Sleep's Role in Human Spatial Learning

971 Editorial—Sheth In this issue of SLEEP, Nguyen and colleagues1 use a novel spatial navigation paradigm to probe the effect of sleep on hippocampus-based spatial learning in humans. The recent surge of research on the influence of sleep on learning and memory began with behavioral studies of perceptual learning2-4 and motor skill learning5 in humans, and neurophysiological studies on rodents showing reactivation of hippocampal neural patterns during slow wave sleep.6 The study of Nguyen et al. provides a cross-species link between mechanism and behavior. A number of studies in recent years have shown hippocampal contributions to human memory during sleep.7-10 Some of these studies found a correlation between hippocampal activity in sleep and post-sleep performance but did not find a benefit of sleep-dependent memory reactivation.10 Studies either did not assess9 or failed to find8 a unique benefit of sleep on spatial memory consolidation. In a study where a unique benefit of sleep on spatial memory consolidation was found,7 factors such as differences in vigilance state between wakefulness and sleep or in the capacity to relearn the memory were not eliminated as alternative accounts. The study by Nguyen et al.1 is a distinct departure in several key methodological aspects that go a long way in addressing these issues. First, it directly compares the effects of sleep following learning with the effects of wake using multiple measures of performance with fine-grained trialby-trial preand post-assessments. Second, it simultaneously assesses vigilance state in participants. Thus, in terms of behavioral assessment, Nguyen et al. is arguably the most thorough to date. In addition, Nguyen et al. measure accuracy of spatial navigation within a virtual three-dimensional environment resembling a maze, an environment ideally suited to engage the hippocampus. Nguyen et al. report overnight benefits of sleep on the accuracy of navigation, providing support for the idea of sleep-dependent human spatial memory consolidation involving the hippocampus. A closer look at the data illustrated in Figure 4 of Nguyen et al.1 (bottom, and especially Figure S1 in supplementary materials) bears hints of a more nuanced interpretation. An argument can be made that Nguyen shows overnight “catch-up” by the participants undergoing sleep following training. Nguyen measured distance traveled and amount of backtracking during the time participants navigated through a virtual three-dimensional EDITORIAL