Phase-amplitude coupling in human scalp EEG during NREM sleep

Slow oscillations and spindle waves are dominant electroencephalographic (EEG) rhythms during non-rapid-eye-movement (NREM) sleep that have been demonstrated to be associated with various brain functionalities. Previous studies have demonstrated that thalamus originated spindle waves are regulated by cortically generated slow oscillations through cortico-cortical and cortico-thalamic connections. To investigate the changes of coupling between slow oscillations and higher rhythms during different NREM sleep stages, we analyzed scalp EEG data measured from two bipolar electrodes (Fpz-Cz and Pz-Oz) in 15 healthy adults, with accompanying expert annotations of sleep stages. A normalized modulation index (MI) of statistical significance was used to measure the phase-amplitude interaction between phase of slow oscillations (0.1-1.5 Hz) and amplitude of spindle waves (12-16 Hz). It is found that the normalized MI increased from NREM1 to NREM4 for both anterior and posterior channels, and statistically significant coupling occurs in slow wave sleep (p<;0.05). Further, the coupling strength is correlated with the power of slow oscillations [Spearman correlation coefficient r=0.79 (Fpz-Cz) and 0.61 (Pz-Oz)], which might contribute to the dominant role of slow oscillations in the neuronal modulation mechanism.