Figure 8.

Frequency dependence of stabilization. All data are from monkey M2. Histogram of complex eigenvalues is plotted in the plane spanned by the damping timescale and frequency. Histograms were normalized such that occupancy spans the range from 0 to 1. The histograms are encoded in the green–red–blue color scheme (green for the awake state before the induction of anesthesia; red for the anesthetized state that ensues after drug administration; blue for the awake state observed after the effect of the anesthetic subsides). The rightmost panels correspond to the overlay of the three plots. This overlay is constructed such that pure red, green, or blue indicate regions in the plane that are occupied preferentially during each one of the three conditions. Locations with similar occupancy in the awake and recovery states appear cyan (mixture of green and blue), whereas regions that are exclusive to the anesthetized state appear red. The absence of pure green or blue in the overlay plot shows that the distribution of eigenmodes is essentially restored after the effect of the anesthetic subsides. The leftward shift from cyan to red in the overlay panels demonstrates stabilization observed with both anesthetic regimens. Although in the case of propofol stabilization is observed across the entire frequency range, with the ketamine–medetomidine regimen, stabilization is most prominent at higher frequencies.