IT IS BECOMING INCREASINGLY CLEAR THAT NREM SLEEP, ESPECIALLY SLOW WAVE SLEEP (SWS), IS IM- PORTANT FOR THE PROCESSING OF HIPPOCAMPUS- dependent declarative memories,1-6 with SWS hypothesized to provide the optimal electrophysiological and biochemical state

197 IT IS BECOMING INCREASINGLY CLEAR THAT NREM SLEEP, ESPECIALLY SLOW WAVE SLEEP (SWS), IS IMPORTANT FOR THE PROCESSING OF HIPPOCAMPUSdependent declarative memories,1-6 with SWS hypothesized to provide the optimal electrophysiological and biochemical state for this type of processing.7,8 When sleep occurs in the form of a short daytime nap, it is very common to obtain only NREM sleep, without entering REM sleep, which would usually occur at least 90 minutes into the sleep period. The few studies examining the effect of daytime naps on memory have made use of this knowledge, demonstrating that daytime naps containing only NREM sleep (including SWS) facilitate verbal declarative memory (semantically related paired associates),9 with one study showing that paired associates improvement is contingent on whether subjects obtained SWS during the nap.10 These findings represent a first step forward in our understanding of how daytime naps benefit declarative memory processing. However, there are many questions still to be explored. To this end, the present study examines the benefits of a daytime NREM nap on a spectrum of declarative memory tasks, and begins to assess the importance of factors related to task acquisition and their potential to modulate sleep-related memory processing. To more broadly assess the declarative memory benefits of NREM sleep obtained during a daytime nap, subjects were trained on 3 well-known declarative (hippocampus-dependent) memory tasks. The first task was an unrelated paired associates task, a more difficult counterpart to the commonly used related paired associates task.3,4 The task comprises pairs of common words that lack an inherent semantic relationship (e.g., shirt–paper). Two nonverbal declarative memory tasks that do not rely strongly on previously learned concepts were also evaluated: the Rey-Osterrieth complex figure test (ROCFT; a measure of visuospatial declarative memory) and a maze learning task adapted from the task used by Brenda Milner on a large sample of hippocampal lesion patients including HM.11 Both of these tasks are void of semantically charged landmarks, objects, or verbal material that would have been previously learned by subjects. To date, no studies have used this particular maze learning task, and only 2 have examined the effect of sleep on memory using the ROCFT. In epileptic patients it was shown that performance on the ROCFT correlated positively with low frequency EEG spectral power (<1.25 Hz) overnight,12 and in schizophrenic patients the amount of SWS correlated positively with overnight ROCFT performance.13 In the present study we also explored the extent to which different methods of information encoding modulate the effect of sleep on memory. The impetus for exploring encoding factors was based on two studies by Smith, et al.14,15 who found that only when rats successfully acquired an operant conditioning or passive avoidance task was there an increase in subsequent paradoxical sleep. These findings suggested for the first time that the extent of task acquisition may be an important modulator of the effect of sleep on memory processing. Support for this general finding comes from a recent PET study demonstrating that the strength of acquisition of a serial reaction time task Sleep and performance