Department of Physiology, University College London, London WC1E 6BT, UK
   Transient and detailed memory for recent events declines eventually to a less detailed, but more robust long-term (LT) memory.  The mechanism of consolidation to generate LT memory is uncertain, but may in part involve the conscious or unconscious repetition, during the period of detailed transient memory, of activity patterns similar to those experienced during learning.  Repetition of patterns that are identical to those learned is, perhaps surprisingly, not in general the best strategy for consolidation.  Where substantial overlap exists between experienced patterns (i.e. there are many features or active cells in common) then interference between LT memories is reduced by consolidation with what we call "enriched" recall:  repetition of distorted versions of the original patterns with relatively greater activation of their distinctive elements.
   We suggest that enriched recall and consolidation may occur during rapid-eye-movement sleep (REMS).  Automatic processing algorithms can generate the enriched patterns.  So far as we can see, however, this requires a two-phase process in which the cells associated with common features are first identified in phase 1 and rendered less excitable for a period that lasts through the subsequent phase 2.  During phase 2 the enriched patterns are recalled (perhaps through random activation) and consolidated.  It is essential that the abnormal patterns generated in both phases are not remembered as if they were normal experience, especially certain hybrid patterns generated as intermediates in phase 1.  The conditions required for phase 1 resemble slow-wave sleep (SWS), and we suggest that SWS may serve to prepare conditions for enriched recall and consolidation in the subsequent period of REMS.
   Some benefit would result if there is a permanent reducton ("unlearning") of LT connection strengths between cells identified in phase 1 as common to overlapping patterns: such effects have previously been shown by other authors.  The benefit of consolidation with enriched patterns is different and in general greater.  It may enhance both LT episodic memory and, more speculatively, discrimination learning.