BEHAVIORAL AND BRAIN SCIENCES (2001), 24 (1), 134
(C) 2001 Cambridge University Press
The nature of forgetting from short-term memory
Department of Psychology, University of Toronto, Toronto, Ontario M5S 3G3, Canada
Abstract: Memory and forgetting are inextricably intertwined. Any account of short-term memory (STM) should address the following question: If three, four, or five chunks are being held in STM, what happens after attention is diverted?
Assuming that the central thesis of Cowan's (2001) paper is correct, the question remains: Given that three, four, or five chunks are in STM, are negligibly registered in long-term episodic memory, and have been erased from sensory memory, what will be the nature of the forgetting when attention is diverted? In the third paragraph of his paper, Cowan raises the issue of the nature of forgetting from STM, and somewhat peremptorily dismisses it as "nearly intractable" (para. 3), and beset with difficulties, such as the "apparent unresolvability of the decay issue" (para. 3). Is the question of the nature of forgetting from STM any more intractable than the question of the capacity of STM?
Of course, some issues regarding the nature of forgetting from STM are covered, both explicitly and implicitly, in Cowan's paper, but the above question is largely ignored, and is a remarkable lacuna in the discussion. Memory and forgetting are always inextricably intertwined.
Over the decades there have been hundreds of attempts to answer approximations to the above question. Many of these attempts have been concerned with the rate of forgetting from STM. A study by Peterson and Peterson (1959) was quite typical: As Cowan mentions in passing, in a serial recall task Peterson and Peterson found severe forgetting of three letters after 18 seconds of distracting activity. This study is often cited as indicating the "duration" of short-term memory (e.g., Solso, 1995). Muter (1980), however, argued that in the Peterson and Peterson experiments and experiments like them, participants were relying on more that STM, because they knew that they were going to be asked to recall the to-be-remembered items after an interval filled with distracting activity. Theory (e.g., Craik & Lockhart, 1972) and data (e.g., Jacoby & Bartz, 1972; Watkins & Watkins, 1974) suggest that if participants know they are going to be tested after a retention interval filled with distracting activity, secondary memory traces are likely to be formed. When subjects expect to be tested after a filled retention interval rarely or never, there is evidence that severe forgetting occurs after approximately 2 seconds, (Marsh, Sebrechts, Hicks, & Landau, 1997; Muter, 1980; Sebrechts, Marsh, & Seamon, 1989), though this finding remains controversial (Cunningham, Healy, Till, Fendrich, & Dimitry, 1993; Healy & Cunningham, 1995; Muter, 1995).
If the experiments of Peterson and Peterson and others like them did indeed tap more than STM, then many questions remain unanswered regarding the nature of forgetting from STM, and are not covered in Cowan's paper. What is the typical rate of forgetting? (This will undoubtedly depend on various circumstances, just as the capacity does, but it may tend to be at a certain level, just as capacity tends to be a certain chunk-size.) What is the shape of the forgetting curve, and how does it compare to the shape in long-term memory (Rubin & Wenzel, 1996)? Does the forgetting curve depend on the nature of the information remembered (e.g., Murdock & Hockley, 1989)? What are the roles of decay, displacement, and interference (Laming, 1992)? What is the role of inter-chunk similarity (Posner & Konick, 1966)? How important are the expectations and needs of the rememberer (Anderson, Tweney, Rivardo, & Duncan, 1997)? What is the effect of the nature of the distracting activity (e.g., verbal versus nonverbal, level of difficulty)? Can forgetting from STM be instant, if an extremely salient multi-modal event occurs? Almost all of the research on such questions has been performed under conditions in which secondary memory contamination was likely, because the participants expected to be tested after a filled retention interval.
Cowan's paper usefully elucidates many issues regarding STM. However, a comprehensive account of STM should surely include treatment of the nature of forgetting after attention has been diverted.
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I thank Bennet B. Murdock, Jr. and Jay W. Pratt for helpful comments.