• Krisha Janaswamy

Split Brain Syndrome

--Krisha Janaswamy

Split-brain is a term used to describe people whose corpus callosum, a large bundle of myelinated nerve fibers connecting the left and right hemispheres of the brain enabling the transmission of signals and communication between the two hemispheres, has been cut (usually as a last resort) to alleviate medically intractable epilepsy. A corpus callosotomy is a surgical treatment for epilepsy, a chronic condition that causes recurrent seizures; it may affect both children and adults. While its normal function is to mediate communication between the two hemispheres, it also gives way for seizures to spread. Although the seizures will still occur on the side of the brain they start, cutting a part or all of the corpus callosum greatly reduces the spread. After surgery the seizures are less severe because they affect only one side of the brain. It also reduces the frequency of “drop attacks” due to certain types of generalised seizures (atonic seizures).

Illustrated by Avani Gupta

Roger Sperry, an American neuropsychologist received the 1981 Nobel Prize in Physiology or Medicine for his split-brain research. Sperry began his research in the late 1950s to determine the function of the corpus callosum. He predicted that there should be major consequences for cutting the brain structure as the corpus callosum was large, connected the two hemispheres of the brain and must have an important function. He noted that humans with severed corpus callosums did not show any significant differences in functioning than a human with an intact corpus callosum.

At the time, it was known that the left hemisphere of the brain was responsible for movement and vision on the right side of the body while the right hemisphere of the brain was responsible for movement and vision on the left side of the body. So Sperry designed experiments in which he could control what each eye saw and thus, what information was going to each hemisphere. To test how severing of the corpus callosum might affect mammals, he cut the corpus callosum of many cats and had them perform tasks that involved their vision and response to a visual stimulus. He closed one of their eyes with an eye patch and showed it two wooden blocks with different patterns, a cross and a circle, one of which had food under it. He then switched the eye patch (he would switch the eye patch to either eye depending on what visual field he wanted the cat to see) to the other eye of the cat and put the food under the other block. The cat memorized these events separately and was unable to distinguish between the two blocks when both its eyes were open.

Sperry performed the same experiment on monkeys but made them open both eyes simultaneously, which was possible with the help of some light filters and special projectors. The split-brain monkeys memorized two mutually exclusive scenarios when a normal monkey memorized only one; this seemed like a benefit and Sperry questioned if there were any adverse effects. He concluded that with a severed corpus callosum, the hemispheres do not communicate and each hemisphere acts as the only brain.

Next, Sperry performed a similar experiment on human volunteers that had a split corpus callosum. He showed a word to one of their eyes and quickly found that the humans could only remember the word they saw with their right eye. He then showed them two different objects, one to their right eye only and the other to their left eye only and asked them to draw what they had seen. All the participants described what they had seen with their right eye and drew what they had seen with their left eye. This supported Sperry’s hypothesis that with a cut corpus callosum, the hemispheres do not communicate, neither acknowledges the existence of the other hemisphere and each acts as the only brain since the description of the object did not match the drawing. Sperry thus concluded that the left hemisphere of the brain was responsible for recognizing and analyzing speech, a function that the right hemisphere could not perform. Sperry discovered that while the right hemisphere could understand words, it could not articulate it and the left hemisphere was responsible for language comprehension and articulation.

Even though there are no apparent signs of disability in people with a severed corpus callosum the inability of the hemispheres to communicate compromises the full function of the brain.

Complete sectioning of the corpus callosum can cause some permanent deficits. When the individual’s eyes are closed, one hemisphere does not cooperate with the other even to perform simple tasks so the left and right extremities may carry out conflicting movements. These complications are called disconnection syndromes. The individual can compensate for this problem by keeping their eyes open.