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On September 1st, 1953,[br]

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William Scoville used a hand crank[br]and a cheap drill saw

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to bore into a young man's skull,[br]cutting away vital pieces of his brain

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and sucking them out through a metal tube.

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But this wasn't a scene from a horror film[br]or a gruesome police report.

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Dr. Scoville was one of the most [br]renowned neurosurgeons of his time,

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and the young man was Henry Molaison,[br]the famous patient known as "H.M.",

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whose case provided amazing insights[br]into how our brains work.

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As a boy, Henry had cracked [br]his skull in an accident

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and soon began having seizures, blacking out[br]and losing control of bodily functions.

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After enduring years of frequent episodes,[br]and even dropping out of high school,

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the desperate young man [br]had turned to Dr. Scoville,

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a daredevil known for risky surgeries.

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Partial lobotomies had been used[br]for decades to treat mental patients

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based on the notion that[br]mental functions were strictly localized

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to corresponding brain areas.

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Having successfully used them[br]to reduce seizures in psychotics,

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Scoville decided to remove [br]H.M.'s hippocampus,

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a part of the limbic system[br]that was associated with emotion

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but whose function was unknown.

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At first glance, [br]the operation had succeeded.

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H.M.'s seizures virtually disappeared,[br]with no change in personality,

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and his IQ even improved.

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But there was one problem: [br]His memory was shot.

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Besides losing most of his memories[br]from the previous decade,

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H.M. was unable to form new ones,[br]forgetting what day it was,

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repeating comments,[br]and even eating multiple meals in a row.

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When Scoville informed another expert,[br]Wilder Penfield, of the results,

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he sent a Ph.D student named Brenda Milner[br]to study H.M. at his parents' home,

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where he now spent his days[br]doing odd chores,

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and watching classic movies[br]for the first time, over and over.

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What she discovered through[br]a series of tests and interviews

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didn't just contribute greatly[br]to the study of memory.

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It redefined what memory even meant.

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One of Milner's findings shed light[br]on the obvious fact

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that although H.M. couldn't form new memories,[br]he still retained information

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long enough from moment to moment[br]to finish a sentence or find the bathroom.

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When Milner gave him a random number,

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he managed to remember it [br]for fifteen minutes

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by repeating it to himself constantly.

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But only five minutes later,[br]he forgot the test had even taken place.

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Neuroscientists had though of memory[br]as monolithic,

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all of it essentially the same[br]and stored throughout the brain.

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Milner's results were not only the first[br]clue for the now familiar distinction

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between short-term and long-term memory,

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but show that each uses [br]different brain regions.

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We now know that memory formation[br]involves several steps.

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After immediate sensory data is temporarily[br]transcribed by neurons in the cortex,

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it travels to the hippocampus,

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where special proteins work to strengthen [br]the cortical synaptic connections.

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If the experience was strong enough,

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or we recall it periodically[br]in the first few days,

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the hippocampus then transfers the memory[br]back to the cortex for permanent storage.

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H.M.'s mind could form[br]the initial impressions,

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but without a hippocampus[br]to perform this memory consolidation,

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they eroded, [br]like messages scrawled in sand.

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But this was not the [br]only memory distinction Milner found.

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In a now famous experiment,[br]she asked H.M. to trace a third star

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in the narrow space between [br]the outlines of two concentric ones

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while he could only see[br]his paper and pencil through a mirror.

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Like anyone else performing such [br]an awkward task for the first time,

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he did horribly.

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But surprisingly, he improved over [br]repeated trials,

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even though he had no memory[br]of previous attempts.

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His unconscious motor centers remembered[br]what the conscious mind had forgotten.

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What Milner had discovered was that the[br]declarative memory of names, dates and facts

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is different from the procedural memory[br]of riding a bicycle or signing your name.

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And we now know that procedural memory

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relies more on the basal ganglia [br]and cerebellum,

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structures that were intact in H.M.'s brain.

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This distinction between "knowing that"[br]and "knowing how"

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has underpinned all memory research since.

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H.M. died at the age of 82 after[br]a mostly peaceful life in a nursing home.

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Over the years, he had been examined[br]by more than 100 neuroscientists,

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making his the most [br]studied mind in history.

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Upon his death, his brain was[br]preserved and scanned

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before being cut into over 2000[br]individual slices

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and photographed to form a digital map[br]down to the level of individual neurons,

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all in a live broadcast[br]watched by 400,000 people.

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Though H.M. spent most of his life[br]forgetting things,

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he and his contributions[br]to our understanding of memory

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will be remembered for [br]generations to come.