The fractionation of human memory

A recurrent theme in the study of human memory over the last 20 years has been the question of whether it should be regarded as a unitary system, or as a collection of two or more subsystems. After a few years of relative quiescence, this topic has begun to move back into the theoretical limelight and, as has often been the case in this area, the stimulus for theoretical development has come from the study of clinical evidence from patients with memory deficits. Before going on to discuss these recent developments, it would perhaps be helpful to outline the earlier background to the controversy. (A more detailed discussion is given in Baddeley, 1983a, chapters 9-11.) Up to and including the 1950s, the study of memory was largely concerned with the learning of lists of words or paragraphs of prose and their retention over periods ranging from a few minutes to a few years. Memory was, implicitly at least, regarded as a single unitary system. During the latter half of the 1950s, however, a number of investigators began to observe that even small amounts of material would show signs of being forgotten over a matter of seconds, provided that the subject was prevented from continually rehearsing it, and they postulated a separate short-term memory system to account for their results (e.g. Broadbent, 1958). The 1960s saw the development of a controversy as to whether it was, in fact, necessary to assume separate systems to account for these new results, or whether existing theories could explain both sets of data (Melton, 1963). This produced a great deal of experimental work attempting to look for clear empirical evidence for or against two separate memory systems. Many apparent differences were observed, with the following three being among the more theoretically cogent.