Human Responses to the Human Split Brain :: Biology Essays Research Papers

When neuroscientists first made direct contact with the right hemisphere of the brain, during neurological tests of "split brain" human subjects, it was as if they had found intelligent, albeit speechless, life on Mars. At a time when brain imaging techniques were crude or nonexistent, the only way to observe and communicate with the brain's right hemisphere unimpeded by the left hemisphere was by testing split brain subjects (1). The right hemisphere, previously supposed mute, illiterate, mentally retarded, and completely subordinate to the left hemisphere, had a mind of its own (1). While it could not speak, it could respond to commands and questions via its contralateral control of the left hand. It had different abilities and even opinions and emotional states than the neighboring left hemisphere (2). These discoveries led to a model of hemispheric specialization of normal human brain function, with an analytic, verbal, problem solving left hemisphere and a visuospatial, syntheti c, creative right hemisphere (1, 2). The formation of this model in turn offers insight into the brains of the observers as well as the observed. The observers' behavior supported some of their own hypotheses about the human brain, split or unsplit. The term "split-brain" is commonly used to describe a person whose corpus callosum has been surgically severed (3). The corpus callosum, comprised of approximately 200 million neuronal fibers connecting the left and right hemispheres of brain, exists only in mammals' brains, and is largest in human brains (1, 3). Until the 1960's neuroscientists were unsure what purpose the corpus callosum served (3). By observing deficits in split brains' functions, scientists could better assess the corpus callosum's function (1). Roger Sperry and his colleagues pioneered the operation severing the corpus callosum, known as callosal commisurectomy, in the 1960s, as a last ditch effort to control the seizures of life threateningly severe epilepsy by creating a fire wall to prevent electrical impulses from traveling between hemispheres (1). This treatment was successful, and after recovering from the surgery, the split-brain patients appeared normal in every day interactions and even during a routine physical exam (1). However, Sperry and his colleagues, after extensive and specific neurological tests of split brain patients, posited that the corpus callosum communicated stimuli and responses between the two hemispheres, each specialized for different cognitive functions (1). Using a tachistoscope, Sperry delivered visual stimuli to a single visual field of the subject (1). He discovered that, with the exception of olfactory stimuli, the hemispheres of the brain receive sensory stimuli and exercise motor control contralaterally (1, 3, 5).