A striking idea emerged from observations of people with damage to the language areas of the brain. Researchers noticed that when damage was limited to areas of the left hemisphere, there were impairments in the ability to use or understand lan-guage. Damage to corresponding parts of the right hemisphere usually did not have these effects. Perhaps, they reasoned, the right and left halves of the brain serve dif-ferent functions.
This concept was not entirely new. It had long been understood, for example, that most sensory and motor pathways cross over as they enter or leave the brain.
As a result, the left hemisphere receives information from, and controls movements of, the right side of the body, whereas the right hemisphere receives input from and controls the left side of the body. However, both sides of the brain perform these functions. The fact that language centers, such as Broca’s area and Wernicke’s area, are found almost exclusively on the left side of the brain suggested that each hemi-sphere might be specialized to perform some functions almost independently of the other hemisphere (Stephan et al., 2003).
●
Split-Brain Studies
As far back as the late 1800s, scientists had wanted to test the hypothesis that the cerebral hemispheres might be specialized, but they had Movement and the Brain Scientists aretrying to understand exactly how smooth movements are coordinated by neural activity in the brain and the spinal cord.
The complexity of the processes involved presents a challenge to researchers working on devices to restore movement in paralyzed individuals. As shown here, delivering computer-controlled electrical stimulation to leg muscles allows walking movements to occur, though they are jerkier than the brain normally produces. It has also been possible to implant a device that records activity from the motor cortex of a completely paralyzed man. When data from the device was fed into a computer that controlled an artificial hand, the man was able to open and close the hand by imagining these movements (Hochberg et al., 2006). A video of this remarkable accomplishment is available at http://www.nature.com/nature/journal/
v442/n7099/extref/nature04970-s7.mov.
The Central Nervous System: Making Sense of the World
● 87
no techniques for doing so. Then, during the 1960s, Roger Sperry, Michael Gazzaniga, and their colleagues began to study split-brain patients—people who had undergone a surgical procedure in an attempt to control severe epilepsy. Before the surgery, their seizures began in one hemisphere and then spread to engulf the whole brain. As a last resort, surgeons isolated the two hemispheres from each other by severing the , a massive bundle of more than a million fibers that connects the two hemispheres (see Figure 3.18).
After the surgery, researchers used a special apparatus to present visual images to only one side of these patients’ split brains (see Figure 3.19). They found that sev-ering the tie between the hemispheres had dramatically affected the way these people thought about and dealt with the world. For example, when the image of a spoon was presented to the left, language-oriented side of one patient’s split brain, she could say what the spoon was; but when the spoon was presented to the right side of her brain, she could not describe the spoon in words. She still knew what it was, how-ever. Using her left hand (controlled by the right hemisphere), she could pick out the spoon from a group of other objects by its shape. But when asked what she had just grasped, she replied, “A pencil.” The right hemisphere recognized the object, but the patient could not describe it because the left (language) half of her brain did not see or feel it (Sperry, 1968).
Although the right hemisphere has no control over spoken language in split-brain patients, it does have important capabilities, including some related to non-spoken language. For example, a split-brain patient’s right hemisphere can guide the left hand in spelling out words with Scrabble tiles (Gazzaniga & LeDoux, 1978).
Thanks to this ability, researchers discovered that the right hemisphere of split-brain patients has self-awareness and normal learning abilities. In addition, it is superior to the left hemisphere on tasks dealing with spatial relations (especially drawing three-dimensional shapes) and at recognizing human faces.
●
Lateralization of Normal Brains
Sperry (1974, p. 7) concluded from his studies that each hemisphere in the split-brain patient has its own “private sensations, perceptions, thoughts, and ideas all of which are cut off from the cor-responding experiences in the opposite hemisphere. . . . In many respects each F I G U R E3.18
The Brain’s Left and Right Hemispheres
The brain’s two hemispheres are joined by a core bundle of nerve fibers known as the corpus callosum. In this figure the hemispheres are separated to reveal the corpus callosum. The two cerebral hemispheres look nearly the same but perform somewhat different tasks. For one thing, the left hemisphere receives sensory input from, and controls movement on, the right side of the body. The right hemisphere senses and controls the left side of the body.
corpus callosum
Corpus callosum
Hemispheres
Language Areas of the Brain Have you ever tried to write notes while you were talking to someone?
Like this teacher, you can probably write and talk at the same time, because each of these language functions uses different areas of association cortex. However, stop reading for a moment, and try writing one word with your left hand and a different word with your right hand. If you had trouble, it is partly because you asked the same language area of your brain to do two things at once.
88 ●
CHAPTER 3 Biological Aspects of PsychologyTHISTRY
disconnected hemisphere appears to have a separate ‘mind of its own.’” But what about people whose hemispheres are connected normally? Are certain of their functions, such as mathematical reasoning or language skills, lateralized? A task is one that is performed more efficiently by one hemisphere than by the other.
To find out, researchers presented images to just one hemisphere of people with normal brains and then measured how fast they could analyze information. If information is presented to one side of the brain, and if that side is specialized to analyze that type of information, a person’s responses will be faster than if the information must first be transferred to the other hemisphere for analysis. These studies have confirmed that the left hemisphere has better logical and language abil-ities than the right, whereas the right hemisphere has better spatial, artistic, and musical abilities (Springer & Deutsch, 1989). Positron emission tomography (PET) scans of normal people receiving varying kinds of auditory stimulation also demon-strate these differences (see Figure 3.20). We know that the language abilities of the left hemisphere are not specifically related to auditory information, though, because people who are deaf also use the left hemisphere, Broca’s area specifically, more than the right for sign language (Horwitz et al., 2003).
The precise nature and degree of lateralization vary quite a bit among individuals.
Functional MRI studies show, for example, that one in ten people show activation of both hemispheres during language tasks, and the brains of another 10 percent appear to coordinate language in the right hemisphere (Fitzgerald, Brown, & Daskalakis, 2002). Both of these patterns are seen mostly in left-handed people (Knecht et al., 2002). Evidence of sex differences in brain laterality comes from studies of the cogni-tive abilities of normal men and women, of the effects of brain damage on cognicogni-tive function, and of anatomical differences between the sexes. Among normal individuals, there are sex differences in the ability to perform tasks that are known to be lateralized in the brain. For example, women tend to do better than men at perceptual fluency tasks, such as rapidly identifying matching items, and at arithmetic calculations. Men tend to be better at imagining the rotation of an object in space and tasks involving target-directed motor skills, such as guiding projectiles or intercepting them, in real or virtual-reality situations (Halperin, 1992; Waller, 2000). However, these sex differences tend to be quite small (Boles, 2005; Frost et al., 1999; Haut & Barch, 2006).
쐌corpus callosum A massive bundle of fibers that connects the right and left cerebral hemispheres and allows them to communicate with each other.
쐌lateralized Referring to the tendency for one cerebral hemisphere to excel at a particular function or skill compared with the other hemisphere.
lateralized F I G U R E
3.19
Apparatus for Studying Split-Brain Patients
When the person stares at the dot on the screen, images briefly presented on one side of the dot go to only one side of the brain. For example, a picture of a spoon presented on the left side of the screen goes to the right side of the brain. The right side of the brain can find the spoon and direct the left hand to touch it.
However, because the language areas on the left side of the brain did not see the spoon, the person is unable to say what it is.
The Central Nervous System: Making Sense of the World
● 89
Damage to just one side of the brain is more disabling to men than to women.
In particular, men show larger deficits in language ability than women when the left side is damaged (McGlone, 1980). This difference may reflect a wider distribution of language abilities in the brains of women compared with those of men. When participants in one study performed language tasks, such as thinking about whether particular words rhyme, MRI scans showed increased activity on the left side of the brain for men but on both sides for women (Shaywitz et al., 1995; see Figure 1.1 in the chapter on introducing psychology). Women appear to have proportionately more of their association cortex devoted to language tasks (Harasty et al., 1997).
Although humans and animals show definite sex differences in brain anatomy (Allen, Hines, et al., 1989; Gur et al., 1995; Juraska, 1998), no particular anatom-ical feature has been identified as underlying sex differences in lateralization. One study reported that the corpus callosum is larger in women than in men (de Lacoste-Utamsing & Holloway, 1982), but more than fifty attempts to replicate this finding have all failed (Morton & Rafto, 2006; Olivares, Michalland, & Aboitz, 2000).
Despite the overwhelming evidence against it, the original report of a sex difference in the corpus callosum continues to be cited, suggesting that scientists are sometimes not entirely unbiased.
Having two somewhat specialized hemispheres allows the brain to more effi-ciently perform some tasks, particularly difficult ones, but the differences between the hemispheres should not be exaggerated. The corpus callosum usu-ally integrates the functions of the “two brains,” a role that is particularly impor-tant in tasks that require sustained attention (Rueckert et al., 1999). As a result, the hemispheres work so closely together, and each makes up so well for what-ever lack of ability the other may have, that people are normally unaware that their brains are made up of two partially independent, somewhat specialized halves (Banich & Heller, 1998; Staudt et al., 2001).