The role of the spindle in human information processing of high-intensity stimuli during sleep

Sleep spindles are 12-14 Hz oscillations in EEG, which are thought to inhibit or 'gate' information processing. Event-related potentials may be employed to probe the extent of information processing during sleep. Previous research indicates that event-related potentials elicited by moderate intensity stimuli show increased positivity (or further removal of negativity) when stimuli are presented concurrent with spindles. However, the effectiveness of spindles to inhibit the processing of much louder stimuli remains unknown. The purpose of the present study was to investigate the extent of this gating, by using a range of stimuli including those that are loud and intrusive. Eight good sleepers were recorded during a single night. Auditory stimuli were delivered randomly at 0, 60, 80 or 100 dB SPL. Trials were sorted off-line by sleep stage, stimulus intensity and spindle characteristic (i.e. spindle absent, spindle present). During the sleep-onset period, the often-reported changes in event-related potentials were observed - N1 decreased and P2 increased in amplitude. In Stage 2 sleep, P2 was affected by the presence of spindles, particularly when stimulus intensity was loud. Its amplitude was greatest when spindles occurred following the onset of the stimulus. Scalp-recorded spindles might, therefore, be a consequence of the prior thalamic inhibition of information processing, especially when confronted by loud, intrusive external stimuli.