The processes of learning and memory are often subdivided into stages of encoding (initial learning of information), storage (maintaining information over time), and retrieval (using stored information). Processes of encoding establish some representation of experience in the nervous system, which is referred to as an engram or memory trace. Memory traces certainly have physiological underpinnings, but cognitive psychologists use the construct as an abstraction to refer to the changed state of the cognitive system before and after some experience. Retrieval processes refer to the means of accessing stored information and can be affected by a variety of factors.

Retrieval is the key process in the act of remembering (Roediger, 2000). Most experiences of life are encoded and stored (at least briefly) but will never be retrieved and thus will have no real consequence for the individual. Encoding and storage are cheap, in the...