The human brain appears bilaterally symmetrical, particularly the cerebrum, which consists of two extremely similar-looking hemispheres. However, it has been known since the early 1900s that this symmetry does not extend to brain function. Much of what is known of the differences in hemisphere function comes from two sources: studies of accident victims with localized damage to one hemisphere and studies of patients who have had the corpus callosm severed. This surgical procedure was performed in rare cases of uncontrollable epilepsy to prevent the spread of seizures through the brain.

Studies based on selective damage to the left cerebral hemisphere had led to the belief that the right hemisphere was relatively retarded, lacking the ability to speak, write, recognize words, or reason. For example, people suffering damage to localized areas of the left hemisphere, but not the right, often became unable to speak, read, or understand spoken language. In addition,the left hemisphere for most people is superior in mathematical ability and in logical problem-solving tasks.

Roger Sperry, of the California Institute of Technology, worked with people whose hemispheres had been surgically separated by cutting the corpus callosum. In his studies, Sperry made use of the knowledge that axons within each optic nerve follow a pathway that causes the left half of each visual field to be projected on the right cerebral hemisphere, and vice versa. Through an ingenious device that projected different images onto the left and right visual fields, he and other investigators have gained more insight into the roles of the two hemispheres. If he projected an image of a nude figure onto the left visual field only, the patients would blush and smile but would claim to have seen nothing, because the image had reached only the nonverbal right side of the brain! The same figure projected onto the right visual field was readily described verbally. These experiments, begun in the 1960s and refined since then, have revealed that the right side of the brain is actually superior to the left in several areas, including musical skills, artistic ability, recognition of faces, spatial visualization, and the ability to recognize and express emotions. For his pioneering work, Sperry was awarded the Nobel Prize in 1981.

Recent experiments indicate that the left-right dichotomy is not as rigid as was once believed. Patients who have suffered a stroke that disrupted blood supply to the left hemisphere typically show symptoms such as loss of speaking ability. Frequently, however, training can partially overcome these speech or reading deficits, although the left hemisphere itself has not recovered. This fact suggests that the right hemisphere have some latent language capabilities. Interestingly, female stroke victims recover some lost abilities more often than males, and females have a larger corpus callosum. These findings suggest a sex difference in the degree of specialization of the two hemispheres and the extent of their interconnections. Further evidence of this difference has recently been provided by sensitive techniques that allow imaging of neural activity in the brains of normal subjects performing various mental tasks. When subjects were asked to compare word lists for rhyming words, a specific region of the left cortex of male subjects became active, but in female, similar areas in both left and right hemispheres were activated.