Most of us learn how to ride a bike during childhood. But as we grow older, many of us stop riding and put those once-beloved bikes in storage. Years later, when we discover these relics and hop on, it’s as if we never stopped biking.
This is surprising because our memories let us down in so many other instances, such as remembering the name of a place or a person we once knew or where we put our keys. So how is it that we can ride a bicycle when we haven’t done so in years?
As it turns out, different types of memories are stored in distinct regions of our brains. Long-term memory is divided into two types: declarative and procedural.
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There are two types of declarative memory: Recollections of experiences such as the day we started school and our first kiss are called episodic memory. This type of recall is our interpretation of an episode or event that occurred. Factual knowledge, on the other hand, such as the capital of France, is part of semantic memory. These two types of declarative memory content have one thing in common—you are aware of the knowledge and can communicate the memories to others.
Skills such as playing an instrument or riding a bicycle are, however, anchored in a separate system, called procedural memory. As its name implies, this type of memory is responsible for performance.
One of the most famous studies showing the separate memory systems was that of an epileptic named Henry Gustav Molaison (aka H. M.). In the 1950s he underwent the removal of portions of his brain, including large parts of his hippocampus. After the operation doctors found that although the number of seizures had decreased, H. M. was unable to form new memories. Many of his memories of the time before the operation were also erased.
To learn more about his amnesia, neuropsychologists carried out various tests with H. M. In one, they asked him to trace a five-pointed star on a sheet of paper while only looking at it and his hand in a mirror—meaning the image was reversed. Although H. M.’s hand–eye coordination skills improved over the several days he performed this task, he never remembered performing it. This meant that he could develop new procedural, but not declarative, memories.
Is procedural knowledge then fundamentally more stable than explicit knowledge? As it turns out, the former is more resistant to both loss and trauma.
Even with traumatic brain injury the procedural memory system is hardly ever compromised. That’s because the basal ganglia, structures responsible for processing nondeclarative memory, are relatively protected in the brain’s center, below the cerebral cortex. However, it’s not clear, beyond brain damage, why procedural memory contents are not as easily forgotten as declarative ones are. According to one idea, in the regions where movement patterns are anchored fewer new nerve cells may be formed in adults. Without this neurogenesis, or continuous remodeling in those regions, it’s less likely for those memories to get erased.
One thing we know for sure, however, is simple sequences of movements we internalize, even far in the past, are typically preserved for a lifetime. Or as the saying goes, it’s “just like riding a bicycle.”