Fig. 8. Midlatency auditory evoked potential waveforms (continuous line ) from two unmedicated subjects. Stimuli consist of 50-Hz tonebursts. Each trace is the average response to 3,000 stimuli. The thick dashed line  is a segment of a 40-Hz sinusoid superimposed on the N^a, P^a, and N^bwaves to illustrate that this portion of the midlatency auditory evoked potential contains substantial 40-Hz activity. (Redrawn with permission from Plourde G: Auditory evoked potentials and 40-Hz oscillations: An opportunity to study mechanisms of action of general anesthetics? Anesthesiology 1999; 91:1187–9.)