Is Rapid Eye Movement Sleep a Paradoxical State of Sleep?
Rapid eye movement sleep (REM) is often considered as a homogeneous state of sleep. However, the frequent occurrence of transient events indicates that it may be separated into two distinct, phasic and tonic, substates. During tonic REM, we found local appearances of spindle waves in the barrel cortex concomitant with strong delta power on the local field potential. Subthreshold spindle oscillations in neurons of the ventral posterior medial nucleus further confirmed the thalamic origin of these cortical spindles. Spindle oscillations were suppressed in phasic REM, while thalamus spike firing increased associated with rapid whisker movements of mice and cortical activity transitioned to an activated state. During REM, sensory thalamus and barrel cortex therefore alternate between high (wake-like) and low (non-REM sleep-like) activation states, possibly allowing transient sensory integration windows to emerge throughout this paradoxical sleep stage.