Precisely phase-locked acoustic stimuli globally enhance slow oscillations, but depress fast spindles

Introduction: Several studies have shown manipulation of slow oscillations (SO) and sigma power through auditory stimulation during sleep. Most of the evidence, however, regards effects immediately following stimulation rather than enduring effects. Moreover, effects on discrete fast and slow spindles have as yet not been assessed. Materials and Methods: Here we use a modeling-based approach to predict upcoming oscillatory activity in the EEG and precisely phase-lock subtle acoustic stimuli to the start of the SO positive deflection. We assess the effects of stimulation on discrete slow oscillations, fast and slow spindles in the seconds after stimulation and on the longer term. We relate our findings to observations at the level of spectral measures and stimulus evoked responses. Results: Our observations show that slow wave measures were consistently increased, as apparent in measures of discrete SO's, SO and delta power and deflections in the ERP. On the other hand, fast spindle measures showed a temporally localized increase during a stimulus-induced SO positive deflection around 1 second after stimulation, but were globally decreased, both on the short and long term. The latter was apparent in measures of discrete fast spindles and PSD across longer periods of sleep. Conclusions: Acoustic stimuli, precisely phase locked to the SO onset, increase SO's and delta power globally, therewith deepening sleep. On the other hand, fast sleep spindles are globally depressed. This appears to be due, in part, to interruption of ongoing spindles by the stimulus and may furthermore reflect a depressing influence of slow oscillations on fast spindle dynamics. Future studies could evaluate the therapeutic potential of sleep deepening with acoustic stimulation in clinical populations who suffer from reduced deep sleep, such as in insomnia or post-traumatic stress disorder.