The American Heritage® Dictionary of the English Language, 4th Edition
n. Surgical incision of a commissure in the body, as one made in the heart to relieve constriction of the mitral valve or one made in the brain to treat certain psychiatric disorders.
n. The surgical division of a commissure or similar structure.
Word Usage
"As support, they appeal to commissurotomy patients who after their brain hemispheres are divided seem to act as if each half were itself a functioning mind."