Evaluation of a filter/integrator system for quantifying delta activity in the sleep electroencephalogram

The integrated output of a delta bandpass filter was evaluated as a means of quantifying delta activity in the sleep electroencephalogram (EEG). Total integrated slow wave amplitude (ISWA) and a density measure (ISWA/20-s epoch) were determined from four complete NREM sleep periods (NREMPs) on each of 2 baseline nights from 15 subjects. Total ISWA displays a linear decline across NREMPs, whereas ISWA/20-s epoch shows a curvilinear decline. Both measures show highly significant correlations with manually staged slow wave sleep and a high degree of intra-individual consistency between nights, for both whole-night values and those of individual NREMPs. The slow-wave density index (ISWA/20-s epoch) demonstrated consistently higher between-night correlations across NREMPs than did the total ISWA index, particularly in the first NREMP, suggesting it may better characterize the nightly distribution of delta activity in an individual's sleep EEG.