Visual processing in the sleeping brain

Bright lights cause us to wake up, unexpected colors divert our attention and threatening creatures trigger a flight response. These instinctual processes in response to visual cues are robust during wakefulness. However, since the study of visual processes of the sleeping brain has been limited by the closed eyelids, we do not know what neuronal activity these stimuli trigger during sleep. Using sleep compatible virtual reality goggles, we aimed to investigate which visual functions persisted during NREM sleep in humans sleeping with eyes open. Here we designed a series of experiments to elicit well established pupillary reactions and visually evoked potentials. In Experiment 1, we successfully demonstrated that pupillary constriction and neural responses during NREM covaried with the brightness levels of presented light flashes without awakening participants. In Experiment 2, we used a color oddball task and showed that the evoked potential typically seen for surprising/unexpected events (i.e., P300) persisted during sleep. In Experiment 3, we found that scared faces elicited stronger, face-specific (i.e., N170) event-related potential and pupil dilation compared to neutral faces. Taken together, our preliminary results suggest that, even during NREM sleep, visual stimuli are processed not only within early visual cortices, but may also engage regions of the ventral visual pathway, subtending more complex visual representations. This work opens a very exciting avenue for further research on visual functions across wakefulness and sleep.