How Many Human Memory Systems?

Appearing in over 125,000 peer-reviewed journal articles[1], the word “memory” may one of the most frequently used terms in the psychology literature. Despite the enormous effort spent researching the concept of memory, a unified picture of the number, type, and defining traits of human memory systems has not yet emerged.

One source of difficulty is that memory can be defined along a variety of dimensions, such as content (e.g., episodic vs. semantic vs. procedural), stage of processing (e.g., consolidation vs. retrieval), accessibility to consciousness (e.g., implicit vs. explicit), and time (e.g., short-term versus long-term) (Brand & Markowitsch, 2006).

A second difficulty is that many of these distinctions are operational, in which a concept is defined in terms of the methods used to demonstrate it. At their extreme, operational definitions can run the risk of describing differences that reflect the operations themselves more than real underlying differences in the systems they measure. So, which of these dimensions capture inherent differences between memory systems, and which capture only the differences between the diverse methods used in psychology?

The answer to this question rests on the definition of “system.” In the next few posts, I will be examining the traditional distinctions made between memory systems according to a new definition, one that I hope will provide much needed clarity to the study of memory as a whole.

I have defined a memory system as any persistent effect of experience which has both
  1. unique computational requirements or characteristics, and is therefore frequently anatomically distinct from other brain regions, and
  2. unique functional characteristics, such that damage to the system results in a complete deficit (or pattern of deficits) that would not be caused by other types of damage.

With this framework in mind, this week's posts will review the evidence for each of the traditional divisions between memory systems, and then account for all of this evidence with a simple three-system theory of human memory.

[1] This statistic comes from a PsycInfo search performed on 4/15/2006, indexing most psychology journals from the last 40 years.


Brand, M., & Markowitsch, H. J. (2006). Amnesia I: Clinical and anotomical issues. In M. J. Farah & T. E. Feinberg (Eds.), Patient-based approaches to cognitive neuroscience (pp. 289-301). Cambridge, MA: MIT Press.


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