The previously mentioned observations, specifically that of the sheer size of the fiber tract and its position as a unique midline structure, were important in suggesting to early scientists that it would be crucial to understand the callosum's role in behavior to fully understand the organization of the brain. In the early literature, it even competed with the pineal gland as a potential seat of the soul. Responding to an increasing belief that the corpus callosum played a major role in the integration of brain activities, Thomas Huxley called the corpus callosum "the greatest leap forward anywhere made by Nature in her brain work.''
However, in the 20th century, studies of the callosum suggested that it was of much less interest than its prominent anatomy might indicate. Work in Pavlov's laboratory by Konstantin Bykov and Aleksei Speransky demonstrated that the transfer of conditioned learning from one side of the body to the other in dogs was abolished after section of the corpus callosum. However, this interesting work failed to gain significant recognition and was overshadowed by other animal work that did not document an important role for the corpus callosum in behavior. In the 1940s, William Van Wagenen and Robert Herren resected the callosum in a series of patients as treatment for epilepsy. These patients were extensively tested for psychological and behavioral changes. Although some patients improved, there did not appear to be consistent benefits in control of epilepsy nor consistent changes in their cognitive ability as a result of the severing of this enormous band of fibers. This series of surgeries did not continue and the interest in callosal function waned. Psychologist Karl Lashley was so unimpressed with the effect of severing the corpus callosum that he concluded that the corpus callosum played only a minimal role in psychological function.
It was the remarkable developments in animal research in the 1950s that paved the way for a greater understanding of the function of the corpus callosum in humans. Roger Sperry and Ronald Meyers, after splitting both the corpus callosum and the optic chiasm in the cat, showed that if one eye of the cat was covered while it learned a task, when that eye was covered and the other eye uncovered the animal acted as if it had no knowledge of the task. When exposed to the same learning trials, it had to learn again from scratch with no significant benefit from the prior exposure. In this case, the hemisphere of the brain that had not been able to see the task being learned had to complete the task and showed no evidence of having learned it previously. The important principle that their approach uncovered was that care had to be taken to introduce information only to one hemisphere of the brain and to test that hemisphere independently as well. For the first time, the discovery that learning of a task could occur independently in the separate hemispheres of the brain was widely appreciated.
In the 1960s, Norman Geschwind published his influential review of disconnection syndromes. Disconnection syndromes are patterns of behavior that occur when the fibers that connect areas of the brain responsible for different aspects of a task are damaged, preventing communication needed to complete complex behaviors. Perhaps the most striking of these syndromes is alexia without agraphia—that is, the ability to write in the absence of the ability to read. Patients with this striking disorder can write words and sentences spontaneously or to dictation, but they are unable to read what they have written. All reading is severely impaired in this group due to a disruption of the fibers that carry visual information to the area of the left hemisphere that decodes words into sound and meaning. However, that decoding area is intact and, hence, the ability to write and spell is intact. The existence of such syndromes suggested that a complete section of the corpus callosum should produce other dramatic behavioral changes.
Geshwind's comprehensive review of the animal and human literature emphasized the importance of the corpus callosum and other fiber tracts in producing complex behaviors. The corpus callosum was obviously the largest of the known fiber tracts and Geshwind's review suggested that it played an important role in transmitting information between the two hemispheres of the brain, despite the discouraging results from the series of split-brain surgeries performed by Van Wagenen. Pursuing this idea, Geschwind and colleague Edith Kaplan were among the first to observe disconnection symptoms in a patient with a naturally occurring lesion of the corpus callosum.
Meanwhile, physicians continued to consider a possible role for the corpus callosum in the spread of epileptic seizures. After a careful review of the Van Wagenen series and with the new observations of Geschwind in mind, Philip Vogel and Joseph Bogen decided once again to attempt to resect the corpus callosum as a treatment for epilepsy, but with some differences in procedure. The corpus callosum is not the only fiber tract that connects the left and right hemispheres. Many smaller tracts, the anterior commissure in particular, that were not severed in the Van
Wagenen series also cross the midline and could serve as an alternative route to spread seizure activity. Bogen and Vogel suspected that some of the original surgeries had not alleviated the epilepsy because these alternate tracts served to spread the seizure activity in the absence of the callosum. Hence, they initiated a new series of surgeries that resected not just the corpus callosum but also additional tracts, including the anterior commissure.
Guided by the exciting observations made in the Sperry laboratory with cats and by the renewed interest in disconnection syndromes, plans were made for a more intensive study of the disconnection effects in humans after commissurotomy. A team consisting of Joseph Bogen, Roger Sperry, and Michael Gazza-niga assembled the tasks and methods of response they thought would be necessary to demonstrate the effect of split-brain surgery in humans. To understand their approach and observations, some knowledge of functional anatomy is necessary.
The most crucial problem the researchers faced was that the left hemisphere in most people is the only one that controls speech. Hence, if only a verbal response to a task is accepted, the mute right hemisphere will not be able to respond and may appear unable to do simple tasks. They realized that if right hemisphere skills were to be probed, a manual response was required. Tasks had to be designed to allow a button press response or some other tactile response.
Second, because there is some ipsilateral control of motor output, knowledge being tested had to somehow be isolated in one hemisphere or the other to prevent interference between the two hemispheres in controlling a tactile response. Because of the unique anatomy of the visual system in humans, it was possible to isolate visual words and pictures to one hemisphere but only under very special conditions. Figure 4 shows the organization of the visual system. Information from the right side of space first reaches the cortex in the back of the left side of the brain or occipital lobe and vice versa. Unlike in the cat in the earlier Sperry experiments, covering one eye does not isolate information to one hemisphere, so more elaborate procedures are necessary. In most people, the information about the two sides of space is quickly woven together into a seamless visual world through the neural transmission across the callosum. Once these fibers are severed, information in one visual field is isolated in the contralateral hemisphere. This only holds true, however, if the eyes remain focussed on a single central location or fixation point. In everyday life, though, our eyes are always in motion and are
drawn quickly to changes in the visual environment. Hence, to an investigator who wishes to have visual information presented in the left visual field seen only in the right hemisphere, this reflexive orienting movement presents a challenge. To prevent eye motion from interfering with lateralization, words or pictures had to be presented for 150 msec or less so that there was not time for the eyes to move from the fixation point to the stimulus display. The early devise used to ensure brief presentations was known as a tachistoscope, but today many investigators control the length of the stimulus display with a computer.
Although the visual system provides clean lateralization of information in a split-brain subject, the auditory system does not. Fibers from the auditory system of one ear reach both hemispheres of the brain, with about 60% of the pathway arriving in the contralateral hemisphere and 40% in the ipsilateral hemisphere. In order to test auditory comprehension separately in the left and right hemispheres, an auditory stimulus had to be compared with some completely lateralized visual stimulus.
One other neural pathway, although it has some ipsilateral representation, provides relative isolation of information to the contralateral hemisphere. Somato-sensory information from the hands is predominently transferred to the contralateral hemisphere. Hence, when real objects are placed in the hands and palpated to aid in identification, the somatosensory information about the object remains in one hemisphere. The ipsilateral fibers do provide the ipsilateral hemisphere with some basic perceptual information but do not generally provide enough cues for identification of the object.
Bogen, Sperry, and Gazzaniga used these basic facts about the anatomy of the nervous system to guide their investigations of the changes that follow section of the corpus callosum. By carefully isolating the hemispheres via these methods, they were able for the first time to map the profound changes that do occur after callosotomy. They examined the subjects in this new series before and after surgery and were able to confirm one of the early observations: These patients appear quite normal after surgery. To the casual observer, there appears to be very little difference in the presentation of the person before and after surgery. After the initial few weeks following surgery in which there may be symptoms such as mutism and intermanual conflict, the split-brain subjects experience the world as unified and converse and interact normally. The subjective response to what might be expected to be a radical and frightening change in one's inner world appears to be minimal. Gazzaniga observed, "Indeed, one would miss the departure of a good friend more, apparently, than the left hemisphere misses the right.''
However, when observations were made under conditions that allowed only one hemisphere access to information and gave the mute right hemisphere a means of response, the now well-known hemispheric disconnection syndrome was elicited. The researchers gave patients objects to palpate in one hand at a laboratory table that screened the hand and the item from the subject's view. When an item was palpated by the right hand, there was no difference from normal behavior. The patient was easily able to name the item because all the tactile information from the right hand was available to the speaking left hemisphere. In contrast, when the same common objects were placed in the left hand (and kept out of view), the talking left hemisphere was not able to identify the item and could not name it. The subject appeared to be anomic. However, despite being unable to name the object being grasped, the left hand of the subject could often demonstrate how the object was used or identify it from a group of objects. The somatosensory information from the left hand allowed the right hemisphere to identify but not to name the object. Because the information about the object could not be transmitted across the callosum, the talking left hemisphere could not help and supply the spoken name of the item. This was a vivid demonstration of what is now generally accepted about lateralization of function in right-handed people. The right hemisphere may possess knowledge about objects and their use in the world, but it lacks knowledge of how to produce the spoken name. This information is represented only in the left hemisphere, and once the callosum is cut the right hemisphere is left mute.
Another way to demonstrate the inability to transfer somatosensory information between the hemispheres in the absence of the callosum is to lightly touch different points on the patient's hand when it is out of view. If the right hand is touched in different positions, the right thumb can accurately point to each of the stimulated positions, but if the patient is asked to respond to the right-sided touch on the homologous area of the left hand, the task is impossible. This is equally true in reverse. The left hand can point to stimulated areas on the left hand but cannot transfer this information to the right hand. This task is trivial for people with an intact callosum, and if you are in doubt, try it. With your eyes closed, have a friend tap different areas on the palm and fingers of each hand. You will be able to respond easily with either the ipsilateral or the contralateral hand to the light touches.
Perhaps the most striking change demonstrated by Gazzaniga and Sperry was the inability to transfer visual information between the hemispheres. When two visual stimuli, either words or pictures, were lateralized one to each hemisphere, simple same/ different judgments were performed at chance levels. That is, the patients could not accurately decide if the two words or pictures were identical or different when one was seen by the right hemisphere and one by the left hemisphere. However, within a hemisphere, both the right and the left hemispheres not only were able to decide if two stimuli were identical but also to match words and pictures. The ability to match words and pictures within the right hemisphere was particularly exciting because it showed that the isolated right hemisphere was able to read for meaning at least at the single word level (although it could not say the words out loud).
These basic observations ignited a period of rapid and productive investigation of the capacities of the two hemispheres. In the following sections, only the work that bears on a closer examination of the functional significance of the callosum will be presented.
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