Figure 2. (Top) The auditory steady state response amplitude (log scale) as a function of the measured end-tidal concentration of isoflurane. Empty circles indicate measures obtained when the volunteers were conscious; filled circles indicate measures obtained when the subjects were unconscious. The triangles show residual electroencephalographic noise estimated from recordings obtained during baseline with the earphone disconnected. These triangles delimit the range of expected amplitudes if the auditory steady state response is abolished. Because many points overlap at the 0.38% isoflurane target concentration, an expanded view of these results is shown in the lower portion of this figure. The 0.001 value at the bottom left is a y-axis (amplitude) label. It does not pertain to the isoflurane concentration. (Bottom) An expanded view of the 0.38% isoflurane target concentration data.