Split brain is a term used to refer to someone who has had the corpus callosum (a white fiber tract connecting the two sides of the brain, discussed in the last post) severed. Therefore, the two sides of the brain cannot communicate with each other. The reason that this would be done is usually in very severe cases of epilepsy. Seizures are caused by brain activity synchronizing at extremely high levels. Therefore, you can prevent seizures by preventing the whole brain from being able to synchronize its neuronal activity.
Split brain patients act entirely normally during normal life. It is only when doing tasks, such as those that only use one side’s visual field, that you can notice abnormal behavior. The left side of the brain contains all of the language centers. Therefore, whatever is in the left visual field (above: “RING”) can be spoken aloud. The right side of the brain is more abstract- relating to more “artistic” things- like spatial reasoning. Therefore, whatever can be seen in the right visual field (above: “KEY”) can be drawn or picked out, but it cannot be verbalized. This becomes extra cool when you ask the patient, “Why did you pick up the key?” In asking verbally, you are talking to the left side of their brain, which did not see the word KEY (They will say they only saw the word RING). Instead, they will come up with some logical reason (For instance, they might say something like “Oh, I was thinking about when I get to go home- I’ll need to grab my keys.” or “I liked the texture of the key.” or “I saw RING, and thought of my key ring.”). The reason depends on the person, but the mind can come up with a reasonable explanation (albeit inaccurate) for any behavior. Split brain research helped us to realize that this sort of reasoning happens in the left hemisphere.
Additionally, the right hand (which receives input from the left brain) is terrible at drawing or spatial reasoning tasks (matching a pattern of blocks for instance) in split brain patients who don’t have the normal communication, which helped us to learn that this behavior is localized to the right hemisphere. I was lucky enough to see a talk by Gazzaniga who is one of the pioneers of split brain research a few years ago. He showed a really interesting video of a patient having to put together a set of blocks in a pattern. He had them sit on the left hand and do the task with the right. The right hand would just make a mess of the blocks. The left hand of one patient kept trying to come in and fix what the right hand was making such a mess of. Kind of like a “No, no, you’re doing it wrong- let me!” response. It was really fascinating. I’ll show more about this in my next post.