What is HM most famous for in psychology?

HM, or Henry Molaison, is most famous in psychology for experiencing severe amnesia following neurosurgery conducted to alleviate his debilitating epilepsy. In 1953, he underwent a surgical procedure that involved the removal of parts of his medial temporal lobes, including the hippocampus. This surgery effectively alleviated his seizures, but it had the unintended consequence of causing profound amnesia.

HM's case has been pivotal in understanding memory because it provided crucial insights into how different types of memory work. Specifically, after the surgery, HM was unable to form new long-term memories while his short-term memory remained intact for brief periods. This distinction between short-term and long-term memory, particularly implicating the hippocampus in the formation of new long-term memories, was a groundbreaking realization in the field of cognitive psychology and neuroscience. His detailed case studies, conducted by various researchers over the years, have contributed significantly to our understanding of memory processes, making him a central figure in psychological research on memory and amnesia.