The Brain And Handedness

Control of the limbs, including the hands, is contra-laterally organized, meaning that the left hemisphere of the brain controls the right side of the body and the right hemisphere of the brain controls the left side of the body. In the 1860s, Paul Broca discovered that the major language production center in the brain is located in the left hemisphere of the brain. This center is now called Broca's area in his honor. There is another speech center called Wernicke's area that is associated with speech comprehension, and it is also located in the left hemisphere of the brain. The fact that both the right-hand and primary speech functions are controlled by the same half of the brain has fuelled speculations attempting to link brain organization, language functions, and handedness. Broca started these speculations by introducing the notion of the ''dominant hemisphere,'' by which he meant the hemisphere that directs not only the movements of writing, drawing, and other fine movements but also language and perhaps even major aspects of logical thought. He then suggested that left-handers might have a brain that is organized in a mirror image to that of right-handers, with language control on the right side of the brain.

The notion that handedness and language control are related in the brain has persisted and is still found in some theories today. The data, however, suggest that this relationship is unfounded. One important source of data on this issue has been the observations of people who are injured on one side of their brain or who acquire a pathology that damages one side of the brain. This often results in the development of aphasia, which is the loss of language abilities. Evidence from such conditions first led Broca to place the language control center in the left hemisphere. Other techniques also provide information on this matter. These involve procedures that effectively ''turn off" one hemisphere of the brain. This can be done through the injection of certain drugs (such as sodium amytol) into one side of the brain. Alternatively, electric shock, such as that which is used in electroconvulsive therapy for severe psychological depression, can be used. Instead of applying the shock to the whole brain, however, it is applied to only one-half of the brain. If the drug treatment or the electric shock result in temporarily impaired language comprehension or production abilities, then we can conclude that we have turned off the hemisphere of the brain that contains the language control center. The composite results of 12 studies that attempted to locate the hemisphere containing the language control centers are shown in Fig. 3. Notice that although virtually all right-handers have language in their left hemisphere, we do not find the mirror-imaged brain that we expected in lefthanders. The data show that the vast majority of lefthanders have the same brain organization as righthanders, with their language control exclusively confined to the left hemisphere. Only about 2 of 10 lefthanders have language exclusively in the right hemisphere, as had been predicted by Broca. There is also a small group of individuals who seem to have their language control split between both sides of the brain or duplicated on both sides of the brain.

The new technology associated with various brain scanning techniques, such as positron emission tomography, computerized axial tomography, and functional magnetic resonance imaging, has been used to explore the organization and functioning of the brains of left- and right-handers. The overwhelming number of studies have found that there is a remarkable similarity between brains, regardless of handedness, with the only major difference being that hand control regions are somewhat larger and more active on the side opposite the dominant hand. If there is any general pattern of results that seems to emerge, it is that right-handers seem to have brains that are slightly more asymmetrical, in terms of both their structure

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Right-handers Left-handers

Figure 3 The side of the brain that contains the centers that control language ability for both right- and left-handers is usually the left hemisphere.

and their functional specialization. A larger proportion of left-handers seem to have more symmetrical brains and are more likely to have functions equally represented in both hemispheres. However, in most instances, a researcher presented with only data based on the structural aspects of the brain, or even the brain's function in a variety of circumstances, would be unable to determine if the brain that he or she were looking at was that of a left- or a right-hander.

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