To sleep, perchance to integrate

One of the most intriguing and lasting proposals in psychological and neuroscience research is that sleep plays an important role in the consolidation of memories from a labile state into a permanent state. In recent years, there has been considerable research on this issue, and the findings have generated both supporters and skeptics locked in heated debate (1–3). A new study by Ellenbogen et al. (4) in this issue of PNAS contributes to this debate and may help us get past the superficial question of whether sleep plays a role by beginning to inform us on how sleep might contribute to information-processing that ultimately underlies memory consolidation.

In this study (4), subjects learned a series of “premises” composed as choices between pairs of visual patterns that contained overlapping elements. Thus, subjects were initially taught to choose stimulus A over stimulus B, then B over C, then C over D, then D over E, and then E over F. These pairs can be integrated to form the relational hierarchy A>B>C>D>E>F (where “>” stands for “should be selected over”). The existence of the hierarchical representation is reflected in the capacity for transitive inferences about the relationships between items that are indirectly connected by a single intermediate item (B>D and C>E; a 1° transitive judgment) and by two intermediate items (B>E; 2° judgment). Immediately after learning all of the premise pairs, subjects could not make either the 1° or 2° transitive judgments, indicating that initial learning had not formed the relational hierarchy but had generated only a distinct representation for each premise pair (Fig. 1 Top). However, after a 12-h active day, subjects could make 1° relational judgments, indicating partial success in the integration of the premises without sleep (Fig. 1 Middle). Moreover, if the subjects had instead slept during the …

*E-mail: hbe{at}bu.edu