There is evidence that sleep supports the enhancement of implicit as well as explicit memories (i.e., two memory systems that during learning normally appear to act together). Here, employing a serial reaction time task (SRTT) paradigm, we examined the question whether sleep can provide explicit knowledge on an implicitly acquired skill. At learning, young healthy subjects (n = 20) were first trained on the SRTT. Then, implicit knowledge was assessed on two test blocks, in which grammatically incorrect target positions were occasionally interspersed by the difference in reaction times between grammatically correct and incorrect target positions. To assess explicit sequence knowledge, thereafter subjects performed on a generation task in which they were explicitly instructed to predict the sequential target positions. In half the subjects, learning took place before a 9-hour retention interval filled with nocturnal sleep (sleep group), in the other half, the retention interval covered a 9-hour period of daytime wakefulness (wake group). At subsequent retesting, both testing on the generation task and the SRTT test blocks was repeated. At learning before the retention interval, subjects displayed significant implicit sequence knowledge which was comparable for the sleep and wake groups. Moreover, both groups did not display any explicit sequence knowledge as indicated by a prediction performance not differing from chance on the generation task. However, at retesting, there was a distinct gain in explicit knowledge in the subjects who had slept in the retention interval, whereas generation task performance in the wake group remained at chance level. SRTT performance in the test blocks at retesting did not indicate any further gain in skill (i.e., unchanged reaction time differences between grammatically correct and incorrect target positions) independently of whether subjects had slept or remained awake after learning. Our results indicate a selective enhancement of explicit memory formation during sleep. Because before sleep subjects only had implicit knowledge on the sequence of target transitions, these data point to an interaction between implicit and explicit memory systems during sleep-dependent off-line learning.

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