Saturday, November 5, 2011

Transection of the Corpus Callosum

The transection of the corpus callosum is the division of the two brain hemispheres by cutting across the corpus callosum, which is the large bundle of myelinated fibers that connects them. There are no changes in personality or intelligence following the transection of the corpus callosum. The patients are completely normal in everyday life. Only special tests of the tactile and visual systems will reveal any shortcomings. Touch sensation of the left hand is registered in the right hemisphere, that of the right hand in the left hemisphere (in righthanded persons, the dominant hemisphere controls the ability to speak). Visual stimuli affecting the left halves of each retina are transmitted to the left hemisphere, while the stimuli for the right halves are transmitted to the right hemisphere.

An examination shows that right-handed persons with a severed corpus callosum can only read with the left halves of the retinas. They cannot name objects perceived with the right halves of the retina. However, they can illustrate the use of these objects through movements with their hands. The same phenomenon occurs when such persons have their eyes covered and receive an object in their left hand: they are unable to describe it verbally but can indicate its use through gestures. Objects perceived with the right hand, or with the left halves of the retina which are connected to the "speaking" hemisphere, are immediately named. Movements performed with one limb cannot be repeated with the contralateral limb, because the one hemisphere has not been informed about the impulses been sent out by the other hemisphere.