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 inhib it or 'gate' information processing. Event-related potentials may be employ ed to probe the extent of information processing during sleep. Previous res earch indicates that event-related potentials elicited by moderate intensit y 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 unkno wn. 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 intru sive. Eight good sleepers were recorded during a single night. Auditory sti muli 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 oft en-reported changes in event-related potentials were observed - N1 decrease d and P2 increased in amplitude. In Stage 2 sleep, P2 was affected by the p resence of spindles, particularly when stimulus intensity was loud. Its amp litude was greatest when spindles occurred following the onset of the stimu lus. Scalp-recorded spindles might, therefore, be a consequence of the prio r thalamic inhibition of information processing, especially when confronted by loud, intrusive external stimuli.