HM, now revealed as Henry G. Molaison, suffered from temporal lobe epilepsy that was not helped by existing drugs and so was referred to neurosurgeon William Scoville in 1953.
Scoville attempted a new type of operation to remove the parts of the brain which triggered the seizures, cutting out the majority of the hippocampus on both sides of the brain, along with the amygdala and parahippocampal gyrus.
This left HM with a dense antereograde amnesia, meaning that while his memory for pre-surgery events was generally very good, he was unable to create new conscious long-term memories.
His ability to learn new skills and obtain conditioned associations remained intact, however, and the differences in his memory abilities and the precise knowledge of which parts of the brain were missing allowed some of the first insights into the neuropsychology of memory.
The initial study on HM and his dense amnesia was first published in 1957 by Scoville and the young psychologist Brenda Milner. It has since become one of the most widely cited and widely taught of all neuropsychology case studies.
However, HM continued to participate in research studies since his initial appearance in the scientific literature and was known among researchers for his warm and easy going personality.
The most recent study on HM was published only this year and examined the linguistic content of his crossword puzzles, of which he’d been a fan of for the whole of his adult life. The study examined whether his language skills had been affected by years of dense amnesia.
They hadn’t, suggesting that once acquired, the maintenance of written language skills doesn’t seem to require intact medial temporal lobes.
Much of the later work with HM was completed in partnership with neuropsychologist Suzanne Corkin, who wrote an article [pdf] for Nature Reviews Neuroscience in 2002 that was part tribute and part research summary, detailing his massive contribution to our understanding of memory.
UPDATE: The New York Times has an excellent obituary for HM.