Sunday, May 11, 2014

What Happens When a Neurosurgeon Removes Your Hippocampus

Excerpts from the book The Tale of the Dueling Neurosurgeons: The History of the Human Brain as Revealed by True Stories of Trauma, Madness, and Recovery:

Her battery of tests confirmed Scoville’s basic observations pretty quickly: H.M. had little memory of the past and no ability to form new memories going forward. This was already a big advance — proof that some parts of the brain, namely the hippocampus, contribute more to forming and storing memories than other parts. And what Milner discovered next redefined what “memory” even meant.

She gave him a piece of paper with two five- pointed stars on it, one nested inside the other: The outer star was about six inches wide, and there was a half- inch or so gap between them. The test required H.M. to trace a third star between the two with a pencil. The catch was, he couldn’t see the stars directly: Milner had shielded the diagram, and he had to look at them in a mirror instead. Left was right, right was left, and every natural instinct about where to move his pencil was wrong. Anyone taking this mirror test for the first time makes a mess — the pencil line looks like an EKG — and H.M. proved no exception.

Somehow, though, H.M. got better. He didn’t remember any of the 30 training sessions Milner ran him through. But his unconscious motor centers did remember, and after three days he could trace the star in the mirror fluently. He even commented near the end, “This is funny … I would have thought it would be rather difficult, but it seems I’ve done pretty well.”

Milner remembers the star test as a eureka. Before this, neuroscientists thought of memory as monolithic: the brain stored memories all over, and all memory was essentially the same. But Milner had now teased apart two distinct types of memory. There’s declarative memory, which allows people to remember names, dates, facts; this is what most of us mean by “memory.” But there’s also procedural memory — unconscious memories of how to pedal a bicycle or sign your name.

Tracing the stars proved that H.M., despite his amnesia, could form new procedural memories. Procedural memories must therefore rely on distinct structures within the brain. This distinction between procedural and declarative memories (sometimes called “knowing how” versus “knowing that”) now undergirds all memory research.

Scientists also discovered that time worked differently for H.M. Up to about 20 seconds, he reckoned time as accurately as any normal person. After that, things veered wildly. Five minutes lasted, subjectively, just 40 seconds for him; one hour lasted three minutes; one day 15 minutes. This implies that the brain uses two different timekeepers — one for the short term and one for everything beyond 20 seconds, with only the latter suffering damage in H.M. Eventually more than one hundred neuroscientists examined H.M., making his probably the most studied mind in history.

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