In a normal brain, the hemispheres cooperate on many cognitive tasks.
Split-brain experiments were first carried out in the 1960s. At the time, epilepsy was commonly cured by separating the hemispheres of the brain, short-circuiting the neural storms that cause epileptic seizures. While this treatment was generally successful, and is still occasionally used to cure severe cases of epilepsy today, cutting the connection between the brain's hemispheres has some significant side effects. By studying patients who had undergone this procedure, a team of scientists led by Robert Sperry were able to make groundbreaking discoveries about how the hemispheres of the brain work together.
Hemisphere Dominance
In a normal brain, each hemisphere is dominant in specific cognitive tasks. The dominant hemisphere will display much higher levels of activity when the brain is dealing with certain tasks. In most people, the left hemisphere is dominant in higher language functions, conscious logic and detail-oriented operations. The right hemisphere is dominant in spacial awareness, interpreting emotion and whole-picture analysis. Each hemisphere is dominant when controlling muscles and receiving sensory input from the opposite side of the body. Therefore, the left hemisphere is primarily involved in moving the right arm and leg and is more attentive to input from the right eye and ear.
The Corpus Callosum
While each hemisphere is dominant in certain areas, the hemispheres also cooperate on many tasks. The hemispheres communicate with one another through a wide band of nerve tissue known as the corpus callosum, which connects the hemispheres at the back of the brain, just ahead of the brain stem. If the corpus callosum is severed, the hemispheres can no longer communicate with one another. Since they are each connected to the brain stem, they can continue to function normally in isolation, but cooperation is no longer possible. The brain has been split.
Split-Brain Experiment Design
The split brain experiments conducted by Roger Sperry worked by exposing one eye at a time to certain stimuli. If a picture is displayed to the right eye in a split-brain experiment, the left hemisphere will perceive it, but the right hemisphere will not. Normally, the image would be communicated to the right hemisphere through the corpus callosum. In a split-brain patient, this communication is not possible, and the right hemisphere fails to process the image.
Results
While split-brain patients appear normal in most respects, they display some odd responses when images are shown to only one hemisphere. If an image of a pencil is shown to the right hemisphere only and the subject is asked to describe the object, he will be unable to, since his language center has not processed the image. However, he will be able to select the object from a collection of other objects, since the part of the brain dealing with spacial awareness has been exposed to the image. Likewise, if the image is exposed to the left hemisphere only, the subject will be able to describe it as a pencil, but would not be able to describe the object in detail or select it from a group of other pencils.
Tags: hemisphere dominant, right hemisphere, left hemisphere, corpus callosum, hemisphere will, able describe