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A K-complex is a waveform that may be seen on an electroencephalogram (EEG). It occurs during stage 2 NREM sleep. It is the "largest event in healthy human EEG". They are more frequent in the first sleep cycles.
K-complexes have two proposed functions: first, suppressing cortical arousal in response to stimuli that the sleeping brain evaluates not to signal danger, and second, aiding sleep-based memory consolidation.
The K-complex was discovered in 1937 in the private laboratories of Alfred Lee Loomis.
Both K-complex and delta wave activity in stage 2 sleep create slow-wave (0.8 Hz) and delta (1.6–4.0 Hz) oscillations. However, their topographical distribution is different, and the delta power of K-complexes is higher.
They are created by the occurrence in widespread cortical areas of outward Dendrite currents from the middle (III) to the upper (I) layers of the cerebral cortex. This is accompanied by a decrease in broadband EEG power including gamma wave activity. This produces "down-states" of neuronal silence in which neural network activity is reduced. The activity of K-complexes is transferred to the thalamus where it synchronizes the thalamocortical network during sleep, producing sleep oscillations such as spindles and . It has been observed that they are indeed identical in the "laminar distributions of transmembrane currents" to the slow waves of slow-wave sleep.
K-complexes have been suggested both to protect sleep and also to engage in information processing, as they are both an essential part of the synchronization of NREM sleep, while they also respond to both internal and external stimuli in a reactive manner. This would be consistent with a function in suppressing cortical arousal in response to stimuli that the brain needs to initially process in regard to whether it is dangerous or not.
Another suggested function is aiding the activation homeostasis of and memory consolidation. The activation thresholds of cortical synapses become lowered during wakefulness as they process information, making them more responsive, and so need to be adjusted back to preserve their signal-to-noise ratio. The down-state provided by K-complexes does this by reducing the strengths of synaptic connections that occur while an individual is awake. Further, the recovery from the down-state they induce allows that "cortical firing 'reboots' in a systematic order" so that memory engrams encoded during neuronal firing can be "repeatedly practiced and thus consolidated".
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