Slow horizontal eye movement at human sleep onset.

Slow eye movement (SEM) at human sleep onset has been studied as a function of EEG and behavioral state but has not been subjected to systematic physical measurement. This study counted the frequency of horizontal SEM and quantified its physical properties during hypnagogic EEG stages 3-8 (stage 1 sleep). SEM amplitudes, peak velocities, and durations were digitally measured. As in previous studies SEM occurred throughout stage 1 sleep, coinciding with all tonic states and with all phasic events that classically define stage 1. Within stage 1 the physical properties of SEM varied significantly. In hypnagogic stage 3, where EEG alpha dominates up to half of any 30-s epoch, the typical SEM was moderate in amplitude, slow, and short-lasting, and SEM peak velocity described a weak function of SEM amplitude. In stages 5 and 6 combined--characterized by a low-voltage delta-theta EEG, sustained alpha suppression, and no more than one vertex sharp wave (VSW) per 30-s epoch--SEM velocity was maximal, and described a strong function of amplitude. Despite low absolute velocities, stage 5/6 SEM could approximate saccade-like morphologies. In stages 7 and 8 combined, characterized by VSW bursts and incipient spindles or K complexes, SEM was maximal in amplitude and duration but not faster than SEM in stage 5/6. These properties of human SEM are discussed in relation to the anatomy and physiology of SEM in macaque.