Many Different Kinds of Memory

Memory is Plural
Different structures in the brain handle different kinds of memory

Long-Term Memory:
Why repetition, rehearsal and practice work so well
The problem with short-term memory is that it’s, well, short-term. The phonological loop component of working memory can only hold something in our “mind’s ear” for a few seconds. As soon as we dial the phone number, it’s forgotten, because we stop repeating it in our mind. If you want to retain the number longer, you have to do something more.

One of the ways to transfer information (like a telephone number) from your mind’s ear to your long-term memory is to return to it again and again (like the phone number of a close friend). Repetition’s ability to consolidate knowledge in long-term memory works not just for declarative memory — memory of facts and events — but also for procedural knowledge of skills such as remembering how to use a computer mouse or (in countries and states where it’s still legal to do so) how to dial a cell phone while driving.

Moreover, the most recent research indicates that these bits of information our brain pays special attention to during the day will also be rehearsed at night when we’re asleep. So in addition to performing “live” repetitions of knowledge or a skill, our brain rehearses things offline as well — which is as good a reason as any to avoid skimping on sleep.

Practice makes perfect
Neuroscientists have only recently figured out how the brain forgets most of the data flooding through our minds every second of every day, and yet allows us to remember those things that we practice and rehearse. But it’s important to bear in mind that consolidation of knowledge in long-term memory does not guarantee it will stay there forever. If the same phone number is dialed every day, it will be memorized for the time being. But stop dialing it for any length of time and it’s back to the phone book again. Cramming can work well if all you’re worried about is passing the test. But it won’t help much if you want to recall that knowledge a year later. In order to retain access to most factual knowledge, we must remind ourselves of the details, and use them, on an ongoing basis.

To sum up: Short-term memory traces fade quickly unless they are maintained by repetition, rehearsal, and practice. By coming back to information time after time, we can transfer it into our long-term memory banks even if it’s as arbitrary as a PIN or combination lock number. But is there any way we can help the memory process along and steepen the learning curve? The answer is yes, but the important thing to remember in this connection is that memory does not work like a camera or tape recorder. We’re making a mistake if we think of our brain as a passive recipient of information. As memory researcher Alan Baddeley (1999; Essentials of Human Memory. Hove, UK: Psychology Press.) puts it, the central feature of human learning is that it is dependent on organization.

SAD The Science: How Researchers Use Sea Slugs and Fruit Flies to Study Human Memory
Since H.M.’s (H.M. was a man who developed amnesia after a part of his brain was removed in an operation to cure his epileptic seizures) case taught researchers about multiple memory systems, a tremendous amount of detailed knowledge has been gathered about how different kinds and stages of memory work in the brain, on a fine-grained chemical and structural level.

Most of this work was done, believe it or not, by studying such creatures as sea slugs and fruit flies. Sea slugs have the advantage of possessing a small number of very large neurons that happen to work by the same principles as ours. So by studying very specific parts of very simple animal systems, scientists were able to learn a lot about much more complex human memory-creating and memory-storing systems.

Of course, there are some kinds of memory that humans have and sea slugs and fruit flies don’t. Sea slugs can learn to stop reacting if you prod them repeatedly in the gill, a change in behavior relying on a simple type of learning and memory called habituation. Humans can handle this kind of learning while they’re still in the womb. Fruit flies can learn to associate an odor with an unpleasant shock if the two occur repeatedly together, a form of learning and memory called classical conditioning. These kinds of memory have been around for hundreds of millions of years, far longer than humans have existed on this earth. They are useful enough that evolution doesn’t dispense with them, it just builds on top of them as new species evolve.

But humans have other kinds of nondeclarative memory, such as skill and habit learning (procedural memory), as well as conscious forms of declarative memory for events (episodic) and facts (semantic). Some of these kinds of memory depend (as H.M. demonstrated) on the hippocampus and other nearby structures that sea slugs and fruit flies don’t have. So how could you study these kinds of memory in an animal model?

Even though non-human species can’t “declare” anything, some of them do have a hippocampus, and they display evidence of episodic memory when their behavior shows that they remember having been in a certain location before. Birds have a hippocampus and spatial memory, too. So researchers have been able to study these other, hippocampus-dependent forms of memory in animals such as monkeys and rodents. Using animals more complex than sea slugs, then, scientists have been able to study declarative memory on a detailed, molecular level as well. It turns out that, even though declarative memory uses different parts of the brain from nondeclarative memory, all kinds of learning share the same essential molecular mechanisms for converting experience into permanent structural changes in the brain.

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