The corpus callosum, a bundle of neurons which connects the right and left hemisphere, is cut, meaning that the electrical overload causing the seizures cannot cross into the other hemisphere. Unfortunately, this means no other information can cross over either.
Vision works like this:
(I rock at microsoft paint =p )
What you see here is an apple in the person's right visual field, and a banana in their left visual field. As you more than likely know, things in the brain are flipped, so that the right hemisphere controls the left side. Similarly, in the eyes, the left visual field in both eyes goes to the right hemisphere to be processed, and the right visual filed of both eyes goes to the left hemisphere, which leads to the right hemisphere processing the banana and the left processing the apple in this picture. (it is NOT the entire right eye going to the left side and vice versa. this is critical.)
Also, recall that the left hemisphere controls language.
As you can see here, this means the banana in the person's left visual field is processed solely in the right hemisphere. If the corpus callosum were cut, the information from the left visual field in the left eye, and from the right visual field in the right eye, would never get to cross over to the proper hemisphere and get processed. This is not a problem, because the information from the left and right visual fields of the other eye do still make it to the occipital lobe as they do not need to cross over. however, since language is controlled in the left hemisphere, information processed in the right occipital lobe would never make it to the left hemisphere to be turned into a word.
This means that a patient with a cut corpus callosum would see the banana and say they do not know what it is (and they're not lying). however, their right hemisphere does know what it is, it just can't put it into words. This means that if you hold up a poster board with a series of pictures, and you allow them to reach out their left hand (controlled by the right hemisphere) and point to one of a series of pictures, they will point to the picture of the banana. Or, if you let them reach their left hand into a bag and feel around amongst different fruits, they'll pull out a banana, all the while professing that they don't know what it is. Their right side knows what they are looking at, and can pick it out from among other fruits, but their left side doesn't know, because the processed information that says "banana" never reached the left hemisphere to be turned into speech.
Similarly, if they turn their head enough to put the banana into their right visual field, meaning the information will be sent to the left hemisphere as well, they will immediately blurt out "banana!"
For all intents and purposes, they have two distinct minds, which cannot communicate. They are two people in one body.
Is that not the coolest thing you've ever heard ever?
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