A patient with a split brain, when shown an image in his or her left visual field (the left half of what each eye sees), will be unable to say 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. Since the two sides of the brain cannot communicate, the patient can't say what he or she is seeing. The person can, however, pick up a corresponding object with their left hand, since that hand is controlled by the right side of their brain.
Most modern callosotomies involve only the anterior portion of the Corpus callosum, reflecting the fact that the frontal and temporal lobes are the most commonly involved in the genesis of seizures. The deficits from this modified procedure are milder; some patients have no side effects whatsoever.
Some of the earliest split brain research was carried out by Roger Wolcott Sperry. Results from this research have led to important theories on Left-Right brain specialisation. However, in analyzing this research, it is necessary to remember that the patients involved in these operations had abnormal brains to begin with.