Grouping of MEG gamma oscillations by EEG sleep spindles.

Studies have revealed an association between EEG sleep spindles and processing of memories during sleep. Here we investigated whether there is a temporal relation between sleep spindles and MEG oscillatory activity in the gamma frequency band (>30 Hz) which is considered to reflect local cortical processing of memory representations. MEG and simultaneous EEG (at Cz) were obtained in subjects during sleep together with standard polysomnography. As expected EEG spindles were correlated with power increases in MEG spindle (12.5-15.5 Hz) power mainly over prefrontal and occipital cortical areas. During EEG spindles we revealed both transient significant increases and decreases in MEG power, with decreases occurring significantly more often than increases. The modulations in gamma power occurred mainly at sites of increased MEG spindle power, and more often during peaks than troughs within the EEG spindle cycle. Cross-frequency coherence analyses confirmed a strong phase-coupling of gamma band activity with the spindle rhythm. The findings are consistent with the idea that spindles provide a fine-tuned temporal frame for integrated cortical memory processing during sleep.