Neuropsychologm, Vol. 18, 435 to 442 0028-3932/80/1001-0435 102.00/0 © PerBamon Press Ltd., 1980. printed in Great Britain

L O S S O F M E N T A L I M A G E R Y : A C A S E S T U D Y

ANNA BASSO, EDOARDO BISIACH and CLAUmO LUZZATT~

Centro di Neuropsicologla delrUniversitA di Milano

(Received 2g February 1980)

Almtract--A case of loss of mental imagery following a vascular lesion of the left occipital lobe is described and discussed. The findings support a twofold (analogue and propositional) theory of neural representations of the external world. It is argued that sanse-specifi¢ representations may he preserved in spite of the reported loss of imagery in the corresponding modality. The possibility that this disorder may reflect a functional disconnection between brain centres is discussed.

INTRODUCTION

IT IS known that people experience, in varying degrees, sense-specific represemations of the external world [1]. In rare instances, patients suffering from brain damage report having lost precisely this kind of representation. Their statements would appear to be related to the breakdown of a definite system of processing of stored information which may be called mental /maoery, without any preconceived implicatiom. The relations between the introspective report and other behavioural aspects in the case we shall describe and in earlier observations may contribute to the understanding of a topic which is a subject of inquiry and debate. Specifically, the hypothesis of a twofold structure of mental representations of the external world, analogue and propositional,is supported by the findings. We will also argue that although the subjective experience of mental images is likely to reflect representations in analogue format, their reported disappearance does not necessarily imply the absence of representations of this kind. We shall also discuss whether this peculiar disorder could be the consequence of a functional disconnection of these representations from language.

CASE HISTORY M.G., fight-handed male with 6-yr schooling, tram-driver by occupation. Over a period of several years his blood

pressure was above normal values. Fourteen years previously he had suffered a coronary occlusion. On the morning of 17 December, 1977, at the age of 63, he had a severe attack of giddiness associated with headache. After a short while he noticed that he could no longer read. Three days later he developed a slight right hemiparesis; neurological examination also disclosed dysphasia and right-sided homonimous hemianopia. The patient was first examined in our aphasia unit on 17 January, 1978.

Picture naming was impaired: the patient could name only 7 out of 20 pictures of famillar objects. Object naming was normal. He said he was able to identify the pictures he could not name; he could accurately point to pictures named by the examiner. Writing of single letters to dictation showed no errors, but he could only read 8 letters out of 10. Writing of syllables, words, meaningless words and sentences was moderately impaired, rate of errors ranging from 10 to 30 ~. It is possible that these errors were at least partly due to th6 slight paresis of the right upper limb and to the inability to cheek his own writing. Reading of syllables and words was severely impaired (45 and 80~ errors r~speetively); reading of meaningless words and of sentences was impossible. Four months later, writing to dictation was almost normal; the patient had only some difficulties with meaningless words. Picture naming had improved slightly; he could name 11 pictures out of 20. He made only rare errors in reading single letters and syllables; reading of words was hesitant but accurate. In contrast, reading of meaningless words was still impaired to a considerable degree and reading of sentences impossible. Though disorders of colour gnosis were not evident, they were detected,

435

436 ANNA BASSO, EDOARDO BISIACH and CLAUDIO LUZT.ATrl

in mild form, by special examination (27 January, 1978): the patient scored 338 on the Farnsworth-Munscll 100 hue test and 41.5 on the coiour-figure matching test [2]. He did not show apractic disorders during any stage of the illness. He never reported, nor did we detect impairment in mental arithmetic. His recollection of past events, both preceding and following the stroke, was normal. He was alert, cooperative and pleasant. He stated his troubles lucidly, infrequently betraying readily controlled emotional reactions.

A C T scan performed on 4 January, 1980 (Fig 1), showed an area of reduced density involving the lower mesial region of the left occipital lobe and part of the juxtaventricular structures of the left temporal lobe with possible inclusion "of the hippocmnpus. The left Sylvian fissure and trigone were markedly enlarged. An area of decreased density was also apparent in the upper portion of the left cervbellar hemisphere, extending to the midline.

Curiously, the loss of mental imagery which, as it will be seen, was experienced as one of the most painful consequences of his brain injury, was never referred to before the time of the first language examination, when we found the patient to be completely unable to describe very familiar places. At this time, the patient had a slight motor but a severe somatoseasory impairment confined to the right upper limb; he also showed a dense right-sided homonymous hemianopia without macular sparing. His general behaviour, as well as cancellation tests and drawing, showed no evidence of unilateral neglect. Initially, the patient had not been aware of his visual field defect. This was gradually realized during the days following the stroke by the patient's wife and then by the patient himself when they found that he failed to notice people and objects in his right visual field.

The disorder which we will subsequently take into consideration remained substantially unchanged up to June 1979, when the patient was last seen in our unit. The loss of visual imngery was first stated by the patient when requested to describe the Cathedral Square of Milan. He was unable to add a single item to the description of the square beyond the Cathedral itself and the equestrian monument in the centre of the square. He asserted that this was not due to an inability to name individual details, but to the fact that he could recall none of them and was totally unable to form a mental image of the Square. When asked whether 10 well-known buildings of the town (5 in the square and 5 outside) belonged to the perspective he had to describe, he was quick and confident in rejecting the wrong ones but hesitant about the others: he accepted one (the Arcade), rejected another (the department store La Rinascente) and abstained from answering in the remaining three instances. Once taken to the Square, he failed to identify all buildings and shops with the exception of the Cathedral and of the Arcade, despite the fact that the buildings that enclose the Square, as well as several of the shops, restaurants and cafes are very familiar to most people in Milan. tt may be added that, because of his occupation, the patient was particularly well acquainted with urban geography. He was also unable to point to individual spots named by the examiner, with the exception of the Royal Palace, which he was able to locate only after having argued that it had to be the second oldest building in the Square after the Cathedral itself, as it had been the residence of a court at one time. He was unable to describe his way home from the hospital, in spite of the fact that he could walk back and forth with no trouble, while admitting that all houses except his own looked unfamiliar. He was also unable to state the route taken by tram No. 15 which he had driven for many years. He was unable to give a description of his kitchen but volunte~ed the information that he could move around in it and find objects quite adequately if he happened to do it absent-mindedly, whereas he got lost whenever performed the same activities more deliberately, following an explicit programme of successive actions. The inability to form mental images was not confined merely to buildings. The patient was also unable to form a visual representation of faces. He said he knew his wife to be "small, grey haired, with almond-shaped eyes" but was unable to conjure up a picture of her in his mind. After the stroke he frequently failed to identify the faces of distant acquaintances, though recognizing them. He was however able to recite the parts of the human body as well as the nomenclature of familiar objects (e.g. of a bicycle). He could also enumerate the provinces of Italy, but showed some hesitancy in locating them on a map; he only succeeded after having verbally stated their geographical relationships. A keen hunter prior to his illness, he now denied having any image of pheasants, hares and other game. When asked to describe a pheasant, he only could say that it was a gallinacean. Asked about differences between the cock and the hen, he replied that the former had more colours. Nonetheless he could draw a rough, still recognizable silhouette of a pheasant. The patient also said that since the stroke his hypnagugic imagery was completely missing. He asserted that he used to enjoy very rich imagery of this kind; especially before and after a day's hunting he experienced very vivid images of game. "Now", he added in a broken voice, "when I am lying sleepless in my bed I cannot even think!" Dreams also had fully disappeared.

The loss of mental imagery did not seem to be confined to the visual modality. The patient complained of being unable to imagine odours, tastes and sounds. In fact, he was no longer able to hum even the most popular tunes. Nevertheless, when asked to identify a target tune from others hummed by the experimenter, his answer was prompt and accurate.

D I S C U S S I O N

As far as we k n o w a loss o f i m a g e r y b e i n g m o r e o r less i n d e p e n d e n t o f a g n o s i a has b e e n

i n t r o s p e c t i v e l y r e p o r t e d by five pa t i en t s . T h e s e a r e C h a r c o t ' s case M.X. d e s c r i b e d by

BERNARD ['3], C~OLDSTEIN a n d GELS'S p a t i e n t Schn. [4] , t he t w o cases r e p o r t e d by BRAIN [5] ,

a n d o u r pa t i en t . F o r all t hese p a t i e n t s w i t h t h e e x c e p t i o n o f t he las t , t he c o m p l a i n t w a s

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LOSS OF MENTAL IMAGERY: A CASE STUDY 439

confined to visual imagery. In Brain's case II, only intentionally evoked images were lacking. Our patient reported a complete disappearance of dreams, whereas in Charcot's case and in Brain's case I only the visual content of the dreaming activity was allegedly lacking. No mention is made in the earlier observations of a loss of hypnagogic imagery as that experienced by our patient. Paradoxically, Brain's case II was able to give a fairly good description of absent objects such as his house or his wife's face,whilst keeping his eyes open, whereas, when he closed them, he was no longer able to do so. Mental descriptions of familiar objects and environments were severely impaired or totally impossible in the other patients of this group, with the striking exception of Goldstein and Gelb's patient, whose good performances however could have relied on motor imagery, as the authors rather convincingly argued. On visual presentation, the verbal identification of objects which could not be visualized or described was comparatively unimpaired in Brain's two patients. Even in those remaining, however, it was considerably less impaired than the visualization and the verbal description of the same objects. In the everyday life of ~.ll these patients, the behavioural identification, i.e. the adequate responsiveness to objects and surroundings which could neither be imagined nor described was even better.

Loss of mental imagery may occur as a comparatively isolated phenomenon, as demonstrated by Brain's cases, of which only case I showed a mild aphasia. A "pure" alexia was however present in the other three observations. A disorder of colour gnosis of a mixed (apperceptive and associative) character was detected in our patient through special testing, whereas it. is not possible to assess exactly to what extent, and in which form symptoms of colour agnosia were present in Charcot's and in Goldstein and Gelb's observations. Prosopoagnosic" disorders are described in Charcot's case; our patient only reported slight difficulty in identifying faces of some distant acquaintances, nonetheless r eco~i~b le to him. A more pervasive visuoagnosic disorder was found in Goldstein and Gelb's patient through special testing.

The anatomical location of the lesion, probably a vascular accident, could not be specified in Charcot's observation. The patient of Goldstein and Gelb had sustained a left occipital wound. Unfortunately, a precise localization, which would have been of considerable interest due to the purity of the syndrome, is lacking in Brain's cases, both of a traumatic aetiology. Only in case I, a mild disorder of responses to photic stimulation suggested a dysfunction of the posterior regions of the left heinisphere.

Several questions arise from these clinical records. To begin with: which inferences, if any, can we draw about the brain-processes rdated to mental imagery? Further: are mental images a necessary stage for further cognitive processing? And last: are the. proce-'~s which give rise to mental images actually lacking in these patients, or, rather, have they merely been disconnected from the verbal system?

Concerning the structure of the brain-processes involved in mental imagery, the high degree of isomorphism between the subjective experience of such representations and the perceptions of the external world would seem to require continuous, point-to-point sense- specific replicas of past impressions. It has however been claimed that we cannot assume an identical structuring for our subjective experiences and their neural embodiment 1.6]. It has also been argued 1"7] that although suggestive of an analogue model of information processing, the results of experiments on imaginal inference 1.8] are compatible with a propositional model of representations, that is with a model where the information to be represented is coded into a finite set of symbols interconnected through manifold relations. On the other hand, evidence from patients showing symptoms of unilateral neglect suggests

440 ANNA BASSO, EDOARDO BISIACH and CLAUDIO LUZZATTI

that, at least in some stages of the representational process, information can be organized in an analogue format [9, 10]. Anyway, the two models are not mutually exclusive; even if it might appear more parsimonious to assume a single procedure for storage and representation of information in the nervous tissue, the coexistence of different representational systems [11] would not constitute the sole instance of neural redundance. Clinical observations corroborate the hypothesis of a twofold representational procedure. Our patient, though firmly denying being able to call up pictures of any kind whatsoever and, as a consequence, to describe the visual appearance of known objects, still in many instances retained the ability to retrieve information about them, even if in a rather peculiar way. Whereas the "blind" description of his wife's appearance would not differ to any great extent from a description of a picture of her, except for its poverty of detail and perhaps the "staccato" enumeration of her features, the description of the pheasant would not go beyond a single taxonomical statement. Even more interesting, however, is the seemingly paradoxical behaviour of this patient with relation to the Royal Palace. Why, in fact, should the words "Royal Palace" key up comparatively marginal fragments of stored information and not others, associated through much more reinforced links? By assuming manifold representations of single objects we could explain why the patient, still able to generate sentences like "the pheasant is a gallinacean" or "the Royal Palace was the residence of a court disappeared long since" (thus retrieving merely conceptual information which is likely to be organized in a propositional format), can no longer say what these items look like. That would betray the selective inaccessibility of an analogue visual medium for the representation oftbe external world. It might be objected that predicates of the kind "beside the Cathedral" are likely to be derivable from a propositional representation as well and yet our patient proved unable to locaee the Royal Palace. A tentative answer would be that whenever a response depends on the representation of sensible data, it is primarily a representation in the analogue format which is called up. Should this format not be available, the response cannot be given, unless by having recourse to, in some instances, a propositional data base, as might have been the case for the patients's awkward description of his wife's appearance. The fact that under the a, bove considered conditions, the natural way of responding is one which proceeds from perceptual simulation, might be related to the evolution of our thought processes from procedures used in perception [12].

As for the second questioh, it has already been observed by Brain that the loss of voluntary visualization does not necessarily entail an impairment of those functions in which it might have been expected to play some part. Clearly, these patients, unlike patients showing loss of topographical memory, or patients in whom loss of mental imagery has been inferred from agnosic disorders [13, 14] retain adequate, sense-specific internal representations of the external world: the existence of these representations is indeed a necessary condition for the perceptual identification of objects and for the ability to find one's way in surroundings whose image is said to be absent and which cannot be described.

That leads us to the last point. We have hitherto avoided the question of whether the self- report of these patients means that mental imagery has really disappeared from consciousness (as would primo fac/e be suggested). In fact, we suspect that the concept of consciousness, ff referred to a damaged brain, may become nonsensical. This is suggested by the behavioural analysis of commissurotomy patients [15] and, notably, by the inherent precariousness of all attempts at localizing consciousness in the left hemisphere on the grounds of clinical observations [16]. If the possibility is acknowledged that some patients can form mental images but cannot report them owing to a functional disconnection which

LOSS OF MENTAL IMAGERY: A CASE STUDY 44.1

isolates the related processes from language, it makes little sense to pose the question of the conscious properties of these images. The denial of visual imagery could be similar to the verbal denial of visual experiences in callosal patients flashed with pictures of objects in their left visual field, sharply contrasting with the retained ability to select the flashed picture accurately when shown in full vision among distractors 1"151. The hypothesis that loss of visual imagery is the result of a functional disconnection is supported by the frequent association with '~pure" alexia and with disorders of colour gnosis, which also have been argued to be dependent upon visuoverbal disconnection 1"17]. Both these disorders were present in our patient, whose lesion, involving the striate area and extending to the juxtacallosal region of the left occipital lobe, could lay the conditions for a partial visuo- verbal cleavage. On a much more conjectural ground one could point to the involvement of the deep temporal structures, and perhaps of the limbic system, to explain the loss of mental imagery outside the visual modality.

Acknowledgement--Research supported by CNR Grant 78.01998.04.115.4935.

REFERENCES 1. GALTON, F. Inquiries into Human Faculty and its Development. McMillan, London, 1883. 2. BASSO, A., FAGUON1, P. and SPINNLER, H. Non-verbal colour impairment of aphasics. Neuropsychologia 14,

183--193, 1976. 3. BERNARD, D. Un cas de suppression brusque et isoi6e de la vision mentale des signes et des objets (formes et

couleurs), proo. Mdd. !1, 568-571, 1883. 4. GOLErSTEIN, K. and GELS, A. Psychologische Analysen hirnpathologischer Fiille auf Grund yon

Untersuchungen Hirnverletzter. Zeitschr. Ges. Neurol. Psychiat. 41, 1-142, 1918. 5. BRAIN, R. W. Loss of visualization, proc. R. Soc. Med. 47, 288-290, 1954. 6. PYLYSHYN, Z. W. What the mind's eye tells the mind's brain: A critique of mental imagery. Psycho/. Bull. 80,

1-24, 1973. 7. ANDERSON, J. R. Arguments concerning representations for mental imagery. Psychol. Rev. 85, 249-277, 1978. 8. StmPAXD, R. N. and POOOOaNY, P. Cognitive processes that resemble perceptual processes. In Handbook of

learning and Cognitive Processes, W. K. ~ (Ed), pp. 189-237. Lawrence Erlbaum, Hillsdale, N. J., 1978. 9. BmAoa, E. and LUZZATn, C. Unilateral neglect of representational space. Cortex 14, 129-133, 1978.

10. BISIACH, E., LUZZAYn, C. and I'ERANh D. Unilateral neglect, representational schema and consciousness. Brain 102, 609-618, 1979.

11. PAlVIO, A. The relationship between verbal and perceptual codes. In Handbook of Perception, E. C. CARTL~-~ rL and M. P. FRn~MAN (Editors), Vol. VIII, pp. 375-396. Academic Press, New York, 1978.

12. W|LTON, R. N. Explaining imaginal inference by operations in a propositional format. Perception 7, 563-574, 1978.

13. NIELSEN, J. M. Agnosia, Apraxia, Aphasia, pp. 188 IT. New York, 1946. 14. MACRO, D. and TaOLLE, E. The defect of function in visual agnosia. Brain 79, 94--110, 1956. 15. GAZZANIGX, M. S. Die Bisected Brain. Appleton-Century-Crofts, New York, 1970. 16. ECCL~S, J. C. Brain and free will. In Consciousness and the Brain, G. G. GLOBUS, G. MAXWELL and I. SAVODNIK

(Editors), pp. 101-121. Plenum Press, New York, 1976. 17. C_~SCHWIND, N. Disconnexion syndromes in animals and man. Brain 88, 585-644, 1965.

NSY 1 8 ~ / 5 - - D

442 ANNA BASSO, EDOAeDO BISIACH and CLAUDIO LUZZAT'n

R6sum6 :

On d 6 c r l t a t on d l s c u t e un t e e de p a r t s de l ' l ~ g e r i e mente le & l a s u i t e

d°une 14ston vascu le t " re du l obe o c c i p i t a l gauche. Ces c o n s t a t a t i o n s sen t en ~aveur

d'une th6oris & 2 plans (analogue at proPosltlonnel] des repr~sentatlone nerveusss

du monde ext6rleur. On edmet qua lee repr~santetlons sp~olflques des modellt~s sen-

eorlslles peuvent I t re pr~eerv~es malgr~ le perte de l'Imagerle dens la modallt6

correspondents. On dlscute le posslblllt~ que oe d6sor~re tredulse une disconnexlon

~onctionnelle entre oentres o6r~breux.

Zula~ne~assu~: Nach e iner vaskulttren L l s i o n im linken Occipital lappen v e r l o r ein Pat ient

die FJdligkeit zur blldlichen Vorstel lung. Die Beobachttmg tmterstt i tzt die

Theor ie e ther zweifachen, analogen und propoeit ioneUen neuronalen

Reprttsentation der Au~enwelt. Es wird e rS r t e r t , da~ s inneespezi f i sche

R e p r b e n t a t i o n e n erhal ten bleiben kOnnen, such wenn die Vors te l lungskraf t

in der entsprechenden Modal i t i t ve r lo r en geht. Die M6glichkeit wird diskut ier t ,

da~ diese Funktionsst6rung eine funktioneLte LeitungsstOrung zwischen Hirn-

zentren anzeigt.