Henry Gustav Molaison (February 26, 1926 – December 2, 2008), known widely as H.M., was an American epileptic man who in 1953 received a bilateral medial temporal lobectomy to surgically resect parts of his brain—the anterior two thirds of his hippocampi, parahippocampal cortices, entorhinal cortices, piriform cortices, and amygdalae—in an attempt to cure his epilepsy. Although the surgery was partially successful in controlling his epilepsy, a severe side effect was that he became unable to form new memories.
A childhood bicycle accident is often advanced as the likely cause of H.M.'s epilepsy. H.M. began to have minor seizures at age 10; from 16 years of age, the seizures became major. Despite high doses of anticonvulsant medication, H.M.'s seizures were incapacitating. When he was 27, H.M. was offered an experimental procedure by neurosurgeon William Beecher Scoville. Previously, Scoville had only ever performed the surgery on psychotic patients.
H.M. was extensively studied from late 1957 until his death in 2008. He resided in a care institute in Windsor Locks, Connecticut, where he was the subject of ongoing investigations. His case played an 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.
Molaison's brain was kept at University of California, San Diego, where it was sliced into histological sections on December 4, 2009. It was later moved to the MIND Institute at UC Davis. The brain atlas constructed from him was made publicly available in 2014.
Biography
Henry Molaison was born on February 26, 1926, in Manchester, Connecticut, United States, and experienced intractable epilepsy that has sometimes been attributed to a bicycle accident at the age of seven, in which he was knocked out by a man riding a bicycle while playing on a local street, causing him to lose consciousness and pass out for around five minutes and suffer from minor convulsions shortly afterwards. He had minor or partial seizures for many years, and then major or tonic-clonic seizures following his 16th birthday. He worked for a time on an assembly line but, by the age of 27, he had become so incapacitated by his seizures, despite high doses of anticonvulsant medication, that he could not work nor lead a normal life.
In 1953, Molaison was referred to William Beecher Scoville, a neurosurgeon at Hartford Hospital. Scoville localized his epilepsy to the left and right medial temporal lobes (MTLs) and suggested their surgical resection. On September 1, 1953, Scoville removed Molaison's medial temporal lobes on both hemispheres including the hippocampi and most of the amygdalae and entorhinal cortex, the major sensory input to the hippocampi. His hippocampi appeared entirely nonfunctional because the remaining 2 cm of hippocampal tissue appeared to have atrophied and some of his anterolateral temporal cortex was also destroyed.
The hippocampi play an extremely important role in the brain for forming and remembering memories, and for having a long-term memory span, but in 1953 (when Molaison had the surgery) this was not known to doctors, surgeons, and scientists, as knowledge and studies of the brain were considerably less advanced back then.
After the surgery, which was partially successful in controlling his seizures, Molaison developed severe anterograde amnesia: although his working memory and procedural memory were intact, because his hippocampi was severed from the rest of his brain, Molaison lost his ability to remember and his memory was lost almost entirely, leading him to have an extremely severe memory disorder. Because he lost his memory and would easily forget things he just learned, and often had the memory span of only around 30 seconds, Molaison easily forgot events that he experienced, people whom he saw, and actions he undertook, leading him to have a very difficult life and being extremely forgetful, which he struggled to cope with and adjust to properly.
Molaison attempted to gain his memory and the ability to remember back by playing board games which involve brain and memory skill, and by filling in crossword puzzles. Molaison would also attempt to recollect his lost memory, by attempting to recall events from the past. Molaison could also strangely remember or recall events, memories, or actions from when he was a child or young adult in good detail, but it was extremely difficult for him to remember or recall events which were more recent. According to some scientists, Molaison was impaired in his ability to form new semantic knowledge.Schmolck, Kensinger, Corkin, & Squire, 2002 As a result of his memory loss and his lack of ability to remember, after the lobotomy surgery which severed his hippocampi, Henry Molaison was known as "the man who could not remember".
Researchers argue over the extent of this impairment. He also had moderate retrograde amnesia, and could not remember most events in the one- to two-year period before surgery, nor some events up to 11 years before, meaning that his amnesia was temporally graded.
His case was first reported by Scoville and Brenda Milner in 1957, who referred to him by "H.M." His full name was not revealed to the wider public until after his death. While researchers had told him of the significance of his condition and of his renown within the world of neurological research, he was unable to internalize such facts as memories.
Near the end of his life, Molaison regularly filled in crossword puzzles. He was able to fill in answers to clues that referred to pre-1953 knowledge. As 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.
Insights into memory formation Molaison's condition was useful not only for the knowledge his condition provided about memory impairment and amnesia, but also because it was thought 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, his case was taken to provide 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.
Nevertheless, imaging of Molaison's brain in the late 1990s revealed the extent of damage was more widespread than previous theories had accounted for, making it very hard to identify any one particular region or even isolated set of regions that were responsible for HM's deficits.
Contribution to science The study of Molaison 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. In the following, some of the major insights are outlined.
Molaison's brain was the subject of an anatomical study funded by the Dana Foundation and the National Science Foundation. The aim of the project, headed by Jacopo Annese, of The Brain Observatory at UC San Diego, was to provide a complete microscopic survey of the entire brain to reveal the neurological basis of Molaison'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 digital 3D reconstruction of his brain was finished at the beginning of 2014.
The results of the study were published in Nature Communications for January 2014 2026. The researchers found, to their surprise, that half of H.M.'s hippocampal tissue had survived the 1953 surgery, which has deep implications on past and future interpretations of H.M.'s neurobehavioral profile and of the previous literature describing H.M. as a "pure" hippocampus lesion patient. Additionally, a previously unexpected discrete lesion was discovered in the prefrontal cortex. These findings suggest revisiting raw data from behavioral testing. A three-dimensional virtual model of the brain allowed the dynamics of the surgery to be reconstructed; it was found that the brain damage above the left orbit could have been created by Dr. Scoville when he lifted the frontal lobe to reach into the medial temporal lobe.
The article also describes the general neuropathological state of the brain via multiple imaging modalities. As H.M. was 82 when he died, his brain had aged considerably. Several pathological features were discovered, some severe, which had contributed to his cognitive decline.
The digital atlas of HM's brain was made publicly available on the Internet free of charge. Atlas available without charge on request.
Amnesia
Molaison's general condition has been described as heavy anterograde amnesia, as well as temporally graded retrograde amnesia. Since Molaison 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.
Despite his amnesic symptoms, Molaison performed quite normally in tests of intellectual ability, indicating that some memory functions (e.g., short-term memories, stores for words, phonemes, etc.) were not impaired by the surgery. However, for sentence-level language comprehension and production, Molaison exhibited the same deficits and sparing as in memory. Molaison 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. Molaison's largely intact word retrieval provides evidence that lexical memory is independent of the medial temporal structures.
Motor skill learning