Specialization of the Cerebral Hemispheres
If you were to hold the human brain in your hands, the two sides would appear remarkably similar. In addition to the basic anatomical similarities, some of the basic functions of each hemisphere are mirrored in the opposite side as well. However, each of the cerebral hemispheres is specialized for certain abilities or functions.
For example, Paul Broca (1824–80) helped determine that the brain's language centers are primarily in the left hemisphere by studying patients with damage to relatively specific areas of the brain — damage that resulted in speech problems. Patients with damage to the lower left frontal lobe, or what is now known as Broca's area, are able to understand written or spoken language, but they have difficulty speaking or writing.
Carl Wernicke (1848–1905) discovered that when another area in the left hemisphere was damaged, patients were still able to speak fluently — but what they said often made little or no sense. Today, this area of the left temporal lobe is known as Wernicke's area.
Wilder Penfield (1891–1976) took a more sophisticated approach called electrical brain stimulation, in which tiny and harmless electrical currents are sent through areas of the brain to see what thoughts or other behaviors result. His participants were people who were undergoing necessary brain surgery.
Roger Sperry (1913–1994) utilized a different approach. His participants were people who had their corpus callosum severed as a treatment for severe epilepsy, which has the effect of isolating the hemispheres. Sperry then presented various types of stimuli to only one hemisphere at a time, demonstrating that the hemispheres aren't exactly mirror images. Each side of the brain performs some functions better than the other.
Research on hemisphere specialization led some to speculate that differences in ability might result from being predominantly a “leftbrain” or “right-brain” person. For example, a left-brain person was said to be better at logical reasoning, a right-brain person at artistic or creative endeavors. However, research generally has not borne out this notion.
In the years since, numerous researchers also have used systematic brain lesions (tiny cuts) to study changes produced in laboratory animals such as rats. This research, for example, yielded evidence that areas of the hypothalamus are involved in homeostatic behaviors such as eating. Rats with lesions to one area won't stop eating even when they become obese. Rats with lesions to another area won't eat at all. Still other lesions to the hypothalamus increase or decrease sexual interest.