Friday, December 5, 2008

H.M. Has Passed Away

In the news today, H.M. has passed away. "H.M.", well known by anyone who has ever studied brain science or neuroscience, are the initials for a patient who had radical surgery in 1953 in order to stop his debilitating epilepsy seizures. The surgery involved removing both Hippocampus and Amygdala on both sides.

This did indeed stop the seizures, but it also gave him profound anterogade amnesia - he could no longer form new memories. He could remember most things up until a few days before his surgery, but could not form any new long-term memories after this point. Think about it: every morning he woke up believing it was still 1953 and he was still 27. Doctors and nurses that had seen him for years had to introduce themselves to him if they left his side for a few hours.

He became one of the most studied people in modern medicine, and his tragic case taught us an enormous amount about how memory actually works. Generations of brain researchers know about him and the research his case has spawned. It is through him, for instance, that we know that semantic, or episodic memory - remembering events, for instance - is different and distinct from short-term memory and procedural memory. You can learn to do something without ever remembering having learned it.

At one time a researcher brought H.M. a Tower of Hanoi puzzle. H.M. cheerfully admitted to never having heard of the puzzle in his life. He worked on the puzzle until he somehow managed to solve it, after which the researcher left, taking the puzzle with him. The next day the researcher returned. For H.M. this was all new again: he had never heard of the puzzle (or the researcher) in his life. He worked on the puzzle until he solved it, and the researcher left.

This went on for some days. And every day, H.M. would manage to solve the puzzle faster and better until he clearly had "got" the way you solve it. But while he clearly had learned the optimal solution to the puzzle - procedural memory - he still had no recollection of ever having heard of the puzzle. Procedural memory and declarative memory are different systems in the brain.

In 2002 Nature Neuroscience published a review article (PDF) that summarizes half a century of research with H.M. and his life during that period. It's well worth reading.

Henry G. Molaison, now no longer anonymous, passed away at age 82. In accordance with his wishes, a detailed autopsy will be performed on his brain.

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