A GROWING NUMBER OF STUDIES SUPPORT THE HY- POTHESIS THAT SLEEP IS INVOLVED IN MEMORY CON- SOLIDATION—A SLOW PROCESS THAT ENABLES labile

A GROWING NUMBER OF STUDIES SUPPORT THE HYPOTHESIS THAT SLEEP IS INVOLVED IN MEMORY CONSOLIDATION—A SLOW PROCESS THAT ENABLES labile recent memories to be incorporated into more stable, restructured, long-term memories (see references 1-3 for reviews). There is substantial evidence to indicate that slow-wave sleep (SWS) is beneficial for consolidation in episodic (or declarative) memory, which refers to a system that stores events set in a specific spatiotemporal context (see references 4 and 5 for examples), whereas rapid-eye movement (REM) sleep would be involved in consolidation in implicit (or nondeclarative) memory.5-7 Nevertheless, some recent studies have provided discrepant results in favor of a beneficial role of both SWS and REM sleep in episodic8 as well as in implicit9,10 memory consolidation. Implicit memory refers to a change in the ability to identify or produce an item as a result of a specific prior encounter with the item.11,12 Assessment of implicit memory mainly concerns the research of priming effects. These latter correspond to the phenomenon whereby the mere presentation of a stimulus will modify (and usually facilitate) the subsequent processing of this item or of an item close to it, despite the absence of any conscious recollection of a preliminary encounter with it.11 Tasks assessing priming are usually composed of 2 distinct phases (study and test) separated by a variable interval. Priming is attested when subjects identify or complete previously experienced items (target items) with greater accuracy or reduced response times than items that have never been studied before (control items). Several studies have been interested in the influence of the interval between the study phase and the test (see references 13-18 for examples). Thus, Mitchell and colleagues15,16 observed a relative preservation of priming effects after a 6-week interval between the 2 phases of a picture-naming task contrasting with a significant decline in recognition performance (see also references 17 and 18 for similar conclusions). So, it appears that priming effects are less affected by the length of the interval between the study and the test than is recognition performance. Relatively few studies have investigated the relationships between sleep stages and long-term priming. Thus, Plihal and Born5 assessed the effect of partial sleep deprivation on performance on a word-stem completion task. In the study phase, subjects were shown a word list and instructed to rate the nouns according to their melodious sounds on a 5-point scale from unpleasant to pleasant. This instruction was considered to induce an incidental encoding of the items. After a 5-minute arithmetic distraction task, subjects were invited to complete a list of word-stems with the first noun that came into their heads. Priming was attested by an increased percentage of completion for previously studied words, compared with new ones. After a 3-hour interval, during the first or second half of the night, filled either with sleep or wakefulness, the authors administered the completion task again, using word-stems derived from the remaining words in the initial list and the same number of items derived from a novel list. The Effects of Partial Sleep Deprivation on Within-Format and Cross-Format Priming