A psychophysiological causal model of pain report validity

The validity of the pain report is vitally important but difficult to assess because pain is a personal experience. Human laboratory research affords an opportunity to investigate validity because one can measure the consistency and sensitivity of pain ratings produced in response to known stimuli. This article presents 2 levels of evidence characterizing the validity of the pain report measure. The within-subject agreement of pain report with known stimulus variation quantifies the criterion validity, or accuracy, of the measure. Causal modeling defines a second, between-subject, level of construct validity by suggesting a psychophysiological mechanism determining the observed individual variation in accuracy. We analyzed pain rating data obtained in a laboratory study where 100 subjects (56 men and 44 women) experienced varied levels of painful fingertip electrical stimulation, delivered in random order across 144 trials. Unknown to the subjects, there were only 3 stimulus intensities. Accuracy, defined operationally as the proportion of variance in pain report explained by stimulus level, ranged from 0.07 to 0.91 with a median of 0.64. Hypothesized determinants of accuracy comprised current intensity, event-related late near field evoked potentials, skin conductance response, heart rate, and pupil diameter change. We limited the evoked potential measures to the amplitude of the negative peak at 150 msec (N150amp) and combined the latter 3 measures to form a single index of overall sympathetic nervous system arousal (Arousal). Although men chose higher stimulus levels for the experiment and had higher Arousal than did women, their mean pain reports and their Accuracy did not differ from those of female subjects. We constructed a sequence of path analysis models designed to clarify the causal contributions of current intensity, N150amp, and Arousal, and to determine whether these relationships differ in men and women. The final model revealed a direct causal chain. Stimulus current determined the amplitude of N150amp (possibly an indicator of attention). N150amp in turn determined Arousal, and Arousal emerged as the sole determinant of the Accuracy of the pain report. In addition, this latter effect differed across the sexes. Men who experienced higher levels of Arousal gave more accurate pain reports than those who had lower levels, but women who had higher levels of Arousal gave less accurate pain reports than those with lower levels. Thus construct validation emerged, not from direct stimulus-response correlation, but from the elucidation of a causal chain that related stimulus to response.