Splitting the Brain
 

From Left Brain, Right Brain:
Perspectives from Cognitive
Neuroscience, 5th Ed. (pp. 39-45)

by Sally P. Springer and Georg
Deutsch1997 by W.H. Freeman and Company.

Editor's note: Left Brain Right Brain is a fascinating examination of the
phenomenon known as hemispheric asymmetry, which describes the differences in function and organization of the two hemispheres of the brain. For example, in general the left side of the brain apprehends and organizes language and speech, while the right hemisphere handles visuospatial comprehension. Research into hemispheric asymmetry, important in itself, also plays a vital role in the larger field of cognitive neuroscience, which seeks, as the authors tell us, "to delve into the biological basis of memory, learning, imagery, emotion, and other higher mental functions and, perhaps, to discover how consciousness itself arises from the workings of the brain."

Hemispheric asymmetry usually goes unnoticed because the brain's two sides share information via several commissures- groups of nerve fibers (the largest being the corpus callosum) that connect corresponding sections of each hemisphere. Those connections can be severed through
injury or by a surgical procedure,commissurotomy, performed on the corpus callosum to treat patients suffering from intractable epilepsy. In the resulting "split brain," while the two halves still function normally, the left side literally does not know what the right is doing, and vice versa. Here the authors explore the questions answered and raised in work with
split-brain patients.

Notes on the text: Neurosurgeons Philip Vogel and Joseph Bogen performed the first series of complete commissurotomies on patients suffering from intractable epilepsy; the series is known by the doctors' names, and also as the "California series." The hippocampus is a subcortical structure that is divided into two parts, which are connected by a band of fibers. It is believed to play an important role in memory.
 

Everyday Behavior after Split-Brain Surgery

It is natural to wonder what disconnection effects be observed in the everyday behavior of split-brain patients. Commissurotomy patients are often mute for a time after surgery, and sometimes they have difficulty controlling the left side of the body, which may even be paralyzed at first. As the patient recovers use of the left hand, competitive movements between the left and the right hands sometimes occur. This syndrome, known as the acute disconnection syndrome, usually passes quickly. It is probably due to the surgical division of the commissures and to general
trauma resulting from the compression of the right hemisphere required to provide access to the nerve tracts between the hemispheres during the operation.

After recovering from the initial shock of major brain surgery, most patients report a feeling of improved well-being. Less than two days after surgery, one young patient was well enough to quip that he had a
"splitting headache." Within a few weeks,  the symptoms of the acute disconnection syndrome subside, thus making it necessary to use carefully contrived laboratory tests to reveal the effects of the operation.

In some cases, however, the effects of disconnection persist and manifest
themselves in bizarre ways. One of the earliest patients, for example, described the time he found his left hand struggling against his right when he tried to put his pants on in the morning: One hand was pulling them up while the other hand was pulling them down. In another incident, the same patient was angry and forcibly reached for his wife with his left hand
while his right hand grabbed the left in an attempt to stop it.

The frequency with which such stories are mentioned in popular articles on split-brain research would lead one to believe that they are commonplace events. In fact, the frequency of such events is low in most patients. One exception is P.O.V., a female patient operated on by neurosurgeon Mark
Rayport of the Medical College of Ohio. This patient reported frequent dramatic signs of interhemispheric competition for at least three years after surgery. "I open the closet door. I know what I want to wear. As I reach for something with my right hand, my left comes up and takes
something different. I can't put it down if it's in my left hand. I have to call my daughter." [

Cases such as these support the concept that the cerebral commissures transmit information that is inhibitory in nature. In other words, activity in one hemisphere leads to callosal transmissions that serve to moderate, decrease, or stop certain activities in the other.

It seems likely that corpus callosum-mediated inhibition is an
important process that is quickly masked by compensatory mechanisms in most split-brain patients. In fact, in a large majority of cases, the two sides of the body appear to continue to work in a coordinated fashion. Perhaps the rarity of patients with persistent disconnection effects indicates that more than callosal damage is necessary to prevent adjustment to the commissurotomy.

There is ample evidence, however, both from reports from the patients themselves and from sophisticated batteries of tests, that there are subtle changes in behavior and ability after surgery. Several patients,
for example, have reported great difficulty in learning to associate names with faces after surgery. Verification of this problem came from a study in which subjects had to learn first names for each of three pictures of young men.  The investigators reported that subjects eventually learned the name-face associations by isolating some unique feature in each picture (for example, "Dick has glasses") rather than by associating the name with the face as a whole. This finding suggests that the deficit in the ability to associate names and faces may be due to a disconnection of the verbal naming functions of the left side of the brain from the facial-recognition
abilities of the right side.

Deficits in the ability to solve geometry problems have been anecdotally linked to the sectioning of the corpus callosum. Patient L.B., a high school student with an IQ considerably above average, was transferred out of geometry into a class in general math after he experienced inordinate difficulty with the course. Another report told of a college student who had exceptional difficulty with geometry despite average grades in other courses. Studies with split-brain patients to determine the ability of each hemisphere to match two- and three-dimensional forms on the basis of common geometrical features showed that the right hemisphere was markedly superior, especially on the most difficult matches.  Thus, as in the preceding example, the patient's deficit may be the result of the disconnection of the speaking left hemisphere from the right-hemisphere regions specialized for such tasks.

Another complaint of some split-brain patients is that they no longer dream. Because dreaming is a process involving visual imagery some investigators speculated that it might be the responsibility of the right half of the brain and thus disconnected from the left hemisphere in split-brain patients. This idea, however, has not been confirmed by research in which split-brain patients were monitored for brain-wave activity while sleeping and were awakened whenever the recording indicated that they were dreaming. When asked to describe the dream they had just been having, the patients provided the experimenter with descriptions of their dreams, a result contradicting the prediction that they would be unable to do so.

Other anecdotal evidence documents poorer memory after surgery. Recent work suggests a physiological basis for these reports. Some patients, specifically those with damage to the hippocampal commissures or other extracallosal structures, display memory deficits, whereas others do not.  In a study involving pre- and postsurgical memory tests, patients with commissurotomies that included the posterior region of the callosum showed memory impairments, with recall more affected than recognition.
Patients with partial callosal sections that excluded the posterior region did not show these deficits.  The investigatorsconcluded that these results were consistent with the earlier work, because the hippocampal commissure is usually damaged during posterior sectioning, but not during anterior sectioning, of the corpus callosum.

Overall, it is not clear why a few patients seem to show persistent patterns of deficit after commissurotomy, whereas the majority of patients do not. Important differences among patients in their preoperative condition and surgical treatment probably exist, although we do not yet know what they are.

                    Language and the Hemispheres

Split-brain research has dramatically confirmed that, in most persons, control of speech is localized to the left hemisphere. But what about other language abilities? How well can the right hemisphere understand language, either written or spoken? What are its capabilities with respect to the various components of language-phonological or sound-based,
syntactic or grammatical, and semantic or meaning? The split-brain patient provides investigators with a unique opportunity to answer these questions, because tests of such patients permit an assessment of the
right hemisphere's abilities in isolation, decoupled from those of the language-rich left hemisphere. In contrast to studying the remaining language in aphasics with left-hemisphere damage as a measure of
right-hemisphere language, split-brain research permits assessment of the positive language competence of the right hemisphere free from any inhibitory effects of the damaged left hemisphere.

A major concern in interpreting work with split-brain patients, however, is how their unique neurological and surgical histories may affect the outcome of the research. In particular, some have argued that research
with split-brain subjects may overestimate the degree of right-hemisphere language function as a result of reorganization of language following early left-hemisphere damage. Those who believe the approach  is a valuable one, however, look for commonalities across patients who vary in their neurological history or who show little evidence of brain damage before their surgery. Subjects are also studied on a case-by-case basis to understand the factors that may be responsible for individual differences.

Eran Zaidel was the first investigator to undertake systematic investigations of right-hemisphere language in split-brain patients.  Working primarily with two patients, L.B. and N.G., from the original
Bogen and Vogel series, Zaidel developed a contact lens device that permits the patient to move his or her eyes freely without a time limit when examining something but at the same time ensures that only one hemisphere of the patient's brain receives the visual information. Zaidel's strategy was to test the abilities of each hemisphere, using a variety of stimuli that had been used previously with children and aphasic patients. His goal was to obtain data that would allow comparisons of the abilities of the right hemisphere of split-brain patients with the abilities of the two other groups.

 Other studies of language function have been undertaken by Gazzaniga and his  associates, working with patients operated on by Donald Wilson of Dartmouth Medical School (e.g., J.W. and P.S.) as well as others (e.g., V.P. and V.J.). Although some of their research has involved the use of an eye tracking device that tracks eye movements and coordinates the position of
the image with the eye movement so that retinal position is maintained, much of it has used the traditional approach of very brief displays of visual stimuli to the left or right of fixation.

A recent review of research on right-hemisphere language in the different
groups of split-brain patients found a surprising degree of consistency in the findings, along with some variability of considerable theoretical interest.  For example, the ability of the left hemisphere to control speech has been one of the most dramatic and consistent findings in split-brain research. While this continues to be the case in most instances, two
 patients, P.S. and V.P., have shown evidence of being able to control speech from the right hemisphere. This ability, while not present immediately following surgery, appeared to develop in the years that followed. J.W. has also demonstrated the ability to respond verbally to stimuli presented in the left visual field, an ability that was first detected 12 years after his surgery.

Overall, split-brain patients have difficulty deriving  phonology, or the
sounds of speech, from words. For example, patient L.B. was able to match pictures with names that sound alike with his right hemisphere (e.g., picture of a flying bat with a picture of a hat), although he could not perform the same task when he had to match a printed name with a picture (e.g., the printed word "bat" with a picture of a hat). This interesting observation suggests that L.B.'s right hemisphere could not evoke the sound image of a word from its orthographic or printed representation, despite the fact that it could match pictures for rhyming.  Patient J.W. was also unable to identify visually presented words that rhyme. Patients V.P. and P.S., however, have shown some ability to
recognize printed rhymes, although they appear to be exceptions, both in this regard and, as we noted above, in their ability to verbally identify visually presented words and pictures presented to the right hemisphere.

Because all the patients have demonstrated the ability to match printed
words and pictures in the right hemisphere, the inability of most patients
to select rhyming words suggests that the right hemisphere moves from the printed word to its meaning without the usual phonological decoding that takes place in the left hemisphere. Patients who show no evidence of controlling speech from the right hemisphere appear to lack the ability
in that hemisphere to decode the printed word into its phonological representation. Having control of speech in the right hemisphere, however, does not appear sufficient for phonological decoding. Patient J.W., who has shown evidence of right-hemisphere speech, cannot identify
words that rhyme, using that hemisphere.

Zaidel's early tests of grammatical competence with N.G. and L.B. led him to conclude that the right hemisphere possesses competence equivalent to that of  a five-year-old child. While equivalent testing has not been conducted with other patients, in general they show an ability to differentiate among nouns, verbs, and function words, and to distinguish
grammatical from ungrammatical sentences that is consistent with Zaidel's
observation. Additional testing of N.G. and L.B. also led Zaidel to conclude that the right hemisphere had an auditory lexicon, or auditory mental dictionary, that included concrete and abstract nouns, verbs, and some spatial prepositions. Its visual lexicon, however, was less extensive. Patients tested by Gazzaniga and colleagues showed results that were
generally consistent with this pattern.

The newest patient to be studied by Gazzaniga, V.J., presents an interesting case of how verbal and written expression may be controlled by different hemispheres.  V.J., who is left-handed, shows evidence from visual field studies and naming of objects held in one hand out of view that speech is controlled by her left hemisphere. However, since her surgery
was completed, she has complained that she is unable to write with either hand. When letters or words are presented in the right visual field, she can name them but cannot legibly reproduce them with her right hand out of view. When the same stimuli are presented in the left visual field, she cannot name them but can write them with her left hand when she cannot
see it. Thus it appears that writing is controlled by her right hemisphere,
whereas speech is controlled by her left hemisphere, an interesting variation that has not been observed previously in split-brain patients. Gazzaniga and colleagues speculate that this atypical pattern is related to the fact tha V.J. is left-handed.

Zaidel has recently reported on additional findings with the California
series of patients that led him to conclude that studies of the disconnected right hemisphere underestimate the language competence the normal right hemisphere. In addition, he argues that the language of aphasic patients also underestimates the language competence of the normal right
hemisphere. The reason for this underestimation, he proposes, is that
under normal conditions, the corpus callosum permit linguistic interhemispheric interaction, including a sharing of resources residing in the left hemisphere that effectively increases the competence of the right hemisphere. He proposes that "the psycholinguistic profile of a left
hemisphere-damaged patient is determined not only by the lost functions
of the left hemisphere and by the residual competence the right hemisphere, but also critically by the balance of activation a system of control that incorporates several levels of facilitory and inhibitory circuits."
Speech following left-hemisphere injury, then, maybe a result not only of
the left- and right-hemisphere competencies that remain, but of the
complex interaction of the two.

Long-standing evidence recently confirmed in neuroimaging studies using positron emission tomography (PET) has demonstrated the recovery from aphasia may involve right-hemisphere compensation.  Zaidel's ideas expand on this and make the case that language following left-hemisphere damage reflects the contributions of both hemispheres in a complex, as yet to be understood, way.

Research on right-hemisphere language illustrates well the power of the cognitive neuroscience approach to brain-behavior relationships, bringing together modern neuroimaging and behavioral techniques and linguistic analysis in the study of clinical and normal populations. Split-brain research has been, and will continue to be, a important contribution to the effort.

Sally P. Springer has been assistant  chancellor at the University of  California at Davis since 1987. Her  research has focused on the brain  mechanisms underlying speech and  language. She has employed  dichotic listening and other  behavioral techniques with normal,  neurologically impaired, and split-brain subjects.

Georg Deutsch is Associate  Research Professor of Radiology and Neurology at the University of Alabama at Birmingham, and is a scientist in the Center for Aging and the Alzheimer's Disease Center at UAB. His investigations focus on cerebral changes and
reorganization in normal aging, dementia, and stroke.