ORIGINAL ARTICLE

Marguerite Cocude á Emmanuel Mellet á Michel Denis

Visual and mental exploration of visuo-spatial con®gurations:Behavioral and neuroimaging approaches

Received: 24 January 1998 /Accepted: 18 April 1998

Abstract Do mental imagery and perception involvecommon processing mechanisms? Imagery researchershave devoted a great deal of e�ort to establishing thefunctional and structural similarities between imagesand perceptual events. Recent studies have focused onthe comparison of images that are reconstructions ofprevious perceptual experience and images constructedfrom verbal descriptions. This article reports the ®ndingsof a research program based on the mental scanningparadigm; they reveal the similarities and di�erencesbetween the two kinds of mental images. Neuroimagingstudies have also provided evidence that the parieto-occipital cortex is involved in the processing of visualimages, whether they are based on perceptual experienceor constructed from linguistic inputs. However, thePET studies conducted by our research groups provideno evidence that the primary visual cortex is engagedin the generation of visual images. As there is con-tradictory evidence about this, further research is neededto clarify the role of the early visual areas in mentalvisual imagery.

Introduction

People interact perceptually with the objects in theirenvironment, but their visual experience is not merelysuspended when objects are no longer available to theirperception. Internal representations may be activated inorder to retrieve ®gural information or to access infor-

mation that has not been explicitly encoded but is in-ferrable from processing these representations. Visualanticipation is another context in which visual aspects ofan object are activated from memory. This paper con-centrates on behaviorally relevant aspects of visualcognition that can be demonstrated in the absence ofvisual perception.

The concept of mental imagery encompasses a set ofpsychological processes that specialize in the creation ofmental events by which absent objects are temporarilymade available for a subject's conscious inspection (cf.Denis, 1991; Kosslyn, 1980, 1994). Some images consistof mere reproductions of past perceptual events, inquasi-photographic or more schematic forms. However,in recent years, imagery researchers have recognized thesigni®cance of a form of imagery that has long beenneglected, in spite of it being ubiquitous in people'scognitive activity. This is the use of imagery constructedfrom discourse or verbal descriptions, when language isused to help a person construct the image of new objectsor con®gurations that he/she has not yet perceptuallyencountered (and may well never encounter). People areable to create new visual knowledge and perform mentaloperations on these internal representations comparableto those executed on images resulting from actual per-ceptual experience.

This is the situation encountered by the person shownin the right-hand part of Fig. 1. The ®gure illustrates atypical communication situation. The person on the leftis in the presence of a visual con®guration, and her taskis to convey to her listener a verbal description whichshould allow him to build an internal representation ofthe con®guration. The listener may have to build thisrepresentation and store it in order to retrieve it later toperform speci®c operations on it (for instance, inspect itin order to answer questions such as ``Is there a blackobject just above a white one?'', incorporate furtheritems into the con®guration, or modify its internalstructure). This communication situation illustratesseveral important cognitive issues. In particular, thevery fact that a mental image can be generated from a

Psychological Research (1999) 62: 93±106 Ó Springer-Verlag 1999

M. Denis (&) á M. CocudeGroupe Cognition Humaine,LIMSI-CNRS, Universite de Paris-Sud,BP 133, F-91403 Orsay, France;e-mail: denis@limsi.fr

E. MelletGroupe d'Imagerie Neurofonctionnelle,GIP Cyceron, Universite de Caen, Caen, France

linguistic input raises the issue of the relationships be-tween cognitive representational systems endowed withdistinct functional properties (cf. Bloom, Peterson, Na-del, & Garrett, 1996; Denis, 1996; Landau & Jackendo�,1993).

These interfacing problems are well documented onthe production side. A well-known problem is thesequencing of outputs selected by speakers duringdiscourse. Discourse has an inherently linear structure.Any entity having two or more dimensions can only bedescribed linearly. Although speci®c linguistic devicesmay be implemented to provide macrostructural infor-mation, speakers must convey information by adoptinga sequence selected from among a wide range of options.Speakers tend to use only a subset of these possibleorders, but the subset remains very large. Some ordersmay prove to be more friendly than others or betteradjusted to the expectations of the listener, resulting inbetter processing conditions (cf. Daniel, Carite , & Denis,1996; Levelt, 1989; Robin & Denis, 1991).

The listener's task is to carry out the reverse trans-formation, starting from a linearly organized linguisticinput and building an internal representation that re-¯ects the structure of the con®guration described. Thereare situations where such transformation need not takeplace (if, for instance, the listener judges it to be ad-vantageous to only perform rote learning of the verbalinput), but we are interested in the situation where thelistener makes use of visual imagery to generate aninternal experience that is, to some extent, similar to theperceptual experience that the speaker has had or that hewould himself have if he were exposed to the originalcon®guration.

This situation raises the issue on which the e�orts ofour research groups have converged over the past fewyears: What is the cognitive status of visual images, anddoes it depend on the conditions in which they areconstructed? We refer here to the contrast between twoforms of visual imagery. In one, the image is constructedfrom a visual experience and, in the other, from verbal

inputs. Both forms of imagery convey some form of®gural information, and both clearly fall within thedomain of visual cognition. However, do they have thesame properties? Are images constructed more fuzzywithout any initial visual experience, or do they lackclarity when compared to images that are essentiallyreconstructions of visual inputs? Could the informationavailable in these images be insu�ciently integrated?Would its internal structure be less respectful of theobject's structure? The situation in Fig. 1 illustrates anideal case, where information has been exhaustively andaccurately transmitted. However, this is just a theoreti-cal case. What is the status of images constructed fromverbal inputs?

The research program described below was based onthe assumption that visual images have a structurewhich is not random, but genuinely re¯ects the structureof the objects represented. This assumption is supportedby a number of empirical investigations on perceptually-based visual images, which are thought to correspond topatterns of activation in a specialized medium (the so-called visual bu�er). The visual bu�er has structuralproperties which constrain the pattern of its activation.Images are thus internal representations that havestructural features similar to those of the objects theyrepresent (cf. Finke, 1989; Kosslyn, 1980, 1994).

The mental scanning paradigm

Mental scanning is undoubtedly the paradigm which hascontributed most to support claims about the structuralproperties of visual images (cf. Kosslyn, 1973; Kosslyn,Ball, & Reiser, 1978). In a typical mental scanning ex-periment, subjects are invited to learn a visual con®gu-ration containing several objects, each at a speci®clocation. For instance, in the island map that popular-ized the paradigm (Kosslyn et al., 1978, Exp. 2), sevengeographical features are located in such a way that thedistances between each pair of objects are di�erent(Fig. 2). Once the subjects have memorized thecon®guration and the precise locations of landmarks,

Fig. 1 A communication situation

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the experiment proper starts. In each trial, the subjectfocuses on a speci®ed object (for instance, the hut).Upon hearing the name of a second object (for instance,the beach), the subject has to mentally scan the distancethat separates the two objects and to indicate (bypressing a button) when scanning is completed. Thesubject's response interrupts a clock that gives the timetaken to perform the mental scanning on each trial. Thesecond named object may also be an object that was notpresent in the island map. If this occurs, the subjectpresses another button, indicating that the object is notpart of the original con®guration. The mental scanningexperiments in fact focus on analysis of the ®rst type ofresponse.

The main ®nding from the experiments using thisparadigm is that scanning times increase linearly withincreasing distance. In addition, when times are aver-aged over subjects, there is usually a signi®cant positivecorrelation between times and distances (Fig. 3). This®nding has generally been taken to re¯ect the structuralisomorphism of mental visual images with the objects orcon®gurations they represent. In particular, an imageexhibiting the mental scanning e�ect is thought to in-corporate the metric information present in the originalobject. No claim is made that visual images have spatialproperties in themselves, or that they occupy metricallyde®ned portions of the brain. The idea is that themechanisms that explore visual images operate on acognitive entity which includes the spatial characteristicsthat were processed during perception. In other words,the chronometric pattern of the scanning process is in-voked to make claims about the structure of the mentalrepresentation on which it operates. Although these in-terpretations elicited considerable discussion in the earlyeighties (e.g., Pylyshyn, 1981), the mental scanning dataresisted interpretations to reduce them to the mere re-¯ection of subjects' metacognitive knowledge about therelationships among distance, time, and speed (cf. Denis& Carfantan, 1985; Pinker, Choate, & Finke, 1984).

Thus, a visual image may genuinely re¯ect the spatialstructure of the con®guration it stands for, in particularthe relative distances among speci®c points. These®ndings do not exclude the possibility that some imagesmay lack such properties and be fuzzy or indeterminate.However, provided enough processing has taken placeduring learning, it seems that images can achieve struc-tural coherence and de®nition, which are re¯ected by themental scanning pattern. In optimal learning conditions,it is possible to show that not only do mental imageshave an internal structure, but that this structure isanalogous to the structure of perceived objects.

At this point, a further issue has emerged. Consid-erable e�ort has been devoted to study the relationshipsbetween images and perceptual experience, but there hasbeen less interest in images resulting from creative pro-cesses. In fact, much of the imagery used in daily cog-nitive activities consists of creating new images, for suchthings as problem solving, spatial reasoning, or reading.For instance, it is common for readers to attempt tocreate internal scenes providing an imaginal substratefor text comprehension (cf. Denis, 1982; Johnson-Laird,1996). If it is recognized that language can be used toconvey instructions for generating new images, possiblyby the recombination of already available units, will theimages constructed from verbal descriptions be of thesame nature as the images of the object constructed afteractually seeing it?

There are many situations suggesting that an imagegenerated by processing a verbal description lacks theclarity or vividness of an image resulting from percep-tion. Let us consider the ideal case where discourse isprecise and explicit enough to allow a reader or listenerto create an image whose geometry is strictly compara-ble to that of a perceptually-based image. This case as-sumes that discourse involves a high degree of codingand a number of descriptive conventions. Some highlyspecialized domains have such linguistic devices. Forexample, heraldry has a ®nite corpus of conventional

Fig. 2 The map used in Kosslyn, Ball, and Reiser's (1978) experi-ments

Fig. 3 Time needed to scan between all pairs of locations on theimaged map (Kosslyn, Ball, & Reiser, 1978, Exp. 2)

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terms that makes it possible to construct con®gurationsthat are unambiguous and practically identical for allindividuals who possess the code. Other examples arespatial descriptions using precise metric and positionalindications (``The vase is 30 centimeters to the left of thecomputer and 15 centimeters behind the telephone''),which allow several listeners to build quite similar im-ages. Such a type of discourse is important in commu-nication contexts that require people to have similarrepresentations, on which depends the coordination ofaction of several operators. This is illustrated in ergo-nomic contexts such as aerial navigation or copiloting incars, where it is common to speak of objects or direc-tions ``at 10 o'clock'' to refer to the direction whichpoints away from an observer by an angle relative to his/her frontal axis that is identical to the position of 10relative to noon on a clock.

The research program summarized in the present ar-ticle essentially consisted of creating situations in whichsubjects were invited to build an image of a spatial con-®guration without any perceptual cues, using only averbal description. Subjects were then asked to executecognitive operations on the newly constructed image.The chronometric patterns of these operations werecompared to those observed when the subjects processedimages derived from perception. This comparison madeit possible to measure the similarities and di�erencesbetween images derived from these two sources. Themental scanning paradigm was selected for this researchprogram, based on the postulate that the analysis of ex-ploration processes provides indirect, but valid, re¯ec-tions of the structure of visual images. The similaritybetween scanning patterns is thought to justify makinginferences about the internal structure of scanned images.

Scanning visual images constructed fromperceptual or verbal inputs

Several constraints had to be respected in order to usecomparable materials in the two learning conditions(con®guration learning and description learning). First,if the structure of the object must be described com-pletely, shapes that can be easily designated verbally arerequired. For instance, the outline shape of the islandused by Kosslyn et al. (1978) is too complex for anyreasonable attempt at precise description. We thereforereplaced this shape with a simpler circular one, whichis easier to refer to verbally. The landmarks were lo-cated on the periphery of the circle, and they had thesame function as the landmarks on Kosslyn's island(Fig. 4a). Secondly, we needed a descriptive devicewhich made it possible to locate each landmark at ametrically de®ned location that could be described veryclearly, in such a way that the subject assigns to eachlandmark an unambiguous location in his/her image.We therefore used the conventional directions used inaerial navigation, stating, for instance, than an object islocated ``at 1 o'clock'' (Fig. 4b). The subjects who par-

ticipated in the experiments were all familiar with theseconventions. Lastly, the successive steps of learning inthe description condition could not be assessed by ask-ing subjects to ®ll in a blank map after each trial, as thiswould have provided the subjects with ®gural informa-tion in addition to the verbal material. Thus, the de-scription was learned in a number of trials ®xed inadvance. Preliminary experiments allowed us to checkthe minimal number of trials needed to draw the mapand to indicate good memory of the con®guration, itslandmarks, and their positions.

The ®rst set of Denis and Cocude's (1989) experi-ments using this material con®rmed the existence of themental scanning e�ect when subjects had learned themap material under conditions identical to those ofKosslyn et al. (1978). They also provided new informa-tion on the variation of scanning times in a visual(perceptual) scanning task. The classic interpretation ofmental scanning experiments is that the linear relation-ship between scanning times and distances re¯ects thestructural similarity of the image to the perceived con-®guration. The tacit assumption is that the linear rela-tionship holds true in a condition where subjectsperform perceptual scanning on the con®guration itself.Surprisingly, however, the literature on mental scanningdid not report controls for this condition. This is why weincluded a perceptual scanning task to ensure that per-ceptual scanning does exhibit the regularities that areassumed to be transferred to imaginal conditions. Theresults, in fact, showed that visual scanning of an actualcon®guration and the mental scanning of the visualimage of this con®guration resulted in very similarchronometric patterns. The time to scan between loca-tions on a visual con®guration and a visuo-spatialmental representation that was learned perceptually in-creased with the distance between the two locations(Figs. 5a and 5b).

Fig. 4a, b Materials used in Denis and Cocude's (1989) experiments.a Map. b Description

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Similar linear time-distance relations also occurred inthe mental scanning of representations formed fromverbal descriptions. However, there were some di�er-ences in the scanning performance under the two im-agery conditions. In the verbal learning condition,subjects were presented with the description three times.Their response times resulted in a signi®cant, butsomewhat lower, time-distance correlation coe�cientthan those of the subjects in the perceptual learningcondition. Their absolute scanning times were alsolonger than in the perceptual learning condition(Fig. 6a). Although the verbal information contained inthe description was memorized accurately after threetrials, the structural qualities of the visuo-spatial repre-sentation were presumably imperfectly consolidated,resulting in a poorer time-distance correlation. In theverbal learning condition, another group of subjectslistened to the description six times. The di�erencespreviously observed disappeared, and scanning perfor-mance was now very similar to performance followingperceptual learning (Fig. 6b). Thus, for a moderate rateof learning, an image constructed from a verbal de-

scription may not attain the coherence of an imagederived from perception. With additional learning of thedescription, the image may reach a structural coherenceand resolution which makes it cognitively similar to aperceptually-based image.

These experiments provided the ®rst demonstrationthat mental images generated from descriptions can havegenuine metric properties, as these are re¯ected in thechronometric pattern of mental scanning processes, justas in scanning tasks involving images constructed fromperceptual inputs. The inclusion of metric properties inimages constructed from verbal descriptions is an espe-cially interesting feature, as it shows that these imagescontain information in addition to the explicit infor-mation given in the descriptions. In our experiments, notonly did images contain information on the nature andlocations of the geographical landmarks (informationthat was e�ectively conveyed by the descriptions), butinformation on the relative distances among landmarkswas actually available in the representations constructedby the subjects, although the descriptions said nothingabout these distances. This was due to the transforma-

Fig. 5a, b Response time (msec) as a function of scanning distance(cm) in conditions where subjects were exposed to the map (Denis &Cocude, 1989, Exp. 1). a Visual scanning of the map. b Mentalscanning following map learning

Fig. 6a, b Response time (msec) as a function of scanning distance(cm) in conditions where subjects heard the description (Denis &Cocude, 1989, Exps. 1 and 2). a Mental scanning following threelearning trials. b Mental scanning following six learning trials

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tion of the verbal inputs to a visuo-spatial representa-tion, using a medium that cannot elude explicit expres-sion of distances.

Alternative explanations of the ®ndings must beconsidered. It may be, for instance, that the subjects inour experiments based their responses on a representa-tion of the verbal description they listened to, ratherthan on the visual image they were supposed to mentallyscan. It is possible that, after listening to the description,subjects kept a propositional representation of the textin their memory in addition to a visual image of thecon®guration. Their responses during the mental scan-ning task could have been based on the informationretrieved from the propositions. If scanning between alandmark located at ``1 o'clock'' and another one at ``2o'clock'' is based on an estimation of the distance sep-arating the landmarks, simple computation may result inan estimate (and corresponding scanning time) thatshould be shorter than, say, when landmarks are locatedat ``1 o'clock'' and ``4 o'clock.'' Some locations mayindeed o�er such alternate ways of computing distances.This explanation, however, is only valid for a subset ofitems. For instance, it can hardly be used to estimatedistances separating landmarks located at ``11 o'clock''and ``2 o'clock.'' Computation based on memory fornumerical values thus does not account for all the dis-tances involved.

However, in our experiments, we systematically try toexclude any risk that the data re¯ect any contributionfrom such alternate strategies by conducting in-depthinterviews of the subjects after the experiment in whichthey are questioned as to whether they used such a``numerical'' strategy. The subjects who report havingrevised the hour-coded location of a landmark beforementally scanning to it are excluded and replaced to besure that none of the response times used in the analysisre¯ect any strategy other than the mental scanning ofimages. The number of subjects excluded is generallyvery low, suggesting that the majority of subjects obeythe instructions to construct a visual image of the con-®guration and base their responses on that image (not itsverbal description). Further questions asked after theexperiment are used to estimate how frequently subjectsfeel they have followed the image scanning instructions.The data from subjects who report having followed theinstructions less than 75% of the time during the testphase are excluded from the analyses. Thus, even thoughthe subjects in our experiments probably encode bothverbal and imaginal information, there are good reasonsto think that the data used in the analyses have beenproduced by subjects who processed an image repre-sentation during the scanning task. By taking similarprecautions in experiments where subjects were asked tocompare pairs of distances after learning verbal de-scriptions of spatial con®gurations, we collected evi-dence that computation was more likely to be based onimages than on numerical or verbal representations. Thefact that subjects scoring high on a test measuring theirimagery capacities performed more accurately and faster

than low scorers lent support to the notion that thesubstrate of their computations was a genuinely visuo-spatial representation (cf. Denis, 1996; Denis & Zimmer,1992).

Structural properties of descriptions and images

The Denis and Cocude (1989) data support the claimthat images generated from descriptions can have someof the structural properties that are characteristic ofimages derived from perceptual experience. However, wedo not claim that both sorts of images are generallyequivalent. The case illustrated in our experiment is, tosome extent, an optimal one. The experimental condi-tions were designed to allow the phenomenon to appear,which indeed occurred. However, the equivalence ofimages generated from perception and from descriptionsis limited to the geometrical properties of the image.Other properties such as subjective vividness, may bedi�erently expressed in perceptually or linguistically-based images. In particular, the time course of learningand the regular sequence used for the description helpedthe image achieve the same structural coherence as whenit was shaped by visually examining the detailed metricsof the object.

For this reason, the subsequent step in our researchprogram was to examine the sensitivity of the mentalscanning paradigm to experimental variations likely tocreate more di�cult conditions for processing informa-tion and building a coherent, integrated image (Denis &Cocude, 1992). There is substantial literature on the ef-fects of discourse structure on the construction of mentalrepresentations of described objects. In particular, theorder in which information is entered in descriptionsa�ects the on-line construction of internal representa-tions and thus their availability for retrieval. Indeter-minacies and referential discontinuity have also beenshown to hinder the elaboration of visual mental modelsof spatial con®gurations (cf. Denis, 1996; Denis & De-nhieÁ re, 1990; Ehrlich & Johnson-Laird, 1982; Mani &Johnson-Laird, 1982).

While a well-structured description was used in theprevious experiments, with geographical details pre-sented in a predictable, consistent clockwise sequence,we thought it relevant to determine whether subjectscould construct e�ective, scannable representations frompoorly structured descriptions. We constructed a versionof the description that was designed to make it di�cultfor subjects to incorporate details into the outlinestructure of the map. Sentences were presented in arandom sequence intended to create more demandingconditions for the formation of an image, while notimpeding its elaboration, so that it should take longer toreach a stable, well-de®ned image than by using a well-structured description.

This new set of experiments was also intended tostudy image construction at di�erent points in thelearning of the description. Unlike the previous experi-

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ments in which di�erent subjects performed the mentalscanning task after three or six learning trials, the newexperiments used the same subjects at each successivestep in the experiment. Subjects took part in threelearning trials before performing the ®rst mental scan-ning task. They then resumed learning for three moretrials and performed the second scanning task.

Analysis of response times revealed that the subjectswho processed the clockwise description produced re-sponses re¯ecting structural coherence of their images inthe ®rst scanning task, as indicated by a signi®cantpositive correlation between times and distances. Afteradditional exposure to the same description, subjects'responses in the second scanning test resulted in a slightincrease in the time-distance correlation and a decreasein absolute scanning times (Fig. 7a). The pattern of re-sults was strikingly di�erent for the subjects who pro-cessed the random description. The results of the ®rstscanning test gave no suggestion that the subjects'mental images possessed any internal structure. Re-sponse times were very long, and there was no signi®cantcorrelation between scanning times and distances. Thesituation changed markedly after three more exposuresto the description. Scanning times were much shorterand there was now a signi®cant positive correlation be-tween times and distances (Fig. 7b).

These data con®rmed that images generated fromverbal descriptions can have metric properties similar tothose of images derived from perceptual inputs. Theyalso demonstrated that the structure of a description can

a�ect the intrinsic structure of images of described ob-jects and hence the mental operations subsequentlyperformed on these images. The subjects still showed thestandard time-distance correlation when the descriptionwas poorly structured, but the manifestation of thise�ect was substantially delayed. In contrast, a well-structured description, which placed minimal require-ments on the subjects' processing capacities, rapidlyyielded accurate, well integrated portrayals of thedescribed objects, as well as a more pronounced corre-lation between time and distance. Therefore, poorlystructured descriptions required additional exposure toachieve an image coherence similar to that produced bywell-structured descriptions. We interpreted these resultsas indicating that the referential validity of images (i.e.,their capacity to re¯ect accurately the objects they referto) is not an all-or-nothing property, but results fromstepwise elaboration.

The process of image elaboration

The gradual construction of visual images was investi-gated in more detail in a subsequent experiment (Denis,GoncË alves, & Memmi, 1995). A model was developed toaccount for the gradual process of image elaborationand the progressive increase in image accuracy. Themodel posits that the location of a landmark mentionedin a description is not represented as a sharp point in themental image, but is instead associated with a region

Fig. 7a, b Response time(msec) as a function of scanningdistance in the ®rst and secondtests (Denis & Cocude, 1992,Exp. 1). Distances are expressedas their ratios to the diameter ofthe island. a Clockwise descrip-tion. b Random description

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around this point. The region represents the possiblerange of the landmark's location at a given stage of thelearning process. Learning the description essentiallyconsists of progressively narrowing each ``region of un-certainty'' associated with a landmark to its exact loca-tion. The size of the unfocused (or ``fuzzy'') regions isexpected to vary inversely with the degree of imageelaboration. The closer the image is to its ultimate stepof elaboration, the more restricted these regions are.

Additional experimental data were collected to pro-vide a more ®ne-grained understanding of image elab-oration. Whereas in the previous experiment the subjectsheard the description six times and received two scan-ning tests (one after three exposures and the other afterthree more exposures), the new experiment used thesame number of exposures, but there was a total of threemental scanning tests, one after each pair of learningtrials. This procedure provided three successive ``views''of the mental representation under construction. Thesubjects were presented with the description in randomorder, which should show the most dramatic e�ects.

The scanning times for the ®rst test were longer thanin the second test, and the times further decreased be-tween the last two tests. In addition, correlational ana-lyses revealed that there was no correlation between thetimes and distances for the ®rst scanning test, but thecoe�cients reached signi®cance for the second and thethird tests (Fig. 8). These results are in agreement withthose of the Denis and Cocude (1992) study. They alsoprovide a more detailed view of the dynamics of theprocess. The results of the ®rst scanning test revealed nostructure at all in the image under construction. Re-sponse times were extremely long. The ®rst two expo-sures were not enough to construct a coherent, accurateimage, since mental exploration revealed no sign of themetric properties of the imagined map. Two more ex-posures changed the situation dramatically. Subjectsperforming mental scanning produced chronometricpatterns which revealed that their images had an internalstructure in which metric information was fairly cor-

rectly represented. The times were also signi®cantlyshorter, a ®nding compatible with the assumption thatthe images had an internal structure which was morereadily available. The process was even more markedafter two additional exposures. The numerical values atthe third test (scanning times as well as correlation co-e�cients) all indicated further improvement in the in-ternal structure of the image.

Our model of image accuracy was applied to the datato see whether the analysis re¯ected a stepwise restric-tion of regions of uncertainty in the mental image, andwhether such restriction was more evident between the®rst and second tests than between the second and third.Individual scanning times for every distance separating apair of landmarks were used to compute values re¯ectingthe sizes of regions of uncertainty. The underlying logicwas that these times could be used in conjunction with ameasure of each subject's individual scanning speed tocompute the distances theoretically scanned during thesetimes. Each of these distances was thus compared withthe actual distance separating the corresponding pair oflandmarks, which made it possible to calculate the errorin the location of the landmark. We ®rst calculated eachsubject's scanning speed, using measures of the thirdscanning test as references. Each individual scanningtime for a given subject in each test was then used tocompute a corresponding estimated distance based onindividual speed values. The distance was compared tothe actual distance and the corresponding error wascalculated. The scanning time used for a given distancewas an average value for both scanning directions (e.g.,``lighthouse±harbor'' and ``harbor±lighthouse''). Theerror was thus distributed evenly between the twolandmarks. A total of ®ve error values were calculatedfor each landmark (since there were six landmarks, anygiven landmark was involved in ®ve distances). The er-ror was expressed in terms of the ratio to the diameter ofthe island. Corresponding values were then entered asradii of circles re¯ecting the region of uncertainty asso-ciated with each landmark. The regions of uncertaintyfor each individual subject were expressed graphically ascircles centered on the exact point of each landmark.Averaging values over subjects for each scanning test ineach condition resulted in graphic representations of the

Fig. 8 Response time (msec) as a function of scanning distance inthree successive tests (Denis, GoncË alves, & Memmi, 1995). Distancesare expressed as their ratios to the diameter of the island

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regions of uncertainty for the three scanning tests(Fig. 9).

Inspection of the data ®rst revealed that the sizes ofregions of uncertainty (i.e., the reciprocal of accuracy)varied widely among landmarks. Overall, the landmarksin the part of the map which was the richest in land-marks tended to be located most accurately (i.e., bysmallest regions of uncertainty). This result is consistentwith the assumption that landmark location is favoredwhen a landmark has close neighbors. Isolated land-marks were less accurately located at the outset. Theother feature of interest is that the size of regions ofuncertainty decreased after each new exposure of thesubjects to the description. This decrease occurred for allregions, although it was more marked for some of them,and the hierarchy among the six landmarks remained thesame throughout the experiment. As expected, there wasa greater decrease between the ®rst and second tests thanbetween the second and third. The fact that there was aslight additional restriction of regions of uncertaintybetween the second and third tests con®rms that therecan still be a further improvement in image coherenceand resolution even when the verbal description is per-fectly memorized.

The robustness of the mental scanning effectand the role of individual imagery capacities

The mental scanning experiments described above fo-cused mainly on the spatial (metric) properties of theobjects represented. We did not investigate the e�ects ofsemantic content of the geographical landmarks on themap. For instance, ``harbor,'' ``lighthouse,'' and so on,were used as instantiations of points for which only to-pological and metric properties were considered. How-ever, in memory of real-world spatial con®gurations, thestructure of mental representations depends to someextent on knowledge, experience, and the value attached

by the subjects to the landmarks. For example, subjectsrequired to estimate distances in natural environmentstend to underestimate distances which separate themfrom a landmark which is well-known or with whichthey have had repeated interactions, whereas they tendto overestimate the distances to less frequently visitedlandmarks. This bias is probably at work in the experi-ments showing that distances are underestimated in thecenter of cities and overestimated in peripheral zones (cf.Byrne, 1979; Moar & Bower, 1983).

The sensitivity of the mental scanning paradigm todescriptions in which some of the landmarks underwentspecial cognitive processing likely to give them particu-lar cognitive salience was examined (Denis & Cocude,1997). We wanted to determine whether a speci®c ma-nipulation of the description could be responsible forsystematic biases in the representation constructed fromit, as in real-world con®gurations. Our expectation wasthat such biases would be revealed through scanningtimes di�ering from those recorded previously.

The ®rst experiment devoted to this issue ensured thatthree of the six landmarks on the periphery of the islandwould be processed in a particular way. The descriptionof each landmark not only gave information on its lo-cation, but also provided a short narration containingmany concrete details, in order to increase the salience ofthis landmark. The other three landmarks were de-scribed in a rather neutral fashion. Subjects learned thedescriptions and were required to perform the mentalscanning task. The relevant aspect of the data here wasthe scanning times towards an important landmark anda secondary one. The results showed that there was nodi�erence between these times, and time-distance cor-relation coe�cients were virtually identical, whatever thetype of scanning. This suggested that the time-distancecorrelation in the mental scanning paradigm is relativelyrobust, since the additional semantic content did nota�ect it.

The importance of the landmarks was then manipu-lated by including additional descriptions referring toimaginary actions of the subjects themselves in thelandmarks. The subjects had to imagine their own

Fig. 9 Graphic representations of regions of uncertainty (Denis,GoncË alves, & Memmi, 1995)

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activity in three landmarks, while the other three land-marks were described in neutral terms and did not implyany associated activity. This manipulation also pro-duced no di�erential e�ect on scanning times or oncorrelations. Lastly, the descriptions of three of the sixlandmarks were enriched by a detailed picture designedto give these landmarks greater cognitive salience. Theresults still revealed no signi®cant e�ect of di�erentialtreatment of the two sets of landmarks. Thus, on thewhole, experimental attempts to modify the salience oflandmarks in materials newly learned by subjects from averbal description did not result in representational bi-ases similar to those which were demonstrated in cog-nitive maps of natural spatial environments.

Lastly, we tested the sensitivity of mental scanning tosubjects' individual characteristics. The studies onmental scanning performed to date were done withoutmuch concern about individual di�erences and thepossible in¯uence of the imagery capacities of subjectson scanning performance (see, however, Dror, Kosslyn,& Waag, 1993; Kosslyn, Brunn, Cave, & Wallach, 1984).The present research was done to identify the e�ects ofindividuals' imagery capacities on the mental scanningof images constructed from a verbal description. Thesubjects of the experiments reported above were asked tocomplete the Minnesota Paper Form Board (MPFB:Likert & Quasha, 1941), a visuo-spatial test widely usedin imagery research. They were split into two groups,those who scored above and those who scored belowthe median of scores. Thus, we compared a group ofsubjects who were apt at generating and manipulatingvisual images and a group of subjects less prone toimaging.

Two contrasting patterns of results emerged. Thesubjects with high visuo-spatial capacities produced thepattern typical of mental scanning (relatively shortscanning times and a signi®cant time-distance correla-tion coe�cient; Fig. 10a). Conversely, the subjects withpoorer visuo-spatial capacities produced responseswhose chronometric characteristics indicated that theirimages had no stable, consistent structural properties.Their scanning times were quite long, and there was noconsistent relationship between scanning times and dis-tances (Fig. 10b). This pattern suggests that these sub-jects had di�culty controlling the generation andexploration of their images. Their images probablycontained a large amount of noise, which may have re-sulted from the di�culty experienced by these subjects inmaintaining their mental representations at a su�cientlyhigh level of activation.

Neuroimaging investigations

The experiments reported in this paper have contributedto document the functional and structural similaritiesbetween images and the perceptual events from whichthey were formed. The images derived from perceptionand those constructed from verbal descriptions were also

found to be very similar. In addition to the behavioralevidence that visual imagery and perception sharefunctional properties, there is a strong suggestion thatthe two systems involve common brain structures. Sev-eral neuropsychological studies support this assumption(e.g., Basso, Bisiach, & Luzzatti, 1980; Farah, 1984;Farah, Levine, & Calvanio, 1988). The involvement ofcortical structures common to visual imagery and per-ception is also indicated by studies on evoked potentials(Farah, 1995; Farah, Weisberg, Monheit, & Pe ronnet,1989) and regional cerebral blood ¯ow (Goldenberget al., 1989; Kosslyn et al., 1993).

However, some ®ndings have raised doubts that vi-sual imagery and visual perception use the very same

Fig. 10a, b Response time (msec) as a function of scanning distance(Denis & Cocude, 1997, Exp. 1). Distances are expressed as their ratiosto the diameter of the island. a High visuo-spatial imagers. b Lowvisuo-spatial imagers

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neural substrate. The most signi®cant are those showingthat brain-damaged patients with severely impaired ob-ject recognition may have fully preserved visual imagery(cf. Behrmann, Winocur, & Moscovitch, 1992). On theother hand, tests on patients with visual imagery disor-ders have shown that these subjects may have no per-ceptual disorder (cf. Guaraglia, Padovani, Pantano, &Pizzamiglio, 1993). The double dissociation betweenimagery and perception in brain-damaged patients maysimply re¯ect the fact that visual imagery depends onbrain structures that perception does not require.However, it is also possible that selective damage toearly visual areas impairs visual perception while leavingvisual imagery intact, if the areas subserving visual im-agery are considered to be a subset of those active invisual perception (cf. Roland & Gulya s, 1994).

Little information is available on the brain structuresthat are involved in visual imagery based on verbal in-puts. It is reasonable to assume that the same neuralsubstrate is activated whatever the origin of an image(derived from visual perception or constructed from adescription of a never-seen object). A neuroimagingstudy on visual imagery of described objects using singlephoton emission computerized tomography (SPECT)was conducted by our research groups, with an addi-tional investigation of the in¯uence of individual dif-ferences in imagery capacities (Charlot, Tzourio,Zilbovicius, Mazoyer, & Denis, 1992). Two visuo-spatialtests, the MPFB and the Mental Rotations Test (Van-denberg & Kuse, 1978), were used to select two groupsof subjects, one with high and the other with low visuo-spatial capacities (corresponding to the upper and lowerthirds of scores). The subjects ®rst learned the map ofthe island used in our previous experiments from apurely verbal description. They then took part in twocognitive tasks (in addition to a rest condition). One taskconsisted of mentally conjugating abstract, irregularverbs. The other task was reconstructing the visual im-age of the island and performing mental exploration ofthe distances separating the landmarks. Upon hearingthe name of one landmark, they were asked to mentally¯y from this landmark to each of the ®ve others andreturn, while maintaining as vivid a visual image aspossible. Because of the technical constraints of this typeof experiment, chronometric measurements of individualscanning performances were not done.

High visuo-spatial imagers showed a selective in-crease in blood ¯ow in the left sensorimotor cortex overthe resting state while performing the verbal task,whereas there was a signi®cant increase in blood ¯ow inthe left temporo-occipital cortex when the subjectsimagined and mentally scanned the visual con®guration(without activation of the primary visual cortex). Thisresult is compatible with the idea that visual represen-tations constructed from visual experience and thoseconstructed from descriptions involve the same corticalareas. In contrast, the low visuo-spatial imagers showedmuch less clearly di�erentiated increases in their cerebralblood ¯ow.

The issue of the involvement of the primary visualcortex in visual imagery remains open. Kosslyn et al.(1993) reported experiments using positron emissiontomography (PET) in a variant of Podgorny and Shep-ard's (1978) paradigm. Subjects were presented with agrid of 5 ´ 5 cells. One of the cells contained an X-mark.In the imagery condition, subjects had to visualize anuppercase letter and decide whether this letter wouldhave covered the X-mark if it were present in the grid. Inthe perceptual condition, the uppercase letter was su-perimposed on the grid and the same kind of decisionwas required. The primary visual cortex was activated inboth conditions, suggesting that imagery and perceptioncall upon common cerebral mechanisms. Activation wasalso greater in imagery than in perception, indicatingthat the generation of a visual image is a more de-manding cognitive task than perception. Other PETstudies have also reported increases in blood ¯ow in theprimary visual cortex when subjects created mental im-ages of places and persons familiar to them and in-spected these images in detail (Damasio et al., 1993).Similar conclusions were drawn from studies usingfunctional magnetic resonance imagery (cf. Le Bihanet al., 1993), but other research programs using the sametechnique found that the visual association cortex, andnot the primary visual cortex, was engaged during thegeneration of mental images (cf. D'Esposito et al., 1997).

The data indicating that early visual areas are acti-vated in visual imagery support the hypothesis that thevisual areas subserving visual imagery are identical tothose subserving visual perception (cf. Kosslyn et al.,1993; Kosslyn, Thompson, & Alpert, 1997; Kosslyn,Thompson, Kim, & Alpert, 1995). However, these areasare not activated during visual imagery in all subjects (cf.Le Bihan et al., 1993; Ogawa et al., 1993), and it ispossible that they are activated only by tasks that re-quire high-resolution images (cf. Sakai & Miyashita,1994). There is also evidence that early visual areas arenot involved in visual imagery, but that neuronal pop-ulations in temporo-occipital and parieto-occipital areasrepresent objects and scenes during imagery (cf. Roland& Gulya s, 1995).

Given the controversial nature of this issue, we de-signed an experiment to compare PET activations inperceptual and imagery conditions (Mellet, Tzourio,Denis, & Mazoyer, 1995). In the learning phase, subjectswere invited to inspect and memorize the map of an is-land similar to those used in previous experiments, withlandmarks around the periphery of the map. The subjectswere then examined in a perceptual or an imagery con-dition. In the perceptual condition, the subjects wereshown the map and required to scan from landmark tolandmark, alternatively in clockwise and counterclock-wise order. In the imagery condition, the subjects wereplaced in total darkness and were instructed to recreate avivid image of the map and then to perform a mentalexploration of the landmarks by following the sameprocedure. Brain activity was recorded during the twoexplorations and compared to a rest condition.

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The results clearly indicated that both tasks involveda common network of cerebral structures, including abilateral superior external occipital region and a leftinternal parietal region (precuneus) (Figs. 11a and 11b).While there are grounds for claiming that the occipitalregion is responsible for the generation and maintenanceof the visual image, the parietal region is likely to beinvolved in the exploration component of the process.Other PET studies have also indicated that memory-re-lated imagery is associated with precuneus activation (cf.Fletcher et al., 1995). The most critical ®nding was thatbilateral activation of the primary visual areas occurredin the perceptual condition, but these areas were notactivated during the mental exploration in the imagerycondition.

The involvement of regions belonging to the ``dorsalroute'' in the mental exploration of a previously learnedvisual con®guration is in line with current assumptionsabout the role of the parieto-occipital cortex in thespatial processing of mental images (cf. Farah, Ham-mond, Levine, & Calvanio, 1998; Levine, Warach, &

Farah, 1985). These ®ndings may indicate that mentalimagery involves the same dichotomy as the one in thevisual system between the dorsal pathway ± which isresponsible for processing spatial attributes of visualstimuli ± and the ventral pathway, which processes ®g-ural attributes (e.g., Haxby et al. 1991). We attempted toinvestigate the capacity of the cognitive system to gen-erate images based on verbal descriptions, conducting anexperiment designed to establish whether a highlyspecialized network for visuo-spatial processing can beactivated by purely verbal inputs (Mellet et al., 1996).

The PET technique was used to monitor variations inregional cerebral blood ¯ow while subjects constructedmental images of objects that they had never seen beforefrom oral instructions. These objects were three-dimen-sional cube assemblies that subjects were trained tomentally construct and visualize by listening to speci®csets of verbal instructions. This task involved a strongspatial component, since the objects to be imagined ex-

Fig. 11a, b Statistical parametric maps corresponding to twocomparisons (Mellet, Tzourio, Denis, & Mazoyer, 1995). a Visualexploration minus rest. b Mental exploration minus rest

Fig. 12a, b Statistical parametric maps corresponding to two com-parisons (Mellet et al., 1996). a Mental imagery minus rest. b Mentalimagery minus word listening

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tended along all three dimensions. This mental imagerytask was contrasted with two control conditions, oneinvolving passive listening to phonetically matched non-spatial word lists, and the other involving silent rest. Allthree tasks were performed in total darkness. The resultsshowed that the mental imagery task activated a bilat-eral occipito-parieto-frontal network, including the su-perior occipital cortex, the inferior parietal cortex, andthe premotor cortex. The right inferior temporal cortexwas also activated in this task, but there was no acti-vation of the primary visual areas (Figs. 12a and 12b).There was bilateral activation of the superior and middletemporal cortex during both mental imagery and passivelistening when they were compared to the rest condition.

These data con®rm that at least some mental imagerytasks may not involve any detectable participation ofearly visual areas. They also provide evidence that thedorsal route, which is known to process visuo-spatialinformation, can be recruited in the absence of any vi-sual input by auditory linguistic inputs only. This is animportant ®nding, since it indicates that the involvementof the dorsal route for spatial processing is not linked tothe modality under which information is presented tothe subject. This network involving visual unimodal andmultimodal association regions can thus operate on non-visual inputs and be activated whatever the nature ofinput. It is thus engaged in both the mental scanning ofvisual images and the creative construction of purelymental objects.

A brief conclusion

It is probably too early to draw a ``conclusion'' in a ®eldwhere so much work remains to be done. A few majorpoints, however, should be mentioned. Imagery researchhas for some time been devoted to assessing the func-tional similarities between mental images and perceptualevents. A new domain of investigation has been recentlyopened by researchers looking for the similarities anddi�erences between two sorts of mental images, thosewhich are reconstructions of previous perceptual expe-rience and those which are constructed anew from verbaldescriptions. We used the mental scanning paradigm,which has been shown to be a sensitive tool for assessingthe analogies between the structures of visual images andvisual percepts, to compare the processes involved in theexploration of visual images derived from direct per-ception and verbal description. This approach provideda more ®ne-grained assessment of the process of imageconstruction, as well as the robustness of the mentalscanning e�ect.

Neuroimaging techniques have recently been used toidentify the regions of the brain that are engaged whenpeople generate and explore mental images, whether theimages are reconstructed from perception or constructedfrom verbal inputs. The assumptions about the role ofthe parieto-occipital cortex in the processing of visuo-spatial images are clearly supported by PET investiga-

tions. Our studies, however, provide no indication thatthe primary visual cortex is involved in the generation ofvisual images, but the fact that there is contradictoryevidence about this issue suggests that it should be oneof the priorities of future neuroimaging investigations ofmental imagery.

Acknowledgements The research reported in this article wassupported by the National Center for Scienti®c Research (CNRSCogniscience Program, National Thematic Project on the Repre-sentation of Space) and the French Ministry of Defence (DRETGrant #89/242 to Michel Denis). Thanks to Maryvonne Carfantanfor her invaluable help in the preparation of the manuscript.

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