In 1953, a 27-year-old man called H.M. had brain surgery to stop serious seizures. Doctors removed parts of his brain, including the hippocampus. After that, he could not remember new events. He met people again and again, but each time he said, "Nice to meet you."

Then he practiced drawing a star while looking in a mirror. He never remembered practicing, but he got better every day.

Henry Molaison, known for decades only as H.M., is one of the most famous patients in brain science. In 1953, at the age of 27, he had surgery to remove parts of both medial temporal lobes, including the hippocampus, to treat severe epilepsy. His seizures decreased, but he lost the ability to form new memories of events.

However, many older memories, his ability to talk, and his intelligence were mostly unaffected. Psychologist Milner then gave him a difficult task: tracing a star while looking only at a mirror. Over three days, he improved steadily, even though he insisted each time that he had never done it before. This showed that memory for skills uses different systems from memory for events.

In 1953, a 27-year-old man with epilepsy that medication could not control had surgery to remove much of the medial temporal lobe on both sides, including the hippocampus. His seizures became less frequent. But he was left with profound anterograde amnesia, greeting people he had met many times as strangers. Until his death in 2008, the world knew him only as H.M. His name was Henry Molaison. Many earlier memories, his intelligence and his conversational skills stayed largely intact.

When Scoville and Milner reported the case in 1957, it established that these structures are essential for forming new memories and overturned the view that memory is spread evenly across the brain. Milner later asked him to trace a star while watching his hand only in a mirror. Over three days his performance improved, though each time he said he had never tried it. Learning a skill and remembering an experience, it seemed, depend on separate systems, procedural and declarative memory.

But a 2014 post-mortem analysis found that considerable hippocampal tissue had survived and that his left frontal lobe was damaged, so the surgery alone may not explain his deficits. He took part in research for about 55 years.