The Case of Henry Molaison (H.M.)

In August 1953 a twenty-seven-year-old man named Henry Molaison, known for decades in the scientific literature only by his initials H.M. to protect his privacy, underwent experimental brain surgery performed by the neurosurgeon William Beecher Scoville to relieve severe, medication-resistant epileptic seizures, in which Scoville removed the inner portions of both temporal lobes, including most of the hippocampus and amygdala on each side. The seizures markedly improved, but the operation left Molaison with a profound and permanent inability to form new long-term memories, a condition known as anterograde amnesia, while sparing his general intelligence, his short-term working memory, and much of his memory for events and people from well before the surgery. He could hold a conversation, but if the other person left the room and returned minutes later, Molaison would have no memory of having met them at all, and he was nonetheless able to learn and retain some new motor skills through repeated practice even though he could never recall having practiced them. Beginning in 1955 the psychologist Brenda Milner, working with Scoville, studied Molaison intensively over decades and used his case, along with several similarly operated patients, to demonstrate that the hippocampus is essential for converting new experiences into lasting long-term memories and that memory is not a single unified system but is instead composed of at least partly separable systems for different kinds of learning. Molaison's case, studied more continuously than perhaps any other in the history of neuropsychology, remains foundational to the modern understanding of the brain's memory systems.