J. Neurol. 209, 75--79 (1975)
© by Springer-Verlag 1975
Editorial
Review of the Split Brain
M. S. Gazzaniga
State University of New York, Department of Psychology, Stony Brook, New York 11794
Received J a n u a r y 14, 1975
Die grundlegende Literatur zur Aufdeckung des Split brain-Syndroms wird diskutiert.
The corpus callosum, the cerebral commissure that interconnects the left and
the right half brain, had been considered an enigma to neurologists and neurosurgeons in the 1940's and 50's. I t was the structure discussed most often when
an example was sought to show how little was known about the brain. Even
though it is the largest nerve tract in the brain, which in man contains more
than two hundred million neurons, it was generally believed it could be sectioned
and destroyed and have no apparent consequences for behavior. People continually
quipped about the general lack of importance of the forebrain commissures.
I t was in this context that the original experiments on the split-brain were
carried out in the cat by Ronald Myers and Roger Sperry at the University of
Chicago. Myers had successfully developed the technique of spliting the optic
chiasm, thereby allowing visual information presented to the right eye to be
exclusively projected to the right hemisphere and visual information projected
to the left eye to be exclusively presented to the left hemisphere. He had discovered in follow-up behavioral tests that there was, nonetheless, interocular
transfer. Therefore the logical structure to next surgically section to see whether
one could block interhemispherie transfer of visual information was the corpus callosum. This was done and this study gave birth to the now classic "split-brain"
experiments first done on the cat showing that discriminations trained to one side
leaves the other half of the brain naive. Later studies carried out in both the
monkey and chimpanzee confirmed these findings.
Still, however, these results stood in marked contrast to the earlier results of
Akelaitis who had studied a series of some 26 patients with the corpus callosum
and anterior commissure sectioned in complete or in part for the interhemispherie
control of epileptic seizures. I n an extensive series of studies he purported to show
that sectioning of these structures did not result in any significant neurological
or psychological sequela. This point was made and emerged as the dominant
view even though there were several contradictory reports in the literature
showing disconnection effects as a result of having the callosum sectioned or
rendered non-functional by a tumor or the like (see Geschwind, 1965). I t was also
generally considered that cutting the callosum did not, in fact, help control
epilepsy.
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M.S. Gazzaniga
Then, in 1960, Dr. Joseph Bogen, who at the time was a resident at White
Memorial Hospital in Los Angeles, proposed, after a careful review of the Akelaitis'
literature, that the brain be split for the purpose of controlling the interhemispheric
spread of epilepsy. His hunch that the surgery should work proved largely correct.
I t was his first patient, W. J., that was extensively studied both pre and postoperatively on a host of psychological tests that we devised at the California
Institute of Technology (Gazzaniga et al., 1962, 1963, 1965; Gazzaniga and Sperry,
1967). In subsequent examination of patients in the Bogen series a variety of
striking and dramatic effects were observed.
Information Exchange between Cerebral Hemispheres
To begin with, the interhcmispheric exchange of information was totally disrupted following commissurotomy such that visual, tactual, proprioceptive, auditory and olfactory information presented to one hemisphere could be processed
and dealt with in that half brain, but these activities would go on outside the realm
of awareness of the other half cerebrum. Thus, the work confirmed the earlier
animal work by Myers and Sperry but was in a sense more dramatic in that all
processes ongoing in the left hemisphere could be easily verbally described by
the patients since it is the left hemisphere that normally possesses the natural
language and speech mechanisms. Information presented to the right hemisphere
went undescribed and it was only through using special testing techniques that
we were able to discover that the right hemisphere also had a rich mental life and
was capable of experiencing most of the phenomena appreciated by the left brain.
Mental Properties of the Right Hemispheres
Using non-verbal tests especially designed for the purpose, it was determined
that the right hemisphere had some language, could initiate its own responses
and could emote, learn, remember and do all the things of normal life without
the left hemisphere knowing the why or what of it all. Thus, for example, a word
could be flashed in the left visual field which is exclusively projected to the right
hemisphere in m a n - - s u c h as "spoon" and the subject would say, " I did not see
anything," but then subsequently would be able with the left hand to retrieve
the object from a series placed out of view. When the correct object was indicated by the subject holding it in his hand, the experimenter would say, " W h a t
is it you have in your hand ?" they would say, " I don't know." Here again "mind
left" does not know. I t did not see the picture nor did it have access to the stereognostic or touch information from the left hand which is also exclusively projected
to the right hemisphere. Yet clearly the right hemisphere knew it because it reacted appropriately to the correct stimulus and made the appropriate response.
These patients, however, allow for even further studies that would allow the
examiner to investigate the separate mental properties of the two half brains. We
showed that the left hemisphere, as would be predicted from early clinical reports,
excelled in verbal processing of information of all kinds. The right hemisphere,
however, was superior in managing visual spatial tasks such as drawing cubes,
arranging blocks to match designs and the like. This distribution of mental work
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77
in the brain highlights a major difference between man and animals and raises
the intriguing possibility t h a t the various modes of consciousness have separate
physical identities in the brain.
These earlier results carried out on Bogen's series of patients have recently
been confirmed and extended on the patients of Dr. Donald Wilson of the Dartmouth Medical School. Wilson, using a different surgical approach t h a n Bogen,
has also carried out both complete and partial commissurotomies with the aim
of preventing interhemispheric spread of epileptic seizures. Neuropsychological
examination of these patients has confirmed m a n y of the earlier findings on the
Bogen series and, in brief, pointed out t h a t visual, taetual, auditory and olfactory
information cannot be transferred following forebrain commissurotomy. Moreover,
the partial sectioned cases showed t h a t a surgical lesion can be made t h a t will
block visual information from transferring between the two half brains but not
tactile and vice versa. Interestingly, it seems to not matter which part of the
callosum is in tact in order to realize transfer of what might be called the superordinate function of the two half brains. Thus, the specialized activities of the
right m a y come to the aid of the left no matter what part of the callosum is
intact ; whereas, more specific information from the right half brain, such as visual
information in the left field, cannot be communicated to the left hemisphere if
the splenium alone is sectioned.
Tests on these patients also revealed t h a t there is a possibility t h a t one of the
costs to an individual for having this seizure-controlling surgery, is t h a t the mental
life of the left hemisphere m a y go on without the aid of the normal imaging
mechanisms which are most likely normally housed on the right. Consistent with
this is one of the complaints of the patients in the Bogen series t h a t they no
longer dream. Dreaming reflects imaging processes and what has been traditionally called the subconscious process. I t would be fascinating to discover t h a t
the right half brain is primarily responsible for this side of our mental life. A
series of sleep research experiments is now being contemplated to confirm this
possibility.
I n the past, the amount of language and mental activity discovered in the
right brain, in our earlier studies, was thought to be the product of early brain
damage. I t is well known t h a t if brain damage occurs early in life, there is a
bilateralization of the language and speech mechanisms. However, a recent case
has been reported of a patient who, at the age of 70, suffered vascular disease
which specifically affected the splenium with the results t h a t the person was a
split-brain with respect to visual functions. This patient also showed the same
language talents t h a t we had outlined earlier in the Bogen series of patients. This
kind of evidence suggests t h a t in the normal developmental process there is some
redundancy in the system such t h a t all of language or all of spatial functions
are not strictly and exclusively lateralized to the respective left and right hemispheres.
Educational Significance
Indeed one of the intriguing possibilities coming from the split-brain research
is the possibility t h a t man can be explicitly specialized for a variety of aspects
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M.S. Gazzaniga
of m e n t a l life with superiorities in t h e v e r b a l a r e n a n o t necessarily m e a n i n g
superiorities in t h e v i s u a l - s p a t i a l a n d vice versa. Thus, when dealing with a
cognitive t a s k it m a y well prove to be the ease t h a t a p a r t i c u l a r child, for instance,
m i g h t well be able to solve t h e p r o b l e m using v e r b a l symbols with g r e a t e r ease
t h a n with visual, b u t t h a t a n o t h e r child m i g h t be b e t t e r off solving the p r o b l e m
using v i s u a l - s p a t i a l relations.
T h e m o t i v a t i o n a l aspect of this observation, of course, cannot be o v e r e m p h a sized. I f a child h a p p e n s to be t a l e n t e d for visual-spatial relations a n d is being
forced into a curriculum t h a t emphasizes t h e v e r b a l a r t i e u l a t o r y modes of solving
a c o n c e p t u a l p r o b l e m , he will e n c o u n t e r enormous f r u s t r a t i o n a n d difficulty which
m a y result in a h o s t i l i t y t o w a r d s the t e a c h e r a n d t o w a r d s the learning process
itself. I f t h e t e a c h e r was aware t h a t t h e child was specialized in these skills, t h e
same p r o b l e m could be i n t r o d u c e d to him using these skills which would obviously
bring t h e child in line with his special talents. Conversely, of course, the child
with high v e r b a l skills m a y quite f r e q u e n t l y be unable to visualize his p r o b l e m s
w i t h a s p a t i a l aspect. He would be f r u s t r a t e d if forced into these academic areas.
I n this regard, of course, t h e more we u n d e r s t a n d a b o u t the b r a i n a n d t h e more
we u n d e r s t a n d a b o u t t h e various kinds of cognitive processing systems we use
in our n o r m a l m e n t a l life, the more clearly we can u n d e r s t a n d t h e c o m p o n e n t s
of w h a t is n o r m a l l y called " i n t e l l i g e n t b e h a v i o r " . W e h a v e r e c e n t l y developed
an a n i m a l m o d e l for intelligence which emphasizes t h e i m p o r t a n c e of the shortt e r m m e m o r y system. A t e s t can be designed t h a t , when c o m b i n e d with p a r t i c u l a r
surgical lesions, can p r o d u c e s h o r t - t e r m m e m o r y deficits which has t h e result of
m a k i n g t h e a n i m a l a p p e a r less b r i g h t t h a n his cage-mates. I n fact, w h a t is at
stake, is m e r e l y how m a n y variables t h e animal can keep in m i n d in a given
space of time. The exciting aspect of a t e s t of this kind, of course, is t h a t it operationalizes t h e q u a n t i t y of intelligence a n d as these tests develop t h e y can be
specifically designed to diagnose a p a r t i c u l a r individual. W h e r e we hope to be
going, of course, is to develop a series of tests t h a t will diagnose an i n d i v i d u a l
with respect to his specific m e n t a l skills. I t is more t h a n likely t h a t we will find
t h a t m a n y people m a y be s p a t i a l l y bright, v e r b a l l y dull or vice versa. Or m a y b e some
of us h a v e a b e t t e r s h o r t - t e r m m e m o r y s y s t e m or h a v e a faster processing c a p a c i t y
when dealing with certain kinds of symbols. Once identified, of course, the t e a c h e r
would be e n o u r m o u s l y a i d e d in how to best p r e s e n t i n f o r m a t i o n ibr its s p e e d y
c o m p r e h e n s i o n a n d use.
Philosophical h n p l i e a t i o n s
The m o r e general p o i n t of s p l i t - b r a i n research, of course, is t h a t t h e surgeon's
knife has r e v e a l e d t h a t t h e brain can be s i m p l y a l t e r e d to produce a s t a t e which
can best be described as allowing for the doubling of consciousness. R e s e a r c h of
this k i n d encourages us to t h i n k in t e r m s of modes of consciousness. W h a t writers,
poets a n d scientists h a v e been telling us fbr centuries seems true a n d t h e only
difference b e t w e e n t h e s p l i t - b r a i n s u b j e c t a n d t h e n o r m a l is t h a t n o r m a l b y virtue
of t h e callosum can switch between t h e various modes whereas t h e split experiences a real decoupling.
L a s t l y , it would a p p e a r t h a t if we s t u d y t h e cerebral commissure t h a t gives
us our n o r m a l s t a t e of conscious u n i t y , a n d grow to u n d e r s t a n d its neural physio-
Review of the Split Brain
79
logical a n d n e u r o - a n a t o m i c a l m e c h a n i s m s m o r e fully, w e m i g h t well be closer t o
u n d e r s t a n d i n g h o w n e u r o l o g i c a l s y s t e m s e n c o d e p s y c h o l o g i c a l i n f o r m a t i o n . I f we
k n e w t h a t we w o u l d be a lot closer t o b r e a k i n g t h e c o d e o f t h e brain.
References
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Prof. Dr. M. S. Gazzaniga
State University of New York at Stony Brook
Department of Psychology
Stony Brook, N. Y. 11794, USA