A cognitive process occurring during sleep revealed by rapid eye movements

Since the discovery of REM sleep, the nature of the rapid eye movements
that characterize this sleep phase has remained elusive. Do they reveal
gaze shifts in the virtual environment of dreams or simply reflect random
brainstem activity? We harnessed the head direction (HD) system of the
mouse thalamus, a neuronal population whose activity reports, in awake
mice, their actual HD as they explore their environment and, in sleeping
mice, their virtual HD. We discovered that the direction and amplitude of
rapid eye movements during REM sleep reveal the direction and amplitude of
the ongoing changes in virtual HD. Thus, rapid eye movements disclose gaze
shifts in the virtual world of REM sleep, thereby providing a window in
the cognitive processes of the sleeping brain.