Split-Brain Surgery and its Effects on Behavior

2/2/03

Split-brain surgery is covered on pages 76-77 of your textbook.

What is a split-brain surgery?

Split-brain (corpus calloscotomy) surgery involves the cutting of the corpus callosum. The corpus callosum is the region of the brain that connects the left and right hemispheres. Millions of axons run from one side of the brain to the other via this region. This allows the left side to communicate with the right side. When a split-brain surgery is performed, the left and right hemispheres can no longer fully send messages to one another.

Why would one have a split-brain surgery done?

Split-brain surgery is done as a radical procedure to treat severe epilepsy. With epilepsy, some abnormal neurons begin firing uncontrollably. By “firing” I mean they are sending signals to other neurons. Eventually this firing will cause other neurons, receiving the signals, to also start firing. Eventually a “storm” of neuron activity results; A seizure. In the worst case scenario the seizure will move from one side of the brain to the other, engulfing the brain in a grand mal seizure. If drugs are ineffective at treating the epilepsy, then surgery is considered.

Separating the left and right hemispheres by cutting the corpus callosum reduces the severity of the seizure by preventing the “storm” from moving from one side of the brain to the other.

This isn’t the only kind of surgery performed to treat epilepsy. In some cases the temporal lobe may be removed (lobectomy) and in very young children an entire hemisphere may be removed (hemispherectomy).

The surgery is usually done in two phases. In the first phase, some of the connections between the left and right sides are cut. If this does not treat the seizures, then a second surgery may be performed to cut the remaining connections. In terms of reducing the severity of the seizures, split-brain surgery has proven successful.

How does this surgery affect behavior? Before answering that question, we need to consider some differences between the left and right hemispheres of the brain and the information received by each side.

What differences between left and right hemispheres are relevant here?

First, it should be noted that the left hemisphere of the brain controls the right side of the body and the right hemisphere controls the left side of the body. So, when you wave at somebody with your right hand, the command to do this is coming from the motor cortex of your left hemisphere.

Second, the scenario with your visual system is similar, although differences exist. The visual cortex in your left hemisphere processes things in your right visual field. The visual cortex in your right hemisphere processes things in your left visual field. In other words, for the things in front of you that you look at, the stuff to your left goes to the right side of your brain and the stuff to your right goes to the left side of your brain. For most of us, however, the left hemisphere tells the right side what it sees and the right hemisphere tells the left what it sees. Also, our eyes are always moving around, so both hemispheres usually see the same thing.

Third, speech is controlled by Broca’s area in the left side of the brain.

How did the surgery affect behavior?

The patients did not demonstrate any abnormal behavior. One even woke up from the surgery and joked that he had “a splitting headache.” A few unusual things occurred. For example, one patient reported that he was putting on some pants with one hand, and the other hand wanted to take them off. Overall, however, no ill effects were reported.

However, some amazing things were discovered when these people were studied in the laboratory. In these studies, the patients remained very still while viewing a screen and, very quickly, visual information was flashed to either their left or right visual fields. By doing this, information was presented to only one hemisphere or the other.

So, when information was presented to the left hemisphere and the person was asked, “what do you see?” they could answer. After all, the left hemisphere can talk. However, when information was presented to the right hemisphere and the person was asked what they saw, they replied that they saw nothing. The left hemisphere was being honest and the right can’t talk.

When information was presented to the left hemisphere and the person was asked to point to what they saw with their right hand they could do so. The left hemisphere controls the right hand. However, when asked to point to what they saw with their left hand, they could not. Likewise, when information was presented to the right hemisphere, they could point to it with their left hand. They could not do this with their right hand, however. When asked, “why did you point to [whatever it was]?” the left hemisphere, which controls speech but did not see what the right hemisphere saw, would either say, “I don’t know” or make up some explanation.