Imagine being a senior in college interested in patients with epilepsy who had had their corpus callosum cut as a means of controlling seizures. Imagine still being so interested in these patients that you designed experiments to test the effects that the callosotomy surgery (i.e., cutting of the corpus callosum) had on cognitive and behavioral performance. Imagine then going to graduate school and, as a first-year graduate student, actually getting to implement the studies that you had designed with actual individuals who had received callosotomies. Although this may sound a bit farfetched, Michael Gazzaniga did not have to imagine this scenario—he lived it. In fact, he made a career out of it and, through his split-brain studies, revealed a wealth of information about the brain and hemispheric lateralization (Gazzaniga, 2005). Importantly, Gazzaniga did not pioneer split-brain research. Rather, he worked with Roger Sperry, the original designer of split-brain experiments. Sperry spent much of his career conducting split-brain studies with animals. With Gazzaniga, however, their focus shifted to human participants.
To understand the results of split-brain experiments, bear in mind that the left hemisphere, which is dominant for most speech functions, receives information from the right visual field and that the right hemisphere receives information from the left visual field. Normally, whether the right or left hemisphere receives the information makes little difference because once the message reaches the brain, the two hemi- spheres freely pass information between them via the corpus callosum. Severing the corpus callosum, however, blocks this sharing of information (Gazzaniga, 1967).
Figure 3.13a depicts a typical split-brain experiment. A patient is seated at a table, and the surface of the table is blocked from view by a screen so the individual cannot see objects on it. The experimenter asks the person to focus on a point in the center of the screen. A word (here, key) is quickly flashed on the left side of the screen (which is therefore processed in the right hemisphere). When information is flashed for only about 150 milliseconds, the eyes do not have time to move, ensuring that
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the information is sent to only one hemisphere. The patient is unable to identify the word verbally because the information never reached his left hemisphere, which is dominant for speech. He can, however, select a key with his left hand from the array of objects hidden behind the screen because the left hand receives information from the right hemisphere, which “saw” the key. Thus, the right hand literally does not know what the left hand is doing, and neither does the left hemisphere. Figure 3.13b illustrates the way visual information from the left and right visual fields is transmit- ted to the brain in normal and split-brain patients.
This research raises an intriguing question: Can a person with two independent hemispheres be literally of two minds, with two centers of conscious awareness, like Siamese twins joined at the cortex? Consider the case of a 10-year-old boy with a split brain (LeDoux et al, 1977). In one set of tests, the boy was asked about his sense of himself, his future, and his likes and dislikes. The examiner asked the boy questions in which a word or words were replaced by the word blank. The missing words were then presented to one hemisphere or the other. For example, when the boy was asked “Who blank?” the missing words “are you” were projected to the left or the right hemisphere. Not surprisingly, the boy could answer verbally only when inquiries were made to the left hemisphere. The right hemisphere could, however, answer by spelling out words with letter tiles with the left hand (because the right hemisphere is usually not entirely devoid of language) when the question was flashed to the right hemi- sphere. Thus, the boy could describe his feelings or moods with both hemispheres.
Many times the views expressed by the right and left hemispheres overlapped, but not always. One day, when the boy was in a pleasant mood, his hemispheres tended to agree (both, for example, reporting high self-esteem). Another day, when the boy seemed anxious and behaved aggressively, the hemispheres were in disagree- ment. In general, his right-hemisphere responses were consistently more negative than those of the left, as if the right hemisphere tended to be in a worse mood.
Researchers using other methods have also reported that the two hemispheres differ in their processing of positive and negative emotions and that these differences
FIGURE 3.13 A split-brain experiment. In a typical split-brain study (a), a patient sees the word key flashed on the left portion of the screen. Although he cannot name what he has seen, because speech is lateralized to the left hemi- sphere, he is able to use his left hand to select the key from a number of objects because the right hemisphere, which has “seen” the key, controls the left hand and has some language skills. Part (b) illustrates the way information from the left and right visual fields is transmitted to the brain in normal and split brains. When participants focus their vision on a point in the middle of the visual field [such as the star in diagram (b)] anything to the left of this fixation point (for instance, point L) is sensed by receptors on the right half of each eye. This information is subsequently processed by the right hemisphere. In the normal brain, information is readily transmitted via the corpus callosum between the two hemispheres. In the split-brain patient, because of the severed neural route, the right and left hemispheres “see” differ- ent things. (Source: Part (a) adapted from Gazzaniga, 1967.)
Intact corpus
callosum Severed corpuscallosum
Key
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hemisphere hemisphereRight
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hemisphere hemisphereRight R L Normal brain Fixation point Split brain (a) (b) kowa_c03_063-106hr.indd 91 9/13/10 10:40 AM
may exist at birth (Davidson, 1995; Fox, 1991). Left frontal regions are generally more involved in processing positive feelings that motivate approach toward objects in the environment, whereas right frontal regions are more related to negative emotions that motivate avoidance or withdrawal.
Current split-brain studies are informing neuroimaging studies by allowing re- searchers to map out neurological functions that are localized to particular areas of the corpus callosum, for example. By studying individuals with lesions to different areas of the corpus callosum, researchers have uncovered the fact that different areas of the corpus callosum are specialized for the transfer of specific types of information. For example, posterior regions of the corpus callosum are specialized for the transfer of sensory information, such as vision and audition (Gazzaniga, 2005). This research has also highlighted the fact that the amount and types of information that can be trans- ferred between hemispheres following a complete severing of the corpus callosum depend on the species being studied (Gazzaniga, 2005).
R E S E A R C H I N D E p T H : A S T E p F U R T H E R
1. What has split-brain research taught us about the different functions of the left and right hemispheres of the brain? What functions do each of the hemispheres specialize in?
2. For what purpose might a person’s corpus callosum be cut?
3. Explain why an individual who has had his corpus callosum severed and who is pre- sented with an image in his left visual field is unable to state what that image is. How would this person be able to identify the object?
4. Functional plasticity refers to the ability of parts of the brain to assume functions previ- ously performed by other parts of the brain that have now become damaged (e.g., through strokes). Research has shown that adults who have had their corpus callosum either partially or totally severed have little functional plasticity but that infants who have had similar callosotomies have much more functional plasticity. Why would this be the case?
5. Gazzaniga has often been asked, “If you could have just one hemisphere, which would it be?” What do you think his answer to this question is and why?
Sex Differences in Lateralization Psychologists have long known that females typically score higher on tests of verbal fluency, perceptual speed, and manual dex- terity than males, whereas males tend to score higher on tests of mathematical ability and spatial processing, particularly geometric thinking (Casey et al., 1997; Maccoby & Jacklin, 1974). In a study of students under age 13 with exceptional mathematical ability (measured by scores of 700 or above on the SAT), boys out- numbered girls 13 to 1 (Benbow & Stanley, 1983). On the other hand, males are much more likely than females to develop learning disabilities with reading and language comprehension.
Although most of these sex differences are not particularly large (Caplan et al., 1997; Hyde, 1990), they have been documented in several countries and have not consistently decreased over the last two decades despite social changes encouraging equality of the sexes (see Randhawa, 1991). Psychologists have thus debated whether such discrepancies in performance might be based in part on innate differences be- tween the brains of men and women.
Some data suggest that women’s and men’s brains may differ in ways that af- fect cognitive functioning. At a hormonal level, research with human and nonhuman primates indicates that the presence of testosterone and estrogen in the bloodstream early in development influences aspects of brain development (Clark & Goldman- Rakic, 1989; Gorski & Barraclough, 1963). One study found that level of exposure
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to testosterone during the second trimester of pregnancy predicted the speed with which seven-year-olds could rotate mental images in their minds (Grimshaw et al., 1995). Some evidence even suggests that women’s spatial abilities on certain tasks are lower during high-estrogen periods of the menstrual cycle, whereas motor skills, on which females typically have an advantage, are superior during high-estrogen periods (Kimura, 1987).
Perhaps the most definitive data on gender differences in the brain come from re- search using fMRI technology (Shaywitz et al., 1995). In males, a rhyming task activated Broca’s area in the left frontal lobe. The same task in females produced frontal activation in both hemispheres (Figure 3.14). Thus, in females, language appears less lateralized.
I N T E R I M S U M M A R Y
Some psychological functions are lateralized, or processed primarily by one hemisphere. In general, the left hemisphere is more verbal and analytic, and the right is specialized for nonlinguistic functions. Split-brain studies have provided a wealth of information about lateralization. Although the differences tend to be relatively small, males and females tend to differ in cognitive strengths, which appear to be related in part to differences between their brains, including the extent of lateralization of functions such as language.