When performing a corpus callosotomy, the surgeon cuts the corpus callosum, the large fiber bundle that connects the two sides of the brain. In contrast with lobectomy, corpus callosotomy does not involve removing any brain tissue. Instead, it usually involves cutting the front two-thirds of this bundle (a procedure called a "partial callosotomy") in the hope that the operation will markedly reduce the seizure frequency. Sometimes the other one-third is cut later ("complete callosotomy").
Corpus callosotomy is most effective for atonic seizures ("drop attacks"), tonic-clonic seizures, and tonic seizures. Seizure frequency is reduced by an average of 70% to 80% after partial callosotomy and 80% to 90% after complete callosotomy.