HM became densely amnesic after an operation removed the hippocampus on each side of the brain to treat his otherwise untreatable epilepsy.
Epilepsy can often be triggered by disturbances in the hippocampus, and removing the site of this disturbance is one way of treating life-threatening seizures.
We know now, largely because of HM, that removing one hippocampus has relatively small impact on memory, while removing both causes a profound antereograde amnesia.
This means HM cannot remember new information, meaning that he has relatively normal memory for the time before his operation, but can remember virtually nothing since.
This was one of the first and only times the operation to remove both hippocampi was conducted because of the effects that were discovered.
However, because the removal of the brain areas was done surgically, it allowed a very precise understanding of how the removed areas might contribute to normal memory processes.
A discipline called cognitive neuropsychology studies damage to the brain to work out normal function, by matching up which areas are damaged by what patients can no longer do.
Using these methods, HM has provided a huge insight into the neuropsychology of memory.
He has been anonymous and kept from the public eye, but his family has now agreed to release audio tapes of him made in the 1990s.
The NPR programme is their first broadcast.
HM is still alive and has been the focus of studies on the neuropsychology of memory until the last paper [pdf] in 2002 although now has reportedly ‘retired’ from research.