Background to H.M

William James, in 1890, distinguished between a primary, or short-term, memory process (STM), of limited capacity, and a secondary process (LTM) by which the long-term retention of information in the brain is achieved.  Clinical studies of amnesic patients with known brain lesions have validated this distinction, with STM preserved but the transition to LTM impaired. Early studies of amnesic patients pointed to a major role for the diencephalon in memory processes, but there were also some clinico-pathological studies suggesting a relationship between memory disorder and hippocampal damage, as well as accounts of post-encephalitic amnesia associated with damage to the medial temporal-lobe region.  A major breakthrough came with Milner and Penfield’s detailed study of two cases of profound anterograde amnesia following a left-temporal lobectomy that invaded the hippocampus, and which they attributed to bilateral hippocampal damage (later to be confirmed at autopsy for patient PB).  This report led Scoville to invite me to study his patient HM, who had become amnesic following a bilateral medial temporal-lobe resection at the age of 27, also for the treatment of longstanding epilepsy. When I first met HM, in April 1955, it was clear that his memory impairment was similar to that of PB and FC, but more severe.  His immediate memory was intact, but information could only be retained by constant verbal rehearsal, and non-verbal material was lost in about 30 seconds.  In contrast, his IQ had actually risen, perhaps because he was having fewer seizures. The next challenge was to discover whether HM was capable of learning a task with repeated practice, and here the exciting discovery was that he could acquire a motor skill (mirror drawing) at a normal rate, although unaware on the last trial that he had ever done the task before.  This was early evidence for the existence of multiple memory systems in the brain.