Non-REM sleep and the neural correlates of consciousness: more than meets the eye

The scientific study of the neural correlates of consciousness (NCC) has long relied on comparing conditions in which consciousness is normally present with others in which it is impaired. Brain lesions offer a unique opportunity to understand which anatomical networks are needed to sustain consciousness, but provide limited insights on the patterns of neural activity that can support conscious processing. Non-REM sleep, on the other hand, has long epitomized the typical case of a non-conscious yet fully active brain. Consequently, the differences in neural activity existing between wakefulness and non-REM sleep have also been used to define the NCC. Recently, however, several studies have started challenging the traditional understanding of neuronal activity during wakefulness and sleep. First, oscillatory dynamics characteristic of non-REM sleep-such as slow oscillations-have been reported to occur during wakefulness. Second, neural dynamics typical of conscious states have also been observed during non-REM sleep. Finally, the disconnection in cortical effective connectivity that has been indicated as one of the hallmarks of the loss of consciousness that occurs during non-REM sleep has recently been shown to be a less general phenomenon than previously thought. Here I will provide an overview of these recent findings, and discuss their implications for understanding the real nature of the NCC.