Magdalena Abel

Retrieval-induced forgetting, delay, and sleep

Retrieval-induced forgetting (RIF) refers to the finding that retrieval of a subset of previously studied material can cause forgetting of related unpractised material. Prior work on the role of delay between practice and test for RIF reported mixed results. Whereas some studies found RIF to be a relatively transient phenomenon, others found RIF to persist over time. We examined whether the persistence of RIF can depend on whether wakefulness or sleep follows retrieval practice. We employed a variant of the retrieval-practice paradigm with short (20 minutes) and long (12 hours) delay conditions. In all conditions participants studied a perceptually categorised list followed by retrieval practice on some of the items from some of the categories. Participants studied and practised the material in the morning or the evening. RIF was present in the short-delay conditions, and it was present in the long-delay condition if the delay included nocturnal sleep; if the long delay was filled with diurnal wakefulness, RIF was absent. Results show that whether delay eliminates RIF or not can depend on whether sleep or wake follows retrieval practice. Connections of the results to recent findings about the role of delay and sleep for RIF are discussed.

Sleep can eliminate list-method directed forgetting

Recent work suggests a link between sleep and memory consolidation, indicating that sleep in comparison to wakefulness stabilizes memories. However, relatively little is known about how sleep affects forgetting. Here we examined whether sleep influences directed forgetting, the finding that people can intentionally forget obsolete memories when cued to do so. We applied the list-method directed forgetting task and assessed memory performance after 3 delay intervals. Directed forgetting was present after a short 20-min delay and after a 12-hr delay filled with diurnal wakefulness; in contrast, the forgetting was absent after a 12-hr delay that included regular nocturnal sleep. Successful directed forgetting after a delay thus can depend on whether sleep or wakefulness follows upon encoding: When wakefulness follows upon encoding, the forgetting can be successful; when sleep follows upon encoding, no forgetting may arise. Connections of the results to recent studies on the interplay between forgetting and sleep are discussed.

Sleep can reduce proactive interference

Sleep has repeatedly been connected to processes of memory consolidation. While extensive research indeed documents beneficial effects of sleep on memory, little is yet known about the role of sleep for interference effects in episodic memory. Although two prior studies reported sleep to reduce retroactive interference, no sleep effect has previously been found for proactive interference. Here we applied a study format differing from that employed by the prior studies to induce a high degree of proactive interference, and asked participants to encode a single list or two interfering lists of paired associates via pure study cycles. Testing occurred after 12 hours of diurnal wakefulness or nocturnal sleep. Consistent with the prior work, we found sleep in comparison to wake did not affect memory for the single list, but reduced retroactive interference. In addition we found sleep reduced proactive interference, and reduced retroactive and proactive interference to the same extent. The finding is consistent with the view that arising benefits of sleep are caused by the reactivation of memory contents during sleep, which has been suggested to strengthen and stabilise memories. Such stabilisation may make memories less susceptible to competition from interfering memories at test and thus reduce interference effects.

Collective memory: a new arena of cognitive study

Collective memory refers to recollection of events shared by a group. The topic has been of central interest in the humanities, but recently researchers have begun empirical studies. Topics such as how people remember a war or how quickly Americans forget their presidents can be studied objectively.

Sleep can reduce the testing effect: it enhances recall of restudied items but can leave recall of retrieved items unaffected.

The testing effect refers to the finding that retrieval practice in comparison to restudy of previously encoded contents can improve memory performance and reduce time-dependent forgetting. Naturally, long retention intervals include both wake and sleep delay, which can influence memory contents differently. In fact, sleep immediately after encoding can induce a mnemonic benefit, stabilizing and strengthening the encoded contents. We investigated in a series of 5 experiments whether sleep influences the testing effect. After initial study of categorized item material (Experiments 1, 2, and 4A), paired associates (Experiment 3), or educational text material (Experiment 4B), subjects were asked to restudy encoded contents or engage in active retrieval practice. A final recall test was conducted after a 12-hr delay that included diurnal wakefulness or nocturnal sleep. The results consistently showed typical testing effects after the wake delay. However, these testing effects were reduced or even eliminated after sleep, because sleep benefited recall of restudied items but left recall of retrieved items unaffected. The findings are consistent with the bifurcation model of the testing effect (Kornell, Bjork, & Garcia, 2011), according to which the distribution of memory strengths across items is shifted differentially by retrieving and restudying, with retrieval strengthening items to a much higher degree than restudy does. On the basis of this model, most of the retrieved items already fall above recall threshold in the absence of sleep, so additional sleep-induced strengthening may not improve recall of retrieved items any further.

The roles of delay and retroactive interference in retrieval-induced forgetting

When subjects study items from different categories and then repeatedly retrieve some of the items from some of the categories, retrieval practice typically improves recall of the practiced items but impairs recall of related but unpracticed items, relative to control items from unpracticed categories. Here, we report the results of three experiments, in which we examined practiced and unpracticed items’ delay-induced forgetting (Exp. 1) and their susceptibility to retroactive interference (Exps. 2 and 3). Control items showed the expected memory impairment after longer delay between practice and test and in the presence of retroactive interference. In contrast, both the practiced and the related unpracticed items showed hardly any forgetting under these conditions. The findings are consistent with the results from recent testing-effect studies, which have reported reduced delay-induced forgetting and reduced susceptibility to interference for retrieval-practiced items, and generalize the results to related unpracticed items. The findings are discussed with respect to the inhibitory and noninhibitory accounts of retrieval-induced forgetting, as well as the possible role of selective segregation processes, which may be induced by retrieval practice.

Adaptive memory: The influence of sleep and wake delay on the survival-processing effect

Processing items in terms of their survival value leads to superior memory relative to many deep (semantic) processing tasks. To date, such survival processing has been investigated with short delay intervals only, ignoring possible effects of longer wake and sleep delay. In this study, subjects performed orienting tasks that induced survival or deep (semantic) processing of single items. A surprise memory test was administered after a short delay or a delay of 12 hours that included either sleep or wake; a cued-recall test was conducted in Experiment 1, and an item-recognition test in Experiment 2. Survival-processing effects were present regardless of delay, and their size was not influenced by delay interval. While wake delay reduced memory for both item types, sleep compared to wake enhanced their memory. These results suggest that the survival-processing effect is fully maintained across longer delay, regardless of whether the interval is filled with wake or sleep.

Retrieval practice can eliminate list method directed forgetting

It has recently been shown that retrieval practice can reduce memories’ susceptibility to interference, like retroactive and proactive interference. In this study, we therefore examined whether retrieval practice can also reduce list method directed forgetting, a form of intentional forgetting that presupposes interference. In each of two experiments, subjects successively studied two lists of items. After studying each single list, subjects restudied the list items to enhance learning, or they were asked to recall the items. Following restudy or retrieval practice of list 1 items, subjects were cued to either forget the list or remember it for an upcoming final test. Experiment 1 employed a free-recall and Experiment 1 a cued-recall procedure on the final memory test. In both experiments, directed forgetting was present in the restudy condition but was absent in the retrieval-practice condition, indicating that retrieval practice can reduce or even eliminate this form of forgetting. The results are consistent with the view that retrieval practice enhances list segregation processes. Such processes may reduce interference between lists and thus reduce directed forgetting.

Collaborative remembering revisited: Study context access modulates collaborative inhibition and later benefits for individual memory

Collaborating groups typically show reduced recall relative to nominal groups, i.e., to the cumulated non-redundant recall of the same number of people remembering in isolation—a finding termed collaborative inhibition. Motivated by the results of several previous studies, this study examined in two experiments whether access to study context at test influences the effects of collaboration. In both experiments, subjects collaborated in triads or recalled previously studied material in isolation. Experiment 1 applied short versus prolonged retention intervals to vary access to study context at test, whereas Experiment 2 used the list-method directed forgetting task and applied remember versus forget instructions to modulate context access. In both experiments, collaborative inhibition was present when access to study context at test was intact (i.e., after the short delay and the remember instruction) but was eliminated when the access was impaired (i.e., after the prolonged delay and the forget instruction). Also, post-collaborative gains for individual recall were greater when context access was impaired and collaborative inhibition was eliminated. The findings demonstrate a critical role of access to study context at test for collaborative inhibition, indicating that impaired context access may reflect a general boundary condition for the recall impairment. The possible role of context reactivation processes for beneficial effects of social recall is discussed.

Sleep reduces the testing effect—but not after corrective feedback and prolonged retention interval.

Retrieval practice relative to restudy of learned material typically attenuates time-dependent forgetting. A recent study examining this testing effect across 12-h delays filled with nocturnal sleep versus daytime wakefulness, however, showed that sleep directly following encoding benefited recall of restudied but not of retrieval practiced items, which reduced, and even eliminated, the testing effect after sleep (Bäuml, Holterman, & Abel, 2014). The present study investigated, in 4 experiments, whether this modulating role of sleep for the testing effect is influenced by two factors that have previously been shown to increase the testing effect: corrective feedback and prolonged retention intervals. Experiments 1a and 1b applied 12-h delays and showed benefits of sleep for recall after both restudy and retrieval practice with feedback, but not after retrieval practice without feedback. Experiments 2a and 2b applied 24-h or 7-day delays and failed to observe any long-lasting benefits of sleep directly after encoding, on both restudied and retrieval practiced items. These results indicate that both corrective feedback and prolonged retention intervals reduce the modulating role of sleep for the testing effect as it can be observed after 12-h delays and in the absence of corrective feedback, which suggests a fairly limited influence of sleep on the effect.