It Is Known That Similar Phenomena to Dreams Arise during the Period of Transition from Wakefulness to Non-rapid Eye Movement (nrem)

IT IS KNOWN THAT SIMILAR PHENOMENA TO DREAMS ARISE DURING THE PERIOD OF TRANSITION FROM WAKEFULNESS TO NON-RAPID EYE MOVEMENT (NREM) SLEEP. This dream-like phenomenon during the sleep-onset period is called hypnagogic imagery. The usual way experimental studies regarding hypnagogic imagery are performed is to awaken a dozing subject and discuss his or her mental experiences that occurred before awakening. However, this method relies on the introspection of the subjects. Therefore, a psychophysiologic method is needed to more objectively study hypnagogic imagery. Although there are several reports on the functional activity of the brain immediately before waking with imagery during the sleeponset period using spectral analysis of electroencephalogram (EEG) records,1-5 these studies do not show a common view corresponding to the presence of imagery. Therefore, it seems worthwhile for intracerebral and intercerebral information processing of hypnagogic imagery to be investigated from another point of view. It is thought that the processing of hypnagogic imagery has an influence on other information processes when imagery is perceived. Components of event-related potentials (ERPs) are useful to examine the information processing of external tone stimuli because these reflect intracerebral information processes. Concerning previous studies about ERPs during the sleep-onset period, changes of ERPs influenced by declines in the arousal level, variation of stimulus frequency, or differences of the task load have been reported.6-14 However, there are no reports on the relationship between hypnagogic imagery and ERPs. Comparison of ERPs to external tone stimuli between the conditions with and without hypnagogic imagery makes it possible to estimate the influence of hypnagogic imagery on the information processing of tone stimuli. It must be noted that the amplitude of the ERP components changes with the fluctuation of the arousal level. According to a review of ERPs during the sleep-onset period,14 amplitudes of the negative wave peaking at 100 milliseconds (N100) and the positive component peaking around 300 milliseconds (P300) gradually decrease as the arousal level becomes lower. Apparent diminution of these 2 components is observed when the subject no longer signals detection of the external stimulus. On the other hand, P200 may increase during the sleep-onset period. The decrease in N100 and the increase in P200 have been explained by the removal of a long-lasting processing negativity associated with attention and consciousness in the waking state. During late stage 1 sleep of the standard sleep criteria (provided by Rechtschaffen & Kales),15 N300 begins to emerge. And finally, in standard stage 2 sleep, which is regarded as definitive sleep, the large N550-P900 components of the K-complex become clearly visible. Therefore, it is necessary to control the arousal level when tone stimuli are presented to investigate differences in the processing of tone stimuli between the presence and absence of imagery. To objectively judge the arousal level during the sleepEffects of Hypnagogic Imagery on the Event-Related Potential to External Tone Stimuli