HM was suffering from incapacitating epileptic seizures that were not helped by any of the medications of the 1950s.
As a last resort, neurosurgeon William Scoville tried an experimental operation to remove 8cms of tissue on both sides of the inner parts of his temporal lobes, including both hippocampi, hopefully also removing the source of his seizures.
Neurosurgery to treat otherwise untreatable epilepsy is still common and highly effective, although this type of operation isn’t used any more.
This is largely because HM’s seizures reduced considerably, but he was left with a severe amnesia, meaning he couldn’t seem to lay down any new conscious memories, although could remember things that occurred before his surgery.
Because of his seemingly unique memory impairment and an exact knowledge of which brain areas were missing, he has become a regular in neuroscience research that has aimed to understand what his impairment tells us about how normal memory is supported by the brain.
This new study is no exception. The researchers, Profs Veronique Bohbot and Suzanne Corkin, guessed on the basis of the existing evidence that the right parahippocampal cortex would be enough to support spatial learning and navigation.
The right side of the brain is known to be specialised for understanding 3D space and some of the parahippocampal cortex, an area adjacent to the surgically removed hippocampus, remained in HM’s brain.
So the researchers used a task where a sensor is hidden under a section of carpet in a room which beeped when it was stepped on.
The participants were asked to find it just by exploration, and subsequently, they were taken to different parts of the room and asked to re-find it.
Despite having no conscious memories of previous tries, HM began to find the sensor quite accurately, much more accurately than if he was just stumbling across it by chance alone.
This suggests that his remaining part of HM’s parahippocampal cortex was enough to support spatial memory, and importantly, that the brain areas missing in HM, although they would help, are probably not essential for navigation.
HM has participated some key studies through the decades and has outlasted many in the field. He probably doesn’t realise it, but he’s been one of the most important people in neuroscience.