Split Brains and Hemisphere Specialization

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One way of seeing differences between the two hemispheres comes from cases of patients with “split-brains”. These patients have had their corpus callosum severed in order to prevent the spread of epileptic seizures. While this process effectively stops the seizures, it also creates a few interesting side effects of having two hemispheres which are no longer able to communicate with one another.

This doesn’t have a major effect on the patients’ lives, as most visual and auditory information is received by both hemispheres, but Roger Sperry and colleague Michael Gazzaniga devised a number of clever experiments to demonstrate some of the specializations of each hemisphere, for which Sperry was awarded the Nobel Prize in 1981. As mentioned when discussing Broca’s work on aphasia, speech production is predominantly located in the left hemisphere. This means that split-brain patients are able to talk about information that is in their left hemisphere, but they are unable to verbalize information that is solely in the right hemisphere.

If a split-brain patient were blindfolded and a key were placed in her right hand, she would be able to recognize it and verbally identify it because that sensory information would be in her left hemisphere (remember contralateral control). If, on the other hand (literally), the key were placed in her left hand, she would not be able to say what it was. She would, however, be able to point to the object or even draw it using the left hand. This shows that the right hemisphere is able to have knowledge that cannot be verbally expressed without the help of the left hemisphere.

If a split-brain patient focuses his vision on the center of a screen, anything to the left of the focal point (left visual field) goes to the right hemisphere, while anything in the right visual field goes to the left hemisphere. Note that this division is by visual field not by eye; each eye sends information to both hemispheres, with the left half of what that eye sees going to the right hemisphere and the right half going to the left hemisphere. By selectively presenting images to a particular hemisphere and then assessing the patient’s response, we can clearly see how some tasks are localized to that hemisphere.

saw + hammer

For instance, if the words above were flashed on a screen (with the cross as the center focal point), the split-brain patient would say that he saw the word “hammer”, yet if he were asked to draw what he saw with the left hand (right hemisphere) he would draw a picture of a saw. In a similar study, Michael Gazzaniga presented images of faces made out of objects and showed that split-brain patients are only able to recognize the faces when they are presented to the right hemisphere, indicating that this hemisphere is specialized for face recognition.

Unfortunately, the popular press frequently exaggerates the importance of these hemispheric differences as though we had to choose which hemisphere to use for each task. It’s certainly not the case that artists are “right-brained” or that mathematicians are more “left-brained”. These differences in skills and preferences cannot be so easily explained by which hemisphere they use. The truth is that we all use both hemispheres all the time, as most of our behaviors are complex enough to require many areas of brain activation in both hemispheres simultaneously. So while it is true that some processes (like speech production or face recognition) are generally confined to particular hemispheres, this doesn’t mean that your left hemisphere is twiddling its thumbs while you look at a face or that your right hemisphere shuts down while you’re speaking. Both hemispheres are constantly working and processing information, and assuming that you have an intact corpus callosum, they’re also constantly letting each other know what’s happening on the other side.

This post is an excerpt from Master Introductory Psychology: Complete Edition