HM is one of the most famous patients in the history of neurosurgery because of what he taught us about memory.
A great many cases of epilepsy are associated with scarring on the brain surface from a variety of causes – stroke, infection, trauma, etc. One brain area commonly responsible for seizures is the temporal lobe, located on each side directly behind the eye and bone of the temple. In the late 1930s surgery was being developed to remove brain scars as one method of seizure control in an era when there were no effective medications. In cases where seizure-causing scars could be easily approached surgically, the results were quite good.
Patient HM developed seizures as a result of a bike accident at age 9. A New England neurosurgeon diagnosed HM’s seizures as originating from both his right and left temporal lobes. In September, 1953, both of HM’s temporal lobes were removed, rendering him seizure free. However, although he could remember how to do previously learned tasks, he was no longer able to commit new events to long-term memory (termed anterograde amnesia). He also suffered moderate retrograde amnesia, and could not remember most events in the 1-2 year period before surgery, and some events up to 11 years before, meaning that his amnesia was temporally graded. However, his ability to form long-term procedural memories was still intact; thus he could, as an example, learn new motor skills, despite not being able to remember learning them.
Until his death, HM was the subject of numerous psychological studies aimed at learning more about the process of memory. Up until his surgery, it was unclear what parts of the brain are crucial for the various processes involved in laying down, storing, and retrieving memories. Even after his death, HM’s brain continues to teach us about the complex physiology of memory.