In 1953, an experimental surgery meant to relieve his severe epilepsy rendered him frozen in time: He could remember events and facts he knew before the surgery, but could retain virtually nothing after. For decades, Molaison (who’s known as H.M. in psychology textbooks and scores of research papers) cooperated with researchers interested in what his strange memory deficit could teach them about how the brain creates a record of faces, facts, and life experiences.

When Molaison died in 2008, his brain was painstakingly preserved. Now it’s available online for scientists (or others who request permission) to explore, right down to the level of its cellular architecture.

At the time of Molaison’s surgery, the conventional wisdom was that memory traces were distributed throughout the brain. But his case showed that certain parts of the brain were essential for certain memory functions. The surgeon, William Beecher Scoville, had removed large parts of the medial temporal lobes, including the hippocampus. If this structure gets taken offline, we now know, a person can’t form new memories of people, places, things, and events. Molaison, for example, would greet researchers who’d worked with him for decades as if he’d never seen them before.