What happens when you remove the hippocampus?

On September 1st, 1953,

William Scoville used a hand crank
and a cheap drill saw

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

and sucking them out through a metal tube.

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

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

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

whose case provided amazing insights
into how our brains work.

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

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

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

the desperate young man
had turned to Dr. Scoville,

a daredevil known for risky surgeries.

Partial lobotomies had been used
for decades to treat mental patients

based on the notion that
mental functions were strictly localized

to corresponding brain areas.

Having successfully used them
to reduce seizures in psychotics,

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

a part of the limbic system
that was associated with emotion

but whose function was unknown.

At first glance,
the operation had succeeded.

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

and his IQ even improved.

But there was one problem:
His memory was shot.

Besides losing most of his memories
from the previous decade,

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

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

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

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

where he now spent his days
doing odd chores,

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

What she discovered through
a series of tests and interviews

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

It redefined what memory even meant.

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

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

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

When Milner gave him a random number,

he managed to remember it
for fifteen minutes

by repeating it to himself constantly.

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

Neuroscientists had though of memory
as monolithic,

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

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

between short-term and long-term memory,

but show that each uses
different brain regions.

We now know that memory formation
involves several steps.

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

it travels to the hippocampus,

where special proteins work to strengthen
the cortical synaptic connections.

If the experience was strong enough,

or we recall it periodically
in the first few days,

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

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

but without a hippocampus
to perform this memory consolidation,

they eroded,
like messages scrawled in sand.

But this was not the
only memory distinction Milner found.

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

in the narrow space between
the outlines of two concentric ones

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

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

he did horribly.

But surprisingly, he improved over
repeated trials,

even though he had no memory
of previous attempts.

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

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

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

And we now know that procedural memory

relies more on the basal ganglia
and cerebellum,

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

This distinction between "knowing that"
and "knowing how"

has underpinned all memory research since.

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

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

making his the most
studied mind in history.

Upon his death, his brain was
preserved and scanned

before being cut into over 2000
individual slices

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

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

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

he and his contributions
to our understanding of memory

will be remembered for
generations to come.