elaboration of representations in a left-handed subject with callosal damage

ELABORATION OF REPRESENTATIONS IN A LEFT-HANDED SUBJECT WITH CALLOSAL DAMAGE

Jeanine Blanc-Garin

(Laboratoire de Psychophysiologie, Universite de Provence)

Although the number of patients with split-brain syndrome is small, a great deal of research has been devoted to them. Much important data has been accumulated since Sperry and his group began formulating questions about the transfer of information within the brain and on hemispheric specialization. These findings constitute one of the ways the problem of conciousness was recently reintroduced into the field of scientific psychology (cf. in particular: Gazzaniga, Le Doux and Wilson, 1977; Sperry, 1961, 1969; Sperry, Zaidel and Zaidel, 1979; Zangwill, 1974, 1976).

In the present study, the elaboration of mental representations was investigated in a patient with total callosal softening associated with a right hemianopia. Also of special interest was the Subject's awareness during perceptual processing. The subject was a left-handed girl and previous observations by Poncet, Ali Cherif, Choux, Boudouresques and Lhermitte (1978) had demonstrated that both hemispheres controlled her expressive language. This gave us the opportunity to investigate what relationships occurred between verbalization and perceptual avareness.

The following summary of the subject's neurological condition was given by Poncet et aL (1978): L.S. "a 14 year old girl, presenting quasiexclusive left-handedness, underwent neurosurgery for a spontaneously occurring hematoma which had destroyed the anterior part of the corpus callosum. The post-operative course was complicated by the appearance of a syndrome of total callosal disconnection associated with right lateral homonymous hemianopia. Fifteen months after surgery, CT revealed the presence of: a left frontal low-density lesion, a left paramedian hypodensity due to acute ischemia in the region supplied by the left anterior cerebral artery, and a low-density lesion in the left medial occipital region due to acute ischemia in the zone supplied by the left posterior cerebral artery. At the time of examination, the patient continued to display a syndrome of total interhemispheric disconnection with the absence of

Cortex (1983) 19, 493-508

494 Jeanine Blanc-Garin

data transfer between left and right occipital, temporal and parietal regions. The existence of right lateral homonymous hemianopia associated with a syndrome of total disconnection of the corpus callosum is undoubtely a unique pathological model". (p.634).

Four experiments were carried out. In the first, we investigated the S's ability to identify and name objects while exploring them with either hand, without the aid of vision. Each hand was used alternately. We repeated these tasks (13 sessions over a period of 18 months) to observe the evolution of processing characteristics, in response to inputs from each hand-hemisphere system (right hand, left hemisphere: r.ha'/L.H.; left hand, right hemisphere: l.ha'/R.H.). The results of this experiment suggested some hypotheses which were tested in three further experiments specifically designed:

-to observe strategies for planning a temporo-spatial search with either hand (Experiment 2);

-to discover if, based on her left visual field/R.H. system, the Subject had access to any of the phonetic characteristics of the names she could not pronounce (Experiment 3);

- to evaluate her memory trace after encoding operations were carried out by either hand (Experiment 4).

EXPERIMENT 1

Haptic exploration: identification and naming of familiar objects

Procedure

Ten sessions (I to X in Figure 1) were conducted during a 7-month period, beginning 13 months after surgery. Control data (sessions XI to XIII) were obtained the following year. Stimuli consisted of a set of objects (30 at the beginning of the experiment, 70 at the end). All the objects were familiar (e.g. cups, belts, tea-pots, bottles, small plastic animals etc.). Their size, which varied from approximately 5 to 20 cm, was such as to facilitate digital exploration. The objects were presented successively, hidden from vision by a screen, and were explored with one hand at a time. A series of 10 to 20 objects for each session was presented to one hand, and then another series to the other hand. (Some sessions consisted only of exploration by one hand).

The Subject was instructed to identify the object. All verbalizations and gestures were recorded. The object was withdrawn when the S wa.s satisfied with her response or after 60 seconds if it was not identified. No feedback was given after each trial. The experimenter often intervened to encourage L.S. to continue searching, to ask her whether she was certain, or to suggest a wrong name. For each object, two responses were noted, each one representing an independent variable:

Representations in a left-handed patient with callosal damage 495

Variable 1. Identification by name: the Subject produced an appropriate noun. Among these naming responses, we distinguished "immediate naming" when the noun was given during the first five seconds and was preceded neither by verbalizations nor by gestures describing the object's use.

Variable 2. Identification by description: gestures or verbalizations describing the object's function and showing, without a doubt, that L.S. recognized the object even if she could not produce the name. (For instance for a tea-pot "You pour tea with it", with the appropriate pouring gesture). This rating criterion was used in a conservative manner, namely, a response was not counted as identification if the Subject simply said she recognized the object, without further elaboration.

A given object mayor may not be identified by name, and/ or by description. For each variable, the percentage of objects that were identified in the manner defined, was calculated.

Results

The Subject's responses differed according to exploring hand. A. Identification with the left hand

During the year of observation, L.S.'s behavior did not change significantly. She would place her hand rapidly on the object, oriented it in the correct position for utilization, and after a few seconds, withdrew her hand announcing that she knew what it was, and that she was searching for the name. She expressed her certainty and made reference to a visual image: "I'm sure ... I can see it. I know what it is". She would try either to draw the object, to mime its utilization, or to point to something similar in the room. She would explain some of its uses or talk about her personal reactions to it. (For example, "I don't like what you put in it", for a coffee cup). She adequately answered any questions, expressing her certainty, and reacted strongly to any inappropriate name suggested by the examiner.

When the percentages of her correct responses are plotted as a function of time (Figure la), it is apparent that she made no definite progress, and that from the beginning there was a distinct difference between the two types of identification. Naming was poor and immediate naming responses were rare. Usually, naming was possible only after a gesture or some verbalization was given, probably after her L.H. was informed through cross-cuing. The ability to use this cross-cuing strategy was her only improvement throughout the experiment. In contrast, identification by description was good from the beginning (and it should be emphasized that the criterion for scoring was conservative). B. Identification with the right hand.

Behavior and verbal expressions were different from those elicited by the left hand. We observed either:


a

L ! I • I II III IV V VI VII VIII IX X XI XII XIII

b

o ! ! ! ,! I '.

I II III IV V VI VII VIII IX X XI XII XIII

Fig. I-Identification of objects by tacti/o-kinesthetic exploration in Exp. 1, with the left hand (upper graph) and the right hand (lower graph). Change over time (13 sessions: I to XIII). Percentages of objects identified by description (diamonds) and by naming (large circles: naming total; small circles: immediate naming). Filled symbols refer to results obtained from the old set of objects; empty symbols to new to that session objects.

- correct naming, after a brief palpation, and then demonstration of a correct use, or

- an "I don't know" -type response, with a slow and incomplete exploration. In these ,cases, and sometimes after half a minute, L.S. stopped and said, with a perplexed look, "I don't know ... I can't see it". She did not give spontaneously any information, but she did correctly answer any questions concerning the sensory qualities of the object (cold vs. warm, hard vs. soft, heavy vs. light, smooth vs. rough). In response to


questions about the object's shape or · use she usually replied, "I don't know". She never made errors in naming, and her responses, though correct, were frequently accompanied by uncertainty. Sometimes, after giving a correct name, L.S. said it was wrong, stubbornly refusing even to repeat it. She was then unable to furnish a description or utilization gesture. This explains the occurrence in some cases of a description score lower than a naming score (for instance, sessions 1, IV and VIII, c.f. Figure lb).

The poor scores on description and naming in early sessions improved over the following three months. The highest scores were obtained in sessions VI and VII for the objects used in previous sessions, but when new objects were introduced during these sessions, she had a great deal of difficulty. Her subsequent impaired performance in session VIII was perhaps due to the increasing size of the reference set. She never had any difficulty in finding the name once the identification was made. There was rarely description without naming, and when naming occurred, it was, often immediate. Throughout, the three curves of Figure 1 b nearly coincide.

Thus, whereas her left-hand impairment is in naming, there is no anomia with the right hand, although in the latter case there are difficulties in forming a visual image from haptic information.

The results of this first experiment raised the following questions, which were investigated in further experiments:

(1) When haptic activities are not directed towards processing sensory information, but rather towards spatial exploration, do we find differences between hands, as we did in Exp. I? In the task of Exp. 2, the Subject had to organize her motor program for spatial exploration.

(2) The last sessions of Exp. 1 showed that while the Subject's descriptive abilities using either hand were comparable, naming (whether immediate or delayed) remained better with her r.ha./L.H. system (cf. Figure 1, sessions XI and XII). However, when exploring with her left hand, L.S. often. said that she had the answer "on the tip of her tongue". Although she was unable to pronounce the name, were some of its phonetic features nevertheless available to her? Experiment 3 was designed to answer this question.

(3) Sensory inputs processed by the r.ha'/L.H. system evoked an adequate noun (cf. Table I). Her performance was good by the end of Exp. 1, but the verbal labels used were drawn from broad categories and were less precise than the ones she might have chosen. In contrast, when working with her l.ha'/R.H. system, L.S. indicated that the visual image was vivid, and gave accurate details of how the corresponding object was used. Her behavior showed that she was satisfied and certain of her response though not having direct access to verbal labels. We can


TABLE I

Examples of Spontaneous Verbalizations during Unimanual Exploration

With the right hand With the left hand

(I) Exploring a small frug flask

- a bottle ... I don't know for what. (IV) - a jar, a bottle; ... it is empty. (VI)

- a bottle ... for perfume? for shampoo? for syrup? (XII)

- no ... it's not for drinking (V). - I don' t like the smell ... it's a bottle

for ether. * (VI) - a bottle for cleaning things.

(XII)

(2.) Exploring a perfume flask

- ... ? .. .it can be broken ... it's not easy... - for perfume (IX) (IX)

- a bottle. (XII) - (lightly slapping her face) ... perfu-me. (XII)

* The flask never contained ether, thus there was no odor, but such a usage was possible; some time later the association made with a cleaning product (XII) was also adequate.

expect, according to Craik and Tulving's model (1975), that memory retention will be better with her l.ha'/R.H. system where encoding is rich and elaborate.

EXPERIMENT 2

Haptic exploration and spatial planning

Procedure

The task was derived from an experiment of Hatwell, Osiek and Jeanneret (1973). L.S. was presented with a square matrix (50 x 50 cm), composed of 25 regularly arranged pegs (diameter: 2 cm; height: 3 cm). She was instructed to haptically explore this matrix, without vision, with the index finger of one hand, in order to find a nail attached to the top of one of the pegs. She was to try to touch as few pegs as possible. The location of the target nail was modified for each trial. Each hand carried out two blocks of five trials. The strategy for the spatiotemporal exploration as well as the time required, within a 100 second limit, were observed.

Results

Her search time is plotted in the upper graph of Figure 2. During the first 2 trials (using her left hand) the target was placed close to the


Fig. 2 - Search for a target in a maze by unimanual exploration (Exp. 2) Upper graph: time (in seconds) taken to find the target, with the left hand (--) and the right hand (- - - -) over 4 blocks of 5 trials. Lowerpart: examples of searchingpatterns of two trials (trial 10 for the right hand; trial 11 for the left hand). The black square indicates the position of the target; the circle, the starting point chosen by the Subject for the search.

sec 100 -

I.h.

60 -

0-1

r.h.

r. h. , I. h.

,~ , , I ' , I ' , I ' , I ' , \ ' , \ '

, I ' , I ' , I ' , I I

I , : , , , , I 1 , , .

• ' I , I , I , I I I 1/

•

r. h.

1 .\ I \

I \ I \

I \

f' \ I \

I \ I • ,

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I I I

I • 2 3 4 5 6 7 8 9 10 11 1213 14 15 16 17 181920

s • 10 11

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Subject's body and she found it quickly, without systematic exploration. Similarly, for the third trial L.S. found the target location after a random exploration, but required more than 50 seconds, since the nail was farther away. Beginning with the fourth trial, she started to construct a systematic way of exploring (cf. Figure 2) that she maintained with her left hand until the end of the experiment. This strategy did not appear when using her right hand (trials 6 to 10). With her right hand, random exploration persisted, some pegs being touched many times, others not at all (For instance, see the route of trial 10, mapped on Figure 2). This method did not always lead to success (e.g. trial 7). When her left hand was re-tested, she again employed the efficient strategy she had discovered (trials 11 to 15), but never used it in the five final trials with her right hand (16 to 20).

Thus, an adequate strategy for spatio-temporal analysis was adopted by the l.ha.lR.H. system in response to the requirements of this task. In contrast, the random movement of the r.ha.lL.H. system did not seem to be directed by any efficient spatial program. In addition, there was no transfer of the successful strategy from the l.ha.lR.H. to the r.ha.ll.H. system.

EXPERIMENT 3

Object identification and access to its phonetic features

When the Subject adequately described an object and claimed to know its name, although being unable to pronounce it, were certain phonetic characteristics accessible to her, as in the "tip of the tongue" phenomenon?

Visual stimuli were used. Since right hemianopia prevents any visual input to the left occipital cortex, only L.S's left visual field (l.v.f.lR.H. system) is actually functioning in free vision. The situation was thus comparable to that in which she palpated objects with her left hand.

Procedure

The stimuli were objects (e.g. tools, flowers, animals) pictured on cards (7 X 7 cm). They were presented successively and L.S. was asked to pronounce their names. When she failed, she was asked whether she was able to identify the object. Following "yes" responses, she was then given orally 2 to 4 sounds chosen from the French phonemes I a/,l el ,liI,I 01 ,/y I. She was required to answer orally (or by gesture): "yes", "no" or "I don't know" to indicate the presence of the given sound in the name of the object. An uncertain response was permitted to ensure that no information was lost (Noizet and Petit, 1970). This was particularly


important with L.S_ who often refused to give a response she was not certain about.

Results

S was unable to provide many names (23,8%) but she claimed she could identify all the objects. (These results are similar to those obtained in the haptic situation, using the left hand, in Exp. 1). The cases, in which no name could be supplied (N = 32 pictures), were used to test the "tip of the tongue" phenomenon. Numerous responses of "I don't know" type were obtained for distractors (41.7%) as well as for phonemes present in the word (31.4%). The false-alarm rate was 5.97% and correct responses were around 50% (cf. Table II).

Thus, it seems that the l.v.f.lR.H. system of L.S. had access to some of the phonetic characteristics of the name she could not supply, but did not have a complete internal verbal representation of it.

TABLE II

Types of Responses in the Tip of the Tongue Phenomenon (Experiment 3)

Stimuli Target phonemes (N = 54) Distractor phonemes (N = 67) Responses ? yes no ? yes no

N 17 26 11 28 4 35

% 31.48 48.14 20.37 41.79 5.97 52.23

correct omission F.A. correct

EXPERIMENT 4

Recognition of objects haptically explored

This task was carried out at the end of Exp. 1, when L.S.'s identification performance was good with either hand. Although immediate naming was better when using her right hand, our hypothesis was that, when using her left hand, right hemisphere encoding operations were more elaborated and used visual images. Thus, retention was anticipated to be higher in the latter case. Particularly, if r.ha/L.H. encodes by broad verbal classes, we should obtain false-alarm (F.A.) responses for objects pertaining to the same category but having different shapes. This type of F.A. would not be found with the l.ha/R.H. system where differentiating cues are extracted in order to specify the precise object.


Procedure Objects were chosen from the reference set of Exp_ 1_ Tests were given over a

six-week period, in four sessions (two series for each hand)_ Hands were alternated across the sessions. The Subject was told that this was a memory experiment, where each session would be composed of two parts: first, silent exploration, and then recognition. She was to explore a series of objects, with one hand and without the aid of sight, ten seconds per object. She was asked to nod or shake her head to indicate whether she identified the object. Following a 2.5 minute delay during which L.S. was to remain silent, the series of objects previously explored were presented mixed with "new" ones (distractors). The Subject indicated, verbally or by gesture, whether the object was "old" or "new". The same objects (22 "memoranda" and 30 "distractors") were used for each hand. Some of the distractors were objects for which L.S. (in Exp. 1) had used the same broad verbal labels as for the memoranda (cf. an example in Table I).

Results L.S. claimed to have identified all objects, in part I of each session. In

the recognition tasks (part 2) carried out with the left hand, 96,7% of her responses were correct (100% hits for "old" objects and 93,4% rejections of distractors), in those carried out with the right hand, 70,7% were correct (78% hits and 63,4% correct rejections).

All the dis tractors which produced F.A.s had a corresponding object in the memoranda series which had been labeled with the same name in Exp. 1. The extent to which distractors producing F.A.s shared physical and functional features with the corresponding memoranda differed according to the exploring hand:

With the left hand only two errors occurred (6,6% F.A.), which concerned distractors very similar to the memoranda (same name, same function, slightly different shape):

- a small cup was confused with a larger one, - a round cigarette-ligther was confused with a flat one. With the right hand, 36,6%. EA. were produced. Errors concerned the

same two objects as above, along with nine others, including: - a bottle of medicine and a bottle of perfume (different shape and

different function); - a china doorknob and a metal door-handle (same function, very

different shape, weight and substance); - a leather glove and a washcloth (both are called "glove" in French,

but are different in function, shape, and substance). Other objects, which pertained to the same semantic field, but were

labeled with different names in Exp. 1, were not confused (e.g. a plastic toy horse and a camel; a flashlight and a flashlight battery).

This pattern of errors suggests that the r.ha/l.H. memory representation of L.S. is based more on the verbal label than on the functional and physical characteristics of the objects. In contrast, her l.ha/R.H. system


leads to a representation in visual format where function and meaning are included and where precise differenciating cues are extracted and used.

DISCUSSION

The hemispheric asymmetry shown by this left-handed patient was in some respects alike to that usually found in right-handed split brain patients:

(1) Inability to pronounce the name of an object (anomia) when input reached the right hemisphere (by the left hand or the left visual hemifield), whereas naming was possible using the left hemisphere (by the right hand).

(2) Better spatial processing with the left hand/right hemisphere system than with the right hand/left hemisphere system.

Other characteristics of her behavior pointed, however, to a cerebral organization different from that typical of right-handed individuals. Let us analyze her verbal and non-verbal activities and then consider the problem of her perceptual awareness.

Verbal processes

While a great deal of evidence has been accumulated showing that the R.H. of right-handed split-brain individuals can understand oral and written linguistic material (Zaidel, 1978), little data exists that shows that the R.H. can produce speech. Most investigators agree that the R.H. of split-brain patients is "mute". Only a few exceptions have been noted. One of Gazzaniga, Le Doux and Wilson's (1977) patients (P.S.) was able "to produce verbal responses by writing as well by selecting and arranging letters" with his left hand/R.H., but "he failed to name all left-field (right-hemisphere) stimuli" (p. 1154). In further experiments (Gazzaniga et aI., 1979) however, data suggested to the authors, that P.S. was actually talking with his R.H. Also the left-handed split-brain patient (P.D.) reported by Levy, Trevarthen and Sperry (1972) exhibited a language pattern different from that of right-handed subjects. Anomia was found for stimuli explored with the right hand and perceived in the right visual field, demonstrating that speech was controlled by the R.H. In our lefthanded S, verbal processes appear to be mediated by both hemispheres as argued by Poncet et ai. (1978). However, each hemisphere seems specialized for different aspects of verbal function.

1. The L.H. appears to be used for "naming". When L.H. processing


leads to object identification, naming is always correct. This naming behavior can operate rather well in a concrete situation where the Subject must find a verbal label for an obj ect manually explored. There appears to be direct access to a lexical store, probably without visual images. Nouns thus obtained are often category names but this does not necessarily imply that a categorization process occurs. Such a process is required in the W.A.I.S. "Similarities" where L.S. manifested a great deal of difficulty in producing a category name when two lower-level nouns were given by the Ex. (For example, the words "table" and "chair" did not bring on the superordinate word "furniture"). However, from this data, we cannot assume the existence of a limitation of L.H. abilities. Indeed, it appears rather certain that, in oral conversation, the R.H. takes over. Moreover, from what we know about L.S.'s past school achievements, her cognitive difficulties existed before the illness.

2. The right hemisphere seems to be devoted to "talking about". In the experimental situations, verbal information about the function of objects is correct but precise names are very rarely pronounced (access to the phonetic features of objects identified but not named is rather limited). The absence of a verbal label does not prevent memory efficiency and it can be supposed that visual images of individual objects play an important role in this task. Similarly, as hypothesized by Poncet et al. (1978), spontaneous speech, which is well adapted to the visual world (not providing input to the left hemisphere, in L.S.), seems predominantly controlled by the right hemisphere; this speech is not devoid of nouns, but they are less frequent than expected.

Non-verbal processes

Our Subject seemed able to process both haptic and visual information with her R.H., and to construct different types of non-verbal representations: spatial strategies and visual images. Only with her l.ha/R.H. system did L.S. find a strategy for spatial analysis (although she could not verbalize it, afterwards), in Exp. 2. It was an economic and efficient way to organize her motor activity in order to systematically explore the area (each peg was touched once).

The right hemisphere answered to haptic or visual inputs (Exp. 1,3,4) by building a concrete and visual image, without a verbal label. This encoding method was more efficient than the verbal one in memory tasks, as it is commonly observed when verbal coding an imaged coding are contrasted (Paivio, 1971).

Spatial strategies and visual images are non-verbal activities, corresponding to different representational levels and associated with discrete


processing systems. The former involves the ability to build up an abstract spatial representational schema, and mayor may not be in visual format. The latter is a more concrete activity in the realm of the visual system. A review of papers on spatial disturbances in brain-injured patients (BlancGarin and Julien-Benichou, 1976) shows that spatial and visuo-imaged disorders are dissociated (see Newcombe, 1970, in particular) although both are predominanlty controlled by the right hemisphere, in left-handers as well as in right-handers (Hecaen et aI., 1981).

Indeed, in all right-handed split-brain patients, the R.H. is better and more competent at handling visual and spatial information (Gazzaniga et aI., 1965; Nebes, 1971, 1973, 1978; Milner and Taylor, 1972; Levy et al., 1972; Levy, 1974: Franco and Sperry, 1977). Also the left-handed splitbrain patient of Levy and Sperry (1972) showed a dominance of the R.H. for perceptual recognition of visual chimeras.

Perceptual processing and awareness '

Although perceptuo-cognitive processing in this subject functions rather well with haptic input to both hemisphere systems and generally leads to correct representations, whether verbal or visuo-imaged, it is evident that these two processes are experienced differently and bring out different behaviors. As proposed by Shallice (1972) it is indeed important to distinguish between "the production of conscious 1 experience and the reporting of the experience" (p. 384). When the right hemisphere is visually or haptically stimulated, the Subject's perceptual awareness is apparent, as demonstrated by the following behavioral cues: a sudden facial expression of discovery and satisfaction, gestures pantomiming the object's use, oral reports of experiencing a visual image, and expressions of certainty, confirmed by the refuse of false suggestions from the examiner. Only naming lacks on many occasions.

With left-hemisphere processing satisfaction and certainty are always less evident and less frequent than with the R.H. processing. These feelings are manifested either by facial expression or by verbalization, and they seem to be contingent on the difficulty to evoke a visual image. In one occasion L.S. manifested her dislike to work with her right hand by saying, "You know that I can't see well with my right hand!" Thus, for L.S., self confidence and visual evocation are always linked together and are weak-

1 The term "conscious" is evidently used by Shallice as a synonym of "aware". We could prefer, in line with Bunge (1980), to make the distinction between "to be aware of (to feel or to perceive)" and "to be conscious of (to think of)" (p. 175).


er with L.H. Moreover, a very puzzling but typical L.H. behavior, that of subsequent denial, was manifested on several occasions. For instance, near the end of Exp. 1 (34 months after surgery), a "new object" (a flash-light battery) was introduced into the set during a right-hand session. After a few seconds and while continuing to palpate it, she said, "A battery ... Is it a battery? .. , No ... it's not a battery ... I don't know what it is ... " This phenomenon was not limited to tactile situations and was also observed in a dichotic listening task (Poncet and Ponzio, 1977). Names given to the 1. ear /R.H. system were not repeated, but gestures or pointing were correct. On the contrary, some of the words given to the r.ear/L.H. were correctly repeated while others gave rise to the first syllable only. In a trial, where the two words were "bouteille" (bottle) to the r.ear/L.H. and "chaussure" (shoe), to 1.ear/R.H., L.S. said "bout ... ", then she pointed to her shoes, and later commented "I was going to say "bouteille" ... it was an error. .. sorry". Thus, this phenomenon is not modality-specific but hemisphere-specific. While input processing is being carried out in the L.H., the R.H.'s activity interferes and takes contro1.

Similar phenomena were noted in two right-handed split-brain patients, by Levy, Nebes and Sperry (1971). After palpating out-of-sight objects with their left hand, subjects were required to write down the object's name with their left hand. The first letters of the written words were correct but then the handwriting changed and incorrect letters were added. As the authors say, "incorrect completions ... are clearly indicative of intervention by the dominant [left] hemisphere" (p. 56).

Thus, when an activity can be controlled by either hemisphere (oral expression in our patient, writing in those of Levy et a1.), the hemisphere that is dominant for the given situation (R.H. for L.S., L.H. for Levy et a1.'s patients), tends "to take the lead" in information processing and motor contro1. Sensory data coming from the other hemisphere which cannot be integrated because of callosal damage, is neglected and only stimuli processed by the leading hemisphere are perceived.

Present findings support Shallice's view that the concept of awareness is "mapped onto an information processing concept" (1972, p. 383) and that the content of awareness depends on what action system is dominant at the moment, only one being "maximally activated at any given time" (p. 385). The switching of hemispheric control (Kinsbourne, 1970; Trevarthen, 1974; Levy and Trevarthen, 1976) probably constitutes, among other ways, a means of passing from one action system to another.

ABSTRACT

Four experiments were carried out in a familial left-handed patient who, following a vascular accident, underwent a callosal lesion and had a right hemi-


anopia. Previous observations by Poncet et al. (1978) demonstrated that both hemispheres controlled expressive language. In the first experiment haptic unimanual exploration showed that each system (right hand/left hemisphere; left hand/right hemisphere) elaborated very different mental representations of the stimulus and gave different types of verbal report of it (naming and talking about). In the second experiment, manual exploration of a maze in the search of a target was performed randomly with the right hand and following a competent spatial strategy with the left hand. The third experiment investigated the ability of the right hemisphere to discriminate the phonetic features of the names of objects that were identified but not named. In the fourth experiment, recognition memory of objects was shown to be better when information was processed by the left hand - right hemisphere system than by the right hand - left hemisphere system. Findings are discussed in terms of verbal vs. non-verbal processes and of perceptual awareness.

Acknowledgements. This work was carried out at the Departement de Neuropsychologie, clinique de Neurologie, C.R.V. La Timone, it Marseille. In am indebted to Professor Michel Poncet for the opportunity to carry out this study in his department and for valuable advice. I thank Vivian Lamongie for her help in translation.

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Jeanine Blanc-Garin, Laboratorie de Psychophysiologie, Universite de Provence, 13397 Cedex 13, France.

elaboration of representations in a left-handed subject with callosal damage

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