539

Fundamental to behavioral neuroscience was the finding by Paul Broca and his contemporaries in the mid-1800s that language is lateralized to the brain’s left hemisphere. But the implications of lateralized brain functions were not really understood until the 1960s, when Roger Sperry (1968) and his colleagues began to study people who, as described in Research Focus 15-1, had undergone surgical separation of the two hemispheres as a treatment for intractable epilepsy. It soon became apparent that the cerebral hemispheres are more functionally specialized than researchers had previously realized. Before considering how the brain’s two sides cooperate in generating cognitive activity, we look at the anatomical differences between the left and right hemispheres.

Building on Broca’s findings, investigators have learned how the language- and music-related areas of the left and right temporal lobes differ anatomically. In particular, the primary auditory area is larger on the right, whereas the secondary auditory areas are larger on the left in most people. Other brain regions also are asymmetrical.

Figure 15-11 shows that the lateral fissure, which partly separates the temporal and parietal lobes, has a sharper upward course in the right hemisphere relative to the left. As a result, the posterior right temporal lobe is larger than the same region on the left, as is the left parietal lobe relative to the right.

Among the anatomical asymmetries in the frontal lobes, the region of sensorimotor cortex representing the face is larger in the left hemisphere than in the right, a difference that presumably corresponds to the left hemisphere’s special role in talking. Broca’s area is organized differently on the left and the right. The area visible on the brain’s surface is about one-third larger on the right than on the left, whereas the cortical area buried in the sulci of Broca’s area is greater on the left than on the right.

Not only do these gross anatomical differences exist but so too do hemispheric differences in the details of their cellular and neurochemical structures. For example, neurons in Broca’s area on the left have larger dendritic fields than do corresponding neurons on the right. The discovery of structural asymmetries told us little about the reasons for such differences, but ongoing research is revealing that they result from underlying differences in cognitive processing by the brain’s two sides.

Although many anatomical asymmetries in the human brain are related to language, brain asymmetries are not unique to humans. Most if not all mammals have asymmetries, as do many bird species. The functions of cerebral asymmetry therefore cannot be limited to language processing. Rather, human language likely evolved after the brain became asymmetrical. Language simply took advantage of processes, including the development of mirror neurons, that had already been lateralized by natural selection in earlier members of the human lineage.

540

The specialized functions of the cerebral hemispheres become obvious in people with damage to the left or right side of the brain. To see these functional differences clearly, compare the cases of G. H. and M. M.

Right Parietal Damage

When G. H. was 5 years old, as he was hiking with his family, a large rock rolled off an embankment and hit him on the head. He was unconscious for a few minutes and had a severe headache for a few days but quickly recovered. Around age 18, however, he started having seizures. Neurosurgical investigation revealed that G. H. had had a right posterior parietal injury from the rock accident. Figure 15-12A shows the area affected. After surgery to remove this area, G. H. had weakness on the left side of his body and showed contralateral neglect. But these symptoms lessened fairly quickly, and a month after the surgery, they had completely cleared.

Nevertheless, G. H. had chronic difficulties in copying drawings; 4 years later, he still performed this task at about the level of a 6-year-old. He also had trouble assembling puzzles, a pastime he enjoyed before his surgery. When asked to perform mental manipulations like the one in Figure 15-6, he became very frustrated and refused to continue. G. H. also had difficulty finding his way around familiar places. The landmarks he had used to guide his travels before the surgery no longer worked for him. G. H. now has to learn street names and use a verbal strategy to go from one place to another.

Left Parietal Damage

M. M.’s difficulties were quite different. A meningioma had placed considerable pressure on the left parietal region. The tumor was surgically removed when M. M. was 16 years old. It had damaged the area shown in Figure 15-12B.

After the surgery, M. M. had various problems, including aphasia, impaired language use. The condition lessened over time: a year after the surgery, M. M. spoke fluently. Unfortunately, other difficulties persisted. In solving arithmetic problems, in reading, and even in simply calling objects or animals by name, M. M. performed at about a 6-year-old level. She had no difficulty making movements spontaneously, but when asked to copy a series of arm movements, such as those diagrammed in Figure 15-13, she had great difficulty. She could not figure out how to make her arm move to match the example. A general impairment in making voluntary movements in the absence of paralysis or a muscular disorder is a symptom of apraxia, the inability to complete a plan of action accurately.

Lessons from Patients G. H. and M. M

541

What can we learn about brain function by comparing G. H. and M. M.? Their lesions were in approximately the same location but in opposite hemispheres, and their symptoms were very different.

Judging from G. H.’s difficulties, the right hemisphere contributes to controlling spatial skills, such as drawing, assembling puzzles, and navigating in space. In contrast, M. M.’s condition reveals that the left hemisphere seems to contribute to controlling language functions and cognitive tasks related to schoolwork—namely, reading and arithmetic. In addition, the left hemisphere’s role in controlling voluntary movement sequences differs from the right hemisphere’s role.

To some extent, then, the left and right hemispheres think about different types of information. The question is whether these functional differences can be observed in a healthy brain.

In the course of studying the auditory capacity of people with a temporal lobe lesion, Doreen Kimura (1967) found something unexpected. She presented her control participants with two strings of digits, one played into each ear, a procedure known as dichotic listening. The task was to recall as many digits as possible.

Kimura found that the controls recalled more digits presented to the right ear than to the left. This result is surprising because the auditory system crosses repeatedly, beginning in the midbrain. Nonetheless, information coming from the right ear seems to have preferential access to the left (speaking) hemisphere.

In a later study, Kimura (1973) played two pieces of music for participants, one to each ear. She then gave the participants a multiple-choice test, playing four bits from musical selections and asking the participants to pick out the bits they had heard before. In this test, she found that participants were more likely to recall the music played to the left ear than to the right. This result implies that the left ear has preferential access to the right (musical) hemisphere.

The demonstration of this functional asymmetry in the healthy brain provoked much interest in the 1970s, leading to demonstrations of functional asymmetries in the visual and tactile systems as well. Consider the visual system. If we fixate on a target, such as a dot positioned straight ahead, all the information to the left of the dot goes to the right hemisphere and all the information to the right of the dot goes to the left hemisphere, as shown in Figure 15-14.

542

If information is presented for a relatively long time—say, 1 second—we can easily report what was in each visual field. If, however, the presentation is brief—say, only 40 milliseconds—then the task is considerably harder. This situation reveals a brain asymmetry.

Words presented briefly to the right visual field and hence sent to the left hemisphere are more easily reported than are words presented briefly to the left visual field. Similarly, if complex geometric patterns or faces are shown briefly, those presented to the left visual field and hence sent to the right hemisphere are more accurately reported than are those presented to the right visual field.

Apparently, the hemispheres not only think about different types of information, they also process information differently. The left hemisphere seems biased toward processing language-related information, whereas the right hemisphere seems biased toward processing nonverbal, and especially spatial, information.

A word of caution: Although asymmetry studies are fascinating, what they tell us about the differences between the hemispheres is not entirely clear. They tell us something is different, but it is a broad leap to conclude that the hemispheres house entirely different skill sets.

The hemispheres have many common functions, such as controlling movement in the contralateral hand and processing sensory information through the thalamus. Still, differences in the hemispheres’ cognitive operations do exist. We can better understand these differences by studying split-brain patients, whose cerebral hemispheres have been surgically separated for medical treatment.

Before considering the details of split-brain studies, let us review what we already know about cerebral asymmetry. First, the left hemisphere has speech; the right hemisphere does not. Second, as demonstrated in Research Focus 15-1, on page 520, the right hemisphere performs better than the left on certain nonverbal tasks, especially those that engage visuospatial skills.

But how does a severed corpus callosum affect how the brain thinks? After the corpus callosum has been cut, the hemispheres have no way of communicating with one another. The left and right hemispheres are therefore free to think about different things. In a sense, a split-brain patient has two brains.

One way to test the hemispheres’ cognitive functions in a split-brain patient takes advantage of the fact that information in the left visual field goes to the right hemisphere and information in the right field goes to the left hemisphere (see Figure 15-14). With the corpus callosum cut, however, information presented to one side of the brain has no way of traveling to the other side. It can be processed only in the hemisphere that receives it.

Experiments 15-3 and 15-4 show some basic testing procedures based on this dichotomy. The split-brain subject fixates on the dot in the center of the screen while information is presented to the left or right visual field. The person must respond with the left hand (controlled by the right hemisphere), with the right hand (controlled by the left hemisphere), or verbally (also a left-hemisphere function). In this way, researchers can observe what each hemisphere knows and what it is capable of doing.

543

544

As illustrated in Experiment 15-3, for instance, a picture—say, of a spoon—might be flashed and the subject asked to state what he or she sees. If the picture is presented to the right visual field, the person will answer, “Spoon.” If the picture is presented to the left visual field, however, the person will say, “I see nothing.” The subject responds in this way for two reasons:

Now suppose the task changes. In Experiment 15-4A, the picture of a spoon is still presented to the left visual field, but the subject is asked to use the left hand to pick out the object shown on the screen. In this case, the left hand, controlled by the right hemisphere, which sees the spoon, readily picks out the correct object. Can the right hand also choose correctly? No, because it is controlled by the left hemisphere, which cannot see the spoon. If the person in this situation is forced to select an object with the right hand, the left hemisphere does so at random.

Now let’s consider an interesting twist. In the Procedure for Experiment 15-4B, each hemisphere is shown a different object—say, a spoon to the right hemisphere and a pencil to the left. The subject is asked to use both hands to pick out “the object.” The problem here is that the right and left hands do not agree. While the left hand tries to pick up the spoon, the right hand tries to pick up the pencil or tries to prevent the left hand from picking up the spoon.

This conflict between the hemispheres can be seen in the everyday behavior of some split-brain subjects. One woman, P. O. V., reported frequent interhemispheric competition for at least 3 years after her surgery. “I open the closet door. I know what I want to wear. But as I reach for something with my right hand, my left comes up and takes something different. I can’t put it down if it’s in my left hand. I have to call my daughter.”

We know from Experiment 15-3 that the left hemisphere is capable of using language, and Research Focus 15-1 reveals that the right hemisphere has visuospatial capabilities that the left hemisphere does not. Although findings from half a century of studying split-brain patients show that the hemispheres process information differently, another word of caution is needed. There is more functional overlap between the hemispheres than was at first suspected. The right hemisphere, for instance, does have some language functions, and the left hemisphere does have some spatial abilities. Nonetheless, the two undoubtedly are different.

Various hypotheses propose to explain hemispheric differences. One idea, that the left hemisphere is important in controlling fine movements, dates back a century. Recall M. M., the meningioma patient with left parietal lobe damage and apraxia (see Figure 15-12B). Although the apraxia subsided, she was left with chronic trouble in copying movements.

Perhaps one reason that the left hemisphere has a role in language is that speaking requires fine motor movements of the mouth and tongue. Significantly, damage to the language-related areas of the left hemisphere almost always interferes with both language and movement, whether the person speaks or signs. Reading Braille, however, may not be so affected by left-hemisphere lesions. Most people prefer to use the left hand to read Braille, which essentially consists of spatial patterns, so processes related to reading Braille may reside in the right hemisphere.

That said, another clue that the left hemisphere’s specialization for language may be related to its special role in controlling fine movements comes from investigating where the brain processes certain parts of speech. Recall that cognitive systems for representing abstract concepts likely are related to systems that produce more concrete behaviors. Consequently, we might expect that the left hemisphere would participate in forming concepts related to fine movements.

Concepts that describe movements are the parts of speech we call verbs. A fundamental difference between left- and right-hemisphere language abilities is that verbs seem to be processed only in the left hemisphere, whereas nouns are processed in both hemispheres. In other words, not only does the left hemisphere specialize in producing actions, it also produces mental representations of actions in the form of words.

545

If the left hemisphere excels at language because it is better at controlling fine movements, what is the basis of the right hemisphere’s abilities? One idea is that the right hemisphere specializes in controlling movements in space. In a sense, this role elaborates the functions of the dorsal visual stream (diagrammed in Figure 15-3).

Once again, we can propose a link between movement at a concrete level and movement at a more abstract level. If the right hemisphere is producing movements in space, then it is also likely to produce mental images of such movements. We would therefore predict impairment of right-hemisphere patients’ ability both to think about and to make spatially guided movements. And they are thus impaired.

Bear in mind that theories about the reasons for hemispheric asymmetry are highly speculative. The brain has evolved to produce movement and to construct a sensory reality, so the observed asymmetry must somehow relate to these overriding functions. That is, more recently evolved functions, such as language, likely are extensions of preexisting functions. So the fact that language is represented asymmetrically does not mean that the brain is asymmetrical because of language. After all, other species that do not talk have an asymmetrical brain.

As we end our examination of brain asymmetry, consider one more provocative idea. Michael Gazzaniga (1992) proposed that the left hemisphere’s superior language skills are important for understanding the differences between humans’ and other animals’ thinking. He called the speaking hemisphere the interpreter. The following experiment, using split-brain patients as subjects, illustrates what Gazzaniga meant.

Each hemisphere is shown a picture of a match followed by a picture of a piece of wood, for example. Another set of pictures is then shown. The task is to pick from this set a third picture that has an inferred relation to the first two. In this example, the third related picture might be a bonfire. The right hemisphere is incapable of making the inference that a match struck and held to a piece of wood could start a bonfire, whereas the left hemisphere easily arrives at this interpretation.

An analogous task uses words. One or the other hemisphere might be shown the words pin and finger then be asked to pick out a third word related to the other two. In this case, the correct answer might be bleed.

The right hemisphere cannot make this connection. Although it has enough language ability to pick out close synonyms for pin and finger (needle and thumb, respectively), it cannot make the inference that pricking a finger with a pin will result in bleeding.

Again, the left hemisphere has no difficulty with this task. Apparently, the left hemisphere’s language capability gives it a capacity for interpretation that the right hemisphere lacks. One reason may be that language serves to label and express the computations of other cognitive systems.

Gazzaniga goes even further. He suggests that the evolution of left-hemisphere language abilities makes humans a “believing” species: humans can make inferences and have beliefs about sensory events. By contrast, Alex, the African grey parrot profiled in Comparative Focus 15-2, would not have been able to make inferences or hold beliefs because he did not have a system analogous to our left-hemisphere language system. Alex could use language but could not make inferences about sensory events with language.

Gazzaniga’s idea is intriguing. It implies a fundamental difference in the nature of cerebral asymmetry—and therefore in the nature of cognition—that exists between humans and other animals because of the nature of human language. We return to this idea in Section 15-7.

546

Cerebral Asymmetry in Thinking

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Answers appear in the Self Test section of the book.