One of the most striking features of the
brain is its bilateralism, or organization into the largely symmetric
left and right cerebral hemispheres. Each
hemisphere is independently capable of processing and storing information.
Evidence for Bilateralism
In humans and other mammals the corpus
callosum is the main pathway of interhemispheric communication. The callosum is the largest fiber tract in the
human brain, containing more than 200 million nerve fibers (axons). Its critical role was demonstrated by Ronald Myers and Roger Sperry in the 1950s, when they showed (in cats) that information reaching one half of the brain was unavailable to the other half when the callosum was absent. In the 1960s in Sperry's laboratory, Joseph Bogen and Peter Vogel cut the callosum in a group of epileptic human
patients in an effort to control their otherwise unmanageable seizures. The psychological follow-up of the patients by Michael Gazzaniga, Sperry, and Bogen confirmed the earlier animal studies.
Hemisphere Specialization
Studies of the left and right hemispheres in human beings have revealed the psychological uniqueness of the separate hemispheres. The left hemisphere is normally dominant for language functions. The right one seems to be better equipped for handling spatial and other
nonverbal relations. Such observations have led to theories suggesting hemispheric specialization through evolution. For example, investigators such as Gazzaniga and Joseph LeDoux suggest that human hemispheric differences can be accounted for in terms of the evolutionary acquisition of language by one hemisphere. The other hemisphere continues to process information essentially as it did in prehumans. The superior performance of the right hemisphere on certain nonverbal tests would then be attributable to the sacrifice of nonverbal processing efficiency by the left hemisphere because of having acquired language.
Research shows that double consciousness exists in split-brain patients. Perhaps a fully integrated consciousness does not develop until a child is several years old. Research shows that the fibers of the corpus callosum do not begin to mature until one year after birth, and that the process continues until the age of ten or older. The corpus callosum has also been found to be about 11% larger in left-handed and ambidextrous than in right-handed people.
More abstracts about the BRAIN BILATERALISM