| Henry Gustav Molaison | |
|---|---|
Molaison in 1953 before his surgery |
|
| Born | February 26, 1926 Hartford, Connecticut |
| Died | December 2, 2008 (aged 82) Windsor Locks, Connecticut |
Henry Gustav Molaison (February 26, 1926 – December 2, 2008), famously known as HM or H.M., was an American memory disorder patient who was widely studied from late 1957 until his death.[1][2] His case played a very important role in the development of theories that explain the link between brain function and memory, and in the development of cognitive neuropsychology, a branch of psychology that aims to understand how the structure and function of the brain relates to specific psychological processes. Before his death, he resided in a care institute located in Windsor Locks, Connecticut, where he was the subject of ongoing investigation.[3] His brain now resides at UC San Diego where it was sliced into histological sections on December 4, 2009.[4]
Contents |
Molaison was born on February 26, 1926 and suffered from intractable epilepsy that has been often—though inconclusively—attributed to a bicycle accident at the age of seven. This accident was initially reported to have occurred at age nine, but was corrected by the patient's mother at a later stage.[5] He suffered from partial seizures for many years, and then several tonic-clonic seizures following his sixteenth birthday. In 1953 he was referred to William Beecher Scoville, a neurosurgeon at Hartford Hospital, for treatment.[2]
Scoville localized HM's epilepsy to his left and right medial temporal lobes (MTLs) and suggested surgical resection of the MTLs as a treatment. On September 1, 1953, Scoville removed parts of HM's MTL on both sides of his brain.[2] HM lost approximately two-thirds of his hippocampus, parahippocampal gyrus, and amygdala. His hippocampus appeared entirely nonfunctional because the remaining 2 cm of hippocampal tissue appeared atrophic and because the entire entorhinal cortex, which forms the major sensory input to the hippocampus, was destroyed. Some of his anterolateral temporal cortex was also destroyed.
After the surgery—which was successful in its primary goal of controlling his epilepsy—he suffered from severe anterograde amnesia: although his working memory and procedural memory were intact, he could not commit new events to long-term memory. According to some scientists (e.g., Schmolck, Kensinger, Corkin, & Squire, 2002), HM was impaired in his ability to form new semantic knowledge, but researchers argue over the extent of this impairment. He also suffered moderate retrograde amnesia, and could not remember most events in the 1–2-year period before surgery, nor some events up to 11 years before, meaning that his amnesia was temporally graded. However, his ability to form long-term procedural memories was intact; thus he could (for example) learn new motor skills, despite not being able to remember learning them.
The case was first reported in a paper by Scoville and Brenda Milner in 1957.[6] Before his death H.M. filled in crossword puzzles.[7] He was able to fill in answers to clues that referred to pre-1953 knowledge. For post-1953 information he was able to modify old memories with new information. For instance, he could add a memory about Jonas Salk by modifying his memory of polio.[1]
He died on December 2, 2008.[2]
HM was important not only for the knowledge he provided about memory impairment and amnesia, but also because his exact brain surgery allowed a good understanding of how particular areas of the brain may be linked to specific processes hypothesized to occur in memory formation. In this way, he provided vital information about brain pathology, and helped to form theories of normal memory function.
In particular, his apparent ability to complete tasks that require recall from short-term memory and procedural memory but not long term episodic memory suggests that recall from these memory systems may be mediated, at least in part, by different areas of the brain. Similarly, his ability to recall long-term memories that existed well before his surgery, but inability to create new long-term memories, suggests that encoding and retrieval of long-term memory information may also be mediated by distinct systems.
The study of the patient HM has revolutionized the understanding of the organization of human memory. It has provided broad evidence for the rejection of old theories and the formation of new theories on human memory, in particular about its processes and the underlying neural structures (cf. Kolb & Whishaw, 1996). In the following, some of the major insights are outlined.
HM's brain is the subject of an unprecedented anatomical study funded by the Dana Foundation and the National Science Foundation. The project, headed by Jacopo Annese, the Director of The Brain Observatory at UC San Diego, will provide a complete microscopic survey of the entire brain and reveal the neurological basis of HM's historical memory impairment at cellular resolution. On December 4, 2009, Annese's group acquired 2401 brain slices with only two damaged slices and 16 potentially problematic slices. The group is currently planning the second phase of the project.[4][8]
HM's general condition has been described as heavy anterograde amnesia, as well as temporally graded retrograde amnesia (Smith & Kosslyn, 2007). HM was unable to form new long-term memories of new events or new semantic knowledge – he basically lived in the present (Corkin, 2002). Since HM did not show any memory impairment before the surgery, the removal of the medial temporal lobes can be held responsible for his memory disorder. Consequently, the medial temporal lobes can be assumed to be a major component involved in the formation of semantic and episodic long-term memories (cf. medial temporal lobes described as a convergence zone for episodic encoding in Smith & Kosslyn, 2007). Further evidence for this assumption has been gained by studies of other patients with lesions of their medial temporal lobe structures.[6]
Despite his amnesic symptoms, HM performed quite normally in tests of intellectual ability, indicating that some memory functions (e.g., short-term stores, stores for words, phonemes, etc.) were not impaired by the surgery (Smith & Kosslyn, 2007; Corkin, 2002). However, for sentence-level language comprehension and production, HM exhibited the same deficits and sparing as in memory (MacKay, James, Taylor & Marian, 2007). HM was able to remember information over short intervals of time. This was tested in a working memory experiment involving the recall of previously presented numbers; in fact, his performance was no worse than that of control subjects (Smith & Kosslyn, 2007). This finding provides evidence that working memory does not rely on medial temporal structures. It further supports the general distinction between short-term and long-term stores of memory (Kolb & Whishaw, 1996). HM's largely intact word retrieval provides evidence that lexical memory is independent of the medial temporal structures (Corkin, 2002).
In addition to his intact working memory and intellectual abilities, studies of HM's ability to acquire new motor skills demonstrated preserved motor learning (Corkin, 2002). In a study conducted by Milner in the early 1960s, HM acquired the new skill of drawing a figure by looking at its reflection in a mirror (Corkin, 2002). Further evidence for intact motor learning was provided in a study carried out by Corkin (1968). In this study, HM was tested on three different motor learning tasks and demonstrated full motor learning abilities in all three tasks. His ability to learn certain problem-solving procedures has also been shown with the Tower of Hanoi task (Kolb & Whishaw, 1996).
Experiments involving repetition priming underscored HM's ability to acquire implicit (non-conscious) memories, in contrast to his inability to acquire new explicit semantic and episodic memories (Corkin, 2002). These findings provide evidence that memory of skills and repetition priming rely on different neural structures than memories of episodes and facts; whereas procedural memory and repetition priming do not rely on the medial temporal structures removed in HM, semantic and episodic memory do (cf. Corkin, 1984).
The dissociation of HM's implicit and explicit learning abilities along their underlying neural structures has served as an important contribution to our understanding of human memory: Long-term memories are not unitary and can be differentiated as being either declarative or non-declarative (Smith & Kosslyn, 2007).
According to Corkin (2002), studies of HM's memory abilities have also provided insights regarding the neural structures responsible for spatial memory and processing of spatial information. Despite his general inability to form new episodic or factual long-term memories, as well as his heavy impairment on certain spatial memory tests, HM was able to draw a quite detailed map of the topographical layout of his residence. This finding is remarkable since HM had moved to the house five years after his surgery and hence, given his severe anterograde amnesia and insights from other cases, the common expectation was that the acquisition of topographical memories would have been impaired as well. Corkin (2002) hypothesized that HM “was able to construct a cognitive map of the spatial layout of his house as the result of daily locomotion from room to room” (p. 156).
Regarding the underlying neural structures, Corkin (2002) argues that HM's ability to acquire the floor plan is due to partly intact structures of his spatial processing network (e.g., the posterior part of his parahippocampal gyrus). In addition to his topographical memory, HM showed some learning in a picture memorization-recognition task, as well as in a famous faces recognition test, but in the latter only when he was provided with a phonemic cue. HM's positive performance in the picture recognition task might be due to spared parts of his ventral perirhinal cortex.
Furthermore, Corkin (2002) argues that despite HM's general inability to form new declarative memories, he seemed to be able to acquire small and impoverished pieces of information regarding public life (e.g., cued retrieval of celebrities' names). These findings underscore the importance of HM's spared extrahippocampal sites in semantic and recognition memory and enhance our understanding of the interrelations between the different medial temporal lobe structures. HM's heavy impairment in certain spatial tasks provides further evidence for the association of the hippocampus with spatial memory (Kolb & Whishaw, 1996).
Another contribution of HM to our understanding of human memory regards the neural structures of the memory consolidation process, which is responsible for forming stable long-term memories (Eysenck & Keane, 2005). HM displayed a temporally graded retrograde amnesia in the way that he “could still recall childhood memories, but he had difficulty remembering events that happened during the years immediately preceding the surgery” (Smith & Kosslyn, 2007, p. 214). His old memories were not impaired, whereas the ones relatively close to the surgery were. This is evidence that the older childhood memories do not rely on the medial temporal lobe, whereas the more recent long-term memories seem to do so (Smith & Kosslyn, 2007). The medial temporal structures, which were removed in the surgery, are hypothesized to be involved in the consolidation of memories in the way that “interactions between the medial temporal lobe and various lateral cortical regions are thought to store memories outside the medial temporal lobes by slowly forming direct links between the cortical representations of the experience” (Smith & Kosslyn, 2007, p. 214).