Neurophysiological evidence for the detection of external stimuli during sleep

Study Objectives: A cognitive evoked potential, labelled "P300," is elicite d when an observer attends to and detects an infrequently delivered "target " stimulus. It is not typically present if the target is ignored or undetec ted. P300 is therefore thought to reflect some aspect of consciousness of t he stimulus. There has been much controversy concerning whether P300 can be recorded in sleep, a state in which information processing of external eve nts is presumably reduced. The present study investigated the effects of bo th pitch and intensity stimuli on information processing, in order to deter mine whether a more salient stimulus might elicit a P300 in sleep that is c omparable to the waking P300. Design: A true P300 will have a parietal maximum peak following a rare stim ulus, and its amplitude will vary inversely with the probability of stimulu s delivery. Participants were thus randomly assigned to one of three probab ility groups, in which the deviant was presented on 20%, 10%, or 5% of tria ls. Setting: Data were collected in the Human Neurophysiology Laboratory at the University of Ottawa. Participants: Twenty-four young adults. Interventions: N/A Measurements and Results: During wakefulness, a parietal P300 was apparent following both pitch and intensity deviants when participants were asked to detect deviant stimuli. A P300 was also apparent following the intensity d eviant when participants were instructed to ignore the stimuli. During non- REM sleep, no P300 could be identified. In REM sleep, very rare (p=.05) lou d deviants elicited a parietal P300. This P300 was attenuated relative to t he waking ignore condition. Moreover, the frontal dispersion of the peak wa s absent. Conclusions: These data provide evidence that participants are conscious (p arietal P300) of very rare and intrusive stimuli during REM sleep, although the frontal aspects associated with this consciousness may be absent.