The experiments with split-brain patients pushed open the door to understanding lateralization. Sensing the importance of this research, many neuropsychologists devised ingenious experiments to test normal people for differences in hemisphere processing activity.
As with the rest of the senses, there is a cross-over of the auditory nerve tracks such that most of what is heard in the left ear is processed in the right hemisphere and vice-versa for the right ear. Doreen Kimura of the University of Western Ontario has done several experiments with a technique called dichotic listening, in which a normal person wearing earphones plugged into a dual channel tape recorder hears a different message in each ear simultaneously. Kimura played a series of digits in each ear that the subject was to remember and report back. She found that with right-handed people, the digits heard by the right ear-left hemisphere were more likely to be remembered than those heard by the left ear-right hemisphere. In other words, there was, for the hearing of sounds that stood for digits, a definite left hemisphere advantage among a normal right-handed population.
When subjects were given tones at different pitches, it was the left ear-right hemisphere that was more accurate in reporting what was heard. To test this even further and to find out whether the left hemisphere was simply specialized for familiar stimuli, such as digits would be, Kimura dichotically played familiar melodies to her subjects. Again she found a clear left ear-right hemisphere advantage for the recognition and repeating of all tested melodies.
These tests seem to show that the left hemisphere is better at processing words, whereas the right hemisphere is better at dealing with pitch and the tone quality of sound. Pitch, tone, and spoken words all have to do with sound. The unique thing about words is not that they are just sounds but that they are patterned sounds that are symbols that stand for something other than themselves. They require interpretation. Dealing with these kinds of sounds seems a left hemisphere function. Pitch and tone on the other hand do not necessarily have anything to do with pattern or anything else other than sound itself, requiring no interpretation. Processing such sounds is usually something the right hemisphere is better at.
How sounds are patterned and turned into meaningful speech has to do with consonants. They provide the varied stops, starts, and transitions surrounding the vowels that make up syllables, words, and sentences. C.J. Darwin of the University of Sussex in Brighton, England undertook several dichotic listening experiments to see whether there is a hemisphere advantage for consonants. He found the right ear-left hemisphere did hear consonants better than the left in normal speech situations. In one test, though, he modified an f sound so as to break it into its distinctive beginning hiss, stop, and then release of air that is the transition to a vowel sound. He then created a sound that had only one of these characteristics of f. In this situation, there was no right ear advantage in hearing this soundóboth ears simply detected a sound. Without the distinctions that turn a consonant into a kind of boundary for various vowel sounds, that is, the components of our words, both hemispheres perceive the sound with equal ability. In other words, the left hemisphere only has an advantage when the consonants are used to create sounds that have meaning linguistically.
We talked earlier about Kimura's research showing the right hemisphere to be more proficient in identifying melody, that is, the pitch of sound as it moves up and down the musical scale. Thomas Bever and Robert Chiarello of Columbia University performed a similar experiment except they played melodies to both musically sophisticated and musically naive listeners. Bever and Chiarello found that the experienced listeners showed a right ear-left hemisphere preference, indicating the analytical nature of their listening. The inexperienced listeners showed a left ear-right hemisphere preference, indicating a focus more on the tonal quality of what they heard. Because they had no experiential base for analyzing music, they simply heard it for the sound it was. However, apparently as individuals become musically sophisticated, they begin to listen for the structure of the melody with its tonal quality becoming secondary. And with this there is a coincident shift to right ear-left hemisphere listening.
Using the tachistoscope as a tool, various researchers have devised experiments to determine how the two hemispheres process visual phenomena in normal people. What they do is flash images to the hemispheres simultaneously and then measure reaction time and accuracy of recall. In one set of experiments, for example, Doreen Kimura and her colleague, Margaret Durnford, found a right hemisphere advantage for location of dots in a two-dimensional field as well for correctly perceiving the number of dots in a field. They also found the right hemisphere performed better in depth perception tests.
What these experiments seem to show is that when images are rapidly flashed so only one hemisphere at a time receives the visual sensations directly, it is those received by the right hemisphere in normal people that, within the context of this experiment, are more correctly processed. In experiments undertaken by other researchers, the right hemisphere in normal people was shown to be faster and more accurate in identifying faces, inverted numbers, gothic lettering, and other complex shapes. Conversely, the left hemisphere had the advantage for identifying letters. Some interesting findings have emerged in the area of emotion and its processing in the hemispheres of normal people. Stuart Dimond, this time working with Linda Farrington of the Department of Ophthalmology at Oxford University, tested normal subjects' reactions to films of varied emotional content. By fitting the subjects with special contact lenses that blocked either one or the other half of their eyes, they showed these people films that they would perceive directly by only one hemisphere or the other. To judge reaction to the films, the researchers measured subjects' heart rates while they were watching. The films shown were a Tom and Jerry cartoon, a travel film, and a film of a surgical operation.
The cartoon and the operation film were both seen as more horrific and unpleasant when viewed by the right hemisphere directly, while the travel film evoked the same response from both sides. Dimond suggests that these results lend "credence to the view that the right hemisphere plays a special role in what is commonly regarded as 'emotion.'"12 In other words, it was the viewing by the right hemisphere that seemed more likely to evoke a bodily response associated with emotional reaction to an experience. This kind of response did not occur with the left hemisphere. It is in the processing of the right hemisphere that emotional connection between the body and the world outside the body seems to be made.
In another study, Gary Schwartz of Harvard University and two colleagues also sought to measure emotion and the role of the hemispheres. Their experiments had to do with asking of various types of questions with emotional and non-emotional content. By careful observation of what is known as lateral eye movement, it can be determined which hemisphere is doing the active processing. If eyes move to the left during the consideration of an answer, this indicates the right hemisphere is active, while movement to the right indicates the left hemisphere is active. Schwartz and his colleagues found that questions with emotional content were more likely to evoke left eye movement, indicating right hemisphere processing.
As with the Dimond and Farrington experiment, the evidence points to the conclusion that the conscious processing of emotion, our feelings of relative ill or well being in any particular situation takes place in the right hemisphere. By the way the brain works, the integration of emotions with situations seems to be the right hemisphere's responsibility. While subject to various interpretations, the research on normal people and how their hemispheres work is in general agreement with that from split-brain experiments. A pattern begins to emerge that seems to indicate that the two hemispheres of the brain do work in certain specified ways and that perhaps in understanding these processes we can detect clues for understanding the human behavior that is the direct reflection of that working.
Studying the differences in hemisphere processing in a normal human being is a statistical task. Researchers present material dichotically to subjects and measure their tendency to better hear letters, for example, in the left hemisphere and melodies in the right. After testing many individuals, the fairly consistent patterns just reviewed show themselves. Another way to study the two modes of hemisphere processing is by "turning off" one of them while the other continues to function. This provides perhaps the most unequivocal picture of how the two brain halves process the world they experience. I have already mentioned how this can be done with an injection of sodium amytal, which shuts down a hemisphere for about five minutes. In the former Soviet Union, where brain research has been enthusiastically pursued, another technique has been used to do the same thing.
Electro-shock therapy is one way of treating depression. Originally doctors placed electrodes on both sides of the skull and induced precisely calibrated current that left the patient unconscious. A British psychologist discovered that it is just as effective and less traumatic if only of the hemisphere is so shocked. This hemisphere is rendered unconscious for several hours, allowing for in-depth investigation of modes of processing in the conscious hemisphere. The treatment creates a one-hemisphere person whose behavior manifests only those skills and memories of the aware half of the brain. Vadim Deglin, an eminent Soviet neuropsychologist, has studied and written of the behavior of these temporary left- and right-hemisphere persons.
The left-hemisphere person retains his power of speech, and, in fact, his vocabulary is richer, and he is more talkative than before. His threshold for hearing speech sounds is greater than when both halves of the brain are functioning. However, the person loses all expressiveness and intonation. The voice takes on a flat nasal quality. There is a corresponding loss of the ability to judge the emotional quality of speech. There is no sense of the words spoken beyond their literal meanings. Such a person cannot even judge on the basis of voice sounds whether a man or woman is speaking.
Similarly, when sounds such as laughter, coughing, or the roar of surf are played on a tape recorder, the left-hemisphere person does not recognize them. Rather than identifying the sound as a person laughing, for example, he will classify it as best he can by saying something like, "that's a person." The same thing happens with music, including well-known tunes. Even if the songs are played, the person cannot hum the correct notes. It is not that his hearing is impaired, since it is enhanced in the perception of speech sounds; it is just that all ability to process the tonal quality of sound is lost.
Other such disabilities are noted in visual processing. For example, the person is unable to detect missing details from familiar objects, such as eyeglasses without ear pieces or a pig without a tail. And if the left-hemisphere person is asked to do a classification exercise such as creating two groups of similar items from cards with the Roman numerals V and X and the numbers 5 and 10, he will always classify the V and 5 together and the X and 10 together, choosing the functional or symbolic similarity over the visual similarity. (This is the same as was described earlier for a Levy experiment.)
Even more striking is the person's comprehension of space and time. Verbally he knows the time and date and the name of the place where he is and thus seems perfectly normal. However, visually he does not recognize places he has been many times and cannot find his way around. Though he knows the date, he has no idea of the season and cannot detect it is winter by looking out the window and seeing snow on the ground. He may infer it's winter from the fact that winter comes in January, but he cannot verify this for himself by seeing snow on the ground and take meaning from that experience.
Finally, the person's seeming emotional outlook is affected. From his behavior, the left-hemisphere person can only be called affable, cheerful, and easy going. He seems to have lost touch with his personal situation as long as the right half of the brain is turned off. Though his behavior is a welcome change, it does not reflect any understanding of the fact that only half of the brain is functioning or recognition of the disabilities that are a consequence of this. In other words, given the reality of the situation, this cheerful demeanor seems inappropriate. Yet, given the way this hemisphere works, this behavior simply emerges.
The same procedure also reveals the characteristics of the right hemisphere. Almost all the incapacities described for the left hemisphere are the strengths of its contralateral partner. However, the use of language is greatly diminished. There is difficulty remembering the names of things, but the person retains some ability to explain or demonstrate how an object is used. The right-hemisphere person generally prefers to point or use mime rather that respond verbally in his interactions with others. Though he uses words with great difficulty, he has no problem with intonation or with identifying nonverbal sounds such as surf or recognizing various pieces of music. However, this person cannot duplicate rhythm patterns (an important component for the composition of music). With unfinished drawings, this person can quickly evaluate them, pointing out the omitted part. And in the classification of Roman and Arabic numerals, he always classifies them by their appearance, that is V and X together and 5 and 10 together.
The right-hemisphere person has much greater difficulty in remembering a series of words and can at best remember two or three out of ten and will forget them within a few hours. However, this person easily remembers images of oddly shaped figures several hours later. In terms of orientation in space and time, the right-hemisphere person has no problem recognizing where he is but cannot give the name of the room. Calendar and clock time have no significance to him, although he is aware of the season and can easily connect the weather with the season by looking out the window.
Emotionally, the right-hemisphere person is sullen and seems depressed and pessimistic about his condition. One might classify this state as a negative outlook on life. However, as with all the behavioral manifestations of right hemisphere processing (which have been characterized by the seemingly correct perception of experiences), this evaluation of his present state also seems proper given the fact that half of the brain has been shut off, and expression is severely repressed.
These studies give perhaps as vivid a picture of the differences between the two hemispheres as any data presented so far. The important thing to remember in considering this information is that when one or the other hemisphere is shut down, the person, in his interaction with Deglin, behaved in these very characteristic ways. This behavior is a direct reflection of how the two individual halves of the brain deal with the world of which they are a part. One seems cheerful but can make no sense of the environment in which it exists nor can it detect emotion in itself or others. The other seems sullen but properly perceives the spatial-temporal quality of the world and the emotional context of his interactions with others. We may speculate that should one or the other of our hemispheres be turned off, we too would behave in these characteristic ways.
Continue to "Damaged Brains"
Return to first page