When he was 9 years old, Henry was hit by a cyclist and developed epileptic seizures as a result of his injuries. In 1953, at the age of 27, he had an operation intended to stop these seizures; his medial temporal lobe (including the hippocampus) was removed on both sides. The surgery did stop the seizures; however, he was no longer able to store new long-term memories, although he could still rehearse and retain information in short-term memory unless distracted.

The case of H.M. has been used to support the multistore model of memory, as it demonstrates a distinction between short and long term memory. Nevertheless, the fact that he was able to learn new skills, such as tracing an outline of a star in a mirror suggests that long-term memory is not just a unitary store as the multistore model of memory suggests.

Henry Molaison has made a massive contribution to the understanding memory, his legacy will never be forgotten. Rest in Peace.

For a more detailed account of Henry Molaison, see this New York Times Article.