Hippocampal theta rhythm during paradoxical sleep. Effects of afferent stimuli and phase relationships with phasic events.

Hippocampal theta rhythm and phasic events (i.e., vibrissa, body and ocular movements and ponto-geniculo-occipital (PGO) spikes) were recorded in rats and cats during paradoxical sleep (PS). Single pulse stimulation of some afferent structures (i.e., medial septum, mesencephalic reticular formation and entorhinal cortex) were performed during PS. Hippocampal afferent stimulation determined the phase resetting of hippocampal theta waves during PS in a similar form to that during wakefulness. All studied phasic events tend to occur at a preferred phase of the theta cycle. Theta waves begin to synchronize some time before movements. These results suggest that PS is an active state during which hippocampal theta rhythm may have functions similar to those occurring during wakefulness, e.g. sensorimotor integration.