The Three Knowledge-Memory Systems That Guide Your Life

When discussing memory systems, it seems the most common point made is regarding the difference between short- and long-term memory. It seems pretty well-known that short-term memory operates over a period of seconds to minutes and has limited operating space (i.e., the famous 7 +/- 2 items), whereas long-term memory has a potentially infinite storage capacity and the material placed in long-term memory is much more stable.

In my experience, people are much less cognizant of the fact that knowledge is stored in three different kinds of long-term memory systems. Yet this is crucial because it speaks volumes about how our minds are organized.

To get a handle on these memory systems, let me ask you three questions:

1. Do you know if wearing a helmet while riding your bike is a law or not?
2. Do you recall when you first were learning how to ride a bike, perhaps the time you skinned your knee?
3. Can you hop on a bike and ride it now without a problem?

The knowledge you have in regard to these questions comes from three very different memory systems. If you knew the answer to the first question about helmet laws, you retrieved it from your semantic memory system. The semantic system stores your factual knowledge about rules, norms, math or logic, and historical events. Answers to such questions as, “Who was Jimmy Carter’s vice president?” “How many protons are in a Helium atom?” “What is six cubed?” and “What is the legal drinking age in Canada?” are all stored in your semantic knowledge system. Think of the semantic system as your stored encyclopedia of definitional and conceptual knowledge.

Now go back to the time you skinned your knee. Maybe you recall your dad running behind your bike, the thrill you felt as he let you go, then the fear as the bike wobbled, the pain as your knee struck the pavement, and the comfort your dad offered as he ran up to you. The episodic memory stores your sensory-perceptual-affective experiences. They are normally stored as visual gestalts (sequenced images), from a particular point of view (yours), and are usually stored based on their affective valence (the stronger the emotional association with the experience, the greater the likelihood you will recall—you almost certainly don’t remember the third time you rode your bike around the neighborhood without falling). Episodic-affective memories are the kind that are inhibited when people are trying to force themselves to forget some distressing experience (e.g., PTSD vets avoiding flashbacks).

Although stored in different systems, both semantic and episodic memories are called “declarative” memory systems. This basically means that they are accessible to self-conscious awareness and people can report (i.e., declare) that they are present (or not).

Now, for the third question. Tell me, exactly, how it is that you are able to ride a bike. People answer this the same way. “I basically get on the bike and ride it," which, of course, does not answer the question at all. At most, people will have some basic conceptual rules (e.g., it is important to get one’s speed up quickly because it is a lot easier to balance while you are moving). But even these basic rules may not be present. Complicated action patterns are stored in a totally different memory system called procedural memory.

Procedural memories are largely nonconscious, as is evident by the fact that you can’t introspect and see them. The fact that procedural learning can take place completely independently of the more conscious, declarative memory systems was brought into very clear relief by one of the most famous patients in psychology, HM.

HM had bad seizures and was operated on in a way that knocked out his ability to lay down new conscious memories. Thus, if you came in and saw HM, asked him a few questions, left for an hour, and came back, he would not recall you at all. However, researchers found they could teach HM procedures, such as drawing in a mirror or playing a game. He would deny he had any recollection of doing such activities, yet he was able to learn them as almost effectively as someone who had full conscious recall abilities, hence the clear separation of procedural from declarative memory systems.

The main point to understand is that you navigate your world by integrating these systems to act with purpose as a coordinated whole.