|
|
SPLIT BRAINS
Unless you are looking for differences in them, the behavior of split brain patients is not readily distinguishable from that of normal people. This is because both hemispheres are receiving the same sensory data, and each has the ability to pick up cues from its partner through means other than the corpus callosum. It takes a special machine called a tachistoscope and special tests to detect processing and perceptual differences in persons with split brains. Sperry and his associates used this machine, which includes a screen on which images can be flashed in the right or left field of vision for receiving and processing exclusively in one hemisphere. It also includes a flap under which patients can manipulate objects out of sight.
What the researchers found with the split brain patients, as in the experiment with the cat, was that information in one hemisphere was not available to the other. In one test, researchers flashed an image, say a picture of an apple, to the left hemisphere, and then the right hand (which connects to the left hemisphere) was always able to point out from a series of pictures on 3x5 cards the correct image. The same occurred with the right hemisphere and left hand. This was true regardless of the stimulus—geometric symbol, single words, letters, numerals, or other objects. It was also true that the hand on the same side as the hemisphere that had received the stimulus could not point to the flashed images.
Further, patients could say the name of those objects flashed to their verbal left hemispheres, while they could not name those flashed to the nonverbal right hemisphere. In fact, early in the experiments, the patients reacted with puzzlement that they should be asked what they had seen when they—that is their verbal left hemisphere—had seen nothing. Yet the nonverbal right hemisphere using the left hand could easily pick out the object. The experimenters were even able to flash different objects to the hemispheres simultaneously, with the opposite hands pointing out what each had seen with equal facility. However, they could only name verbally the object flashed to the left hemisphere.
They achieved the same results in what were called visual-tactile tests and tactile-visual tests. In the first type, researchers flashed a picture of a familiar object, such as a ball, a can-opener, or other such items to one of the hemispheres. The opposite (or contralateral) hand was then required to pick it out by touch from a series of such objects out of sight. In the tactile-visual test, the patient would handle an object out of sight and then point it out from a visual display. In both of these tests, the two hemispheres and their contralateral hands performed well. Again, the patient could only name those objects felt or seen with the left hemisphere. The left hemisphere had no access to the information of its contralateral partner nor the right to its partner.
While these experiments confirmed that both hemispheres were consciously processing data and that with a split brain there was no communication between the two, the only demonstrated processing difference between them was the verbal-nonverbal dichotomy. Michael Gazzaniga is the one who devised many of these tests. In another one, he discovered that after surgery there was a definite difference in the ability of the two hemispheres to perceive and copy figures.
In fact, the drawings by the right hand demonstrated the almost total inability of the left hemisphere to deal with the perceived object in any meaningful way. The left hemisphere was able to direct the right hand to draw but did not seem able to keep track of what it was drawing at the same time. So here we have another indication that, though the same sensory data is being received by the hemispheres, it is not processed in the same way.
Other experiments were devised to get at these processing differences. Jerre Levy, another of Sperry's students in the 1960s and now at the University of Chicago, was responsible for many of them. In one, several three-dimensional blocks of various shapes and surface characteristics were manipulated by the split brain patients out of sight. They were then to match these blocks with two dimensional patterns of what the blocks would look like if laid out flat.
Levy found there was a great advantage for the right hemisphere-left hand in this task. It worked quickly and quietly feeling and pointing to the appropriate two-dimensional figure. With the right hand (and left hemisphere) doing the feeling, there was a simultaneous verbalizing as the hand searched out specific characteristics of the felt blocks to look for in the figures for matching. By this analytical mode, the right hand was occasionally able to make the match but never at the same level of accuracy, speed, or complexity of the right hemisphere. Dr. Levy states, "We concluded that not only was the right hemisphere superior to the left in performing the mental operations necessary for this task, but the hemispheres utilized different and incompatible strategies."3
Chimeric Tests
In another experiment, Dr. Levy and associates created what they called chimeric figures. Chimeric means unreal and existing only as the product of wild, unrestrained imagination. These figures consisted of visual stimuli, such as faces cut down the middle and recombined with another half-face or half-object. The idea was to flash these figures to the split brain patient such that each hemisphere would see only one half of it. Levy would then ask the patients to point what they had seen from a series of normal images.
When the split faces were flashed, the patients almost exclusively pointed to the whole normal face that represented the half processed by the right hemisphere. Though it saw only one-half of a face, the right hemisphere filled in the other half automatically. Without the help of the other half of the brain, both hemispheres have no choice but to believe that both halves of the picture are the same and act on this belief. In this experiment, it did not matter which hand did the pointing nor was there any confusion about the correctness of the choice as selected by the right hemisphere.
In an attempt to evoke a left hemisphere response, the researchers taught the patients the names of the faces, which could only be done by associating certain physical features with the name, such as "Dick has glasses." The patients then had a way to classify verbally what they saw. After this training, Dr. Levy was able to get the left hemisphere to state verbally which face it had seen. But when using the left hemispheres, the patients were not as fast or as accurate in their selections. How the left hemisphere chose was not by the whole form of the face but by verbally identifying certain features. Note that before training, the left hemisphere was lost in performing this task, while the right did it naturally and easily.
Because faces have the same characteristics yet are all different at the same time, it is not easy to identify them by verbally analyzing features, yet that was the only way the left hemisphere could respond in this test. The right hemisphere, however, somehow sensing the face as a whole, has no problem with the task. These findings agree with reports from brain damage studies showing that it is the right hemisphere that most effectively recognizes faces.
Using this principle of chimeric figures, Levy undertook other tests to determine the differences between the hemispheres. In one such test, she split and then recombined into chimeric figures the images of a bee, a rose, and an eye. When the patients were asked to point out what they had seen, they again almost exclusively chose the right hemisphere image while when asked to report verbally, they usually chose the left hemisphere image. The relative ease with which the left hemisphere introduced itself here as compared with the face test has to do with the familiarity of these objects, the fact that they have distinct names, and their generality. While it may be no more true that a rose is a rose is a rose than a face is a face is a face, it was easy enough for the left hemisphere to tell a rose from an eye or a bee. Nevertheless, the left hemisphere would respond only when the request was for a verbal response. When asked to point, it was nearly always to the image the right hemisphere saw.
To demonstrate that a patient would only use the left hemisphere when an analysis of features was called for in a visual task, Levy again flashed the same split images to patients. However, this time she asked them to point out not what they had seen but to a picture that would rhyme with the image perceived. The rhyming pictures were toes, a pie, and a key. In this situation, the patients almost exclusively pointed (a nonverbal action but based on verbal analysis) to the picture that rhymed with what the left hemisphere had seen. For example, if the left hemisphere saw a rose, the patient would point to a picture of toes.
An even more interesting test was one done with three words, deed, noon, and sees, which were recombined to form chimeric stimuli such as "deon" and "noes." When these stimuli were flashed, and the researcher asked the patients to point to the word they saw from the original list, they pointed to the word perceived by the right hemisphere. In this situation there was no request to attach meaning to the image, only see it and point, something the right hemisphere does easily.
In a second test, the words, lady, ball, and shoe were split and recombined chimerically and flashed just as before, and, as before, the right hemisphere responded. However, when the patient was requested to select a picture that represented the word seen, then the left hemisphere quickly took over. The task required giving meaning, making a connection between word and picture, that is, analyzing and naming the stimulus, and this is an ability not seemingly possessed by the right hemisphere.
In yet another demonstration of hemisphere specialties, Levy created a set of images with half of each image portraying a different object. These sets were to be compared on the basis on functional/conceptual similarities or on the basis of visual/structural similarity. A split image of, say, the scissors with the cake was flashed to the patient. When asked to point to a similar item, the left hemisphere always selected the item that was similar in function. For example, the left hemisphere always matched scissors with thread. The right hemisphere, conversely, always selected the item that was similar visually. When it saw the cake, the left hand pointed to the hat. Neither hemisphere seemed capable of making any other choice but the one that was consistent with its mode of processing.
Given the results of these investigations and those of others, Jerre Levy concluded:
In considering the two sets of functions, it appears that they may be logically incompatible. The right hemisphere synthesizes over space. The left analyzes over time. The right hemisphere notes visual similarities to the exclusion of conceptual similarities. The left hemisphere does the opposite. The right hemisphere perceives form, the left hemisphere detail. The right hemisphere codes sensory input in terms of images, the left in terms of linguistic descriptions. The right hemisphere lacks a phonological analyzer; the left hemisphere lacks a Gestalt synthesizer.4
In other words, the left hemisphere uses language to classify details to deal with its perceptions. The right hemisphere simply perceives images as a whole and responds to them.
Let's look at some more of the evidence to get a better picture of these logical incompatibilities. Professor Sperry, working with another of his students (B. Preslowski), found that the right hemisphere was superior to the left in split-brain patients in pointing out short words made of wooden letters that had been felt out of sight. Competing words were presented to the two hands simultaneously, one letter at a time. Then, in free vision, patients were to point to what they had felt. In this freedom of choice situation, it was the left hand-right hemisphere word image predominated. However, as soon as researchers requested a verbal response, there was a prompt shift to what the right hand-left hemisphere had felt.
As with Levy's chimeric words, where there was a shift to the left hemisphere when the word had to be interpreted as a symbol for a picture of something, so it was with this experiment in which the letter image had to be interpreted as a symbol for a sound. In other words, when what is perceived is simply dealt with for what it is, the right hemisphere responds; when the stimulus must be interpreted (analyzed) as a symbol for something else, such as letters that stand for a sound or an image, then the left hemisphere comes into play.
Bent Wires
Brenda Milner of the Montreal Neurological Institute at McGill University is another major researcher of brain lateralization. She devised an experiment in which she asked split-brain patients to manipulate 10 inch pieces of wire bent into irregular shapes. The goal was to see whether, after feeling one of these shapes out of view, the patient could choose the one felt from a group of four such wires in plain view. As in the other tests, the left hand-right hemisphere had no trouble with this take even where there was up to a two minute delay between the feeling and the choosing. With the right hand-left hemisphere, the match could not be made, even with no delay and considerable practice.
One explanation for this performance is that, as with faces, irregularly bent wires do not have characteristics that are easily put into words. We can imagine the split-brain patient holding one of the shapes in his right hand trying perhaps to memorize the sequence of bends, only for his speaking left hemisphere to be confused when the four choices are in front of him. We can also imagine the left hand holding the shape and simply recording it in its totality by the way it lays on the palm and fingers. One irony of this and similar experiments is that it was no doubt Dr. Milner's right hand that formed the wires into their shapes, but it took the left hand of the patients to sense the shape out of sight.
Roger Sperry also reports a pronounced memory impairment in the commissurotomy patients. In the first year and a half after the operation, the patients had trouble remembering appointments, messages, where they put things, how to get back to a parked car, and other everyday tasks. They also had a tendency to explain these gaps by confabulation, that is by verbally rationalizing their mistakes in a way to make them seem normal and not the consequence of impaired thinking. Such verbalizing in the left hemisphere expressing itself, trying to explain that it was not responsible for the problem. We may take this as another example of what the left hemisphere does, and as an indication that, to the left the hemisphere, a problem did not exist.
The memory deficit, Sperry explains, is not in the left hemisphere's ability to remember verbally nor in the right hemisphere's ability to remember spatial relationships. It is rather in the interhemispheric integration of these memories. The memories are there, but the patients often cannot use them purposefully. In addition, Dr. Sperry reports severe impairment in the performance of new movements that require interdependent regulation of speed and timing between the right and left hands, as in knitting for example. As with the inaccessibility of memories for purposeful action, the coordination of movements of the two hands by the two hemispheres is halted by the severing of the corpus callosum.
Other tests indicated that the split-brain patients could not remember more than three patterns perceived sequentially by touch. Sperry points out that these patients had trouble persevering on tasks that are mentally taxing and did not grasp the long-term implications of a situation. He states, "Their conversation tends to be restricted to what is immediate and simple. Undue repetition of the same information or anecdote is common. There indications of a tendency to fantasize, and a mild logorrhea [talkativeness] seems to be present in some cases."5 In other words, the split-brain patients do not seem able to grasp the true nature of situations in which they are involved as these situations affect them, and, by the behavior of these individuals, we can assume neither hemisphere seems aware of this inability.
The late Stuart J. Dimond was a British researcher at the University College in Cardiff, Wales and was an especially prominent contributor in the field of brain lateralization. His examination of split-brain patients fills out our behavioral picture of hemisphere differences even more. Dimond tested one split-brain patient who was bilingual in Spanish and English. This person, regardless of the language used, could only name items flashed in the right field of vision and thus perceived by the left hemisphere. Dimond notes, "This simple demonstration dispenses with the notion that the bilingual makes use of separate hemispheres for the two languages."6
Dimond also studied the concept of attention in split-brain individuals. Donald Broadbent, the head of the British government's Applied Psychology Research Unit at Cambridge University, describes attention as the "bit of machinery which decides from moment to moment what it is we're going to notice and therefore what we shall do."7
Dimond tested the split-brain patients to try to get some insight into this machinery. Using the tachistoscope, he asked patients to keep watch for signals flashed to the right or left hemisphere. Both hemispheres performed badly, but the left was much worse than the right. The reason the left hemisphere performed so badly, Dimond said, is because it appeared to pass out of contact for periods of 15 seconds or more, and the signal flashing would bring it back. A signal flashed to the right hemisphere would evoke a response, though. It became clear that the two were attending to the world in two different ways. Dimond concluded that the left hemisphere has selective, sequential attention, and the right has sustained attention. In other words, the left attends first to one thing and then another through time, while the right attends to the world continuously through time. He suggests that both of these modes are necessary for purposeful attention by an individual and that disconnection of the hemispheres affects a person's attention span.
Dimond also tells of a patient performing a task quite competently for a period of 10 to 15 minutes who suddenly stopped and, despite the competence of performance, asked, "How do I continue? What am I supposed to do now?"8 The person, that is the speaking left hemisphere, did not seem to have awareness of the implications of these questions in terms of its ability to keep at a task. It was looking for cues outside itself to determine what it should do next.
Dimond remarked on the fact that when he encountered the split-brain patients after two years, they were still saying the same things in the same way as during their previous meeting, and none of them remembered meeting him before (another confirmation of Sperry's observation about memory difficulties). What he also noticed was that their speech was somehow mechanical and unempathic. He stated, "The generative productive capacity is present, but the speech emitted is often devoid of genuine interaction with the situation. This feature is difficult to describe. It is as if a radio has been switched on."9 The persons were talking, but the message they gave and the way they gave it were often inappropriate to the situation.
Lateralization and Emotion
The connection between emotions, feelings, and the perception of those feelings in a meaningful way by the two hemispheres also interested Stuart Dimond. Before describing this research, a few words about emotions and feeling are necessary. Human emotions and emotional behavior help us to associate our actions—our own and those of others—with our feelings and beliefs about the circumstances in which we find ourselves. Emotions like fear, rage, pleasure, and happiness are registered in the limbic system, a deeper, older part of the brain than the cerebral cortex. The cortex has awareness of these feelings through its connections with the limbic system. In this way, our emotions are associated with our other sensory perceptions of the environment. Our emotions provide us with the backdrop for how we behave in a particular situation. For example, if, for whatever reason, we come to associate feelings of fear with being in dark places, these feelings will help determine how we will act when the lights go out.
H. Chandler Elliott in the book The Shape of Intelligence defines emotion as the quality of personal involvement in any experience.10 In other words, the association of various innate emotions with the larger situation in which we find ourselves will provide the cues for our behavior. If we are happy or sad or frustrated, we will act in ways that are personally consistent with these emotions. However, how a person acts when in various emotional states may be quite different from another person depending on individual experience. For example, when frustrated, I may simply sit down and give up while the same feeling in you may trigger even more determination to succeed in a particular situation.
Patients who are scheduled for neurosurgery are usually given an injection of sodium amytal through the carotid artery. This drug turns off the hemisphere into which it flows for a few minutes. This procedure is used to determine in which hemisphere speech is lateralized. Dimond reported distinctly different behavioral responses of the hemispheres during this procedure that can be interpreted in emotional terms:
When the dominant hemisphere [usually the left] is inactivated it is reported that a depressive type of response is produced whereas inactivation of the opposite side produces a euphoric or maniacal reaction.11
So we can see the right hemisphere without the left is depressed as if it knew something was wrong, but the left hemisphere without the right seems to lose its inhibitions (not to mention its rationality).
The same kind of euphoric speech was also found in split-brain patients. A dramatic example of this seeming inability to "feel" in but one way came when a split-brain patient reported the just heard news that his mother was near death. There was no hint of sadness in his voice, and the voice seemed to regard the event as one of humor rather than sadness. This patient could talk about the situation (that is, his left hemisphere could about it), but there was no expression in the voice of sadness. It seems there was some basic inability in the left hemisphere to connect up emotionally with this event.
Recalling Dimond's observation about the unempathic and mechanical nature of the left hemisphere speech and the idea that emotion is the quality of personal involvement in an experience, it seems not that the left hemisphere is unempathic but that it has no ability for empathy in the first place. The idea of emotionality—the connection of oneself with the larger situation—and how emotion relates to hemisphere function is important, and we will look at it again as we continue our examination of research on the left and right brain.
Continue to "Normal Brains and Single Hemispheres"
Return to first page
|