Figure 5. Power spectral analysis of pectoralis shivering in the ninth volunteer. Data are presented in three dimensions, with the x axis showing frequency in Hertz, they axis representing power (in relative units), and the z axis showing time in terms of 1-min acquisition series (S1, S2, and so on). The fraction of power between 5 and 14 Hz that was in the 5–7 Hz clonic range was 25% on the control day (A) and 48% during isoflurane (B). This is not a typical example but was chosen to illustrate the saw-tooth response and the shift to clonic frequencies during isoflurane administration.