Temporal correlation between two channels EEG of bipolar lead in the head midline is associated with sleep-wake stages

Whether the temporal correlation between inter-leads Electroencephalogram (EEG) that located on the boundary between left and right brain hemispheres is associated with sleep stages or not is still unknown. The purpose of this paper is to evaluate the role of correlation coefficients between EEG leads Fpz-Cz and Pz-Oz for automatic classification of sleep stages. A total number of 39 EEG recordings (about 20 h each) were selected from the expanded sleep database in European data format for temporal correlation analysis. Original waveform of EEG was decomposed into sub-bands δ (1–4 Hz), θ (4–8 Hz), α (8–13 Hz) and β (13–30 Hz). The correlation coefficient between original EEG leads Fpz-Cz and Pz-Oz within frequency band 0.5–30 Hz was defined as r_EEG and was calculated every 30 s, while that between the two leads EEG in sub-bands δ, θ, α and β were defined as r_δ, r_θ, r_α and r_β, respectively. Classification of wakefulness and sleep was processed by fixed threshold that derived from the probability density function of correlation coefficients. There was no correlation between EEG leads Fpz-Cz and Pz-Oz during wakefulness (|r| < 0.1 for r_θ, r_α and r_β, while 0.3 > r > 0.1 for r_EEG and r_δ), while low correlation existed during sleep (r ≈ −0.4 for r_EEG, r_δ, r_θ, r_α and r_β). There were significant differences (analysis of variance, P < 0.001) for r_EEG, r_δ, r_θ, r_α and r_β during sleep when in comparison with that during wakefulness, respectively. The accuracy for distinguishing states between wakefulness and sleep was 94.2, 93.4, 89.4, 85.2 and 91.4 % in terms of r_EEG, r_δ, r_θ, r_α and r_β, respectively. However, no correlation index between EEG leads Fpz-Cz and Pz-Oz could distinguish all five types of wakefulness, rapid eye movement (REM) sleep, N1 sleep, N2 sleep and N3 sleep. In conclusion, the temporal correlation between EEG bipolar leads Fpz-Cz and Pz-Oz are highly associated with sleep-wake stages. Moreover, high accuracy of sleep-wake classification could be achieved by the temporal correlation within frequency band 0.5–30 Hz between EEG leads Fpz-Cz and Pz-Oz.