The Extent of Processing of Near-Hearing Threshold Stimuli during Natural Sleep

Objectives: Event-related potential (ERPs) provide an exquisite means to monitor the extent of processing of external stimulus input during sleep. The processing of relatively high intensity stimuli has been well documented. Sleep normally occurs in much less noisy environments. The present study therefore employed ERPs to examine the extent of processing of very low intensity (near-hearing threshold) stimuli. Design: Brief duration 1000 Hz auditory tone bursts varying in intensity at random from -5 to +45 dB from normative hearing level (nHL) in 10 dB steps were presented every 1.5 to 2.5 s when the subject was awake and reading a book and again during all-night sleep. Subjects: n = 10 healthy young adults. Measurements and Results: In the waking state, the auditory stimuli elicited a negative-going deflection, N1, peaking at about 100 ms, followed by a smaller positivity, P2, peaking at about 180 ms. N1-P2 gradually decreased in amplitude with decreases in stimulus intensity and remained visible at near-hearing threshold levels. During NREM sleep, the amplitude of N1 was at baseline level and was reduced to only 15% to 20% of its waking amplitude during REM sleep. P2 was much larger in sleep than in wakefulness. Importantly, during sleep, P2 could be reliably elicited by the auditory stimuli to within 15 dB of threshold. During NREM, a large amplitude negativity peaking at about 350 ms was elicited by the higher intensity stimuli. This N350 was much reduced in amplitude during REM sleep. A significant N350 was not, however, elicited when stimuli intensity levels were below 25 dB nHL. Conclusions: Auditory stimuli that are only slightly above hearing threshold appear to be processed extensively during a 200 to 400 ms interval in both NREM and REM sleep. The nature of this processing is, however, very different compared to the waking state.