A closer look
To figure out exactly which fold of grey matter controls your left thumb, Dr. Wilder Penfield took a terrifyingly direct approach: electrocuting the exposed brains of fully conscious patients. While performing epilepsy surgeries in the 1930s, he applied electrical probes to the cerebral cortex to map out sensory and motor functions. The patients' bizarre twitching and tingling allowed him to draw the 'cortical homunculus'—that famously grotesque anatomical map showing how much of the brain is dedicated solely to our massive hands and lips.
