Fig. 3. Line graphs showing mean spontaneous activity of ON cells (A ) and OFF cells (B ) at different isoflurane concentrations relative to each animal’s minimum alveolar concentration (MAC) value. Each thin gray line  represents a single neuron’s spontaneous activity summed over 7 min at each isoflurane concentration. The thick black line  represents the mean spontaneous activity at each isoflurane concentration. Isoflurane depressed spontaneous activity of ON cells and facilitated spontaneous activity of OFF cells. Error bars = SEM. * Significantly different from 0.65 and 0.85 MAC (P < 0.001). + Significantly different from 0.65 MAC when ON and OFF cells were pooled (P < 0.003).