The initial orienting response during human REM sleep as revealed by the N1 component of auditory event-related potentials

The large N1 wave of the auditory event-related potentials (ERPs) typically occurring to the first stimulus after a long silent interval seems to be a ssociated with the involuntary initial-orienting response. Since the mechan isms involved in the generation of this brain response are assumed to be ac tivated automatically, the present study aims at determining whether this e lectrophysiological response can also be elicited during human REM sleep, t he sleep stage considered most sensitive to external stimuli. To achieve th is goal, the auditory N1 wave was analyzed in wakefulness and REM sleep for frequency deviant tones delivered in several positions (1, 2, 4 and 6) wit hin homogenous stimulus trains separated by different intervals of silence (3, 6 and 9 s), the intra-train stimulus interval being 600 ms. A significa nt increment in the amplitude of the NI component for the first deviant ton e, as compared with deviants delivered in remaining positions, was observed in both brain states, independently of the inter-train interval length. Th is result cannot be explained by a release-from-refractoriness effect, sinc e only one deviant was presented in each train and the inter-deviant interv al hardly changed from one train to another. The increase in N1 to the firs t stimulus of the train, probably due to the contribution of the neuronal e lements responsible for the supratemporal and non-specific components, may be explained by changes in the silent interval, rather than by variations i n the stimulus frequency. The enhanced NI could be reflecting a general inc rease in sensory sensitivity associated with the arousal factor of the orie nting response. These findings suggest that the brain maintains the potenti al ability to trigger the brain events responsible for the OR elicitation, even during REM sleep. (C) 2001 Elsevier Science B,V. All rights reserved.