KEY THEME

Although they have many functions in common, the two hemispheres of the cerebral cortex are specialized for different tasks.

KEY QUESTIONS

If you hold a human brain in your hand, the two cerebral hemispheres would appear to be symmetrical. Although the left and right hemispheres are very similar in appearance, they are not identical. Anatomically, one hemisphere may be slightly larger than the other. There are also subtle differences in the sizes of particular structures, in the distribution of gray matter and white matter, and in the patterns of folds, bulges, and grooves that make up the surface of the cerebral cortex (Ocklenburg & Güntürkun, 2012).

What about differences in the functions of the two hemispheres? In many cases, the functioning of the left and right hemispheres is symmetrical, meaning that the same functions are located in roughly the same places on each hemisphere. Examples of such functional symmetry include the primary motor cortex and the somatosensory cortex, which we discussed in the previous section. With regard to other important processes, however, the left and right cerebral hemispheres do differ—each cerebral hemisphere is specialized for particular abilities.

Here’s a rough analogy. Imagine two computers that are linked through a network. One computer is optimized for handling word processing, the other for handling graphic design. Although specialized for different functions, the two computers actively share information and can communicate with each other across the network. In this analogy, the two computers correspond to the left and right cerebral hemispheres, and the network that links them is the corpus callosum.

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As you’ll see in this section, the first discoveries about the differing abilities of the two brain hemispheres were made more than a hundred years ago by two important pioneers in brain research, Pierre Paul Broca and Karl Wernicke.

THE EARLY WORK OF BROCA AND WERNICKE

By the end of the 1700s it had already been well established that injury to one side of the brain could produce muscle paralysis or loss of sensation on the opposite side of the body. By the early 1800s, animal experiments had shown that specific functions would be lost if particular brain areas were destroyed. And, as discussed in the Science Versus Pseudoscience box on page 61, phrenology triggered scientific debates about cortical localization, or localization of function—the idea that particular brain areas are associated with specific functions.

In the 1860s, more compelling evidence for cortical localization was presented by a French surgeon and neuroanatomist named Pierre Paul Broca. Broca treated a series of patients who had great difficulty speaking but could comprehend written or spoken language. Subsequent autopsies of these patients revealed a consistent finding—brain damage to an area on the lower left frontal lobe. Today, this area on the left hemisphere is referred to as Broca’s area, and it is known to play a crucial role in speech production (Figure 2.21).

About a decade after Broca’s discovery, a young German neurologist named Karl Wernicke discovered another area in the left hemisphere that, when damaged, produced a different type of language disturbance. Unlike Broca’s patients, Wernicke’s patients had great difficulty understanding spoken or written communications. They could speak quickly and easily, but their speech sometimes made no sense. They sometimes used meaningless words or even nonsense syllables, though their sentences seemed to be grammatical. In response to the question “How are you feeling?” a patient might say something like, “Don’t glow glover. Yes, uh, ummm, bick, bo chipickers the dallydoe mick more work mittle.” Autopsies of these patients’ brains revealed consistent damage to an area on the left temporal lobe that today is called Wernicke’s area (see Figure 2.21).

The discoveries of Broca and Wernicke provided the first compelling clinical evidence that language and speech functions are performed primarily by the left cerebral hemisphere. If similar brain damage occurs in the exact same locations on the right hemisphere, these severe disruptions in language and speech are usually not seen.

The notion that one hemisphere exerts more control over or is more involved in the processing of a particular psychological function is termed lateralization of function. Speech and language functions are lateralized on the left hemisphere. Generally, the left hemisphere exerts greater control over speech and language abilities in virtually all right-handed and the majority of left-handed people.

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The language disruptions demonstrated by Broca’s and Wernicke’s patients represent different types of aphasia. Aphasia refers to the partial or complete inability to articulate ideas or understand spoken or written language because of brain injury or damage. There are many different types of aphasia.

People with Broca’s aphasia find it difficult or impossible to produce speech, which is why it is often referred to as expressive aphasia. Despite their impairments in speaking, their comprehension of verbal or written words is relatively unaffected.

People with Wernicke’s aphasia have great difficulty comprehending written or spoken communication, which is why it is often referred to as receptive aphasia. Although they can speak, they often have trouble finding the correct words.

At the beginning of this chapter, we described the symptoms experienced by our friend Asha in the weeks before and the months following her stroke. Asha, who is right-handed, experienced the stroke in her left hemisphere. About three days after her stroke, an MRI brain scan showed where the damage had occurred: the left temporal lobe. Asha experienced many symptoms of Wernicke’s aphasia. Talking was difficult, not because Asha couldn’t speak, but because she had to stop frequently to search for the right words. Asha was unable to name even simple objects, like the cup on her hospital dinner tray or her doctor’s necktie. She recognized the objects but was unable to say what they were. She had great difficulty following a normal conversation and understanding speech, both in English and in her native language, Tulu.

Asha also discovered that she had lost the ability to read. She could see the words on the page, but they seemed to have no meaning. Her husband, Paul, brought some of their Christmas cards to the hospital. Asha recalls, “When I realized I couldn’t read the Christmas cards, I thought my life was over. I just lost it. I remember crying and telling the nurse, ‘I have a doctorate and I can’t read, write, or talk!’”

When we visited Asha in the hospital, we brought her a Christmas present: a portable music player with headphones and some albums of relaxing instrumental music. Little did we realize how helpful the music would be for her. One album was a recording of Native American flute music called Sky of Dreams. The music was beautiful and rather unusual, with intricate melodies and unexpected, complex harmonies. Although it was very difficult for Asha to follow normal speech, listening to Sky of Dreams was an entirely different experience. As Asha explained:

I tried cranking up the music very high and it soothed me. I could sleep. At the time, the flute music seemed to be just perfectly timed with the way my brain was working. It was tuning out all the other noises so I could focus on just one thing and sleep. So I would play the music over and over again at a very high level. I did that for a long time because my mind was so active and jumbled that I couldn’t think.

Asha’s language functions were severely disrupted, yet she was able to listen to and appreciate instrumental music—even very complex music. Why? At the end of the next section, we’ll offer a possible explanation for what seems to have been a disparity in Asha’s cognitive abilities following her stroke.

THE SPLIT BRAIN

Since the discoveries by Broca and Wernicke, the most dramatic evidence illustrating the independent functions of the two cerebral hemispheres has come from a surgical procedure called the split-brain operation. This operation is used to stop or reduce recurring seizures in severe cases of epilepsy that can’t be treated in any other fashion. The procedure involves surgically cutting the corpus callosum, the thick band of axons that connects the two hemispheres.

What was the logic behind cutting the corpus callosum? An epileptic seizure typically occurs when neurons begin firing in a disorganized fashion in one region of the brain. The disorganized neuronal firing quickly spreads from one hemisphere to the other via the corpus callosum. If the corpus callosum is cut, seizures should be contained in just one hemisphere, reducing their severity or eliminating them altogether. This is exactly what happened when the split-brain operation was first tried in this country in the 1940s (Wolman, 2012).

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Surprisingly, cutting the corpus callosum initially seemed to produce no noticeable effect on the patients, other than reducing their epileptic seizures. Their ability to engage in routine conversations and tasks seemed to be unaffected. On the basis of these early observations, some brain researchers speculated that the corpus callosum served no function whatsoever. One famous psychologist, Karl Lashley, joked that the primary function of the corpus callosum seemed to be to keep the two hemispheres from sagging (Hoptman & Davidson, 1994).

In the 1960s, however, psychologist and neuroscientist Roger Sperry and his colleagues began unraveling the puzzle of the left and right hemispheres. Sperry and his colleagues used the apparatus shown in Figure 2.22 to test the abilities of split-brain patients. They would direct a split-brain subject to focus on a point in the middle of a screen, while briefly flashing a word or picture to the left or right of the midpoint.

In this procedure, visual information to the right of the midpoint is projected to the person’s left hemisphere, and visual information to the left of the midpoint is projected to the right hemisphere. Behind the screen several objects were hidden from the split-brain subject. The subject could reach under a partition below the screen to pick up the concealed objects but could not see them (Sperry, 1982).

In a typical experiment, Sperry projected the image of an object concealed behind the screen, such as a hammer, to the left of the midpoint. This is shown in Task 2, Figure 2.22. Thus, the image of the hammer was sent to the right, nonverbal hemisphere. If a split-brain subject was asked to verbally identify the image flashed on the screen, she could not do so and often denied that anything had appeared on the screen. Why? Because her verbal left hemisphere had no way of knowing the information that had been sent to her right hemisphere. However, if a split-brain subject was asked to use her left hand to reach under the partition for the object that had been displayed, she would correctly pick up the hammer. This was because her left hand was controlled by the same right hemisphere that saw the image of the hammer.

Sperry’s experiments reconfirmed the specialized language abilities of the left hemisphere that Broca and Wernicke had discovered more than a hundred years earlier. But notice, even though the split-brain subject’s right hemisphere could not express itself verbally, it still processed information and expressed itself nonverbally: The subject was able to pick up the correct object.

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Over the past decades, researchers have gained numerous insights about the brain’s lateralization of functions by studying split-brain patients, using brain-imaging techniques with normal subjects, and employing other techniques (Gazzaniga, 2005; Hugdahl & Westerhausen, 2010). On the basis of this evidence, researchers have concluded that—in most people—the left hemisphere is superior in language abilities, speech, reading, and writing.

In contrast, the right hemisphere is more involved in nonverbal emotional expression and visual-spatial tasks (Corballis, 2010). Deciphering complex visual cues, such as completing a puzzle or manipulating blocks to match a particular design, also relies on right-hemisphere processing (Gazzaniga, 1995, 2005). And the right hemisphere excels in recognizing faces and emotional facial cues, reading maps, copying designs, and drawing. Finally, the right hemisphere shows a higher degree of specialization for musical appreciation or responsiveness—but not necessarily for musical ability, which involves the use of the left hemisphere as well (Springer & Deutsch, 2001).

Figure 2.23 summarizes the research findings for the different specialized abilities of the two hemispheres for right-handed people. As you look at the figure, it’s important to keep two points in mind. First, the differences between the left and right hemispheres are almost always relative differences, not absolute differences. In other words, both hemispheres of your brain are activated to some extent as you perform virtually any task (Toga & Thompson, 2003). In the normal brain, the left and right hemispheres function in an integrated fashion, constantly exchanging information (Allen & others, 2007). Thus, Figure 2.23 indicates the hemisphere that typically displays greater activation or exerts greater control over a particular function. Misconceptions about the roles played by the left and right hemispheres are common in the popular media, and even in education (Howard-Jones, 2014). The Science Versus Pseudoscience box, “Brain Myths,” explores some of the most common misperceptions about the brain. Second, many functions of the cerebral hemispheres, such as those involving the primary sensory and motor areas, are symmetrical. They are located in the same place and are performed in the same way on both the left and the right hemispheres.

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Given the basic findings on the laterality of different functions in the two hemispheres, can you speculate about why Asha was unable to read or follow a simple conversation but could easily concentrate on a complex piece of music? Why were her language abilities so disrupted, while her ability to focus on and appreciate music remained intact after her stroke?

A plausible explanation has to do with the location of the stroke’s damage on Asha’s left temporal lobe. Because language functions are usually localized on the left hemisphere, the stroke produced serious disruptions in Asha’s language abilities. However, her right cerebral hemisphere sustained no detectable damage. Because one of the right hemisphere’s abilities is the appreciation of musical sounds, Asha retained the ability to concentrate on and appreciate music.