Henry Molaison, or patient H.M. as he is popularly known, provided much of the basis of modern neuroscience.
H.M. is known to have suffered a head injury at 9 years old. That, along with a family history of epilepsy, he started to develop debilitating seizures at the age of 10. In 1953, at 27 years old, in order to curb the effects of his seizures, he underwent a bilateral (both right and left) medial temporal lobe resection. The procedure was performed by neurosurgeon William Scovillem in Hartford, Connecticut. In 1993, after some scrutiny of the MRI scans, it was found that Scovillem had in fact removed H.M.’s amygdaloid complex, perirhinal, entorhinal and parahippocampal cortices, the anterior portion of the hippocampal formation and the uncus, as shown below.
The effects of the surgery were almost immediately evident. Although he was not having seizures, the consequences had a severe impact on his memory retention. H.M. was unable to create any new memories and he was forgetting events as fast as they occurred. For instance, H.M. could be having a short, proficient conversation with his doctor but if they then left the room, upon their return H.M. would be unable to recognize them. However, his memories prior to the surgery were easily accessible. From a scientific point of view, it has been regarded that his tragic memory loss, clinically referred to as anterograde amnesia, offered an unprecedented opportunity to study the human brain. Studies, that would go on to garner global attention.
Scientists began to investigate this more extensively, and further testing revealed he was able to learn and retain new motor skills, despite not remembering completing the task previously. Unusually, he was also able to recall the events of World War II and the president at that time but was unable to recall the current president.
His case also provided impetus for scientists to distinguish between different types of memories; it was found that he was incapable of storing new information in his explicit memory (the type of memory allowing conscious remembering of new information and experiences), while his procedural memory was intact (allowing him to perform motor activities). It also highlighted the difference between memory encoding and consolidation along with memory retrieval.
Another lesson worth mentioning was the now common understanding of the human brain being modular; specific brain areas are responsible for specific functions. Scoville, along with neuroscientist Brenda Milner who followed H.M.’s case for years, also concluded that the hippocampus and adjacent structures were crucial for formation of new memories, but not essential for short-term or procedural memory.
In 2008, Henry died of respiratory failure. His brain was then moved to San Diego where it was dissected into 2,401 slices, which were subsequently scanned and digitized to create an atlas (which can be accessed, upon registration, here)
The post-mortem studies also revealed the consequences of H.M.’s surgery and went on highlight the role of the hippocampus in memory formation.
H.M. is arguably the most studied human and his tragedy was perhaps the single most significant advance in understanding memory and the human brain. “It’s a funny thing”, Henry once quipped, “you live and learn. I’m living and you’re learning.”
If you are interested in learning more about his life and his ever-fascinating case, here are some further resources you may find useful:
Spencer Lowell https://www.discovermagazine.com/mind/the-art-and-science-of-slicing-up-a-human-brain
Edited by Malavika Ramanand