Name: 【TED-Ed】What happens when you remove the hippocampus? - Sam Kean
Uploaded: 2014-11-30T12:56:06.000Z
Duration: 5 min 26 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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,

Partial lobotomies had been used for decades to treat mental patients

based on the notion that mental functions were strictly localized

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

At first glance,  the operation had succeeded.

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

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.

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.

he managed to remember it  for fifteen minutes

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

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,

where special proteins work to strengthen  the cortical synaptic connections.

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,

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.

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.