Corpus callosotomy
A surgery that cuts the corpus callosum — the thick bundle of fibres joining the brain's two hemispheres — usually to stop epileptic seizures spreading from one half of the brain to the other. The striking part is how little it disturbs everyday life, which is exactly what made the patients such a clean window onto how the two halves divide their labour.
The surgical severing of the corpus callosum (and in the complete operation, the smaller commissures too), performed to treat intractable epilepsy by stopping seizure activity from crossing between the hemispheres. The modern operations were carried out by the neurosurgeons Philip Vogel and Joseph Bogen from 1962; the patients kept most of their everyday function, and it is the resulting disconnection of the hemispheres that creates the "split brain."
Callosotomy is the operation that made the rest of this topic possible. Cutting the corpus callosum stops a seizure that starts in one hemisphere from recruiting the other, which controlled epilepsy in patients no drug had helped. The surprise was how ordinary the patients seemed afterwards — they walked, talked, and held conversations — so that the disconnection only showed itself under the careful lateralised testing that Sperry and Gazzaniga designed. The first modern case, known as W.J., was a former paratrooper operated on in 1962; the small number of fully operated patients since are the entire empirical base of the field, which is part of why a two-patient study like Pinto's can still move it.