How A Severely Amnesic Woman Is Helping Us Discover A New Type Of Memory

A new type of memory?

Memory and knowledge are typically broken down into two types: knowing “what” and knowing “how.” The first is called explicit or declarative memory, such as memories of a person’s own life and general facts about the world. This is knowledge you are very aware of and can talk about. The other type of memory, non-declarative, is commonly called implicit memory. This is information that’s difficult to verbalize but enables someone to ride a bike or, in case of an artist like Johnson, to bow a viola or apply skilled brushstrokes.

When Landau and her colleagues met Johnson about six years ago, they were struck by how profound and severe her memory loss was. She couldn’t remember much of her past life. She had lost most of her general knowledge, even in areas where she was once an expert.

“Yet we found this really interesting contrast when we spoke to her about how to do things that she knew how to do. She was suddenly very articulate and full of information,” said Michael McCloskey, a cognitive scientist who worked on Johnson’s case and a co-author of the new study. “We thought it was quite striking that that category of knowledge seemed to be spared, despite her severe losses of other kinds of general knowledge.”

These findings pointed to a special category of memory: One that is explicit, dealing with facts, but is closely related to skills — in other words, a type of “skill-related knowledge.” And this category may be treated differently inside the brain, the scientists thought. 

So they decided to test Johnson’s skill-related memory. They tested her memory for facts related to performing her top skills — art, music, flying and driving.  

For each test, scientists asked her about 80 questions on techniques and equipment in each field. For example, “How is a crescendo represented in sheet music?” or “What three major steps must you complete in order to get a pilot’s license?” 

They then compared Johnson’s results with those of other people taking the same tests. In art and driving, Johnson scored nearly as high as people with the same level of expertise as hers. In music and aviation, she performed worse than the experts but significantly better than amateurs. 

“We found that although she had losses in these areas, they were much more spared than all the other sorts of general knowledge,” McCloskey said.

Where memories live in the brain 

In many ways, Johnson’s case resembles that of Henry Molaison, a man who had his hippocampus removed in 1953 to relieve him of debilitating seizures. After the surgery, Molaison’s seizures stopped, but he was no longer able to form new memories. He became an icon in neuroscience history by participating in follow-up studies for the rest of his life, which taught researchers about not only the importance of the hippocampus but also the nature of memory itself.

Molaison lived in the now. Doctors had to reintroduce themselves to him every time they met. But despite his inability to register new information, Molaison showed signs of learning. For example, if he practiced a drawing test, he did better next time — even though he had no memory of having ever done the test. This suggested that there’s a kind of implicit memory, a “muscle memory,” that doesn’t rely on the hippocampus to be registered by the brain.