To wake or not to wake? The two-sided nature of the human K-complex

Sleep fosters performance but likewise renders creatures insensitive to environmental threat. The brain balances between sleep promotion and protection during light sleep. One associated electrophysiological hallmark is the K-complex (KC), the sleep promoting versus arousal inducing role of which is under debate. We examined 37 subjects using EEG-combined fMRI and found KC-associated positive BOLD signal changes in subcortical (brainstem, thalamus), sensory and motor, midline and regions which form part of the default mode network, and negative changes in the anterior insula. Connectivity analysis revealed the primary auditory cortex as the first region to be influenced during the KC and that midline regions activated successively from front to back in association with the sleep protecting part of the KC. Our findings support thalamic involvement in KC mediation and an association of KCs with subcortical arousal mechanisms: activations in sensory areas suggest the existence of low level information processing during KC limited by anterior insula disengagement suggesting a two-sided nature of the KC: it embodies an arousal with subsequent sleep-guarding counteraction that might on the one hand serve periodical monitoring of the environment with basic information processing and on the other hand protect the continuity of sleep and thus its restoring effect.