Multiple Processes Strengthen Memory during Sleep

Abstract

For decades, enhancement of memory consolidation by sleep was investigated using tasks of declarative memory. Karni and coworkers (1994) employed for the first time a nondeclarative visual discrimination task for the investigation of sleep related memory consolidation. Improvement in this task can be seen only following a consolidation period, hours after practice has ended. A series of experiments from different groups examined the influence of sleep and wakefulness on the consolidation process and showed a strong sleep-dependency of task improvement. Enhanced performance on such discrimination tasks was found to occur only after a two-step consolidation process, at first dependent on periods of slow-wave sleep (SWS) rich sleep and, in a later phase, on sleep with a high amount of rapid eye movement (REM) sleep. Other data speak also in favour of a multi-step consolidation hypothesis of sleep. Experiments in rats show that sleep containing specific sequences of SWS and REM sleep positively affects memory performance. Several electrophysiological, neuromodulatory, and neuroendocrine mechanisms related to different sleep stages appear to contribute to memory processing during sleep.