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.

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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. (PsycINFO Database Record

We assessed the knowledge of 1,338 people from 11 countries (8 former Allied and 3 former Axis) about World War II. When asked what percentage their country contributed to the war effort, across Allied countries, estimates totaled 309%, and Axis nations’ estimates came to 140%. People in 4 nations claimed more than 50% responsibility for their country (Germany, Russia, United Kingdom, and United States). The overclaiming of responsibility reflected in these percentages was moderated when subjects were asked to consider the contributions of other countries; however, Russians continued to claim great responsibility, the only country that remained well over 50% in its claim of responsibility for the Allied victory. If deaths in the war are considered a proxy of a nation’s contributions, the Soviet Union did carry much of the burden. This study points to sharp differences in national memory even across nations who fought on the same side in the war. Differing national perspectives shape diverse memories of the same complex event.

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.