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
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
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
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
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.