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On September 1st, 1953,


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William Scoville used a hand crank
and a cheap drill saw

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to bore into a young man's skull,
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
or a gruesome police report.

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

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

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whose case provided amazing insights
into how our brains work.

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

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

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

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the desperate young man 
had turned to Dr. Scoville,

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a daredevil known for risky surgeries.

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Partial lobotomies had been used
for decades to treat mental patients

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based on the notion that
mental functions were strictly localized

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to corresponding brain areas.

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Having successfully used them
to reduce seizures in psychotics,

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Scoville decided to remove 
H.M.'s hippocampus,

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a part of the limbic system
that was associated with emotion

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but whose function was unknown.

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At first glance, 
the operation had succeeded.

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

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and his IQ even improved.

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But there was one problem: 
His memory was shot.

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Besides losing most of his memories
from the previous decade,

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

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

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When Scoville informed another expert,
Wilder Penfield, of the results,

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

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where he now spent his days
doing odd chores,

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and watching classic movies
for the first time, over and over.

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What she discovered through
a series of tests and interviews

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didn't just contribute greatly
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
on the obvious fact

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

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long enough from moment to moment
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 
for fifteen minutes

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by repeating it to himself constantly.

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

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Neuroscientists had though of memory
as monolithic,

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all of it essentially the same
and stored throughout the brain.

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Milner's results were not only the first
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 
different brain regions.

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We now know that memory formation
involves several steps.

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After immediate sensory data is temporarily
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 
the cortical synaptic connections.

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If the experience was strong enough,

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or we recall it periodically
in the first few days,

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

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

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but without a hippocampus
to perform this memory consolidation,

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they eroded, 
like messages scrawled in sand.

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But this was not the 
only memory distinction Milner found.

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

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in the narrow space between 
the outlines of two concentric ones

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

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

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he did horribly.

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But surprisingly, he improved over 
repeated trials,

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even though he had no memory
of previous attempts.

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

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

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is different from the procedural memory
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 
and cerebellum,

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

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This distinction between "knowing that"
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
a mostly peaceful life in a nursing home.

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

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making his the most 
studied mind in history.

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Upon his death, his brain was
preserved and scanned

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before being cut into over 2000
individual slices

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

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

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

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he and his contributions
to our understanding of memory

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will be remembered for 
generations to come.