Tonic and phasic components of eye movements during REM sleep in the rat

Phasic oculomotor activity is one of the features identifying rapid eye movement (REM) sleep. Recently, it has been shown in cats that, despite bursts of complex two-component rapid eye movements, the eyes tend to maintain a nasal and downward rotation during REM sleep. Although the function of eye movements during sleep remains elusive, it is important to know whether the characteristics of eye movements during sleep are species-specific in mammals. In this work, quantitative characteristics of eye movements, recorded by the scleral search coil technique, were studied during wakefulness and sleep in rats. During wakefulness, rats performed conjugated saccades at a very low rate and some eye movements associated to blinking and gnawing. Throughout non-REM sleep, eye movements were slow, mostly unconjugated and the eyes maintained a divergence in the horizontal plane. The beginning of REM sleep was characterized by a convergence and downward rotation of the eyes, which tended to persist until the end of REM sleep. Rapid eye movements, isolated and monocular at the beginning, became complex and organized in high-frequency bursts. These results demonstrate that, despite the difference in extraocular anatomy and visuomotor strategies between frontal- and lateral-eyed species, eye movements during REM sleep in rats are very similar to those described in cats. This suggests that the mechanisms generating eye movements during REM sleep are largely conserved in mammals.