The corpus callosum is the bundle of never fibers that connect the two hemispheres of the brain. It’s the largest single structure in the brain, with some two hundred million fibers. As a last resort for epilepsy, this bundle can be cut in a procedure known as a callostomy. When this happens, a split-brain patient can occur.

What is interesting is that these split-brain patients do not appear outwardly abnormal. There is not any indication that they have a severed corpus callosum if you saw them walking down the aisle at your local supermarket. They do not make weird facial expressions or odd gestures, they do not walk or speak ‘funny.’ They seem just like you or me.

But the evidence for their disorder abounds in the lab.

When the corpus callosum is severed, the two hemispheres of the brain, the left hemisphere (LH) and the right hemisphere (RH), cannot communicate well (this is why cutting the corpus callosum works in epileptic patients: the electrical discharges remain confined to one hemisphere instead of spreading between the two). Language is usually localized in the LH, while abstract thinking is the domain of the RH. (The whole thing about left brain is logic and right brain is creativity is true, but not really. The hemispheres are important for certain tasks, but overall, the whole left-brain-right-brain thing is overgeneralized and exaggerated. Our brains are far more complex than that).

One experiment involved a patient named VP. VP sat in front of a screen that played movies to the RH. The movie was a violent one, with people getting pushed off balconies and firebombed. But after watching the movie, VP could only remember this much: “a white flash, perhaps a few trees but definitely no people.” But when asked about feelings or emotions, VP said this: “I don’t really know why, but I’m kind of scared. I feel jumpy. I think maybe I don’t like this room, or maybe it’s you, you’re getting me nervous.”
The RH experienced the emotions (nervousness, fear) and processed them, but with the severed corpus callosum hindering interhemispheric communication, the left hemisphere was unable to figure out the source of the emotion.

Another experimenter flashed laugh to the RH of a split-brain patient. The patient laughed. Remember, language is a LH task. When the experimenter asked the patient what they were laughing at, the patient responded, “Oh, you guys are really something.” The LH gave a verbal explanation for the laughing. In this case, the LH was trying to interpret the laughing and trying to understand its cause, even though it actually did not. The LH, as the experimenter explained, is aware of what the person is doing and tries to interpret from there, rather than understanding the actual cause for the behavior.

Split-brain patient ‘Joe’ was tested in an experiment about visual fields. In the experiment, Joe sits in front of a computer. The computer screen has a dot in the center of it where Joe is told to stare at. When Joe focused on this central dot, everything on the right side of the screen is processed in his LH (remember, the right side of the brain controls the left side of the body, and vice versa). Since the LH is dominant for language, Joe should be able to read or say what he sees. The rest of the screen is a blank white screen. In intervals, words and images are flashed on the right side of the screen (i.e. to the right of the central dot). When Joe sees a word or an image, he reads or says it aloud to the experimenter.

For example, one of the words was tree. Joe would read car aloud. One of the images was a bundle of purple grapes. Joe told the experimenter grapes.
What’s interesting is when a word, for example, pan, is flashed to the left of the dot, Joe isn’t able to say what it is. He just says that he didn’t see anything. What gets even more interesting, though, is when he is told to draw what he says he didn’t see. He does so, drawing with his left hand, which is controlled by his RH. The result is a drawing of a pan. Then when Joe is asked what it is that he drew, he says pan.
What happened is that when the words or images were flashed to the left side of the dot, the information would go to his RH, which is not involved in language. Therefore, Joe was not able to say what an object was, because his RH from disconnected from the language-oriented LH.

Don’t worry, it gets stranger.

At one point, a picture of a saw was presented on the left side of the screen, and a hammer was shown on the right side if the computer screen. Joe says he saw a hammer.
He never says anything about the saw. But when the experimenter tells Joe to close his eyes and draw with his left hand, guess what he draws? The saw. And guess what he saws when he is asked what it is that he drew? That’s right, a saw.

Experimenter: “What did you see?”
Joe: “Hammer.”
Experimenter: “What did you draw?”
Joe: “Saw.”
Experimenter. “What did you do that for?”
Joe: “I don’t know.”

The effects are not just seen with patients staring at computer screens. Take a blindfolded non-split-brain individual and put a ball in their left hand. Their RH knows what the ball is, and this information goes to the LH, which is able to verbalize and say that a ball in is the person’s hand. But do the same thing with a split-brain patient, and they will not be able to say what the object in their hand is. They know what it is, what to do with it, can draw it, but they cannot give you its name.
Lingual description is out of their reach.