HM (patient)
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A memory impaired patient known as HM (an acronym used to keep his identity confidential) has been widely studied since the late 1950s and has been very important 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 studies brain injury to infer normal psychological function.
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History
HM suffered from intractable epilepsy that has since then been often—though inconclusively—attributed to a bicycle accident at the age of seven. He suffered from minor seizures for many years, and then several grand mal fits following his 16th birthday. In 1953, HM was referred to William Scoville, a surgeon at Hartford Hospital, for treatment.
Scoville localized HM's epilepsy to his medial temporal lobe (MTLs) and suggested surgical resection of the MTLs as a treatment. On August 25, 1953, Scoville removed parts of HM's medial temporal lobe on both sides of his brain. HM lost approximately two-thirds of his hippocampal formation, parahippocampal gyrus (all his entorhinal cortex was destroyed), and amygdala. We can safely assume his hippocampus is entirely nonfunctional because the remaining 2cm of hippocampal tissue appears atrophic and because the entire entorhinal (which forms the major sensory input to the hippocampus) was destroyed. Some of his anterolateral temporal cortex was also destroyed.
After the surgery he suffered from severe anterograde amnesia: although his short-term memory was intact, he could not commit new events to long-term memory. According to some scientists HM is impared 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 3-4 day period before surgery, and some events up to 11 years before. However, his ability to form long-term procedural memories was still intact; thus he could, as an example, learn new motor skills, despite not being able to remember learning them.
Insights into memory formation
HM has not only been important for the knowledge he has provided about memory impairment and amnesia, but also because his exact brain surgery has allowed a good understanding of how particular areas of the brain may linked to specific processes hypothesised to occur in memory formation. In this way, he has provided vital information about brain pathology, as well as having helped form theories of normal memory function.
Particularly, the fact that he seems to be able 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, the fact that HM cannot create new long-term memories, but can recall long-term memories that existed well before his surgery suggests that encoding and retrieval of long-term memory information may also be mediated by distinct systems.
The case was first reported in a paper by Scoville and Brenda Milner in 1957.
See also
External links
- The Day His World Stood Still (http://www.brainconnection.com/topics/?main=fa/hm-memory) - Article on HM from Brain Connection
References
- Scoville WB, Milner B. (1957) (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10678523&dopt=Abstract) Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery and Psychiatry, 20, 11–21. Full text as pdf (http://homepage.mac.com/sanagnos/scovillemilner1957.pdf)
- Corkin, S. (2002) (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11836523&dopt=Abstract) What's new with the amnesic patient H.M.? Nature Reviews Neuroscience, 3(2), 153-60. Full text as pdf (http://homepage.mac.com/sanagnos/corkin2002.pdf)