Evoked K-complex generation: the impact of sleep spindles and age

Objective: It has been proposed that spindles and spontaneous K-complexes reflect two sides of a coin, with the spindle reflecting an inhibitory microstate and the K-complex reflecting an excitatory or aroused microstate [Physiol Behav 1993;54(4):795]. This hypothesis predicts that the presence of a sleep spindle at the time of stimulus presentation would decrease the likelihood of a K-complex being elicited by that stimulus. The present study sought to test this hypothesis in young and elderly subjects. Methods: Ten young and 7 elderly adults who were neurologically healthy and free from medications spent one night in the sleep laboratory. EEG was recorded from 6 gold plate electrodes (Fz, FCz, Cz, CPz, Pz and O2) referenced to A1+A2. Tone clicks (1000 Hz) at 80 dB above measured awake detection thresholds were presented binaurally either during a spindle (SP+) or in the absence of a spindle (SP−). This was achieved by viewing a central EEG channel filtered to pass only the frequencies between 12 and 14 Hz. Trials were further classified based on whether (KC+) or not (KC−) they produced a K-complex. K-Complex probability and the amplitude and latency of the N550 component of the averaged evoked potential for KC+ trials were assessed using a two-way analysis of variance with main effects of age and spindle presence/absence. Results: There were significant reductions in K-complex probability and N550 amplitude and a significant increase in N550 latency, as a function of age. However, no variable displayed a significant effect of spindle presence/absence, or an age×spindle interaction effect. Conclusions: The data failed to support the hypothesis that sleep spindles are antagonistic to the production of K-complexes, both in terms of the likelihood of K-complexes being elicited or in their amplitude when elicited (N550). The absence of spindle effects on K-complex generation argues against them being two sides of a coin and supports the notion of K-complexes having an extra-thalamic or non-specific generation mechanism.