Post-mortem analysis of the brain of Patient H.M., whose case played an important role in the development of theories relating to the link between brain function and memory, reveal new details that may affect our understanding of the role of the hippocampus in memory. The work, published in Nature Communications, reveals that within Patient H.M.’s brain a significant portion of the posterior hippocampus, which was thought to have been removed in the 1950s, is actually intact.
Henry Molaison or Patient H.M. is the most famous human neuroscience case study involving memory. Due to unknown causes, he began experiencing minor seizures at the age of 10, which developed into major seizures by age 15. Anticonvulsant drugs at the time were not very effective and so in 1953, at the age of 27, he underwent a neurosurgical procedure to remove portions of the hippocampus in the brain. Following this surgery, although the frequency of seizures declined, Patient H.M. could not consolidate and store information in long term memory. Brain imaging studies in the 1990s confirmed that most of the hippocampus - a region that is heavily implicated in processing learning and memory - had been removed. However, these techniques lacked the resolution to define the exact boundaries of the lesions.
After his death in 2008, Jacopo Annese and colleagues sectioned Patient H.M.’s brain and created a three-dimensional model of the lesioned region. What they found, in contrast to earlier imaging studies while Patient H.M was alive, was that a substantial portion of the hippocampus was actually retained. This means that the role that certain areas of the hippocampus play in memory may need to be reassessed.