Seeing the Forest through the Trees

1029 Editorial—Payne It is often said that a person is the sum of her memories. Memory allows us to relive our past, and in so doing provides us with a sense of our previous and present selves. But new evidence suggests that our memories are as important for the future as they are for the past.1 This requires thinking of a memory less as a high fidelity and static representation of prior experience, and more as a powerfully flexible construct that is nonetheless prone to errors and inaccuracies. A study by Sterpenich and colleagues2 in the current issue of SLEEP brings this important insight into the field of sleep and memory research, and suggests a role for REM sleep in memory flexibility. We often talk about sleep “consolidating” memories, or fixing them in the brain. Several studies have demonstrated that the neural activity observed during wakeful learning can be reactivated during subsequent periods of sleep. This in turn aids memories, with reactivation during deep, slow wave sleep (SWS) being especially beneficial if one wants to recall information in veridical format—or in exactly the same way one originally encountered it.3 A clever group of studies,4 including the Sterpenich study,2 provide causal evidence for sleep’s role in memory processing by demonstrating that a sensory cue, if originally paired with a memory at the time of learning, can improve memory retention when that cue is re-delivered during sleep. For example, in a recent study by Diekelmann et al., participants were trained on a task similar to the memory game “Concentration” either in the presence or absence of an odor that would later serve as a memory cue. They then either stayed awake or went to sleep, during which time they were re-exposed to the odor. Odors applied during SWS stabilized memories in hippocampal-cortical networks, which in turn benefited subsequent task performance.5 Such studies support the traditional view of memory consolidation as a stabilizing or “stamping in” of newly learned information. In contrast to traditional consolidation theory, however, are studies showing that seemingly permanent memories can be returned to a labile state following reactivation, at which point they can be modified.6 Indeed, in the study mentioned above,5 if the odor cue was re-presented during wakefulness (instead of SWS) memories were labilized, allowing them to be modified and updated with similar information. Although EDITORIAL