How does memory work?

Dr. William Snider, director of the Neuroscience Center at UNC-Chapel Hill School of Medicine, explains how memory works and why some people are better at remembering things than others. Questions and answers have been edited.

Q: Are different types of memories stored differently?

Yes, so first of all you have to consider that there are different kinds of memories. One example is if somebody asks you to remember a phone number. That kind requires you to remember things that can be expressed in language, and is called explicit memory or declarative memory. Another kind of memory would be the memory of how you ride a bicycle, which is asking you to recall something that you may not even be consciously aware of. That kind is called implicit memory.

The other thing that we usually talk about is short-term or working memory and long-term memory. Short-term memory would be if I asked you to remember a phone number for five minutes; long-term would be if I asked you to remember that phone number a year from now.

Q: How does the brain store memories?

There is a structure in the brain called the hippocampus that is key to short-term memory. If you were to have damage to your hippocampus from a stroke, for instance, you could be talking to somebody, and literally five minutes later you will forget everything about that conversation. You essentially can't commit anything to memory. Same thing with Alzheimer's disease; memory problems are one of the first manifestations of the disease because one of the major areas that is damaged is the hippocampus.

Long-term memory involves many parts of the brain called association cortices, which are large regions of the cerebral cortex not directly responsible for sensation or movement. For example, the motor cortex has a narrow part that is directly involved in moving a limb and then a much bigger association cortex that integrates a movement with sensory information, processes memories of the movement, and so on.

One idea is that the hippocampus encodes short-term memories and then sort of "hands them off" to the association cortices for long-term memory storage. How it takes place or the biochemical details of that are not entirely clear.

Q: Is there a way to look at the brain and actually see where memories are stored?

People are starting to think that there may in fact be these "structural correlates" of long-term memory, in something called dendritic spines. Our neurons are just studded with these little protrusions or spines, and there is a growing belief that large dendritic spines make the connections between neurons more effective and may help to store long-term memories. Short-term memories are more likely due to biochemical or electrical changes that can make connections stronger.