A patient with a split brain, when shown an image in his or her left visual field (the left half of what both eyes take in, see optic tract), will be unable to vocally name what he or she has seen. This is because the speech-control center is in the left side of the brain in most people, and the image from the left visual field is sent only to the right side of the brain (those with the speech control center in the right side will experience similar symptoms when an image is presented in the right visual field). Since communication between the two sides of the brain is inhibited, the patient cannot name what the right side of the brain is seeing. The person can, however, pick up and show recognition of an object (one within the left overall visual field) with their left hand, since that hand is controlled by the right side of the brain.
The same effect occurs for visual pairs and reasoning. For example, a patient with split brain is shown a picture of a chicken and a snowy field in separate visual fields and asked to choose from a list of words the best association with the pictures. The patient would choose a chicken foot to associate with the chicken and a shovel to associate with the snow; however, when asked to reason why the patient chose the shovel, the response would relate to the chicken (e.g. "the shovel is for cleaning out the chicken coop").
My head, let me show you it.
Now: If you're not a brains person, this won't mean jack to you. It's a very big deal, though, for a couple of reasons: first, the corpus callosum has a huge blood supply coming from (if memory serves) three different arteries. Second, infarcting (cutting off the blood supply) it means that you end up with something called split-brain syndrome.
See, the C.C. is the part of your brain that ties everything together. It's a big ropy mass of white matter--the stuff that has the thinking bits of your brain in it--that acts as a communication corridor between the right and left sides of your brain. It allows you, for instance, to be able to recognize a chicken, describe that chicken, name that chicken (sounds like a bad game show title), and talk about what sorts of implements you'd need to take care of said chicken. Besides a knife and fork, I mean.
Occasionally we'll cut the corpus callosum as a last-ditch attempt to stop a really horrible case of epilepsy. That's usually done on children who simply can't stop seizing, no matter what we do pharmacologically, and it's usually done at a very young age, because the ability to overcome that callosotomy, as it's known, decreases as you get older. If it's done too late, or if it happens later in life as the result of a stroke or other injury, you end up with split-brain syndrome.
I'll bring back the chicken and let Wikipedia explain what split-brain syndrome does:
Obviously, it's going to be a drag if your C.C. gets all jacked up. It's hard, given the other problems this dude has (like an almost total lack of white matter, thanks to diabetes and hypertension and a bunch of other stuff), to get a really clear read on how it's affecting him and whether or not he even notices. We thought that he had some left-sided neglect (more on that, per Pens's request, in a later post) combined with a major psychiatric overlay (given his history). It wasn't until we saw the MRI, with the corpus callosum lighting up like Christmas, that we realized what, exactly, the problem was. Golly!
(Yes, I actually said "Golly!" upon seeing the guy's MRI. Sometimes I say "Gee whiz!" as well. Ironically, of course. Because I'm hip.)
Now I'm left with a wet, cold chicken who's been in a snowy field and has to be returned to her cozy coop over at Wiki.