The role of memory strength for processes of sleep-associated memory consolidation
Final Report Abstract
Experiments 1∗-10∗ of this research project examined sleep effects for restudied and retrieval practiced items on a behavioral level by investigating both time-dependent forgetting and memories’ susceptibility to interference. Experiments 1∗-6∗ employed intentional learning using either the paired-associate learning task, the list-learning task, or educational text material. The results consistently showed that, relative to a 12-h delay that included diurnal wakefulness, a 12-h delay that included nocturnal sleep improved recall after restudy trials but did not improve recall after retrieval practice cycles. This pattern arose regardless of experimental task, employed material, the presence of interference at test, and recall levels after the 12-h wake delay. Because after a 12-h wake delay, recall after retrieval practice is typically higher than after restudy cycles - the socalled testing effect - the present results indicate that sleep can reduce or even eliminate the testing effect. Experiments 7∗-8∗ examined sleep effects for incidentally encoded items, using traditional deep processing tasks as well as more recent survival processing tasks and time-dependent forgetting. The results showed that both types of incidental memories benefit from sleep and do so to about the same extent. Together with the results of Experiments 1∗-6∗ , these findings suggest that, in the absence of retrieval practice, sleep benefits memory of both intentionally and incidentally encoded items. All of these findings are consistent with the bifurcation model of the testing effect, 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 that additional sleep-induced strengthening may not improve recall of retrieved items any farther. Finally, Experiments 9∗ und 10∗ examined sleeps effects for restudied and retrieval practiced items after retention intervals of 24 hrs, 48 hrs, and 7 days. The results of these experiments support the previous finding that sleep does not benefit recall of retrieval practiced items. In contrast to the previous finding, however, they also indicate that, after longer retention interval, sleep does no longer benefit recall of restudied items. The effect of sleep on restudied items, as observed in Experiments 1∗-8∗, thus seems to be a relatively transient effect. Experiments 11∗ -13∗ of this research project examined whether the manipulation of memory strength leads to different initial neural patterns at memory formation, how these neural patterns relate or influence neural patterns of consolidation processes during sleep and how neural patterns of successful memory retrieval change over a period containing wakefulness compared to a period containing sleep. Therefore, EEG and fMRI was recorded throughout all steps of the memory process: while subjects engaged in different strength manipulations during encoding before sleep, during sleep-associated memory consolidation, and during final cued-recall tests. Behaviorally, results support recent results showing that sleep benefits newly acquired memories, with this benefit being however highly selective in nature. In contrast to earlier believes, results of this project strongly suggest that sleep’s benefit on new memories does not seem to depend on the mere level of items’ initial strength, but more on how items have been strengthened before sleep. On an electrophysiological level, the crucial process of “tagging” new memories during memory formation before sleep for subsequent “offline” consolidation during sleep, appears to be reflected by event-related changes in oscillatory theta activity. Furthermore, fMRI data (Experiment 12∗) indicates that sleep, as compared to wakefulness, prevents fast forgetting of relatively weak memories by “replay” or strengthening in hippocampal structures. Yet, the transfer to more remote cortical sites for permanent storage seems to be reserved for stronger representations.
Publications
- (2013). Adaptive memory: The influence of sleep and wake delay on the survival-processing effect. Journal of Cognitive Psychology, 25, 917-924
Abel, M. & Bäuml, K.-H. T.
- (2014). Sleep can reduce the testing effect - it enhances recall of restudied items but can leave recall of retrieved items unaffected. Journal of Experimental Psychology: Learning, Memory, and Cognition, 40, 1568-1581
Bäuml, K.-H. T., Holterman, C., & Abel, M.
(See online at https://doi.org/10.1037/xlm0000025) - (2015). Oscillatory theta activity during memory formation and its impact on overnight consolidation: A missing link? Journal of Cognitive Neuroscience, 27, 1648-1658
Heib, D. P. J., Hoedlmoser, K., Anderer, P., Gruber, G., Zeitlhofer, J., & Schabus, M.
(See online at https://doi.org/10.1162/jocn_a_00804) - Sleep reduces the testing effect-but not after corrective feedback and prolonged retention interval. Journal of Experimental Psychology: Learning, memory, and cognition, 2018
Magdalena Abel, Valerie Haller, Hanna Köck, Sarah Pötschke, Dominik Heib, Manuel Schabus, KT Bäuml
(See online at https://doi.org/10.1037/xlm0000576)
