The Split Brain: Making Two Hemispheres Whole
Admit it! You’ve taken one of those online quizzes to see if you’re more “right-brained” or “left-brained.” Too bad it’s all bunk! Popular culture would have you believe that creative and artistic people are right-brain dominated, while logical, methodical, and analytical people are left-brain dominated. The fact of the matter is that you use all parts of your brain equally, no matter your personality traits! So, how did this myth even come to be?
Much of what we know about brain function comes from the work of Roger Sperry, who studied how the human brain’s hemispheres work independently. Normally, the two halves of your brain are connected via the corpus callosum, a huge bundle of nerve fibers. So, how do you even study the hemispheres separately? Well, in the 1960s, there was no cure for people with severe types of epilepsies except to cut the corpus callosum, thus disconnecting any cross-talk between the two halves of the brain. After their surgery, patients who underwent this procedure were able to live a normal life, aside from some odd behaviors…
Dr. Sperry would ask a patient to fixate on a dot on a screen directly in front of them, and then he would flash an image on either the right or left side of the screen. Because each half of the visual field projects to opposite sides of the brain, crossing at the optic chiasm, it is possible to project a picture to either the right or the left hemisphere. So, here’s where it gets interesting! When Dr. Sperry flashed the image of a toothbrush on the right screen, the split-brain patient quickly responded that it was a toothbrush, but when the same image was flashed on the left screen, the patient said that he didn’t see anything at all! Then, if he was asked to pick out an object among an assortment in front of him using only his left hand, he could pick out the correct object every time! How? This is because the image of the toothbrush on the left screen was processed by his right hemisphere, which controlled his left hand! Moreover, if the patient was asked again what the object was, he could not vocally name what it was! These experiments showed that language processing is organized in the left hemisphere. Indeed, years later, two language areas of the brain were identified in the left hemisphere: Broca’s and Wernicke’s areas, but this is a discussion better left for another article.
Now, what happened when the patient was presented with stimuli in both visual fields? Dr. Sperry chose to flash “key-ring” across the whole screen, such that “key” was on the left side and “ring” was on the right side of the screen. When asked what they saw, the patient said “ring”, but his left hand gestured at a key in front of him, and not a ring or a key-ring! Both hemispheres were trying to answer the question, but they were not able to put it together.
So, what does this mean for the state of consciousness of these split-brain patients? It is undoubtedly disunited, making them seem like two separate individuals within one being. More interestingly, does that mean that we humans with intact cospus callosi actually have two separate streams of consciousness? Perhaps the communication between the two hemispheres effectively masks them, so we can experience the world as a whole. I leave that philosophical debate for you. Let us know what you think by leaving us a comment below!
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For more information about The Split-Brain Experiments, play the game at NobelPrize.org!
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Images from Wikimedia Commons, adapted from CNS NYU, and made by Jooyeun Lee.
