What are split-brain studies?

Introduction

The study of laterality, or the specialized asymmetric functions throughout the body, is not a new and novel field, as might be suggested by the popularization of “left brain-right brain” dichotomies. Lateralization of functions in the brain, sometimes referred to as hemispheric asymmetries, was demonstrated in 1861 by Paul Broca, a well-known physician at the time. He found that patients suffering from damage to certain regions of the left cerebral hemisphere exhibited more frequent speech and language disorders than did those with right cerebral hemisphere damage. Based on these findings, Broca correctly reasoned that the left hemisphere is specialized for speech and language in the vast majority of people. However, these results were quickly transformed into an overly simplistic dichotomization of cerebral functioning in which the left hemisphere was conceptualized as the dominant hemisphere and the right hemisphere as a rather minor, perhaps even unimportant, hemisphere. From split-brain studies performed since 1940, it has become obvious that the right hemisphere is essential for normal visuospatial functioning.

Commissurotomy Effects

Split-brain surgery, sometimes referred to as commissurotomy, was first performed on a human patient by the neurosurgeon William Van Wagenen in 1940 to reduce the severity of life-threatening epileptic seizures. Other early commissurotomies were performed by two neurosurgeons, Philip Vogel and Joseph Bogen. The rationale for commissurotomies is rather simple: By severing the cerebral commissures, the major interconnecting fiber bundles that allow communication between the cerebral hemispheres, surgeons can prevent epileptic seizures from spreading beyond their focal hemisphere. Commissurotomies are performed only as a last resort, after traditional drug therapy fails to control seizure activity.

Surprising as it may seem, commissurotomy patients show few long-term alterations in behavior. All subjects suffer from acute disconnection syndrome, in which they are mute and partially paralyzed on the left side of the body for an interval ranging from a few days to a few weeks. Otherwise, commissurotomy patients exhibit relatively normal behavior. Moreover, the severity and frequency of seizure activity decline, sometimes quite dramatically, in response to this surgical procedure.

Hemispheric Asymmetries in Information Processing

On closer examination with a tachistoscope (an experimental apparatus for presenting visual information very briefly to the right or left visual field, sometimes called a T-scope) of split-brain patients, however, hemispheric asymmetries in information processing are evident. These asymmetries are investigated in T-scope or divided visual field studies. The split-brain patient is required to fixate on a central point, while visual stimulation is presented to the right or left visual field. Assuming that the patient is fixated on the central point, stimulation in the right visual field is projected to the left hemisphere, and left visual field stimulation to the right hemisphere. Once the information is available to the left or right hemisphere of a split-brain patient, it is not able to cross the cerebral commissures, principally the corpus callosum, because those fibers have been partially or completely severed.

The pioneering studies of the divided visual field in split-brain patients are described in The Bisected Brain (1970) by Michael Gazzaniga, who was a coprincipal investigator with Roger Sperry in those studies. In one of their investigations, pictures of common objects were presented to either the right visual field or the left visual field of split-brain patients. All patients were able to identify the information verbally when it was presented in the right visual field (left hemisphere), but not in the left visual field (right hemisphere). These results suggested specialization for verbal tasks in the left cerebral hemisphere but did not address functioning in the right cerebral hemisphere.

To assess the psychological functions of the right cerebral hemisphere, the researchers repeated the procedure described in the previous experiment, except that subjects were asked to reach under a curtain with their left or right hand to select the object from among several alternatives (rather than verbally identifying the picture of the object). Subjects were able to perform this task competently with their left hand, which is controlled primarily by the right cerebral hemisphere. Therefore, stimulation presented to the left visual field is projected to the right cerebral hemisphere, which controls the left hand. The opposite is true for stimulation presented to the right visual field. Correct identification of objects with the left hand indicated right cerebral hemisphere involvement in recognition of nonverbal stimuli.

Support for the superiority of the right hemisphere on visuospatial tasks came from a study in which split-brain patients were required to assemble patterned blocks into particular designs. Even though all patients were right-handed, they were much better at this task with the left hand, presumably because the right cerebral hemisphere controls that hand. Yet another test of the abilities of the left cerebral hemisphere was performed by requesting subjects to copy pictures of line drawings. Again, despite being right-handed, all subjects performed better with the left hand. Their left-handed efforts were rather clumsy, but the spatial dimensions of the line drawings were proportionally correct. Overall, split-brain studies seem to indicate left-hemisphere superiority on verbal tasks and right-hemisphere superiority on nonverbal, visuospatial tasks.

Further proposals for differences between the right and left hemispheres have been suggested from split-brain research. For example, it now appears that the left hemisphere is specialized for verbal tasks, but only as a consequence of its analytical, logical, information-processing style—of which language is one manifestation. Similarly, the right hemisphere is specialized for visuospatial tasks because of its synthetic, holistic manner of processing information. Support for these hemispheric asymmetries was derived from a 1974 study conducted by Jere Levy in which split-brain patients were given ambiguous instructions; they were simply to match similar pictorial stimuli. These pictures could be matched either by their functions, such as a cake on a plate matched with either a spoon or a fork, or by their appearance, such as a cake on a plate matched with a hat with a brim. When the pictures were presented to the right visual field (left hemisphere), matching was accomplished by function, while pictures projected in the left visual field (right hemisphere) were matched according to appearance. Matching by function was construed to involve logical, analytical information processing; matching by appearance was interpreted as involving holistic, synthetic information processing.

Most of the basic findings on hemispheric asymmetries in split-brain patients have been extended to normal subjects whose cerebral commissures are intact, with the exception that right-hemisphere superiority for visuospatial tasks seems to be slightly weaker in normal subjects. Investigations with normal subjects require measurement of reaction time, because information projected to one visual field can quickly and easily transfer to the opposite hemisphere.

In the twenty-first century, Gazzaniga and others have also begun to research how the hemispheres interact with one another in subjects with intact brains. These studies have found that right-hemisphere regions of the brain are more likely to interact across hemispheres, while left-hemisphere regions often interact primarily with each other, and that inter-hemispheral communication is greater when a stimulus is introduced to the side of the brain that is not specialized for it (e.g. if verbal information is introduced to the right hemisphere instead of the left).

Real-World Phenomena

When generalizing basic laboratory research findings to real-world situations, it is important to note that information transfer across the cerebral commissures is nearly instantaneous in normal subjects. In addition, the real-world environment provides prolonged visual stimulation, which is typically scanned with continuous eye movements. In these situations, environmental stimulation is available to both cerebral hemispheres. Therefore, one must be cautious not to overstate the case for a relationship between hemispheric asymmetries and real-world phenomena. The two cerebral hemispheres do work in combination as a unified brain in normal subjects. Even in split-brain patients, the prolonged availability of environmental stimulation and continuous eye scanning movements result, for the most part, in unified overt behavior. Behavioral, perceptual, and motor differences in split-brain patients are evident only with highly specialized and artificial laboratory testing with such instruments as the tachistoscope. Generalizations, then, from divided visual field studies of asymmetry to everyday situations require actual research evidence rather than the speculation that is popular among some segments of both the scientific and lay community.

Stuttering Research

Stuttering
is one real-world phenomenon for which laterality research has practical implications. There is some evidence that stutterers are bilaterally represented for speech and language to a greater extent than are nonstutterers. In one investigation, R. K. Jones, a neurosurgeon, was presented with four stutterers who had blood clots or tumors located near the normal speech center in the left hemisphere. Because of concern that removal of the blood clots or tumors would produce muteness in his patients by damaging the speech center, Jones performed the Wada test to determine where the speech center was located in each patient. This test involves the injection of an anesthetic agent, sodium amobarbital, into the right or left carotid artery. The carotid arteries provide the frontal regions of the brain, where the speech center is located, with oxygenated blood. The sodium amobarbital anesthetizes the particular hemisphere into whose carotid artery the drug is injected. If speech is disrupted by this procedure, either the speech center is located in the opposite hemisphere or the patient is bilaterally represented for speech. Additional testing of the opposite hemisphere will reveal whether the patient is bilaterally represented.

Using this procedure, Jones found that all four stutterers possessed bilateral speech representation. After the surgery, all four patients stopped stuttering and began to speak normally. These findings raise the question as to why stuttering is related to bilateralization of speech functions. One explanation is that stuttering occurs in these patients because, unlike normal people, they have a speech center in one hemisphere that is competitive with the speech center in the opposite hemisphere. Neural impulses from the two speech centers arrive out of synchrony at the muscles that control speech, which produces stuttering. What are the practical implications of these findings? It is quite obvious that producing irreparable damage to the brain for the sole purpose of eliminating a speech disorder, such as stuttering, would be highly unethical at current levels of medical technology and knowledge about the brain. Additional research on the hemispheric basis of stuttering will need to be conducted, and technological advances will be required before stuttering can be eliminated in bilaterally represented patients by means of neurosurgery; however, findings such as these may be increasingly useful in future applications of laterality research.

Dyslexia Research

Yet another phenomenon linked with laterality research is dyslexia, a disorder of reading that is not associated with sensory impairment, retardation, or emotional disturbances. In 1937, a physician by the name of Samuel T. Orton was the first to propose a link between hemispheric asymmetries and dyslexia. He observed mirror-image reversals of letters and words in reading and writing among children with reading problems. Orton also noted that many of these children exhibited unstable hand preferences, often accomplishing tasks normally reserved for a preferred hand with either hand on a given occasion. To account for these observations, Orton proposed that these children were insufficiently lateralized for speech and language functions. In other words, neither hemisphere was specialized for speech and language.

Evidence to support the hypothesis that dyslexia is attributable to incomplete lateralization was generated in 1970 by E. B. Zurif and G. Carson, who compared the performance of fourteen normal readers in the fourth grade with fourteen dyslexic fourth graders on a dichotic listening task. Dichotic listening involves presenting simultaneous, competing verbal stimuli of differing content to each ear through headphones. The subjects’ task is to identify the words, letters, or digits presented to each ear. Since the right ear primarily transmits auditory input to the left hemisphere, and the left ear to the right hemisphere, detectable differences in the processing of verbal stimulation can be used to suggest hemispheric asymmetries. In the foregoing study, presentation of a dichotic digits task showed a significant right-ear (left-hemisphere) advantage for the normal children and a weak, insignificant left-ear (right-hemisphere) advantage for the dyslexic children. Failure to find a significant hemispheric advantage in processing dichotically presented verbal stimulation suggests that dyslexic children may, indeed, be incompletely lateralized for speech and language. Before practical applications of this finding are realized, further explorations on the development of hemispheric asymmetries will be necessary to determine whether lateralization of functions can be influenced by environmental manipulation. Only if such modifications are possible can the development of incomplete lateralization be altered in dyslexics.

Early Roots of Research

Modern research on hemispheric asymmetries has its origins in a short paper read at an 1836 medical conference in Montpellier, France. Marc Dax, an obscure country physician, reported that aphasia
(any loss of the ability to use or understand language) is related to left-hemisphere brain damage and concluded that each hemisphere is specialized for different function. Unfortunately, the paper received little attention and Dax died the following year, never knowing that he had anticipated one of the most exciting and productive research fields to emerge in the twentieth century. Because Dax’s paper was not widely known, credit for the discovery of hemispheric asymmetries was incorrectly given to Broca, who presented a similar paper in 1861 to a meeting of the Society of Anthropology in Paris. Broca does deserve some of the credit for the discovery of hemispheric asymmetries in that he suggested an exact area of the left frontal lobe that produces an expressive aphasia when damaged. Furthermore, Broca presented a much more impressive case for left-hemisphere lateralization of speech and language; his paper was received with enthusiasm and controversy.

In 1868, British neurologist John Hughlings Jackson proposed the idea of a “leading” hemisphere, which preceded the modern concept of “cerebral dominance,” the idea that one hemisphere is dominant for psychological functions over the other hemisphere. By 1870, Carl Wernicke, a German neurologist, had presented evidence that a specific region of the temporal lobe in the left cerebral hemisphere is essential for comprehending language and, when damaged, produces a receptive aphasia. In combination, these findings led to a widely held position that one hemisphere, usually the left, is dominant for verbal tasks and other higher functions, while the opposite hemisphere, usually the right, possesses no special function or only minor, limited functions. Even though the term “cerebral dominance” is still used today, it is generally recognized that there are no “major” or “minor” hemispheres; they are simply specialized for different tasks and information-processing styles.

The strongest early evidence for a specific function mediated primarily by the right hemisphere came from widespread assessment of brain-damaged patients on spatial relationship tests. After testing more than two hundred brain-damaged patients, T. Weisenberg and K. E. McBride concluded in 1935 that the right hemisphere is specialized for spatial relationships. These results refuted the notion of a single dominant hemisphere for all psychological functions.

Modern contributions made by Sperry, Gazzaniga, and their colleagues have been, perhaps, most instrumental in establishing the functions of the cerebral hemispheres. Their results, as well as those of neuropsychologists, have been incorporated into such areas as biological psychology, cognition, and perception. Biological psychologists are concerned with establishing the functions of various brain structures in normal subjects, including the cerebral hemispheres. Neuropsychologists contribute to laterality research by specifying the cognitive, motor, and behavioral deficits that arise following brain damage to a specific region in the cerebral cortex. Laterality research also provides information about hemispheric specialization for cognitive and perceptual processes.

Future explorations on laterality will continue to examine performance for specific tasks and information-processing strategies in each hemisphere, but with greater emphasis on localizing functions to specific brain structures. In addition, more effort will be expended on developing practical applications of laterality research in clinical and educational settings.

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