Cortex-specific inversion of visual responses during sleep
During sleep, we functionally disengage from our external environment. Our eyes close, profoundly reducing visual input to the brain. However, some light passes through the eyelid, and luminance changes are perceived even through closed eyes during wakefulness. Although the relay of sensory information is thought to be gated by the thalamus during sleep, sensory information can still reach the cortex. To elucidate how visual inputs are modulated at each stage of thalamic and cortical processing during sleep, we used simultaneous EEG-fMRI while presenting luminance-modulated visual stimuli to sleeping humans. We discovered that responses to light remained intact in the visual thalamus during N1 and N2 sleep. However, stimulus-evoked responses in early visual cortex were profoundly suppressed, exhibiting an inverted pattern in which high-intensity visual stimulation evoked visual cortical deactivation. These findings suggest a cortical mechanism where inhibitory circuits regulate stimulus-driven deactivation in visual cortex, facilitating sensory isolation during early stages of sleep.