HM is one of the most famous patients in neuroscience (and perhaps all of medicine, for that matter). His case essentially started research into the neurobiological basis of memory in humans.

The case starts when HM was about 18 years old. As a teenager, HM suffered from severe epilepsy that couldn't be treated with medications. He was on just huge amounts of anticonvulsants--doses at the very top of the therapeutic window--and he still had grand mal seizures several times a day.

Now, in epilepsy, seizures often start at a particular point in the brain (known as the epileptic focus) and then spread out to the rest of the brain from there. Neurosurgeons had known for many years that if they cut out the epileptic focus, the patient often improved. Even better, the surgery has little or no effect on the patient's mental abilities, perhaps because the brain tissue they're removing was damaged in the first place. (Besides, even if the surgery did cause some problems, which would you rather have: a slightly reduced IQ, or constant seizures?)

HM's seizure focus was in the medial temporal lobes. His surgeon, a doctor by the name of William Scoville, went in and destroyed the medial temporal lobes on both sides of HM's brain, removing the amygdala, part of the hippocampus, and a reasonable chunk of the surrounding brain tissue.

HM woke up from the surgery, and lo and behold, he was pretty much cured. He wasn't having anywhere near as many seizures as before, so the surgery--at least to that extent--was a success. Unfortunately, he was also severely and profoundly amnesic. He just couldn't remember much of anything at all. You could go in, introduce yourself, shake hands, and talk for a while, and if you left the room and came back in he'd have absolutely no idea who you were. His memory deficit seemed to be as severe as that of an Alzheimer's patient.

Later on, though, some tests showed that he could learn some things. One such task was the mirror-drawing task. On this task, the patient sits down in front of a mirror and is given a pencil and a line drawing of a star. Then he's asked to trace the star while looking at the reflection of his hand and the paper in the mirror.

This test messes with your mind. When people try to do this, they move the pencil left when they mean to go right, up when they mean to go down, and so on. When HM first tried this task, he did quite poorly, just as most people do. Then, as he did it over and over again, day after day, he got better and better.

Nonetheless, when the doctors asked HM about this test, he would insist that he had never seen it before.

Thus, HM can learn to do this task even though he can't consciously remember it at all. True, he doesn't learn as quickly as a healthy person, but he can still learn it, even if he doesn't know he learned it. These results suggested that there were two different kinds of memory: explicit memory, which involves conscious recall for events and requires the medial temporal lobes, and implicit memory, which involves unconscious recall and doesn't require the medial temporal lobes.

Think about the reasoning that led to this claim, though. We have a patient whose medial temporal lobes were removed, and we observe that he has certain memory problems. From that, researchers concluded that the medial temporal lobes must carry out those functions in healthy humans. But HM's medial temporal lobes were abnormal in the first place--that's why he had them removed! Thus, HM's case does not by itself prove that the medial temporal lobes are involved in explicit memory.

That caveat aside, there is now a wealth of other evidence that shows that the medial temporal lobes have an important role in memory. The medial temporal lobes degenerate in Alzheimer's disease, for example, and other patients with medial temporal damage--from strokes or head injuries or, in one case, a fencing foil through the nose--also have amnesia.

As of 2000, HM was still alive in a hospital near MIT. People still conduct research on him, although he's apparently no longer very healthy.

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