Corpus callosotomy (or less frequently, callotomy) is a surgical procedure that disconnects the cerebral hemispheres, resulting in a condition called split-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; side-effects have not been found in some patients (although it is very unlikely that none exist).

The corpus callosum is usually severed in order to stop epileptic seizures. Once the corpus callosum is cut, the brain has much more difficulty sending messages between the hemispheres. Although the corpus callosum is the largest white-matter tract connecting the hemispheres, some limited interhemispheric communication is still possible via the anterior commissure and posterior commissure. When tested in particular situations, it is obvious that information transfer between the hemispheres is reduced.

From http://en.wikipedia.org/wiki/Corpus_callosotomy