IONTOPHORETICALLY APPLIED POTASSIUM-IONS AS AN EXPERIMENTAL PAIN STIMULUS FOR INVESTIGATING PAIN MECHANISMS

The present study investigated the psychophysical characteristics of p otassium iontophoresis and its suitability as an experimental pain sti mulus. Experiment 1 investigated the optimal duration of the pain stim ulus for reliable reporting across repeated trials and whether the rel ationship between stimulus and subject response was linear, logarithmi c, or a power function. In Experiment 2, the optimal interstimulus int erval (ISI) was determined for reliable pain reporting, and stimulus h istory effects, both in terms of session effects and the effects of im mediately preceding stimuli, were evaluated. In Experiment 3, potassiu m iontophoresis was compared with a sodium iontophoresis control. Line ar functions described the stimulus-pain relationship best. No signifi cant differences in the goodness-of-fit coefficients of determination, correlations, or coefficients of variation were found for the stimulu s durations of 1, 2, and 4 sec. Significant stimulus history effects w ere found across a session, with adaptation and enhancement of respond ing for low- and moderate-intensity stimuli, respectively The effects of the immediately preceding stimuli were suppression or enhancement o f pain response, depending on the ISI, the preceding stimulus intensit y, and the present stimulus intensity. Potassium iontophoresis was a s ignificantly more effective pain stimulus than was sodium iontophoresi s. It was concluded that potassium iontophoresis is a convenient and r eliable experimental pain stimulus, which can be presented rapidly and repeatedly with minimal loss in consistency of subject pain report. P otassium iontophoresis provides a tool for investigating the neural mo dulation of pain in the relative absence of inflammation processes and tissue damage.