Fig. 1. Relation between subjective pain rating (x-axis ) and normalized stimulus intensity (y-axis ). The subjective pain ratings of 0, 3, and 7 correspond to trial types P1, P2, and P3, respectively. For each subject, the stimulus intensity for each trial type was normalized with respect to the overall grand mean (mean over all trials). The ordinate value for any particular dot  indicates the amount of deviation from this mean. The inserted y-axis  on the right  is representative of the average stimulus intensity (in milliamperes) across all six subjects. The mean stimulus intensities across all six subjects were 54.8, 103, and 153.5 for trial types P1, P2, and P3, respectively. The dotted line  represents the linear regression over all subjects.