“It’s like riding a bicycle, you can’t unlearn it!”
Have you heard that before? For some reason, there are things that stick with us our whole life, which we don’t seem to forget. Other things, like your friend’s birthday, seem to slip your mind a lot faster. What’s the difference between these two memories?
Implicit Memory
When you learn an instrument, wake up 30 seconds before the alarm rings every day or get hungry at the sound of a metronome, that’s all connected to the implicit or procedural memory.
These memories are accessed without really thinking about it, your body just seems to do it. Associative and non-associative learning, which I talked about yesterday, is also a part of the implicit memory. 
These memories are built through repetition. Did you ever hear of “muscle memory”? It’s basically that. Your muscles don’t remember, your brain does.
Did you ever have to think about how to walk? I’m sure most of you didn’t. It happens automatically, you don’t need to make a conscious effort, not like a child who is just learning to walk.
Explicit Memory
What did you have for dinner yesterday? When did Napoleon die? How many grains of sand are in the Sahara Desert? Did you know that Sahara already means desert so we’re calling this one “desert desert”?
While the implicit memory stores skills, habits, reflexes and similar, the explicit or declarative memory stores facts and events.
So if you forget your mother’s birthday, forget to bring out the trash on the correct day, don’t blame your whole brain. Just blame your explicit memory. That bitch has probably been ruining your life since you were born.
The Human Experiment
No good research without experiments, am I right? How about experiments on humans?
When someone had epileptic seizures, doctors used to cut out parts of the brain or separate the two hemispheres. In the case of patient H.M., they cut out a big chunk (the hippocampus, amygdala and the parahippocampal gyrus for those who want the details).
H.M. had been suffering from seizures since he was 10 which became worse after the age of 16. At 27, in 1953, it impacted his life so much that he agreed to an experimental surgery. Both he and his family had to sign off on this.
After they had cut out the beforementioned pieces of his brain, H.M. lost about 10 years of memories - and he couldn’t form new memories. You could talk to him and the moment he turned away, he had already forgotten you.
But!
He was able to train himself for a task.
The researchers gave H.M. a pencil and he had to draw a star but could only see his hand through a mirror. Drawing like this is really hard, but you can practice and become better at it. And H.M. did improve over time. In fact, he could still do it after three days without practicing.
He just didn’t remember that he had done this before, ever.
Removing these specific parts of his brain seemed to have removed his explicit memory, but his implicit memory was still working fine!
This is often the case with anterograde (= “directed forwards in time”) amnesia. Those who suffer it might not know what a violin is, but if they were able to play it before and you give it to them, they might still manage to play it.
H.M. died in 2008 after having experiments performed on him for most of his life.
The Animal Experiments
H.M. was only one point of data; proper research needs far more than that. But you can’t just go around and cut out people’s brains. There are laws against it.
Animals are a bit less protected, although the rules are still pretty strict. Researchers took two groups of mice (and rats). One with their brain intact, the other with the same regions removed that had been taken away in the case of H.M.
Then they threw (gently placed) them in a pool of water.
To quote the animal welfare officer: “Mice swim - but they don’t like it”.
Swimming is stress for a mouse. Even more so if it doesn’t know if it will ever get out. In this experiment, there was a platform placed inside the pool, just below the water surface. But the mice couldn’t see this. They had to swim around until they managed to find it.
This experiment was repeated in two ways:
In the first case, the normal mice needed less time to find the platform after several tries. They seemed to remember the position of the platform inside the pool. The mice with the brain surgery … they needed about the same time in every case. They didn’t remember.
But in the second version of the experiment, even the mice with brain surgery improved their times! It seemed like having to swim the same path over and over again is something controlled by the implicit memory. The mice probably didn’t consciously remember (but how are we supposed to find out, ask?), but their “muscle memory” lead them on the correct path.
The Legacy of Patient H.M. for Neuroscience
Patient HM review – a botched lobotomy that changed science
Declarative Memory: Definitions & Examples
Lecture “Bewusstes und Unbewusstes Lernen” by Uli Müller
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