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

The traditional view that non-REM sleep is a uniform state of unconsciousness is being challenged by recent findings. Slow oscillations, once considered exclusive to sleep, have been observed during wakefulness, and neural dynamics typical of conscious states have been detected during non-REM sleep. Additionally, the disconnection in cortical effective connectivity, long thought to be a hallmark of unconsciousness in non-REM sleep, appears less general than previously assumed. These findings complicate the search for neural correlates of consciousness (NCC) and suggest that the boundaries between conscious and unconscious brain activity may be more fluid than standard models propose.

Study at a glance

Abstract

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.