Patient H.M. and the Science of Memory

Henry Molaison lost the ability to form new memories after brain surgery in 1953 — and over five decades of testing taught science that memory is not one thing and depends on the hippocampus.

Henry Molaison (1926–2008) developed severe epilepsy as a child, possibly after a bicycle accident. By his late twenties, despite heavy medication, he was having many seizures a week. In 1953 the neurosurgeon William Scoville performed an experimental operation, removing tissue from the inner (medial) part of both temporal lobes, including most of the hippocampus and the amygdala on each side.

A life in the present

The seizures became much less frequent. But Molaison was left with profound anterograde amnesia: he could no longer form new lasting memories of events or facts. He could hold a conversation, but a few minutes after a distraction he would not remember it — or the person he'd been talking to. He also lost many memories from the years just before surgery, while older memories from childhood were largely intact. His intelligence and personality were unchanged.

Milner's discovery

The psychologist Brenda Milner began testing him in 1955 and, with Scoville, published a landmark paper in 1957. In one famous experiment, she asked him to trace a star while seeing only its mirror image — a tricky skill. Day after day he got better, yet each day he insisted he had never done the task before. He was learning without remembering learning.

What H.M. taught us

His case showed that memory is not a single faculty: declarative memory (facts and events) depends on the medial temporal lobe, while procedural memory (skills) uses other systems; that short-term memory is separate from long-term memory; and that old memories are eventually stored elsewhere in the brain. The psychologist Suzanne Corkin studied him for over 40 years. After his death in 2008, his brain was cut into 2,401 thin slices and digitised for researchers.