THERE IS GROWING AMOUNT OF EVIDENCE THAT MEMORY CONSOLIDATION IS CONNECTED TO SLEEP. SLEEP DEPENDENT MEMORY CONSOLIDATION HAS been shown in humans for declarative as well as non-declara-

302 THERE IS GROWING AMOUNT OF EVIDENCE THAT MEMORY CONSOLIDATION IS CONNECTED TO SLEEP. SLEEP DEPENDENT MEMORY CONSOLIDATION HAS been shown in humans for declarative as well as non-declarative material.1,2 It is less clear which sleep stages or electroencephalographic microstructures are relevant for different learning tasks.3-7 The relevant sleep structures have been studied mostly by indirect means. Some concepts are based on the observation of changes of the sleep structure after learning experiences. For example, for a procedural motor learning task a correlation between the performance improvement and the amount of stage 2 sleep was shown.2 The amount of REM sleep appears to be increased after a motor-learning task.8 Other studies have taken advantage of the uneven distribution of sleep stages over the 2 halves of a night. Declarative memory consolidation was associated with the first half of the night, when SWS is known to be most prominent. In contrast, procedural memory tasks were connected with the second half of the night when REM sleep dominates.9 The associations with the specific sleep stages were deduced from the experiment even though all sleep stages were present in both halves of the night, and therefore remain quite speculative. There have been contradictory findings on the verbal paired associates task,9 which tests declarative memory. Some results favor a connection with SWS,9-12 others a REM sleep dependency.13-15 One explanation is that small differences in the degree of difficulty or emotionality can have a large influence on the outcome.4,16 It is not only emotionality and difficulty that affect sleep-dependent memory consolidation; no task is process pure, meaning that all tasks have different explicit and implicit demands. This is presumably reflected by different sleep mechanisms being associated with memory consolidation. Small differences in test design can cause large discrepancies in the study results of the sleep dependence of memory processes. This may explain why some results are regularly found only by some groups. As a consequence, clear and consistent test design is even more important. It has also been found that REM sleep deprivation impairs consolidation of some procedural tasks but had no effect on declarative memory consolidation.4 A few more recent studies have utilized selective REM sleep deprivation (REMD). 17,18 However, there have been no studies comparing SWS deprivation (SWSD) with REMD with respect to motor and declarative memory consolidation. Selective sleep deprivation has been criticized for evoking additional unspecific effects like arousal, emotional irritation, and concentration deficits. Therefore we chose 2 different deprivation conditions as additional controls and measured stress, concentration, and sleepiness throughout the experiment. Selective sleep deprivation appears to be the most direct method to investigate the impact of specific sleep stages on memory consolidation. Taken together, most experimental results suggest a connection between procedural memory and REM sleep; declarative memory appears to be dependent on SWS, even though numerous studies found no direct relationships between word-pair recall and SWS.19 The aim of the present study was to clarify the MeMory Consolidation