The hemisphere of the brain that specializes in spoken and written language, logic, number skills, and scientific concepts.
The left-brain hemisphere neurologically controls the right side of the body and is connected to the right-brain hemisphere by an extensive bundle of over a million nerve fibers called the corpus callosum. Scientific study of the brain hemispheres dates back to the 1800s. In the 1860s, French physician Paul Broca (1824-1880) observed speech dysfunction in patients with lesions on the left frontal lobes of their brains. Initially, the discovery of specialized functioning of the right and left sides of the brain led to the assumption that all higher reasoning ability resided in the left-brain hemisphere, which was thus regarded as dominant overall. The right-brain hemisphere was thought to possess only lower-level capabilities and was considered subordinate to the left-brain hemisphere.
Interest in the functions of the brain hemispheres was revived in the 1960s, with Roger Sperry's studies of patients who had the corpus callosum severed to control epileptic seizures. It was discovered that each hemisphere of the brain specialized in performing certain types of functions, a phenomenon now known as lateralization . While the left-brain hemisphere performs functions involving logic and language more efficiently, the right-brain hemisphere is more adept in the areas of music, art, and spatial relations. Each hemisphere processes information differently; the left-brain hemisphere is thought to function in a logical and sequential way; the right appears to synthesize material simultaneously. These differences can also be investigated in normal patients (in whom the hemispheres are connected) by temporarily disabling a single brain hemisphere with sodium amytal, a fast-acting barbiturate, and by other means.
Lateralization varies considerably among individuals. Two factors known to affect it are handedness and gender. In one experiment, almost all right-handed persons were unable to speak when their left-brain hemispheres were disabled. In contrast, the incidence decreased to 20 to 40 percent among left-handed people, indicating that only this percentage had their speech centers located in the left-brain hemisphere. Other left-handed subjects appear to use both hemispheres for speech. In general, each gender is known to excel at certain lateralized functions: women are more adept in language-based skills, perceptual fluency tasks (such as identifying matching terms rapidly), and arithmetic calculations. Men are generally more proficient in envisioning and manipulating objects in space. It has also been found that brain function in males is more lateralized than in females. Men who have had one brain hemisphere disabled are more debilitated than similarly affected women. In particular, men display more language difficulties than women when the left hemisphere is damaged. However, it is also known that the sexes are more dependent on different areas of each hemisphere, so the assessment of function after damage also depends on where the damage is localized. In addition, conclusions about lateralization and gender are complicated by the fact that those functions at which members of a particular gender appear to be more adept are often those they are likely to have done more of (such as men manipulating tools), raising the question of environmental as opposed to biological factors.
When the left-brain hemisphere is damaged, the result is often severe aphasia—difficulty using or understanding spoken or written language. Damage localized in the left temporal cortex can cause Wernicke's aphasia, which disturbs the ability to comprehend language. A different condition, called Broca's aphasia, results from damage to the left frontal cortex and interferes with a person's ability to produce language. Persons affected with this disorder experience halting speech, and they often have difficulty recalling even the most familiar words.
Additional methods for studying brain hemispheres include autopsies of cadavers that reveal the location of brain lesions, observation of dysfunction in living patients with known brain lesions, and electrical stimulation of various areas of the brain. Biofeedback instruments have also contributed to the body of knowledge about brain hemispheres; when wired to a research subject, they show a higher electrical discharge from whichever hemisphere is active at a given point in time, while recording alpha rhythms from an inactive hemisphere. Researchers have also made use of the discovery that the eyes will typically move away from the more active hemisphere and toward the side of the body controlled by that hemisphere.
See also Brain