Cyclic EEG and motility patterns during sleep in restrained infant rats.

When restrained in a quasi-fetal position, infant rats (2--3 weeks old) remain for an extended period in a sleep-like state as judged by polygraphic criteria. Moreover, they become very difficult to arouse by sensory stimuli, and show a level of spontaneous motility considerably higher than is normally found at this age during sleep. Phasic generalized body movements, which are often quite stereotyped, occur in trains at regular intervals, against a low background level of neck muscle tonus. The amplitude of the cortical EEG, especially in the delta band, is negatively correlated with the frequency of spontaneous motor burst activity. It is suggested that these REM-like "rapid body movements" in immature mammals may well represent a transitional stage between fetal behavior patterns and sleep motility in the adult organism.