Brain (1987), 110, 1155-1171

POSTERIOR CALLOSAL INFARCTION
CLINICOPATHOLOGICAL CORRELATIONS
by J. D. DEGOS, F. GRAY, F. LOUARN, J. C. ANSQUER, J. POIRIER
and J. BARBIZET
(From the Departement de Neurosciences, CHU Henri Mondor, Creteil, France)

SUMMARY

INTRODUCTION

The best documented syndromes of interhemispheric disconnection are in patients
subjected to commissurotomy for the treatment of severe epilepsy (Sperry et ai,
1969; Gazzaniga, 1970) or for the surgical exploration of the third ventricular
region (Trescher and Ford, 1937; Maspes, 1948; Gazzaniga and Freedman, 1973;
Iwata et ai, 1974; Jeeves et al., 1979; Damasio et ai, 1980). When the postoperative
syndromes in these cases are considered, it is often difficult to determine the exact
contribution from extracallosal damage as opposed to that from the commissurotomy itself. To the best of our knowledge no case of an interhemispheric
disconnection syndrome associated with an anatomically verified lesion confined
to the corpus callosum of an otherwise healthy human brain has been reported.
Our present observation approaches this ideal. The interhemispheric disconnection
syndrome was partial and related to a lesion that was clearly limited to the posterior
corpus callosum; consequently, we were able to correlate anatomical and clinical
Correspondence to: Professor J. D. Degos, Hopital Henri Mondor, 94010 Creteil, France.

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Clinicoanatomical observations on a patient with partial interhemispheric disconnection associated
with complete ischaemic destruction of the splenium and of the posterior part of the body of the
corpus callosum are presented. Neuropathological examination of the areas containing degenerated
white matter indicated that the lesions affected the transcallosal fibres that link the cortex of the
occipital lobes and the superior parietal lobules (SPL). The white matter situated in the temporal
lobes was intact. This suggests that in man, and contrary to what has been described in the monkey,
the callosal pathway followed by the temporal fibres is rostral to the one followed by the parietal or
at least by the SPL fibres.
The most prominent disconnection syndrome elements were left tactile anomia (in spite of a rather
good tactile-motor integration between the hemispheres), left visual anomia, agraphia of the left
hand and 'diagonistic' apraxia. The fact that the anterior part of the corpus callosum was intact
accounts for the preservation of interhemispheric transfer of somatic sensory information and for
the absence of left extinction during the dichotic listening test. An attempt is made to give a more
detailed explanation of the results obtained during the different tests.

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J. D. DEGOS AND OTHERS

findings. Furthermore, since the lesion was old, the location and the resulting white
matter degeneration provided data concerning the callosal connections of the
human cortex. Previous publications pertaining to the clinical study of this case
include a prediction of the anatomical lesions (Barbizet et al., 1974) and a complementary study of visual performance (Lhermitte et al., 1976).
CASE

REPORT

From June 1971, the patient's state of consciousness, gait and vision improved progressively until
her condition stabilized itself in October, 1971. Slight problems with gait and fine finger movements,
as well as brisk tendon reflexes and impaired position sense persisted on the right. On the left, only
somatosensory extinction existed. Strikingly, although she palpated objects with her left hand in a
rapid and precise manner, she named and described them incorrectly (tactile anomia). Gaze movements were normal; optokinetic nystagmus was elicited in all directions. Goldman perimetry showed
a bilateral reduction of the peripheral visual field; macular vision, part of the right inferior quadrant
and most of the left visual field were spared. Visual acuity was 6/10 for both eyes. The patient's oral
language capacities, her memory and her ability to execute transitive and intransitive gestures-were
intact. She showed no attention deficit; in fact she was very cooperative and persevering during the
tasks. In December 1973 a repeat CT 99 brain scan disclosed no abnormal uptake.

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S. was a right-handed 75-year-old woman. On February 26, 1971, while driving her car she suddenly
experienced a severe headache and blurred vision. She managed to return home but during the days
following this incident her family and doctor noticed that her gait and memory were impaired.
However, she did not enter hospital until one month later. At this time neurological examination
revealed an alert patient with reduced spontaneous movement of the right arm and leg. Although no
segmental motor weakness was disclosed she was unable to walk unsupported without immediately
falling towards her right. Fine finger movements were normal. The plantar reflexes were both flexor.
Hypalgesia and defective position sense were detected on the right side of her body. On the left, the
only problem was the misnaming of objects that she palpated with her left hand. Her vision was
restricted to the central zone. When an object was presented to her, eye fixation was obtained only
by chance and visual pursuit was impossible. S. could move her eyes correctly to the right and to the
left whenever she was requested to do so; however, she was unable to raise them above the horizontal.
Her handwriting was poor with micrographia. No language disorder was disclosed and, in particular,
the patient continued to be able to name colours. There was no evidence of acalculia, difficulty in
distinguishing right from left, asomatognosia, anosognosia, or of visual neglect. The motor deficiency
interfered with the assessment of right-sided apraxic disorders that later became evident. On the left,
although objects were put to correct use, at times symbolic or mimed gestures seemed to be disturbed
by a searching movement of her hand. There was no orofacial apraxia. The patient was unable to
retain questions that she had been asked 5 min earlier, nor could she describe the various homes in
which she had lived over the years or arrange in chronological order facts that she had been able to
recall. She was noticeably disorientated in time and space but showed no tendency for erroneous
recognition. She stated that the onset of her illness coincided with the disappearance of dream
activity. The EEG carried out on March 23, 1971, was normal; on April 16 bilateral occipital-slow
monomorphic delta waves were observed on an irregular background. Between May 7-10, 1971, the
patient's condition deteriorated rapidly. Drowsiness and incontinence appeared and the motor and
somatosensory deficit affecting her right arm and leg became accentuated. A loss of gaze movement
and an inability to recognize objects and people were also noted, although the patient remained able
to recognize a few colours. The EEG showed polymorphic delta slow wave activity over the posterior
part of the left hemisphere. One month later a CT 99 m brain scan revealed an area of early uptake
in the left temporo-occipital zone and an area of delayed uptake in the right parieto-occipital zpne.

POSTERIOR CALLOSAL I N F A R C T I O N

1157

From 1974 until 1979, S. lived in a home for the aged and, during this time, no further neurological
disorder appeared. Her diminished vision was partially accounted for by glaucoma in her right eye.
Left tactile anomia was observed each time it was sought. In September 1979, S. died of septicaemia.

STUDY OF INTERHEMISPHERIC EXCHANGE OF SENSORY AND MOTOR
INFORMATION

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Clinical observations
We personally examined S. on a regular basis from March 1972 to November
1973. Throughout this entire period a partial interhemispheric disconnection syndrome was consistently observed.
S. was able to name and to describe objects that were hidden out of sight behind
a screen if she was permitted to feel them with her right hand. Left hand palpation
was less successful in that she almost always misnamed the object: the name chosen
often corresponded to an object that had been presented to her earlier or to one
that had been recently mentioned. When she was asked to describe an object held
in her left hand, S. was either incapable of doing so or else she described the object
whose name she had just incorrectly given.
When the patient held an object in each of her hands she invariably claimed that
her left hand was empty and sometimes she made large searching movements with
her left arm. However, this did not prevent her from manipulating the object
spontaneously in a correct manner. For example, she would rub the bristles of a
toothbrush and open and close a clothes peg or a pair of spectacles even though
she verbally disclaimed any awareness of the presence of these objects in her left
hand.
When she was asked to use the object that she was holding in her left hand, S.
usually did so in a way that was adapted to the name she had just given it, whether
this name was correct or not. However, at times correct usage was observed in
spite of incorrect naming. For example, even though she mistook a pair of spectacles
for a pair of scissors she put them on correctly; she called a spoon a needle but she
brought it to her mouth as if feeding herself; she decided to give the name 'lighter'
to a hairbrush but, although she mimed the action of lighting a cigarette with her
right hand, she brushed her hair with her left hand; she was incapable of naming
a thimble but placed it on her finger correctly.
When she was blindfolded the patient had no difficulty in naming areas of the
right side of her head and body that the examiner touched. When the situation was
reversed to examine areas of the left side of her head (i.e., her hair, eyebrow,
forehead, nose, upper lip, cheek, ear and chin), 20 attempts at verbal identification
led to only two errors: 'the side of my nose' when her upper lip was touched
and 'my eyebrow' when her forehead was touched. Both of these errors were
subsequently corrected. These results contrast with those obtained through stimulation of the left side of her body as opposed to her face: she could name none of
the regions that were being stimulated, although she could use either hand to

1158

J.D

DEGOS AND OTHERS

Left hand
Dictation: 'Je suis venue a I'hopital'
(I came to hospital)

Dictation of letters

9
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Right hand
Dictation

Spontaneous writing

FIG. I Left hand agraphia

indicate them. When the examiner touched two fingers of one of her hands she
could use the opposite hand's thumb to indicate the two fingers of that hand that
corresponded to those being touched. Regardless of the arm chosen to carry out
the task, S. usually had more success at reproducing a position that had previously
been imposed on the same arm than at imitating a position that was actually being
imposed on the opposite arm.
The patient could write correctly with her right hand but when she used her left
hand the results were lacking in form (fig. 1). She could identify letters and numbers
that were traced on the palm of her right hand but this was impossible when her
left hand was stimulated in the same way. However, she could name simple letters

POSTERIOR CALLOSAL INFARCTION

1159

Formal testing
Two sets of 7 objects were used. (1) objects that were considered 'significant' (SO): a champagne
cork, a clothes peg, a sea shell, a spoon, a ball-point pen and a key. (2) Objects of different geometrical
shapes that were considered 'nonsignificant' (NSO).
The patient was given time to acquaint herself with the SOs so that, from the beginning, she was
thoroughly familiar with the objects among which she would have to choose. Each test began either
with visual presentation, tactile presentation or the naming of each of the 7 objects that belonged to
one of the sets described above (input). The patient was then asked to name these objects or, in
accordance with the multiple choice method, to point to or to retrieve I of the 7 objects (output).
Success frequencies were calculated after 3 to 24 tests (21 to 169 trials). The tests are summarized
below according to their respective modes of entry (input) and their numbering corresponds to output
and to the numbering of the squares in Tables 1 and 2.
Input: left hand palpation
1: left-handed retrieval of the object just palpated by the same hand.

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that she was made to draw through passive movements of either of her arms and,
whether the movements used proximal or distal joints, the results were identically
successful.
Tasks concerning the copying of figures with cubes and matches as tools were
incorrectly performed by her right hand although some of the same figures were
correctly copied by her left hand.
S. could use both sides of her body to execute meaningful gestures (symbolic or
mimed gestures) and she could imitate gestures that were presented visually to her.
However, when the examiner merely described these gestures orally (e.g., raise your
arm in front of you, to the side, above your head; bend and straighten out your
forearm; use your thumb and one other finger to form a circle) many errors were
observed on the left side of her body. These errors were particularly frequent for
hand and finger gestures, less frequent for the forearm and practically nonexistent
when the movement called for the use of the proximal part of the arm. In September
1973, two years after the initial experiment, only finger movement errors persisted.
The patient was unable to describe postures that were imposed on her forearm,
hand or fingers unless she was offered a choice between two simple alternatives
such as 'up' or 'down'.
During a test that required the cross-matching of objects, S. said 'My hands
don't agree with each other'. To explain the meaning of this remark she told the
story of herself frying a steak, turning it over in the frying pan with her right hand
and, immediately after, finding herself turning it over once again with her left hand.
She also explained that she was often unaware of an object that she had in fact
been holding in her left hand for several minutes: 'I can be holding a knife in my
left hand while I cook but it is only when I cut myself with it that I am astonished
to discover its presence'. Misadventures such as these made her fear for her sanity.
The patient also declared that she often had to interrupt her activities before being
able to resume and finish them successfully. However, even when blindfolded she
could accomplish tasks, such as tying a knot, that required coordinated hand
activity.

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J D. DEGOS AND OTHERS

Results. The ratios of correct responses to the number of trials can be summarized
as follows. The results obtained during tests 1-14 are presented in Tables 1 and 2;
those for tests 15-18 are given in Table 3. These results mainly show a left tactile
and visual anomia. However, the left tactile anomia, in contrast to the left visual
anomia, is not complete; the proportion of correct responses, although low, is
different from chance (0.256 vs 0.143, P< 0.001). Left-handed retrieval of an object

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2: right-handed retrieval of the duplicate of the object just palpated by the left hand.
3: naming the object just palpated by the left hand.
4: pointing to the duplicate of the object just palpated by the left hand. The patient was usually
asked to use her right hand since her left hand was slower and its use led to identical results.
Input: right hand palpation
5: left-handed retrieval of the duplicate of the object just palpated by the right hand.
6: right-handed retrieval of the object just palpated by the same hand.
7: naming of the object just palpated by the right hand.
8: pointing to the duplicate of the object just palpated by the right hand. For the same reason as
in Test 4 the patient was asked to use her right hand.
Input: the examiner naming an object
9: left-handed retrieval of the named object.
10: right-handed retrieval of the named object.
11: pointing to the named object. Identical results were obtained whichever hand was used.
Input: the sight of an object placed in free view
12: left-handed retrieval of the duplicate of the object.
13: right-handed retrieval of the duplicate of the object.
14: naming the object.
Tests for visual processing were conducted by tachistoscopy in both fields (Prof. F. Lhermitte's
Laboratory, Hopital de la Salpetriere, Paris). A photograph of an object or letter was flashed in the
right or left visual field. The patient was then asked to name or to point to the object or letter whose
image she had just seen. Success frequencies were obtained after 4 tests (24 trials). The number of
each test described below indicates the corresponding square in Table 3.
15 and 16: naming the object or capital letter flashed in .the left (Test 15) or the right (Test 16)
visual field.
17 and 18: pointing to the duplicate of the photograph that had just been flashed in the left (Test
17) or in the right (Test 18) visual field. The duplicate was to be found among 6 photographs all of
which were placed in free view.
A dichotic listening test using the material elaborated by Michel and Peronnet (1975) was performed
in November 1973. Two different digits (between 1 and 13) were simultaneously presented, one to
each ear, at equal sensation levels. The patient was asked to repeat all digits heard. Twenty pairs of
digits were presented in this manner. The same experiment was performed with 10 pairs of monosyllabic and 30 pairs of disyllabic words. The patient had previously shown herself to be able to
repeat items correctly when only one ear was stimulated at a time.
Statistical methods. For each test the binomial distribution was used to calculate the probability
of a significant difference between success frequency and success frequency due to chance. When the
results of two tests were compared a fourfold x1 t e s t permitted the assessment of the statistical
difference between success frequencies. A partition of x2 was used when the results of more than two
tests were compared (Fleiss, 1981). When we wished to study, for example, the influence of input
(left or right hand) and output (ipsilateral or contralateral) on SO or NSO retrieval, a factorial testing
design was used and, in this case, the influence of each separate factor and the influence of their
interactions was determined by analysis of variance after angular (arc sine) transformation (Snedecor
and Cochran, 1980).

POSTERIOR CALLOSAL INFARCTION

1161

named by the examiner led to better results (0.612 vs 0.256, P< 0.001). A certain
amount of interhemispheric exchange for stereognosis is maintained, with better
results for NSO than for SO. When the retrieval was changed from ipsilateral to
TABLE 1. FREQUENCIES OF SUCCESS ACCORDING TO DIFFERENT INPUT/OUTPUT
COMBINATIONS FOR SIGNIFICANT OBJECTS (SO) OR NONSIGNIFICANT OBJECTS (NSO)
Input

LH

RH

0.810
(51/63)

0.571
(48/84)

Ovtpul
SO

LH

0 333
(7/21)

0.612
(49/84)

LH i ,.(2.3,4) /><0.001
3*4 /><0.001

0 371
(39/105)

RH

I 000
(63/63)

1.000
(21/21)
R H 5#(6.7J)P<0.001

1.000
(21/21)

0.512
(86/168)

1.000
(21/21)

I

NSO

LH

1 000
(21/21)
1 000
(21/21)

5

0.786
(66/84)

0810
(51/63)

0.726
(61/84)

*:

P<0.00\

i 2) /*<0.05
2

RH

6

0.405
(34/84)

1 000
(21/21)

0 905
(57/63)

RH j,.(6.1)/•<0.05

S

0600
(63/105)

I 000
(21/21)

LH-left hand, RH-right hand, D-denomination, S — light. The test numbers are used lo indicate all the significant paired
comparisons between the results for both the LH and the RH inputs, respectively, furthermore the score obtained by the
denomination of a SO palpated by the LH (3) is different from chance level (1/7 — 0 143, /»< 0.001) and from the score of the lefthanded retrieval of a named object (3 *i, P < 0 001)

TABLE 2 INTERACTION BETWEEN IPSILATERAL OR CONTRA LATER A L RETRIEVAL
(FACTOR A) ACCORDING TO THE SIGNIFICANCE OR NONSIGNI FICANCE OF THE OBJECTS
(FACTOR B ) A N D T H E S I D E O F T H E ENTRY (FACTOR C)
Output

Ipsilatfral retrieval

Input

LH

Contralateral retrieval

RH

LH
2

SO
NSO

0.810
0.786

1.000
0.905

RH
5

0 371
0 405

0.571
0 726

LH = left hand; R H - n g h t hand; SO — significant object. NSO-nonsignificant object. Frequencies of success presented in
Table 1 are redistributed using such a factorial design to demonstrate (1) a significant factor C contribution, i.e , better results
with the right hand entry than with the left hand entry (/ > <0.00l), and (2) a significant A-B interaction, i.e. a fall in success rate
significantly smaller for NSO than for SO when retrieval was changed from the ipsilaleral to the conlralateral (/"<0.05).

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0.256
(43/168)

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J. D. DEGOS AND OTHERS

contralateral, the fall in the NSO results was significantly less than that for the SO
results (Table 2).
The dichotic listening test results given in Table 4 show no left ear extinction.
TABLE 3 LEFT VISUAL ANOMIA*
Output

Pointing

16

15

SO
L

RVF

LVF

•nit

Denomination
LVF

RVF
II

17

0.833

1.000

0.167

0.750

1.000

0.000

1.000
1 000

•There is a significant interaction ( P < 0 0 0 1 ) between the retrieval method (pointing or naming) and the side of entry, i.e., the
fall in success rate was significantly smaller for pointing than for denomination when input was moved from the nght visual field
to the left LVF = left visual field entry; R VF •=rightvisual field entry; SO — significant object; NSO = nonsignificant object

TA BLE 4. N U M B E R OF CO R R E C T ( + ) OR I N C O R R E C T ( - ( R E P E T I T I O N S OF PA IRS OF D I G I T S
OR W O R D S NO LEFT EAR E X T I N C T I O N
Left ear -

Total

Total

7
11
18

8
4
12

15
15
30

Total

3
17
20

12
8
20

15
25
40

Digits
Right ear +
Right e a r -

Words
Right ear +
Right ear -

NEUROPATHOLOGICAL EXAMINATION
Methods
After horizontal section of the mesencephalon, the brain was cut in coronal slices. The cerebellum
and unseparated brainstem were cut in horizontal slices. Nine verticofrontal bihemispheric brain
slices and 6 horizontal slices of the mesencephalon, pons and medulla oblongata were embedded in
celloidin; the blocks of the posterior hemisphere were cut semiserially. The sections were stained with
haematoxylin and eosin, and with the Loyez stain for myelin. Small specimens of the posterior corpus
callosum and parieto-occipital white matter were embedded in paraffin and stained with haematoxylin
and eosin, Massons trichrome and by Bodian silver impregnation combined with Luxol fast blue.

Results
Four types of change were observed.
1. The lateral ventricles, the foramen of Monro and the third ventricle were
dilated (figs 2, 3, 6). The aqueduct was slightly dilated. The volume of the fourth
ventricle was normal. The cerebral cortex was macroscopically normal, with no
evidence of atrophy or of infarcts.
2. The splenium and the posterior part of the body of the corpus callosum were

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Left ear +

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INFARCTION

1163

FIG. 2. A, coronal section of^the cerebral hemispheres through the mamillothalamic tracts showing normal
corpus callosum and ventricular dilatation, B, coronal section of the cerebral hemispheres through the pulvinar;
the shrunken splenium is viewed from behind. The lateral ventricles are dilated.

the site of old necrotic lesions (figs 2B, 3B, 4). This type of lesion also existed
bilaterally in the white matter surrounding the occipital horn of the lateral ventricles
at the level of the calcarine sulcus (fig. 6c, D). All these regions appeared to be
atrophic. They all showed extensive degeneration involving axons and myelin, and
also astrocytic gliosis with areas of complete cavitating parenchymatous disintegration. Macrophages were rare. No inflammatory infiltrates were observed.
3. Bilaterally symmetric degenerative lesions involved the white matter of the
superior parietal lobules and occipital gyri. They consisted in incomplete patchy
myelin loss with astrocytic gliosis; axons were reduced in number. The U fibres
were spared (fig. 6). Slight neuronal loss was found in the third layer of the
corresponding cortical regions. The distribution of these lesions (fig. 5) led us to
consider them as resulting from the degeneration of the callosal forceps major
radiations secondary to the necrosis of the posterior part of the corpus callosum.

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FIG. 3. A, coronal section of the cerebral hemispheres through the mamillary bodies showing dilatation of the
lateral and third ventricles, and normal corpus callosum, frontal and temporal white matter, and cortex, B, coronal
section of the cerebral hemispheres through the posterior part of the thalamus showing ventricular dilatation and
necrosis of the corpus callosum Loyez stain.

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J. D. DEGOS AND OTHERS

FIG. 4. Topography of the necrosis of the corpus callosum sfg = superior frontal gyrus; cg = cingular gyms;
prcg = precentral gyrus; pocg = postcentral gyms; spl = superior parietal lobule; prc = precuneus; c = cuneus.

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FIG. 5. Topography of the degeneration of the parieto-occipital white matter. sfg = superior frontal gyrus;
mfg — middle frontal gyms; ifg = inferior frontal gyrus; prcg = precentral gyms; pocg - postcentral gyrus; spl = superior parietal lobule; smg = supramarginal gyms; ag = angular gyrus; log = lateral occipital gyri; stg = superior
temporal gyrus, mtg = middle temporal gyrus; itg = inferior temporal gyrus.

4. The basilar artery, the supraclinoid carotid arteries, the circle of Willis, the
anterior, middle and posterior cerebral arteries and all of the meningocortical
arteries were examined. These extracerebral vessels were all histologically normal;
they showed no atherosclerosis and no occlusion. Hyalinization of the walls of
small arteries and arterioles was often noted when sections of intraparenchymatous
blood vessels were examined. Moreover, many perivascular spaces were distended

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POSTERIOR CALLOSAL INFARCTION

spl
spl

ag

ag

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FIG. 6. Coronal sections of the cerebral hemispheres (A, C: left; B, D: right) through the posterior part of the
temporal lobes (A, B) and posterior horn of the lateral ventricles (c, D) showing ventricular dilatation, necrosis of
the white matter surrounding the occipital horn of the lateral ventricle involving the optic radiations, and
degeneration of the white matter of the superior parietal lobules and occipital gyri sparing the U fibres. Loyez
stain, spl =» superior parietal lobule; ag = angular gyms; stg = superior temporal gyms; mtg = middle temporal
gyrus; itg = inferior temporal gyms; log-lateral occipital gyri; c = cuneus; cg = cingular gyms.

and some of these contained a few macrophages. These lesions were particularly
numerous in the lentiform nucleus (especially on the right) as well as in the thalamus
and in the white matter of both cerebral hemispheres. A lacune (small deep infarct)
was observed in the posterior region of the right putamen. The vessel walls in the
globus pallidus were mineralized, especially in the right hemisphere.
5. Except for rather numerous grey matter heterotopias within the right frontal
and parietal white matter, the other cerebral structures were macroscopically and
microscopically normal. In particular this was true for the rostrum and the anterior
region of the body of the corpus callosum (figs 2A, 3A, 4). It was also true of the

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J. D. DEGOS AND OTHERS

fimbriae, the fornix, the hippocampal commissure, the anterior commissure, the
mamillary bodies, the cerebellum, the brainstem, the temporal isthmus and
the cerebral cortex, including the hippocampus.
DISCUSSION

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The clinical study of this case permitted an accurate prediction of the neuropathological findings (Barbizet et al., 1974). The diagnosis of infarction of the
vertebrobasilar territory, which seemed clinically probable, was confirmed by
the finding of necrotic lesions in the occipital regions and in the posterior part of the
corpus callosum which are in the territory the posterior cerebral arteries (Foix and
Masson, 1923; Stephens and Stilwell, 1969). These lesions are in accordance with
the final clinical data (bilateral hemianopia and signs of callosal disconnection).
Several points are of particular interest: the posterior part of the corpus callosum
was totally destroyed so that no nervous structures remained in this area; the limits
of this destruction were neatly circumscribed; the original lesion was several years
old and therefore the subsequent degeneration allowed us to define the cortical
zones involved in the disconnection; the other commissures were intact; and there
were no cortical lesions.
All the white matter lesions, except for those situated around the occipital horns
of the lateral ventricles had the typical characteristics of degeneration secondary
to an interruption of the callosal fibres: they were symmetric, they spared the U
fibres (fig. 6) and there was neuronal loss in the corresponding cortical regions.
This degeneration was confined to the occipital and to the superior parietal lobule
white matter (figs 5,6). The fact that the temporal lobes were unaffected is noteworthy. In view of these observations it seems possible to conclude that, in the
human brain, the temporal lobe fibres follow a rostral pathway as compared with
that followed by fibres from the parietal lobe, or at least with the pathway taken
by the superior parietal lobule fibres. This pattern differs from that described in
the most recent anatomical study concerning the cortical termination of callosal
fibres in the Rhesus monkey (Pandya et al., 1971) which showed that within the
corpus callosum the parietal lobe fibres remain rostral to those from the temporal
lobe. Results as different as these illustrate the risks incurred when animal data are
extrapolated to the human brain, even when the data are obtained from primates.
Recently de Lacoste et al. (1985) correlated the distribution of wallerian degeneration in the corpus callosum with the anatomical sites of focal lesions in man. Their
results and ours agree with each other in that they are both in favour of a splenium
pathway for fibres arriving from the superior parietal lobules as opposed to a more
anterior pathway for those originating in the temporal lobes.
Our patient shared a certain number of clinical characteristics with patients
having undergone forebrain commissurotomy (Sperry et al., 1969; Gazzaniga,
1970): left-sided tactile (Table 1) and visual (Table 3) anomia, inability to name
areas of the left side of her body (apart from facial areas) as they were being

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touched by the examiner and agraphia with the left hand (fig. 1). However, contrary
to split-brain patients (Sperry et al., 1969), she continued to be able to indicate
with the opposite thumb the two mirror fingers that corresponded to two fingers that
were being touched on one side. This interhemispheric exchange of somaesthetic
information can be explained by the integrity of the part of the body of the corpus
callosum which seems to be responsible for the transfer of intermanual information
(Jeeves et al., 1979; Bentin et al., 1984). However, this transfer was insufficient for
satisfactory cross-stereognostic recognition, since an object palpated by the right
hand was less consistently retrieved by the left hand than by the right (Table 1, 5
vs 6, P< 0.001 for SO and P<0.05 for NSO) and vice versa (Table 1, 2 vs 1,
P< 0.001). This somaesthetic transfer was even less efficient for the naming of
objects palpated by the left hand, since the score differs only slightly from chance
(0.206 vs 0.143, P< 0.001). It seems that we are in the presence of a gradient as far
as the possibilities of interhemispheric transfer are concerned. Presumably the
more the task calls for a posterior callosal transfer, the more the quality of the
performance diminishes.
When our patient was blindfolded and asked to make a binary verbal choice
('up' or 'down') to describe a position held by a part of one of her left-sided limbs
(fifth finger and hallux included), she was able to comply with ease. However,
no description was possible when the position involved more than one joint or
when no verbal alternative was offered. The results are identical to those observed in split-brain patients (Sperry et al., 1969; McCloskey, 1973). As it has
been described in the split-brain patients (Sperry et al., 1969), our patient was
able to reproduce positions that had been imposed on one of her limbs but
was less successful at imitating positions that were being imposed on the opposite
limb.
The maintained somaesthetic interhemispheric exchanges could furnish a reasonable explanation for the immediate activation of left hemisphere language activity
on arrival of somaesthetic information originating in the right hemisphere, the end
result of this being incorrect verbalization. Actually verbal activity was frequently
observed even when it was specifically forbidden. According to our patient, silent
verbalization followed palpation. She said that it was particularly frequent when
she knew that the object belonged to the group of significant objects (SO) but that
it also occurred, although to a lesser degree, with nonsignificant objects (NSO) that
she had learned to identify during the course of the tests through a detail or the
object's general shape. It was quite obvious that this silent verbalization created a
disturbance that influenced the tests' results. To eliminate it as much as possible
the observer placed himself in front of the patient, spoke to her and encouraged
her to concentrate on the object's shape. However, this method was only partially
and irregularly effective. The statistical study of the results confirms that incorrect
verbalization did indeed play a disturbing role. When the patient was asked to
search for an object that she had just palpated, whether this search was undertaken
by the hand that had originally manipulated the object or not, left hand entry

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proved to be less effective than right hand entry (Table 2, P< 0.001); when retrieval
was shifted from the ipsilateral to the contralateral side, the fall in the score
associated with NSO retrieval was significantly less than that for SO retrieval
(Table 2, / ) <0.05). Moreover, the success rates associated with SO identification
after left hand palpation were 0.810 for left hand retrieval (Table 1, 1), 0.512 for
visual identification (Table 1,4), 0.371 for right hand retrieval (Tablel, 2) and 0.256
for naming (Table 1, 3). In view of these results it seems reasonable to presume
that the disturbance created by incorrect verbalization was relatively small when
the left hand was confronted with the task of searching for an object that it had
just palpated; the disturbance should have been maximum when the right hand
searched for an object palpated by the left hand and, in fact, the score is not
significantly different from the one associated with the naming of an object palpated
by the left hand. The score for visual identification after left hand palpation is
intermediate, significantly lower than the one obtained through left hand retrieval
(P< 0.001) and higher than that for naming (P< 0.001). This intermediate result
probably depends on which occipital cortex commanded the pointing out of the
object: the left occipital cortex, which was possibly disturbed by erroneous naming,
or the right occipital cortex which had processed somaesthetic information at its
disposal.
As it is the case with split-brain patients (Sperry et al., 1969), S. was more
successful at using her left hand to find an object that had been named by the
examiner than she was at naming an object that she had palpated with her left
hand (Table 1, 9: 0.612 vs Table 1, 3: 0.256, P< 0.001). Whether the SOs were
named (Table 1, 9) or palpated by the right hand (Table 1, 5) the scores associated
with left hand retrieval were identical (0.612 and 0.517). This result is hardly
surprising since immediate verbal identification always followed right hand palpation and, what is more, the patient then actually refused to continue to feel the
object since, as she said 'It is useless to do so since I already know what all this is
about'. Thus right hand palpation was almost equivalent to naming.
This right hemisphere comprehension capacity for oral language was also found
for written language (Lhermitte et al., 1976): when a short printed word was flashed
in the left visual field our patient was able to indicate the corresponding picture or
word written in cursive script.
It has been remarked that after forebrain commissurotomy, patients can no
longer use their left hand and foot to reproduce positions that have just been
described to them even though they continue to be able to imitate these positions
(Sperry et al., 1969). This was also true of our patient although, conversely, she
remained capable of using her left arm to execute symbolic gestures and to mime
the use of objects. In other words, there was no ideomotor apraxia of the left arm.
One can surmize that these two types of gestures bring different callosal connections
into play (Watson and Heilman, 1983). It is pertinent, however, to note that the
former type of gesture, unlike the latter, is composed of several unrelated items
and that the right hemisphere, although capable of grasping a simple order, seems

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incapable of articulating more than two or three different verbal elements (Zaidel,
1978).
After forebrain commissurotomy, verbal description of movements that are to
be carried out by the proximal part of the left arm prompt correct performance
(Gazzaniga et al., 1967). This was also true of our patient. It is in favour of
an ipsilateral motor command for the proximal arm. This hypothesis is further
supported by behaviour that we observed in our patient at several different times:
when S. held an object in each of her hands she disclaimed any awareness of the
object that she was in fact skilfully manipulating with her left hand and, at the
same time, her left arm made large exploratory movements as if to search for this
object. It is plausible to think that in this competitive situation, the amount of left
side tactile information reaching the left (speaking) hemisphere was so small that
this hemisphere made the patient express her unawareness of the object held in her
left hand; in view of the fact that it was misinformed, the left hemisphere activated
the left arm's ipsilateral driving system and, consequently, this arm was thrust out
in search of the object; the right hemisphere, unaware of what was happening in
the left hemisphere, simultaneously continued to exert efficient control over the left
hand's palpation of the object thanks to the afferent information that it was
receiving.
Left hand agraphia contrasting with an absence of ideomotor apraxia is a
characteristic not only of our patient but also of other cases of partial lesions of
the corpus callosum (Sugishita et al., 1980; Gersh and Damasio, 1981).
'Diagonistic apraxia' has been described in patients who had undergone forebrain commissurotomy (Van Wagenen and Herren, 1940; Akelaitis, 1945). It has
occasionally been reported in other cases of commissurotomy (Zaidel and Sperry,
1977; Bogen, 1985) and in cases of spontaneous lesions of the corpus callosum
(Barbizet et al., 1978; Watson and Heilman, 1983). Approximately the same
description is always given: each hand interferes with the other and, as in the 'frying
pan' incident described by our patient, the left hand often undoes what the right
hand has just accomplished. It is as if the right hemisphere was less concerned with
motor intention of deliberate actions than the left (Barbizet et al., 1978).
The dichotic listening test showed no left channel extinction (Table 4) although
this is known to be one of the features of the interhemispheric disconnection
syndrome (Milner et al., 1968; Sparks and Geschwind, 1968; Springer and
Gazzaniga, 1975). The word and digit material that we used could possibly explain
these results (Springer et al., 1978), although it seems pertinent to remark that this
material succeeded in revealing a left ear extinction in other cases of interhemispheric disconnection (Michel and Peronnet, 1975; Barbizet et al., 1978).
Furthermore, the absence of left ear extinction is in accordance with the integrity
of the anterior portion of the corpus callosum that seems to have a preponderant
influence on the right-left transfer of dichotic information (Springer and Gazzaniga,
1975) and with the integrity of the temporal callosal fibres.

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J. D. DEGOS AND OTHERS
ACKNOWLEDGMENTS

The authors wish to express their gratitude to Dr F. Michel for his help with the dichotic listening
test and to Mrs D. Mirabel-Jobin for her help with the English translation of this text.
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