Signs of REM sleep dependent enhancement of implicit face memory: a repetition priming study.

Faces are processed and stored in distinct neuroanatomical systems. Based on evidence of a critical role of sleep in memory processes, we investigated the impact of nocturnal sleep on implicit memories for faces in healthy men. Face repetition effects in reaction times were compared across sleep periods early in the night, which are dominated by slow wave sleep (SWS), and late in the night, where rapid eye movement (REM) sleep prevails, as well as across corresponding nocturnal intervals of wakefulness. An inverse priming effect was found selectively across REM sleep rich late sleep, as indicated by distinctly prolonged response latencies to previously presented faces compared with novel faces after this period of sleep (P<0.05). We assumed this inverse priming to reflect a facilitated identification of previously presented faces after extended REM sleep periods, thereby producing interference with the response generation in our task which did not require face identification but rather required recognizing formal features of the faces. This interpretation was supported by a supplementary experiment where enhanced positive repetition priming was found across late, REM sleep dominated sleep in a task requiring face identification. Together, these findings indicate that implicit face memories particularly benefit from REM sleep associated brain mechanisms.