Sleep-dependent consolidation of contextual learning

Memory consolidation is facilitated by sleep [1-4]. Specifying the functional domain of sleep-dependent consolidation (SDC) is important for identifying the neural mechanisms underlying this phenomenon. Previous work indicates that SDC may be limited to conditions in which learning is explicit [5]. In the present study, we tested the hypothesis that SDC may also occur with implicit learning when learning benefits from the formation of contextual associations, a function associated with the hippocampus. Three versions of the serial-reaction-time task (SRTT) [6] were examined, and SDC was assessed by comparing performance after 12 hr breaks that included or did not include sleep. SDC was observed in the Explicit condition. Two implicit conditions were compared. In the Implicit Noncontextual condition, participants performed a concurrent tone-counting task with the pitch of each tone selected at random, precluding cross-dimensional associations. In the Implicit Contextual condition, participants responded to the color of the cues while the spatial location of the cues followed a correlated sequence. Whereas learning was observed in both implicit conditions, SDC Was restricted to the contextual condition. Given that the formation of contextual associations is dependent on the hippocampus [7-9], we suggest that SDC is a hippocampus-mediated process.