A Theory About Why We Forget What We Once Knew

Traditional theories of forgetting assume that everyday forgetting is a cue-overload phenomenon, and the primary laboratory method used for investigating that phenomenon has long been the A-B, A-C paired-associates procedure. A great deal of research in psychology, psychopharmacology, and neuroscience suggests that this approach to the study of forgetting may not be very relevant to the kind of interference that induces most forgetting in everyday life. An alternative interference theory holds that recently formed memories that have not yet had a chance to consolidate are vulnerable to the interfering force of mental activity and memory formation, even if the interfering activity does not involve material similar to what was previously learned. This account helps to explain why sleep, alcohol, and benzodiazepines all forestall forgetting of a recently learned list, and it is consistent with recent work on the variables that affect the induction and maintenance of long-term potentiation in the hippocampus. KEYWORDS— forgetting; interference; consolidation; retrograde facilitation; long-term potentiation (LTP) In common parlance, the verb ‘‘to forget’’ refers to the inability to remember a prior event whether or not that event was ever committed to memory. Thus, for example, I might claim to forget where I placed my keys, but the truth may be that I neglected to encode the relevant information in the first place. People with amnesia resulting from damage to the brain’s medial temporal lobes typically experience this kind of forgetting. That is, they fail to encode new information, so they seem to forget everything quickly. In the field of psychology, theories of forgetting are typically concerned with the loss of information that was once successfully encoded. Thus, although you once remembered the name of your first-grade teacher, thereby proving that the information was encoded, you may now discover that the name has slipped away (forever, perhaps). Why does that happen? Traditionally, the answer has been that forgetting occurs either because memories naturally decay or because they succumb to the forces of interference. The story that most students of psychology learn is that a classic sleep study conducted by Jenkins and Dallenbach (1924) tilted the balance in favor of interference theory. In that study, subjects remembered more of a previously learned list when they slept before taking the memory test than when they remained awake. Because interference is much less likely to be encountered while sleeping, whereas a natural decay process should unfold whether one is asleep or awake, the results pointed to a dominant role for interference. In a recent review of a century of work on forgetting, I argued that the field accurately interpreted Jenkins and Dallenbach’s study (i.e., the results suggest a role for interference) but then modeled the interference process in a way that is not likely to be very relevant to ordinary forgetting (Wixted, 2004). In the early part of the 20th century, two views of interference struggled for supremacy. According to one view, interference consists of new memories degrading previously established memory traces that have not yet had a chance to consolidate. For such interference to occur, the new memories do not need to be especially similar to the ones they impair (Skaggs, 1925). The formation of new memories, per se, is the interfering force. According to the other view, interference consists of what later came to be called a cue-overload effect (Robinson, 1920; Watkins & Watkins, 1975). Cue overload occurs when more than one memory is associated with the same retrieval cue. Thus, for example, if I memorize ‘‘Learned Hand’’ as the name of a famous judge, the retrieval cue ‘‘famous judge’’ will become ever less effective in retrieving that name as I memorize the names of more famous judges. From this point of view, similarity of the interfering material is critical because similar items tend to be subserved by the same retrieval cue (whereas dissimilar items tend not to be). Thus, instead of interference being viewed as the degradation of a memory trace, it is viewed as a competition phenomenon that occurs at the time of retrieval. Address correspondence to John T. Wixted, Department of Psychology, 0109, University of California, San Diego, La Jolla, CA 92093-0109; e-mail: [email protected]. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 6 Volume 14—Number 1 Copyright r 2005 American Psychological Society The study of cue-overload interference is exemplified by the well-known A-B, A-C paired-associates design. In this procedure, the experimental group of subjects learns an A-B list of cue-target word pairs (e.g., hero-prison, water-valley, tiger-image, etc.) followed by a second, A-C, list involving the same cue words but different target words (e.g., hero-women, water-salad, tiger-infant, etc.). A control group learns the A-B list followed by a C-D list, which involves a completely different set of words. Both groups are then tested by giving the A cues and asking for recall of the B targets. The typical finding is that the experimental group performs more poorly than the control group, and this implies that the more items are attached to the A cue, the less likely that cue will be to occasion the retrieval of any one of its associates. Because paired-associates procedures generate powerful effects in the laboratory, the cue-overload view of forgetting came to dominate the trace-interference account during the latter half of the 20th century. Specific theories of cue-overload interference died an inglorious death in the late 1960s, in part because concepts such as unlearning (the idea that forgetting is caused by the unlearning of previously learned associations) and spontaneous recovery (the idea that unlearned associations spontaneously reestablish themselves) failed to find much empirical support (Tulving & Madigan, 1970). Still, the general notion that interference is mainly due to cue overload at the time of retrieval lived on, despite the fact that multiple efforts to demonstrate that cue-overload effects apply to everyday forgetting (not just laboratory forgetting) generally suggested just the opposite (Underwood & Ekstrand, 1967). In fact, the only reason to believe that cue-overload effects play any role at all in everyday forgetting is that almost everyone can point to a few examples from their own lives where it surely does. But the mere fact that cue-overload effects sometimes play a role does not mean that the role is a substantial one. Summarizing the state of the art late in his career, Underwood (1983) said: ‘‘A relatively few years ago it seemed that a fairly comprehensive theoretical account of forgetting was close at hand, but that has slipped away. Some investigators have lost confidence in interference as a major cause of forgetting, but none of the proposed replacements thus far has created a feeling that things are on a productive new track. But that will surely come’’ (p. 262). In search of a productive new track, I recently reviewed a century of research on forgetting and concluded that an idea long ago abandoned by the field—namely, that everyday forgetting is largely the result of trace interference—was ahead of its time (Wixted, 2004). The evidence bearing on that claim derives from the fields of psychology, psychopharmacology, and neuroscience, and I briefly review that evidence next. PSYCHOLOGY: THE TEMPORAL GRADIENT OF RETROACTIVE INTERFERENCE An idea that the field of psychology has never fully embraced (even though it has become the ‘‘standard story’’ in the field of neuroscience) holds that memories consolidate for a period of time after they are formed. During the consolidation period, memories are especially vulnerable to disruption. Perhaps the most compelling piece of evidence in favor of consolidation theory is the temporal gradient of retrograde amnesia (Ribot, 1882). The temporal gradient is observed when a brain structure known as the hippocampus is damaged. Bilateral hippocampal damage induces anterograde amnesia (which is the inability to form new memories), but it also impairs, to some degree, previously formed memories, with memories formed just prior to brain injury being more impaired than memories that were formed longer ago (i.e., retrograde amnesia is temporally graded). Older memories are assumed to be relatively spared because they are more completely consolidated than newer memories (and, therefore, are less dependent on the hippocampus). If memories do consolidate over time, and if the formation of new memories does interfere with previously formed memories, then one should also expect to see a temporal gradient of retroactive interference. That is, the encoding of new information should interfere more with recently formed memories than with older memories. This issue was first addressed by Müller and Pilzecker (1900), and they found clear evidence for the predicted temporal gradient (i.e., retroactive interference was more pronounced if the interfering material occurred early rather than late in the retention interval before the memory test). However, most other studies did not. In fact, all reviews of the relevant literature conducted after 1930 concluded that there is little or no evidence for the predicted temporal gradient of retroactive interference. To many psychologists, this meant that the process of consolidation, even if it is real, is a physiological process that has little to do with the psychological processes that are responsible for forgetting (such as unlearning and spontaneous recovery). Such findings may help to explain why the field of psychology has never really embraced the notion of memory consolidation. I have argued that this interpretation of the literature is inaccurate and that it results, in part, from a failure to distinguish between interference based on trace degradation (a storage phenomenon) and interference based on cue overload (a retrieval phenomenon; Wixted, 2004). No consolidation theory predicts that the retrieval competition induced by cue overload will be more pronounced if the retrieval cue is overloaded early in the retention interval. Even so, manipulating the timing of cue overload was the preferred method of investigation. The few studies that varied the temporal point of memory formation itself against a background of mental quietude (e.g., Skaggs, 1925), which are the kinds of studies that speak more directly to the issue, found evidence for the temporal gradient that is predicted by consolidation theory. In light of such evidence, the long-neglected notion that memories consolidate may be an important part of the story of why we forget. Volume 14—Number 1 7 John T. Wixted