HM (patient)

HM (also known as "H.M." and "Henry M.," born 1926 in Connecticut) is an anonymous memory impaired patient who 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. He is still alive today and resides in a care institute, where he remains in ongoing investigation.

History
HM suffered from intractable epilepsy that has since then been often&mdash;though inconclusively&mdash;attributed to a bicycle accident at the age of seven. He suffered from partial seizures for many years, and then several tonic clonic seizures (seizures with a loss of consciousness and convulsions) following his sixteenth 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 2 cm 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 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 3-4 day period before surgery, and some events up to 11 years before, meaning that his amnesia was temporally graded. 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 be 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.