Withdrawal reflex, skin resistance reaction and pain ratings due to electrical stimuli in man.

Simultaneous measurements of pain rating, withdrawal reflex, and skin resistance reaction with non-painful and painful electrical stimuli were performed on 15 healthy male volunteers. Eight different intensities were delivered in standardized randomized order. Each intensity appeared 10 times. There was no influence on the preceding stimulus on responses to the following stimulus. Neither skin resistance reaction nor withdrawal reflex were specific for pain in the sense that they appeared solely when pain was reported; the two reaction threshold currents were significantly smaller than the pain threshold. The reaction amplitudes, however, showed a close correlation to the intensity of sensation. The relations between all reactions measured and stimulus strength could be described best by power functions, with an exponent less than or equal to 1 if least square fits in linear scales were performed. Graphical evaluation in double logarithmic scales led to systematic errors causing higher exponents. Compound relations, like skin resistance reaction or withdrawal reflex amplitude as function of subjective estimation, could also be approximated by power functions, with parameters predictable from stimulus-reaction functions. No change in exponent was observed when subjective estimation turned from pre-pain to pain.