close attention.

Notes1As will emerge later in my discussion, Kosslyn has provided more evidence that bears on theseproblems since the publication of his book, and he has offered further analyses of the issues whichfit well with an emphasis on strategies. But he does not yet explicitly emphasize the role of strategyor account in detail for some of the specific counterevidence, nor does he develop the implicationsfor explanation, as I will try to do here.2This is not true, on Kosslyn’s current view, of the other phenomenon that was central to the earlierimagery debates, viz, image rotation. That and other image transformations are now said to consistin changes in the pattern codes stored in the ventral system, the results of which are then displayedin the visual buffer for scanning.3This conflation is perhaps not surprising, given Dennett’s instrumentalist view of science.4Kosslyn treats the methodological problem of perceptual strategy as one of explaining individualdifferences. That may be misleading. It tends to confuse the problem of predicting actual performancewith difficulties in making the lawlike generalizations that are required for explanation. The latterdoes not require predictions of performance.5That is, they do not serve the performance of the task at hand well by economizing on resources,requiring a proper amount of time, etc. When utility and nonutility are so construed, a variety ofstrategies can be said to be useful, even if they are nonoptimal. This construal is of course distinctfrom the trivial sense in which anything that happens to be used is de facto useful.6On Kosslyn’s account, generic parts are not ordinarily held in long-term storage. He argues that aslong as we assume the operation of aviewpoint consistency constrainton the matching of nonacci-dental properties (so that the matching is compatible with a single point of view), then a top-downsearch in the testing of visual hypotheses can essentially generates the requisite parts. Note thatKosslyn also maintains that one will be likely to fixate on parts if they have distinctive features (p.167). In that case, they can be encoded via covert attention. Thus the interpretive bias due to partstorage need not be due to verbal coding.7Of course, there are perspective reversals and figure-ground changes in ordinary perception, too,and in such perceptual cases, as well as in imaging, reinterpretations can often be very difficult. Thusthe microproperties the imaging studies reveal pertain to perception as well, at least to some extent.Imaging experiments bring them out largely by restricting in various ways the roles they play in theperformance of a task.

In more recent work (Kosslyn et al., in press [a]; Rouw et al., in press [b]), Kosslyn also emphas-izes the fact that different tasks require different types of images and argues that this is consistentwith the perceptual similitude thesis. However, he does not offer a specific analysis of Reisberg’sresults, of the sort I have laid out here. I discuss this work below.8As I have noted, Reisberg often speaks as if perceptual organization is what constrains the type ofconcepts that can be applied to an image; but, as he recognizes, that does not pertain to ambiguousfigures like the duck–rabbit, in which both interpretations have the same perceptual organization.Whether perceptual organization is assigned before a concept-based interpretation is made or viceversa is a separate issue from the point I am now making; which is that both might depend on thedirection of attention.

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9On Chambers and Reisberg’s view, it is not the case that a binding interpretation is imposed duringperception; rather the interpretation and organization that prevents reinterpretation is made during theconstruction of images from memory. It is at that point, on their account, that information is lost. ButI claim that their own research shows that the relevant information is somehow retained in memory,because subjects are able to later redraw the complete original figures in which the ambiguity isintact (see Rollins, 1994). It must therefore be the case that information then deleted from the imagein the visual buffer is available in memory but for some reason cannot be retrieved. The effect ofinterpretation is thus both a loss of portions of the contour from the image in the buffer (if Chambersand Reisberg are right in that respect) and the creation of a barrier to retrieval of any description ofthe original contour that might be contained in memory. My view is that organization is imposedduring perception, but some information that has not been explictly encoded as compatible with theorganization still remains available in memory.10This would explain why reinterpretation sometimes does and sometimes does not occur, at least insome cases. Not all selective organization has to depend on a capture strategy. Thus, one might arguethat, where reinterpretation has been prevented, a capture strategy has been used; whereas wherereintrepretation occurs, such a strategy has not been used. The question then is, why should the useof a capture strategy be likely in the Chambers and Reisberg experiments? The answer, I believe,is that, in some of the cases at least, the use of drawings with certain pictorial properties – e.g. thecaricature-like emphasis of diagnostic features in the duck–rabbit – promotes the use of incompletemental representations in which the diagnostic features are exaggerated even further. The Chambersand Reisberg experiments were then able to recreate the original figures in their own drawings in thesame way that the captured spots on a moving leopard can be used to identify it. The correspondenceassumed between the represented and the unrepresented information allows for a reconstruction.11The second part, which amounts to a theory of pictorial content, may or may not be a function ofthe first part. That is, pictorial content might be individuated in a way that is characteristic only ofpictures (e.g. by resemblence), or it might not (e.g. if the content of both pictures and words wereindividuated by causal relations).12 One could also argue that resemblence is a necessary condition for depiction, to which is addedsome other psychological conditions that would be distinctive of picture perception, but a percep-tual equivalence between a picture and its object does not entail that one experience a resemblencebetween them; whereas such an experience does presuppose their perceptual equivalence. This sug-gests that resemblence is really neither necessary nor sufficient, but only a common characteristic ofpictures and images. Even where there is resemblence, the component of the definition of ‘mentalpicture’ that will be most important for explaining performance on imaging tasks will still be the setof perceptual functions involved; i.e. the perceptual equivalence between imaging and perceiving.

I set aside here the question of whether the right sort of causal history is also required forsomething to count as a picture. This idea is congenial to Kosslyn’s views, but it is not requiredby them.

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