Identification of OB gamma as a reliable marker of sleep/wake states.

(A) EMG, HPC, and OB activity during wake, NREM, and REM sleep. Wake is characterised by high EMG activity and high gamma power in the OB, whereas sleep is characterised by low EMG activity and low gamma power in the OB. HPC LFP shows regular theta activity during REM sleep. Filtered signals from the OB in the gamma band (OB-γ, 50–70 Hz) illustrate the remarkable decrease in gamma power during sleep states. (B) Low-frequency (top) and high-frequency (bottom) spectra from different brain regions during NREM, REM, and wake states as classified using movement-based scoring (EMG or filmed activity). Note the strong increase in gamma activity in the OB in the wake state (pink bar). (n = 9 for OB and HPC, n = 6 for PFCx and PaCx, error bars: s.e.m.). Note that differences in HPC activity between vigilance states vary strongly according to recording depth; see S5 Fig. (C) Gamma power in OB drops as the animal transitions from wake to sleep. The data are smoothed with different window lengths (0.1, 1, and 4 s, as indicated by colour). Inset: histogram of gamma values after different smoothing windows have been applied. The fast fluctuations present in the awake state are smoothed out as the window size increases, yielding a more clearly bimodal distribution with a larger smoothing window. (D) Ashman’s D (bimodality index, significant if larger than 2) for gamma power increases (top), and the overlap between the two Gaussians fit to the bimodal distribution of gamma power decreases (bottom) with the length of the smoothing window. The stars show window sizes illustrated in C. EMG, electromyography; HPC, hippocampus; NREM, non-REM; OB, olfactory bulb; PaCx, parietal cortex; PFCx, prefrontal cortex; REM, rapid eye movement.