Stimulus generalization, context change, and forgetting

Forgetting is often attributed to retrieval failure caused by background contextual cues changing over time. However, generalization between stimuli may increase over time and make them increasingly interchangeable. If this effect occurs with contextual cues, it might cancel any effect of a changing context. The authors review the evidence and suggest a resolution of this paradox. Although generalization gradients can change over time, the effect is not always strong. Increased responding to nontarget stimuli is not often shown, and few studies have demonstrated such changes with contextual cues in a way that rules out other interpretations. Even this example of forgetting may be caused by retrieval failure. The physical contexts manipulated in learning and memory experiments themselves occur within a superordinate temporal context and can thus be forgotten with no inherent challenge to a context-change account of forgetting.     

Context-dependent impairment of recollection in list-method directed forgetting

In list-method directed forgetting, people's ability to forget one of the sets of learned material is examined. Research shows that memory for to-be-forgotten items is impaired when assessed by a recall test and by recognition tests reliant on recollective processes. Retrieval inhibition and context-change mechanisms have been proposed to account for the directed forgetting effects and both of them account for the results obtained with recognition tests. However, the context change account makes a specific prediction that recollection is impaired by directed forgetting only if it makes use of contextual associations. In the present study, directed forgetting was examined with two types of recollection-based tasks making use of different types of associations, namely a list discrimination task utilising contextual associations and an associative recognition task utilising interitem associations. Consistent with the context change account, the costs of directed forgetting were observed in a list discrimination task and were not observed in an associative recognition task. The results indicate that impairment in recollection due to directed forgetting is not general and provide converging evidence to support the context-change account.     

Direct remembering,mediated remembering,and atypical forgetting functions

Atypical forgetting functions have been demonstrated in several recent studies of delayed matching to sample, in which experimental conditions are altered partway through the retention interval. The forgetting functions are atypical in that accuracy or discriminability is not always a negatively accelerated monotonic function of increasing retention interval duration, but may increase at later times in the retention interval. Atypical forgetting functions reflect changes in levels of discrimination. A switch from a lower level to a higher level of discrimination, or vice versa, can occur at any time in the retention interval. The behavioral theories of remembering proposed by Nevin, Davison, Odum, and Shahan (2007), and White and Brown (2014), offer quantitative predictions of forgetting functions that differ in intercept or slope. Both theories are able to account for atypical forgetting functions, by assuming time‐independent changes in the mediating effect of attending to sample and comparison stimuli (in Nevin et al.'s model) or in the direct effect of the context of reinforcement of the conditional discrimination (in White & Brown's model). Despite differences in their main assumptions, the theories have an edge over any theory that assumes that forgetting is time‐dependent.     

The contextual change paradox is still unresolved: comment on Bouton, Nelson, and Rosas (1999)

According to the contextual change theory of memory loss, spontaneous forgetting reflects a retrieval impairment due to subtle and unprogrammed shifts in environmental cues over a retention interval. However, Riccio, Richardson, and Ebner (1984) noted an apparent paradox in this model; specifically, laboratory studies inducing explicit shifts in contextual cues found less disruption of performance as retention intervals increased. Bouton, Nelson, and Rosas (1999) critiqued several of the claims made by Riccio et al. and concluded that the contextual cue theory is still a valid account of spontaneous forgetting. In this comment, the authors address the 3 major criticisms offered by Bouton et al., point out an inconsistency in their argument, and conclude that the original paradox still poses problems for the contextual change theory of forgetting.     

Combined effect of context change and retention interval on interference in causality judgments

Four experiments studied the effects of context change and retention interval on retroactive interference in human causal learning. Experiment 1 found evidence of retroactive interference. Experiment 2 found that either a 48-hr retention interval or a change in the context after the interference treatment decreased retroactive interference. An interaction between context change and retention interval effects was also found, eclipsing the context change effect after the 48-hr retention interval. Experiments 3 and 4 found additivity between context change and retention interval effects when participants were remained of the difference between physical contexts before the test, independently of whether the context change involved a return to the original acquisition context. These results add to the evidence suggesting that spontaneous forgetting is caused by a change in either the physical or the temporal contexts where information is acquired.     

Differential effectiveness of various prior-cuing treatments in the reactivation and maintenance of memory

In 3 experiments, changes were examined in the characteristics of newly acquired and reinstated memories over time in preweanling rats. Experiment 1 indicated that forgetting after conditioning was monotonic, with the upper limit of retention at approximately 120 min posttraining. In Experiment 2, Ss were exposed to various elements of the training episode before testing, after either a 3- or a 24-hr retention interval. The results indicated that the prior-cuing treatments were differentially effective and that the effectiveness of a reactivation treatment may change as a function of the retention interval. Experiment 3 indicated that Ss expressed a conditioned aversion at much longer intervals following reactivation treatments than after initial conditioning. Furthermore the susceptibility of the reinstated memory to forgetting was dependent on the prior-cuing treatment used. The results suggest a change in the memorial representation of the conditioning episode over time.     

Time-dependent changes in inaccessible memory

The present experiment examined whether forgetting of contextual attributes—the loss of discriminability of background stimulus features over time—also occurs when a memory remains inaccessible (amnestic) during the retention interval. To examine this issue, rats received an acute hypothermia treatment that caused reversible anterograde amnesia for a passive-avoidance task and were tested 1 or 7 days later in either the same or a different context. At the short retention interval, the subjects discriminated between the contexts, as evidenced by the finding that reversal of amnesia occurred only in the training context. However, at the 7-day interval, recovery of the avoidance response was induced in either context, reflecting the forgetting of differentiating characteristics. These findings indicate that inaccessible memory remains susceptible to processes of modification and distortion that typically influence intact memories.     

Wiping out memories: New support for a mental context change account of directed forgetting

Costs and benefits of directed forgetting are observed when a between-list instruction to forget List 1 impairs List 1 recall while enhancing List 2 recall. These effects are often ascribed to intentional inhibition of List 1. Contrary to this inhibition account, we found that a forget instruction did not produce costs unless an explicit instruction to concentrate on List 2 was used (Experiment 1). Alternatively, costs may be ascribed to a shift in mental context between encoding and retrieval. Consistent with this mental context-change account, an unexpected task (wiping the computer screen and one's hands) produced costs comparable to a forget instruction, as did as a brief chat between lists (Experiment 2). A number-search task between lists produced neither costs nor benefits (Experiment 3), suggesting that mere distraction is insufficient for inducing mental context change. Our findings support the claim that mental context change underlies both intentional and unintentional forgetting.     

The Influence of Retention Interval on the US Preexposure Effect: Changes in Contextual Blocking over Time

A number of studies manipulating the length of the interval between conditioning and testing indicate spontaneous recovery from overshadowing, suggesting that certain instances of overshadowing represent a deficit in memory retrieval rather than a failure of animals to form an association between the overshadowed stimulus and the US. The present series of experiments examined the influence of lengthening the retention interval on blocking, another stimulus selection phenomenon that is typically interpreted as an acquisition deficit. The results indicated that when subjects were tested shortly (3 days) after training conditioning to a taste blocked subsequent conditioning to an odor conditioned in compound with that taste (Experiment 1), whereas prior conditioning to an odor did not block subsequent conditioning to a taste conditioned in compound with that odor (Experiment 2). This pattern of results was essentially unchanged when testing occurred at a longer (21-day) retention interval. However, there was evidence of a US preexposure effect in Experiment 2 when subjects in the US ONLY control condition were tested at the 3-day retention interval, but not when testing occurred 21 days after conditioning. Experiments 3 and 4 examined whether this loss of the US preexposure effect over time might actually represent a change in the degree of contextual blocking as the retention interval is lengthened. Exposure to the conditioning context either during the interval between Phase 1 and Phase 2 of conditioning (Experiment 3) or prior to Phase 1 of conditioning (Experiment 4) alleviated this US preexposure effect suggesting that the loss of the US preexposure effect as the retention interval is lengthened observed in Experiment 2 is due to changes in the degree of blocking by contextual stimuli over time. The results are discussed in terms of differential susceptibility of forgetting of two functional roles played by a contextual stimuli in the current situation-context as a CS and context as a retrieval cue for other CS-US associations.     

Context preexposure prevents forgetting of a contextual fear memory: implication for regional changes in brain activation patterns associated with recent and remote memory tests

Contextual fear conditioning was maintained over a 15-day retention interval suggesting no forgetting of the conditioning experience. However, a more subtle generalization test revealed that, as the retention interval increased, rats showed enhanced generalized fear to an altered context. Preexposure to the training context prior to conditioning, however, prevented this enhanced generalized fear from developing. These results support the hypothesis that the memory representation of the context degrades as the memory ages and is responsible for enhanced generalization. The implications of these results for systems consolidation versus forgetting interpretations of regional changes in neural activation patterns that occur as memories age are discussed.     

Assessment of retention capacities in old rats

Young (3-6 month) and old (24-27 month) barrier reared Wistar rats were tested for their ability to retain an inhibitory (passive) avoidance, acquired immobility, and a conditioned taste aversion response as a function of time. Old rats exhibited accelerated forgetting of both the inhibitory avoidance and acquired immobility response in comparison to young rats. In contrast, old rats displayed good retention of the conditioned taste aversion response at all time intervals tested. It appears that the dynamic aspects of retention are altered in aged rats depending on the task, and in some instances may be expressed as accelerated forgetting.     

Effects on memory of early testing and accuracy assessment for central and contextual content

ABSTRACT

Memory for an event is influenced by many factors including retention interval, frequency of assessment, and type of information assessed concerning the event. We examined the usefulness of observer memory for contextual information in assessing accuracy of memory for central information. Participants viewed a video of a purse being stolen and were asked questions concerning the perpetrator and surrounding context of the event, including where and when the event occurred and who else was present. Participants tested immediately after seeing the video exhibited better memory than those tested for the first time 48 hours after the event. Additionally, testing immediately after viewing the video reduced forgetting over the 48-hour delay (i.e. early testing attenuated subsequent forgetting). Moreover, memory for the context of the event correlated positively with memory of the central information (i.e. perpetrator), and memory concerning other people at the event tended to have the highest correlation with perpetrator memory.     

Dissociation of short-term forgetting from the passage of time

In many theories, forgetting is closely linked to the passage of time. In the present experiments, recall in a short-term memory task was less accurate when the retention interval included a difficult arithmetic addition task, compared with an easy task. In a novel condition, the interfering task was switched from hard to easy partway through the retention interval. Recall accuracy at long retention intervals recovered from the level consistent with the hard interfering task to a more accurate level consistent with an easy interfering task. Such recovery is not in agreement with theories that link forgetting to the passage of time, such as trace decay. The reversed forgetting functions are more consistent with an account in which different levels of nonspecific retroactive interference in the retention interval just before the time of retrieval influence different levels of performance accuracy. With the uncoupling of interference levels from time, forgetting is dissociated from the passage of time.     

Reversing the course of forgetting

Forgetting functions were generated for pigeons in a delayed matching-to-sample task, in which accuracy decreased with increasing retention-interval duration. In baseline training with dark retention intervals, accuracy was high overall. Illumination of the experimental chamber by a houselight during the retention interval impaired performance accuracy by increasing the rate of forgetting. In novel conditions, the houselight was lit at the beginning of a retention interval and then turned off partway through the retention interval. Accuracy was low at the beginning of the retention interval and then increased later in the interval. Thus the course of forgetting was reversed. Such a dissociation of forgetting from the passage of time is consistent with an interference account in which attention or stimulus control switches between the remembering task and extraneous events.     

Acquisition of a conditioned taste aversion becomes context dependent when it is learned after extinction

A conditioned taste aversion experiment tested context-switch effects on retrieval of conditioned stimulus (CS)–unconditioned stimulus (US) acquisition performance in rats. A context switch impaired performance when the target flavour was trained in a context where a different flavour underwent extinction. Conditioned taste aversion in the absence of previous extinction of the alternate flavour was not context dependent. It is suggested that the ambiguity in the meaning of the extinguished cue leads animals to pay attention to the context, so that the information learned in that context becomes context dependent.     

Acquisition of a conditioned taste aversion becomes context dependent when it is learned after extinction

A conditioned taste aversion experiment tested context-switch effects on retrieval of conditioned stimulus (CS)-unconditioned stimulus (US) acquisition performance in rats. A context switch impaired performance when the target flavour was trained in a context where a different flavour underwent extinction. Conditioned taste aversion in the absence of previous extinction of the alternate flavour was not context dependent. It is suggested that the ambiguity in the meaning of the extinguished cue leads animals to pay attention to the context, so that the information learned in that context becomes context dependent.     

Taste-aversion retention: An animal experiment with implications for consummatory-aversion alcoholism treatments

Inquiries into the longevity of illness-induced aversions (TAs) in animals are relevant to consummatory-aversion (CA) treatments of human alcoholism. The range of nausea reactions that accompanied the relapses of some alcoholics who had acquired alcohol aversions during covertsensitization (verbal aversion) alcoholism treatment has implicated CA forgetting as one probable contributor to recidivism. CA forgetting is operationalized as aversion diminution during postconditioning periods in which Ss abstain from contact with the target substance. TAs of varying strengths were induced in groups of Sprague-Dawley rats that received low, medium or high doses of the illness-inducing drug cyclophosphamide following saccharin-solution ingestion. TA retention was assessed following saccharin-free intervals of 2–40 days. Each Ss' retention interval was followed by 30 days of two-bottle preference testing, thereby additionally permitting an assessment of TA extinction following differing degrees of TA forgetting. Low-dose Ss displayed moderate strength TAs that were forgotten within 20 days and that had little resistance to extinction when testing began shortly after conditioning. Medium-dose Ss displayed stronger TAs having greater resistance to both forgetting and extinction. Unlike these low- and medium-dose TAs, high-dose TAs were impervious to aversion degradation as a result of forgetting. This finding is interpreted as supporting the attempted induction of intense nausea during covert-sensitization and chemical aversion (emetic therapy) alcoholism treatment. Other related conditioning procedures that may contribute to effective treatment are also discussed.     

Short-term forgetting and the articulatory loop

Two experiments explored the role of subvocal articulatory rehearsal in the Peterson short-term forgetting task. In the first of these, subjects recalled consonant trigrams after an interval of 0, 5 or 15 s during which they either counted backwards in threes, suppressed articulation by continuously uttering the word “the”, or in a third control condition continuously tapped on the table. While counting backwards caused the usual dramatic forgetting, tapping caused no forgetting, and articulatory suppression only minimal forgetting at the longest delay. A second study used the same procedure but included only two conditions, articulatory suppression during the retention interval and articulatory suppression during both input and retention. Neither showed evidence of forgetting over the 15 s delay. These results suggest that covert speech is not necessary for rehearsal in short-term verbal memory. As such they call for a re-evaluation of the nature and function of rehearsal.     

Dissociating familiarity from recollection in human recognition memory: Different rates of forgetting over short retention intervals

Two functionally distinct forms of recognition memory have been identified in human and nonhuman species—the ability to recollect qualitative information about previous events, and the ability to differentiate between familiar and novel stimuli. Separate dual-process theories have been developed in the animal and human literatures to account for these findings. However, it is not clear whether these theories describe the same two underlying memory processes. On the basis of animal studies of medial temporal lobe function, familiarity is expected to exhibit disproportionately fast forgetting compared with recollection over short retention intervals. We tested this prediction in healthy human subjects by examining recognition forgetting rates across a range of 8–32 intervening items and found significant forgetting in the accuracy of familiarity-based discriminations and no evidence of forgetting in the accuracy of recollection-based discriminations. In agreement with the results from nonhuman species, the present results indicate that item familiarity decreases more rapidly than recollection over short retention intervals.     

Short-term forgetting without interference

In the 1st reported experiment, we demonstrate that auditory memory is robust over extended retention intervals (RIs) when listeners compare the timbre of complex tones, even when active or verbal rehearsal is difficult or impossible. Thus, our tones have an abstract timbre that resists verbal labeling, they differ across trials so that no "standard" comparison stimulus is built up, and the spectral change to be discriminated is very slight and therefore does not shift stimuli across verbal categories. Nonetheless, performance in this nonverbal immediate memory task was better at short (1-, 2-, or 4-s) than long (8-, 16-, or 32-s) RIs, an outcome predicted by temporal distinctiveness theory whereby at long RIs, tones are closer in time to tones on previous trials. We reject this account in the 2nd experiment, where we demonstrate that the ratio of RI to intertrial interval makes absolutely no difference to performance. We suggest that steady forgetting is consistent with a psychoacoustically derived conception of an auditory memory (the timbre memory model) that embodies time-based forgetting in the absence of feature-specific interference.