Different brain regions have specific functions. Before performing any surgery on the brain, including surgery for the treatment of epilepsy, the surgeon seeks to understand the functions of the areas affected by the seizures or of the lesion (abnormal tissue). All of the surgical planning is done to preserve important functions such as speech, comprehension, sight, movement, or sensation, and to lessen the risk of loss of function from the surgery.

The task of identifying the functions of different regions of the brain is called brain mapping. Over about the last hundred years, scientists and physicians have gathered large amounts of information, or maps, of the brain regions generally involved in specific functions. At first they learned from studying the functioning of people who had injuries in certain areas of their brain (from accidents or strokes, for instance). The exact location of various functions differs quite a bit from person to person, however, and the presence of tumors, seizures, or other brain abnormalities may distort these maps so that general rules do not apply. Therefore, surgeons and physicians need a way to identify the precise locations of functions within each patient’s brain. This is now done by using direct electrical simulation, a process called electrical brain mapping.