The bisected brain
- M. S. Gazzaniga
The corpus callosum, the cerebral commissure that interconnects the left and the right half brain, had been considered an enigma to neurologists and neurosurgeons in the 1940's and 50's. I t was the structure discussed most often when an example was sought to show how little was known about the brain. Even though it is the largest nerve tract in the brain, which in man contains more than two hundred million neurons, it was generally believed it could be sectioned and destroyed and have no apparent consequences for behavior. People continually quipped about the general lack of importance of the forebrain commissures. I t was in this context that the original experiments on the split-brain were carried out in the cat by Ronald Myers and Roger Sperry at the University of Chicago. Myers had successfully developed the technique of spliting the optic chiasm, thereby allowing visual information presented to the right eye to be exclusively projected to the right hemisphere and visual information projected to the left eye to be exclusively presented to the left hemisphere. He had discovered in follow-up behavioral tests that there was, nonetheless, interocular transfer. Therefore the logical structure to next surgically section to see whether one could block interhemispherie transfer of visual information was the corpus callosum. This was done and this study gave birth to the now classic "split-brain" experiments first done on the cat showing that discriminations trained to one side leaves the other half of the brain naive. Later studies carried out in both the monkey and chimpanzee confirmed these findings. Still, however, these results stood in marked contrast to the earlier results of Akelaitis who had studied a series of some 26 patients with the corpus callosum and anterior commissure sectioned in complete or in part for the interhemispherie control of epileptic seizures. In an extensive series of studies he purported to show that sectioning of these structures did not result in any significant neurological or psychological sequela. This point was made and emerged as the dominant view even though there were several contradictory reports in the literature showing disconnection effects as a result of having the callosum sectioned or rendered non-functional by a tumor or the like (see Geschwind, 1965). I t was also generally considered that cutting the callosum did not, in fact, help control epilepsy.