FORGETTING

It is a common experience to forget what one has learned. Usually, forgetfulness increases with with the length of the retention interval, the time elapsed since the material was last studied or thought about. A graph of the amount remembered (as measured by tests of recall or recognition or relearning) as a function of increasing retention intervals produces a forgetting curve, the slope of which represents the overall rate of forgetting. The first forgetting curve was published in 1885 by Hermann Ebbinghaus, the pioneer in the scientific study of memory. His curve showed the now-familiar monotonic and negatively accelerated form, where the momentary rate of forgetting decreases over time.

Trace Decay

Perhaps the earliest and simplest attempt to account for forgetting was the idea of trace decay, which postulated that memorizing something lays down a neurochemical imprint or record in the brain, called a memory trace or engram, whose later reactivation is responsible for remembering. This trace was assumed to fade away spontaneously over time if it was not refreshed by some reacquaintance with the learned material; hence forgetting would ensue. With very few exceptions (noted below), trace-decay theory has been abandoned. This is not only because the hypothetical memory trace has never been identified but also because of subsequent research that undermined the thesis: for example, the findings that forgetting is influenced by other activities taking place both before and after the original learning and that forgetting is also greatly affected by the kinds of cues given at the time of test. Providing different retrieval cues in situations where the trace was supposed to have decayed often results in successful recall of the supposedly forgotten material (Capaldi and Neath, 1995); in many instances memory performance can improve with the passage of time (Bjork, 2001). Thus, although forgetting takes place over time, it is probably not because of some inexorable, autonomous fading of a memory trace.

Forgetting is attributed to decay in only two areas of memory research: pro forma decay parameters in abstract mathematical models of memory and the invocation of an unspecified decay process in certain accounts of short-term memory. The former can be seen as mathematical conveniences rather than as strong theoretical statements. The latter suffer from two major problems: ambiguity in the explication of the decay process (i.e., it is unclear exactly what aspect of memory is decaying, what parts of the memory remain) and the reevaluation of the data supporting such assertions. In principle, decay can be empirically demonstrated if one rules out alternative, better-established causes of forgetting, such as interference. When this is done, the evidence for decay is not appreciable; in the words of Cowan, Saults, and Nugent (2001), "no clear evidence of decay has emerged in, lo, these many years."

Interference

One theoretical approach to forgetting that has inspired an enormous amount of experimental effort over several decades is interference theory. As the name implies, it focuses upon forgetting caused by interference. Among its beginnings was the influential demonstration by Jenkins and Dallenbach (1924) that the forgetting of a list of verbal items was markedly reduced if subjects passed the retention interval in sleep rather than in their usual waking activities. This result suggested that the experiences of daily life somehow interfered with the memory of the original material. Eventually, interest narrowed upon other learning experiences as the sources of that interference.

The empirical cornerstones of interference theory are to be found in two kinds of laboratory-produced forgetting. These are schematized in Table 1, and each defines a source of interference by comparing the memory performance of one experimental group, which acquires two lists in succession, with that of a control group, which acquires only one. Retroactive inhibition is the forgetting of the first set of materials, which is caused by the subsequent learning of a second set during the retention interval. Proactive inhibition is the forgetting of the second set, which is caused by the prior learning of a first set. It may seem strange that a preceding list would reduce the memory for a subsequently acquired one, but it can, especially when a retention interval precedes testing. Interference theory construes retroactive and proactive inhibition to be the basic models for its approach to forgetting, inside or outside of the laboratory. It is the task of the theory to devise an experimentally testable description of the processes of retroactive and proactive inhibition.

Whenever two sets of materials are acquired in succession, two processes can impair memory of them. One is the dynamic competition of responses for emergence at the time of the test. To the extent that both sets of materials are activated, a response from the first list may be blocked from consciousness by a stronger competing response from the second list, or vice versa. Two competing responses of equal strength may even block each other. This process is set into motion by the demands of the memory test, and it reduces performances on both lists compared with the single-list control condition. The other process, which takes place during the learning of the second list, consists of the temporary suppression of the contents of the first list, to the extent that they conflict with new response requirements. For example, if the first set of materials contains an A-B association and the second set requires an A-C association, then A-B may be suppressed. With both factors taken into consideration, the memory situation immediately after second-list acquisition is that of a suppressed first list competing against a dominant second list. This correctly predicts that retroactive inhibition will be much stronger than proactive inhibition. Another important observation to be explained is that with a delayed memory test, pro-active inhibition increases in magnitude. This fact is incorporated by postulating a gradual dissipation of first-list suppression, wherein the list regains its strength and competes more effectively, producing increased interference. This implies that a suppressed set of materials should be better recalled after some time has passed, which is the opposite of forgetting. Such an effect has been experimentally verified.

Cue-Dependency Theory

Cue-dependency theory is an alternative approach to forgetting. Although quite different from interference theory, it supplements rather supplants it. Cue-dependency stresses the importance of the reminders, or retrieval cues, that operate at the time of test. It emphasizes that the act of remembering requires not only the stored products of original learning but also an appropriate testing environment in which to make contact with that learning. The successful interaction of stored information and retrieval information produces recollection. Forgetting is proportional to the inadquate accessibility of retrieval cues. Remembering depends on the interaction between the conditions at encoding and the conditions at retrieval.

An experiment reported by Thomson and Tulving (1970) emphasizes the importance of the encoding/retrieval. The researchers asked subjects to recall the target words from a list they were shown. At encoding the target word was sometimes presented by itself, and sometimes with a weak cue. At retrieval there were three cue conditions: Sometimes, no retrieval cue was given, sometimes a weak cue was given, and sometimes a strong cue was given. The terms strong cue and weak cue refer to a cue's ability to elicit a target. For example, if you were asked to respond with the first word that popped into your head when you heard bloom, you are very likely to respond with flower. So bloom is a strong cue for flower. In contrast, you are likely to respond with flower only about 1 percent of the time to the weak cue bloom. The results, shown as the probability of recalling the correct target word, are displayed in Table 2.

When there was no cue at encoding, the weak and strong cues at retrieval functioned as expected: More target items were reported when given a strong cue (0.68) than when given a weak cue (0.43). However, when there was a weak cue at encoding, the weak cue at retrieval elicited the target word more than 80 percent of the time, whereas the strong cue elicited the target word barely 20 percent of the time.

What matters is the extent to which the conditions at retrieval uniquely specify the target information. Thus, if you study in the presence of a particular odor (say, freshly baked chocolate-chip cookies) but are tested in the absence of that odor, your ability to remember is impaired. A change in mood, environment, or pharmacological state can result in similar decrements. Most of the things you have memorized were not accompanied by the smell of chocolate-chip cookies; having the odor at retrieval provides a cue that greatly narrows down the possible items to be recalled. According to this view, the way to overcome forgetting is to envision what cues will be uniquely available at the time the information is going to be needed. Once those cues are identified, then the material can be encoded with reference to those cues.

Normal Forgetting

Are there any conditions of original learning that influence the rate of forgetting of a single set of materials learned in the laboratory? Curiously, normal forgetting rates appear to be independent of the materials memorized. They also seem impervious to the levels of processing (superficial versus meaningful) employed at study. There is little evidence that they yield to deliberate mnemonic strategies. There is even serious doubt about whether the degree of original learning has any effect upon the rate of loss. The present picture suggests a remarkable resistance to experimental manipulation. Surely no theory or model of memory can be expected to account satisfactorily for normal forgetting short of resolution of this question.

Bibliography

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Capaldi, E. J., and Neath, I. (1995). Remembering and forgetting as context discrimination. Learning and Memory 2, 107-132.

Cowan, N., Saults, S., and Nugent, L. (2001). The ravages of absolute and relative amounts of time on memory. In H. L. Roediger III, J. S. Nairne, I. Neath, and A. M. Surprenant, eds., The nature of remembering: Essays in honor of Robert G. Crowder. Washington, DC: American Psychological Association.

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Jenkins, J. G., and Dallenbach, K. M. (1924). Obliviscence during sleep and waking. American Journal of Psychology 35, 605-612.

McGeoch, J. A. (1932). Forgetting and the law of disuse. Psychological Review 39, 352-370.

Neath, I. (1998). Human memory: An introduction to research, data, and theory. Pacific Grove, CA: Brooks/Cole.

Postman, L., and Underwood, B. J. (1973). Critical issues in interference theory. Memory & Cognition 1, 19-40.

Slamecka, N. J. (1985). Ebbinghaus: Some associations. Journal of Experimental Psychology: Learning, Memory, and Cognition 11, 414-435.

Slamecka, N. J., and McElree, B. (1983). Normal forgetting of verbal lists as a function of their degree of learning. Journal of Experimental Psychology: Learning, Memory, and Cognition 9, 384-397.

Thomson, D. M., and Tulving, E. (1970). Associative encoding and retrieval: Weak and strong cues. Journal of Experimental Psychology 86, 255-262.

Underwood, B. J. (1957). Interference and forgetting. Psychological Review 64, 49-60.

Norman J.Slamecka

Revised byIanNeath