How do you make memories?
Inspirational websites will tell you to spend time with the people you love.
Travel websites will tell you to travel to beautiful places.
I’m just going to tell you to watch this episode of SciShow over and over and over
again until it actually sticks in there.
But if you ask a neuroscientist, they might tell you about a man named Henry Molaison.
In 1953, Molaison had surgery to remove certain parts of his brain, and lost his ability to
form most kinds of long-term memory -- changing our understanding of the human mind and memory
forever.
Up until the 1950s, we really had no idea how the human brain could convert an experience
into a memory that could be retrieved and relived.
Scientists had been trying to figure it out, but they didn’t have access to fancy technology
like fMRI scanners to let them look inside living human brains.
Based on what they did find out, from animal studies and the brains of people who had died,
they thought that memories might be stored throughout the brain.
That seemed to make sense, because patients with injuries to different parts of their
brains would sometimes develop amnesia.
So they had no clue what they were about to do to Henry Molaison.
When Molaison was a kid, he hit his head while riding his bike -- and after that, he started
having seizures.
A lot of seizures -- and severe ones.
His doctors tried all kinds of different treatments, but in 1953, he was 26 years old and none
of those treatments had woked.
But there was one more procedure that they thought might help: a surgery to remove the
part of his brain where the doctors thought they seizures were coming from.
So, they did the surgery, removing two finger-sized pieces of brain tissue from Molaison’s left
and right medial temporal lobes.
The specific parts they took out?
The hippocampus, amygdala, and part of the entorhinal cortex.
You might recognize some of those names, because we now know that they’re really important
parts of the brain.
But at the time, doctors had no way of knowing just how big a deal removing them was.
When Molaison woke up, he could remember his name and things that happened in his childhood,
but he had what’s known as anterograde amnesia.
Basically, he couldn’t form new memories anymore.
The doctors weren’t going to be able to give him his memory back.
But for the rest of Molaison’s life, they tried to learn as much from him as they could.
The main thing they found out was that the hippocampus plays a big part in the formation
and retention of certain kinds of memory.
Researchers also learned that there are multiple kinds of long-term memory, controlled by different
parts of the brain.
Long term memories are the memories we store for long periods of time - basically, anything
your brain retains after about 30 seconds.
And there are two kinds of long term memories: declarative, or explicit, and non-declarative,
or implicit.
Declarative memories are memories that require conscious processing, and the kind that Molaison
lost the ability to form.
These include episodic memories -- like the memory of your eleventh birthday party -- as
well as semantic memories, which are the facts and ideas.
Non-declarative memories, on the other hand, are memories of habit, like riding a bicycle
or tying your shoes.
After Molaison’s surgery, he couldn’t learn new facts or remember new events.
He’d meet a person and forget them as soon as they walked out the door.
But his doctors discovered that the different kinds of long-term memory must depend on different
brain structures, because Molaison could still form non-declarative memories.
For example, he could learn new motor skills, like tracing a drawing he was looking at in
a mirror, and his reaction times improved with practice.
For the rest of his life, Molaison was studied by dozens of doctors.
To protect his identity, he was referred to as H.M. in publications.
But after his death in 2008, his name was publicly released for the first time, and
the rest of the world began to understand just how much he’d taught us.
Even in death, Molaison continues to help us learn more about the brain.
His brain was donated to science, so researchers could examine his brain more closely and better
understand the effects of his surgery.
At his death, his brain was removed and flash-frozen before being cut into 2,401 microsections
- super thin slices to be mounted on slides for experimentation.
These sections were used to make a 3D recreation of his brain in 2014.
From that, we’ve already discovered that Molaison hadn’t actually lost his entire
hippocampus - just most of it.
But because it was cut off from the rest of the memory systems by his injury, this small
part of his hippocampus couldn’t help him regain his memory.
So, Molaison may not have been aware of just how important he was to science.
But his life and death are still teaching us all about memory and the human brain.
Thanks for watching this episode of SciShow.
I hope you’ve got it mostly in your memory, though things that happen more than 30 seconds
ago - can you remember how we started this episode really?
Can you?
Check, try and remember.
Something, . . . I can’t even remember.
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