INTRODUCTION ONE OF THE GREAT PUZZLES OF SLEEP IS THE EXTENT TO WHICH THE SLEEPER IS AWARE OF EVENTS IN THEIR EXTERNAL ENVIRONMENT. Information processing

ONE OF THE GREAT PUZZLES OF SLEEP IS THE EXTENT TO WHICH THE SLEEPER IS AWARE OF EVENTS IN THEIR EXTERNAL ENVIRONMENT. Information processing capabilities at sleep onset and across various depths of sleep are of interest to a general audience, including psychologists, philosophers, and those in the medical field (particularly anesthesiologists). Awareness during sleep is relevant for studies of sleep maintenance in good and poor sleepers, the effects of environmental noise on sleep quality, and memory and learning during sleep. The extent of such information processing and consciousness during sleep has been debated for some time. One view is that awareness of external events is not possible, particularly during REM sleep when mental resources are focused toward internal dream activity.1 On the other hand, in the case of lucid dreaming, consciousness must be possible in order for the sleeper to be aware of their dreams and to subsequently signal to the experimenter that they are dreaming.2 Investigating information processing during sleep may also further our understanding of functions of sleep. For instance, Snyder’s sentinel theory proposed that REM sleep served to periodically prepare the brain for arousal in the event that the sleeper might need to respond to threatening stimuli (e.g., predators) in the environment.3 Although data did not support this theory (i.e., prey species did not have more REM sleep than their predators), it may be that enhanced information processing capabilities in REM sleep may be needed or may have developed to provide sensory stimulation to the brain over a long period of continuous non-REM sleep. This position is in line with the view that REM sleep is more like a state of wakefulness than sleep. Indeed, REM sleep is similar to the waking state in many respects. A major difficulty encountered in the study of awareness during sleep is that the experimenter does not have direct access to the sleeper’s mental state. For instance, the sleeper is usually unable to provide either a verbal or behavioral response. Thus, even if the sleeper is conscious of the external environment, he or she may be unable to signal awareness to the experimenter. It is, of course, possible to awaken the subject and ask him or her to recall mental activity. The reliability of such subjective recall is questionable. A failure to recall events immediately prior to awakening may reflect a failure of memory storage or retrieval rather than a lack of awareness of the external stimulus.4 Eventrelated potentials (ERPs) offer a means to probe the extent of information processing during sleep, independently of subjective reports or behavioral responses. The long-latency auditory ERPs are particularly affected by manipulation of the subject’s level of attention, arousal, and consciousness.5-9 The vertex complex of the ERP consists of an “N1” (negative wave peaking between 80-100 ms) and a “P2” component (positive wave peaking between 175-225 ms). Näätänan7 has suggested that N1 may act as a transient-detector system that triggers an internal attentional system. N1 may subserve “conscious perception of auditory stimuli in general. . . without indicating what the stimulus is or what its precise features are” (p.212). As might be