Learning-dependent increases in sleep spindle density.

Declarative memory consolidation is enhanced by sleep. In the investigation of underlying mechanisms, mainly rapid eye movement (REM) sleep and slow-wave sleep have been considered. More recently, sleep stage 2 with sleep spindles as a most prominent feature has received increasing attention. Specifically, in rats hippocampal ripples were found to occur in temporal proximity to cortical sleep spindles, indicating an information transfer between the hippocampus and neocortex, which is supposed to underlie the consolidation of declarative memories during sleep. This study in humans looks at the changes in EEG activity during nocturnal sleep after extensive training on a declarative learning task, as compared with a nonlearning control task of equal visual stimulation and subjectively rated cognitive strain. Time spent in each sleep stage, spindle density, and EEG power spectra for 28 electrode locations were determined. During sleep after training, the density of sleep spindles was significantly higher after the learning task as compared with the nonlearning control task. This effect was largest during the first 90 min of sleep (p < 0.01). Additionally, spindle density was correlated to recall performance both before and after sleep (r = 0.56; p < 0.05). Power spectra and time spent in sleep stages did not differ between learning and nonlearning conditions. Results indicate that spindle activity during non-REM sleep is sensitive to previous learning experience.