Two Brains?

Split-brain is a condition that is a result of a highly dangerous surgery known as a corpus callosotomy, where the corpus callosum (the structure containing the connections between the two hemispheres of the brain) is cut as a response to cure severe epilepsy. Epilepsy is a neurological condition where seizures are frequently experienced as a result of “recurrent and unpredictable interruptions of normal brain function” [1], mainly in the form of unregulated electrical activity, especially in the cerebral cortex. It is important to note that corpus callosotomies are done to help control the frequency or severity of the seizures experienced, especially after drug treatment has proven unsuccessful [2], however is not a procedure done to cure a person’s epilepsy [3]; the procedure is most commonly done for “atonic seizures (‘drop attacks’ [a sudden loss of muscle tone is lost, and the person, usually a child drops to the floor] [2]), tonic-clonic seizures, and tonic seizures” with frequency of seizure reduced nearly 80-90% [4].

Although corpus callosotomies involve severing the corpus callosum, it does not halting all communication between the two hemispheres; there are various other commissures, neuron bridges, that connect the two hemispheres [5]. (The corpus callosum is the largest commissure containing over 200 million nerve fibers connecting the two hemispheres [6].) Because the corpus callosum is the largest of the commissures, the brain’s ability to communicate becomes restricted. This inability is created because the brain has developed in such a way, where specific functions of the brain are localized to one side of the brain. This localization is exhibited best when considering language; the Broca’s area and the Wernicke’s area, the two major language centers in the brain, are both located in the left hemisphere of the brain [7]. Conversely, the right brain is attributed with the functions of problem perception and spatial relationships. Another important distinction that should be known about the brain organization when considering split-brain patients is that each hemisphere receives input and provides outputs for the opposite side of the brain; therefore the left hemisphere controls the right side of the body, and the right hemisphere controls the left side of the body [6].

Because of the localization between functions between hemispheres, split-brain patients have been studied greatly to help understand the unconscious mind and the relationship between the two hemispheres of the brain. In studies done by Michael Gazzaniga, he asked split-brain patients to focus on a point and proceeded to flash images or words on either the left of the right side of the point, allowing for the right brain or the left brain, respectively, to see the images. If the image or word was flashed on the right side, the patient could easily tell Gazzaniga what had just been displayed; however if the word or image was displayed on the left side of the point, the patient could not verbalize what he saw, yet was capable of drawing the image correctly, then properly identify the item once the left brain had seen the image, yet the patient was completely unaware of why he drew the item. Frequently the patient reasons why he drew a certain item, but only after the left brain has seen it, because it is in the left brain that reasoning takes place [8]. Gazzaniga reports similar findings, of the right brain being able to understand the stimulus, but the patient being incapable of verbalizing among other senses, namely: touch, smell and sound [5].
Split-brain research, similar to the one just mentioned by Gazzaniga, has revealed a great deal of specialization found in each hemisphere, as within each hemisphere there is a degree of localization of all of the structures for certain tasks in one area. For example, there is a cluster of functions in the left hemisphere that relate to problem solving, whereas the right hemisphere is lacking in comparison [5]; this might not come as a surprise because problem solving is more related to language than it is to spatial perceptions, but the consolidation within the left hemisphere shows the spatial organization allowing for more efficient message sending, as it makes sense to have structures with the same purpose to be in the same general region.

Split-brain patients are frequently the subjects of research as their unique condition can reveal a lot about the mysteries of the brain. Because each brain is essentially operating independently, we can discover a great deal of information about the brain that before was not capable. As technologies advance and drugs become more effective at treating epilepsy, fewer and fewer split-brain procedures are being performed, so it seems as though the window for split-brain research may soon be closing. Nonetheless, the surgery and the experiments we have performed on split-brain patients prove to be telling of the great deal of sophistication of medical practice and present interesting questions about where the field of neurology can go as a result of the groundbreaking discoveries about brain specialization as a result of split-brain procedures and experiments.

[1] Fisher R., van Emde Boas W., Blume W., Elger C., Genton P., Lee P., Engel J. (2005). "Epileptic seizures and epilepsy: definitions proposed by the International League Against Epilepsy (ILAE) and the International Bureau for Epilepsy (IBE)." Epilepsia 46 (4): 470-2.
[2] Robinson, R. (2007). Corpus callosotomy. http://www.surgeryencyclopedia.com/Ce-Fi/Corpus-Callosotomy.html (cited 10 May 2008).
[3] Carcicone, J. R. (1 Feb 2007). Epilepsy health center. http://www.webmd.com/epilepsy/corpus-callosotomy (cited 10 May 2008).
[4] Weiner, H. L. (8 March 2004). Corpus callosotomy. http://www.epilepsy.com/epilepsy/corpus_collostomy (cited 10 May 2008).
[5] Gazzaniga, M. S. The split brain revisited. http://cwx.prenhall.com/bookbind/pubbooks/morris4/medialib/readings/split.html (cited 10 May 2008).
[6] veronchiquita (Poster.) (29 Sept. 2007). Split-brain patients. http://www.youtube.com/watch?v=MZnyQewsB_Y&feature=related
[7] (8 May 2008). Cerebral hemisphere. http://en.wikipedia.org/wiki/Cerebral_hemispheres (cited 10 May 2008).
[8] Neuroslicer (Poster.) (18 Apr 2007). Split-brain behavioral experiments.
http://www.youtube.com/watch?v=ZMLzP1VCANo