Coupling of gamma band activity to sleep spindle oscillations – a combined EEG/MEG study

•Fast and slow EEG sleep spindles couple to MEG gamma activity with variable phase.•MEG-only spindles not present in EEG do not couple to gamma band activity.•Spindle and low gamma are modulated by different infraslow-oscillation frequencies. Sleep spindles are crucial to memory consolidation. Cortical gamma oscillations (30–100 Hz) are considered to reflect processing of memory in local cortical networks. The temporal and regulatory relationship between spindles and gamma activity might therefore provide clues into how sleep strengthens cortical memory representations. Here, combining EEG with MEG recordings during sleep in healthy humans (n = 12), we investigated the temporal relationships of cortical gamma band activity, always measured by MEG, during fast (12–16 Hz) and slow (8–12 Hz) sleep spindles detected in the EEG or MEG. Time-frequency distributions did not show a consistent coupling of gamma to the spindle oscillation, although activity in the low gamma (30–40 Hz) and neighboring beta range (