Figure 1.
Cortical activity across the sleep–wake cycle is characterized by intermittent transitions between distinct dominant brain rhythms. a, Representative 5 min EEG trace for a control rat during a 12 h dark period. b, A 16 s segment from the 5 min EEG trace shown in a. The time evolution of the EEG signal is analyzed by evaluating the spectral power in several frequency bands on non-overlapping windows of length w, as shown in d–f. c, Average power spectra for control and VLPO-lesioned rats during Wake, brief wake periods (<1 min), REM, and NREM sleep. d, Top, Spectrogram obtained from cortical EEG signal of a control rat over a 2 h segment of 12 h lights-on period. Spectral power is calculated in non-overlapping time windows w = 4 s, and is color coded over a range (0–20 Hz) of physiologically-relevant frequencies. Segments in red indicate bursts of prominent activity in the low-frequency band (0- 4 Hz, corresponding to δ waves) and in the intermediate frequency band (4–8 Hz, corresponding to θ waves). Bottom, Ratio Rθδ = S(θ)/S(δ) of the spectral power in the θ and δ band in logarithmic scale obtained from the spectrogram at the top. Values Rθδ above log(Th) = 0 (Th = 1) indicate predominance of θ rhythm (red), whereas values below log(Th) = 0 correspond to predominance of δ rhythm (blue). e, Top, Spectrogram obtained from cortical EEG signal over 2 h of concatenated wake segments. Spectral power is calculated in non-overlapping time windows w = 4 s, and is color coded as in explained in d. Segments in red, indicating bursts of prominent activity, are mostly concentrated in the θ band (4–8 Hz). Bottom, Ratio Rθδ of the spectral power in θ and δ band in logarithmic scale obtained from the spectrogram at the top. Values Rθδ above log(Th) = 0 (Th = 1) indicate predominance of θ rhythm (red), whereas values below log(Th) = 0 correspond to predominance of δ rhythm (blue). The ratio Rθδ is almost always larger than 1, indicating that wake periods are dominated by bursting activity in the θ band. f, Smoothed ratio Rθδ of the spectral power in the θ and δ band during 30 min segment of 12 h dark (lights-off) period for a control rat (top) and a VLPO-lesioned rat (bottom). Rθδ is calculated on non-overlapping windows w = 4 s and the smoothing is performed using a 5 point moving average. θ- and δ-bursts are defined as sequences of consecutive windows where either the power in θ or δ band is dominant.