Sperry and Gazzaniga's Split-Brain Experiments

Beginning in the 1960s the neuroscientist Roger Sperry and his graduate student Michael Gazzaniga at the California Institute of Technology conducted a series of experiments on patients who had undergone surgical severing of the corpus callosum, the thick band of nerve fibers connecting the brain's two hemispheres, typically performed as a last-resort treatment to control severe epilepsy by preventing seizures from spreading between the hemispheres. Because the visual system is wired so that information from the left half of each eye's visual field reaches only the brain's right hemisphere and information from the right half reaches only the left hemisphere, Sperry and Gazzaniga were able to present words, images or objects to a split-brain patient's left or right visual field in isolation and observe that each hemisphere could respond only to what it alone had seen, since the surgery had cut off the direct route by which the hemispheres would ordinarily share this information with each other. Their experiments showed that in most patients only the brain's left hemisphere could verbally name or describe what had been presented, since language is typically concentrated there, even when the right hemisphere had clearly perceived the stimulus and could indicate its response nonverbally, such as by pointing with the left hand. The split-brain research demonstrated that the two cerebral hemispheres are substantially specialized for different kinds of processing and, more controversially, led Sperry and Gazzaniga to argue that each disconnected hemisphere might support its own separate stream of awareness, a finding that helped establish the field of cognitive neuroscience and earned Sperry a share of the 1981 Nobel Prize in Physiology or Medicine.