Acoustic Stimulation Following Sleep Spindle Activity May Enhance Procedural Memory Consolidation During a Nap

ABSTRACT
Acoustic and electrical brain stimulations are techniques well known to enhance memory consolidation by driving slow oscillations (SO, < 1Hz) and sleep spindle activity. Recent studies have suggested that the temporal relationship between SO and sleep spindle activity may be an important key to understanding memory consolidation mechanisms. We hypothesized that evoking SO after sleep spindle activity may enhance memory consolidation. To derive these spindle-SO pairs, we delivered acoustic stimulation after sleep spindle detection and investigated its effects on memory consolidation with behavioral tests and analyses of neurophysiological features. Thirteen healthy male subjects (mean ± SD age: 26.3 ± 2.4 years) participated in this study. Subjects took a nap with acoustic stimulation after spindle activity detection and a sham nap without acoustic stimulation. All subjects performed word-pair memorization and finger tapping tasks before and after their nap. We found phase-locked SO and delta (1–4 Hz) activity during the stimulation nap in response to acoustic stimuli, and the subjects had a greater improvement in finger tapping tasks after the stimulation nap than after the sham nap (p = 0.014). We found strong motor learning enhancement after the stimulation nap, but this effect was limited to the subjects who did not demonstrate evoked spindle activity after their acoustic stimulation. Evoked spindle activity occurred in the up-state following the negative peak in auditory evoked potential (AEP), and this activity was observed only in subjects who had a greater AEP amplitude than normal SO. Based on these results, we suggest that subject-specific stimulation parameters, such as acoustic amplitude and timing, improve motor learning, and are appropriate to drive SO without causing a spindle response.