An examination of evoked K-complex amplitude and frequency of occurrence in the elderly

The elderly consistently display lower levels of slow wave sleep (SWS), primarily because of the small amplitude of their delta activity. Given that delta electroencephalogram (EEG) and K-complexes are thought to be generated by the same mechanisms, it was hypothesized that K-complex amplitude and rate of production would be lower in the elderly. K-complex amplitude was assessed by averaging K-complex responses to auditory stimuli, and measuring the amplitude of the N550 component of the averaged evoked response. Ten young (five males and five females; mean age 23.10 +/- 5.36 years) and 10 elderly adults (six males and four females; mean age=75.60 +/- 4.48 years) who were neurologically healthy and free from medication spent two non-consecutive nights in the sleep laboratory. EEG was recorded from six gold plate electrodes (Fz, Fcz, Cz, Cpz, Pz and O2) referenced to A1 + A2. Tone clicks (1000 Hz), of varying intensity from 70 to 100 decibels above measured awake detection threshold, were presented binaurally during stage 2 sleep. Responses were classified according to whether they produced: a K-complex, a vertex sharp wave (VSW), both of these responses or neither response. They were then averaged separately for each response type. The elderly showed a substantially smaller N350 (averaged VSWs) and N550 amplitudes compared with the young subjects. The elderly also showed an augmented but delayed P2 component, followed by a long-lasting positive EEG shift. The smaller amplitude of the averaged K-complex N550 component is consistent with lower delta amplitudes previously reported in the elderly and with the hypothesis that K-complexes and delta activity share the same generator mechanisms. The enhanced P2 component and the long-lasting positive deflection in the EEG in the elderly indicate the existence of age differences other than smaller EEG amplitude.