Before H.M., the medical literature contained clues that MTL structures were important for “remembering,” but it was H.M.’s case that firmly established this area’s critical role in long-term memory (LTM). He had a severe and lasting deficit in consciously retrieving events and facts, with poor recall and recognition of stimuli in all modalities, and he did not benefit from practice. In contrast, his short-term memory (STM) was intact; he could hold information online for about 30 sec. His deficit was in converting information held in STM into permanent LTM traces, indicating that different brain circuits support STM and LTM.
H.M.’s amnesia was pure, in the sense that his intellect was spared. Testing showed a gross disparity between his above average IQ and his subnormal memory performance. His memory disorder was uncontaminated by other cognitive deficits. His language and perceptual capacities were normal, and he had no psychiatric symptoms.
Milner’s finding that H.M.’s errors on a mirror-tracing task decreased over 3 days despite his having no conscious recollection of doing the test was evidence that LTM processes are dissociable. Subsequent research showed convincingly that H.M.’s nondeclarative learning (motor and perceptual skills, repetition priming) remained intact, but he could not form new declarative memories for events and facts. This distinction between preserved nondeclarative and impaired declarative processes indicates that MTL circuits are needed for declarative but not for nondeclarative memory. Further, he could not recall autobiographical episodes from before his operation, but his general knowledge of his preoperative world was normal. It is hard to overrate the impact of this extraordinary case on psychology and neuroscience. H.M. left lasting traces on memory research and on humanity.