SPLIT BRAINS: EFFECTS of DIVIDING THE CORPUS CALLOSUM
The corpus callosum is a wide, flat bundle of axons that run between the two hemispheres and allows each side of the brain to communicate with the other. It consists of over 200 million axons and is the main commissure of the brain. This tutorial concerns the consequences of cutting through the whole of the corpus callosum.
In the 1960s, surgical section of the corpus callosum was performed on some epileptic patients in an attempt to stop epileptic seizures spreading from one side of the body to the other.
The complete division of the corpus callosum resulted in some interesting observations regarding how the two hemispheres function when they have to work independently of each other.
In the diagram, the left hemisphere sees the word 'Ring' and the voice, controlled by that hemisphere says that is sees 'ring'. In contrast the right hemisphere sees the word 'key', and the left hand, controlled by the right hemisphere, searches around behind the screen for a key, and is unaware of the fact that the visual input to the left hemisphere is 'ring'.
Normally both sides of the brain would know what is being fed into the opposite side, through the callosal connections.
The experiment made use of the fact that speech is controlled by the left hemisphere in most people. So, following section of the corpus callosum, events in the left hemisphere might be examined by asking the individual to speak about the objects in the right visual field
Similarly, visual events in the left visual field are transmitted to the right hemisphere, which also controls the left hand. So, following transection of the corpus callosum, the left hand can be used to seach for objects introduced to the left visual field.
It is known that many people have a speech controled dominantly by the left hemisphere. The logic was to present different information to each hemisphere and get responses from each of them. As the left hemisphere deals with speech, the spoken voice would respond to stimuli to that hemisphere.
The response of the right hemisphere was indicated by the left hand. So visual stimuli appeared on a screen in the left and right sides of the visual field. Words in the right of a screen feed into the left hemisphere; those in the left visual field feed into the right hemisphere.
In the diagram, the subject is asked to focus both eyes on the black spot on the join in the photo.
The left hemisphere sees a man and says so. The right hemisphere uses the left arm to chose between the sexes, and indicates that the image is of a woman's face.