Memory evolves over sleep

Schönauer, M. 1 , Pargent, F. 1 , Konrad, B. 2 , Czisch, M. 2 & Gais, S. 1, 3

1 LMU Munich
2 MPI for Psychiatry Munich
3 Eberhard Karls University Tübingen

It has been suggested that beyond stabilization and enhancement of memories, sleep also changes the quality of memory traces, enabling multi-item rule extrapolation. We performed three experiments where subjects learned a feedback-driven classification task in which an explicit (item knowledge) and an implicit trace (rule proficiency) were acquired. By nested structural equation modelling we show that over sleep, but not over wakefulness, the structure of memory changes, making explicit and implicit representations independent from each other. This change is reflected in a better performance of the sleep group on implicit rule memory transfer. Functional imaging data shows that improved performance in sleepers is related to stronger recruitment of implicit memory-associated structures, like the caudate nucleus, during an explicit item recognition memory task. Stronger recruitment of the hippocampus, which is associated with explicit memory, is found in implicit rule memory transfer. In a cooperative task, where both explicit item knowledge and implicit rule proficiency contribute to the solution, the sleep group showed higher activation in both the hippocampus and the caudate nucleus. Thus, a sleep-induced cooperation of memory systems and the concurrent change in memory structure lead to improved performance.