Henry Gustav Molaison (February 26, 1926 - December 2, 2008), known widely as H.M., was an American memory disorder patient who had a bilateral medial temporal lobectomy to surgically resect 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. H.M. was widely studied from late 1957 until his death in 2008. 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.

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 (cf. Kolb & Whishaw, 1996). 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. The researchers found, to their surprise, that half of H.M.'s hippocampus 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 lobes.  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; its "permanence on the web relies on contributions from users".

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