H.M.’s memory impairment has generally been taken as reflecting a failure to convert transient, immediate memory into stable long-term memory. A key insight about the organization of memory, and medial temporal lobe function, came with a consideration of his capacity to remember information that he had acquired before his surgery. The first exploration of this issue with formal tests asked H.M. to recognize faces of persons who had become famous in different decades, 1920-1970 (Marslen-Wilson and Teuber, 1975
). As expected, H.M. was severely impaired at recognizing faces from his postmorbid period (the 1950s and 1960s), but he performed as well as or better than age-matched controls at recognizing faces of persons who were in the news before his surgery. This important finding implied that the medial temporal lobe is not the ultimate storage site for previously acquired knowledge. The early descriptions of H.M. conform to this view. Thus, H.M. was described as having a partial loss of memory (retrograde amnesia) for the 3 years leading up to his surgery, with early memories “seemingly normal” (Scoville and Milner, 1957
, p. 17). Similarly, about 10 years later it was remarked that there did not appear
to have been any change in H.M.’s capacity to recall remote events antedating his operation, such as incidents from his early school years, a high-school attachment, or jobs he had held in his late teens and early twenties (Milner et al., 1968
, p. 216).
Subsequently, a particular interest developed in the status of autobiographical memories for unique events, which are specific to time and place, and methods were developed to assess the specificity and the detail with which such recollections could be reproduced. In the earliest efforts along these lines, as summarized by Suzanne Corkin (Corkin, 1984
), H.M. produced well-formed autobiographical memories, from age 16 years or younger. It was concluded that H.M’s remote memory impairment now extended back to 11 years before his surgery. The situation seemed to change further as H.M. aged. In an update prepared nearly 20 years later (Corkin, 2002
), H.M. (now 76 years old) was described as having memories of childhood, but his memories appeared more like remembered facts than like memories of specific episodes. It was also said that he could not narrate a single event that occurred at a specific time and place. Essentially the same conclusion was reached a few years later when new methods, intended to be particularly sensitive, were used to assess H.M.’s remote memory for autobiographical events (Steinvorth et al., 2005
). These later findings led to the proposal that, whatever might be the case for fact memory, autobiographical memories, i.e., memories that are specific to time and place, depend on the medial temporal lobe so long as the memories persist.
There are reasons to be cautious about this idea. In 2002-2003, new MRI scans of H.M. were obtained (Salat et al., 2006
). These scans documented a number of changes since his first MRI scans from 1992-1993 (Corkin et al., 1997
), including cortical thinning, subcortical atrophy, large amounts of abnormal white matter, and subcortical infarcts. These findings were thought to have appeared during the past decade, and they complicate the interpretation of neuropsychological data collected during the same time period. Another consideration is that remote memories could have been intact in the early years after surgery but then have faded with time because they could not be strengthened through rehearsal and relearning. In any case, the optimal time to assess the status of past memory is soon after the onset of memory impairment.
Other work has tended to support the earlier estimates that H.M.’s remote memories were intact. First, Penfield’s two patients described above, P.B. and F.C., were reported after their surgeries to have memory loss extending back a few months and 4 years, respectively, and intact memory from before that time (Penfield and Milner, 1958
). Second, methods like those used recently to assess H.M. have also been used to evaluate autobiographical memory in other patients, including patients like E.P. and G.P. who have very severe memory impairment (Kirwan et al., 2008
). In these cases, autobiographical recollection was impaired when memories were drawn from the recent past but fully intact when memories were drawn from the remote past.
Memory loss can sometimes extend back for decades in the case of large medial temporal lobe lesions (though additional damage to anterolateral temporal cortex may be important in this circumstance). In any case, memories from early life appear to be intact unless the damage extends well into the lateral temporal lobe or the frontal lobe. These findings are typically interpreted to mean that the structures damaged in H.M. are important for the formation of long-term memory and its maintenance for a period of time after learning. During this period gradual changes are thought to occur in neocortex (memory consolidation) that increase the complexity, distribution, and connectivity among multiple cortical regions. Eventually, memory can be supported by the neocortex and becomes independent of the medial temporal lobe. The surprising observation that H.M. had access to old memories, in the face of an inability to establish new ones, motivated an enormous body of work, both in humans and experimental animals, on the topic of remote memory and continues to stimulate discussion about the nature and significance of retrograde amnesia.