The Effect of Conditioned Pain Modulation on Tonic Heat Pain Assessed Using Participant-Controlled Temperature

Descending pain modulation can be experimentally assessed by way of testing conditioned pain modulation. The application of tonic heat as a test stimulus in such paradigms offers the possibility of observing dynamic pain responses, such as adaptation and temporal summation of pain. Here we investigated conditioned pain modulation effects on tonic heat employing participant-controlled temperature, an alternative tonic heat pain assessment. Changes in pain perception are thereby represented by temperature adjustments performed by the participant, uncoupling this approach from direct pain ratings. Participant-controlled temperature has emerged as a reliable and sex-independent measure of tonic heat.

Thirty healthy subjects underwent a sequential conditioned pain modulation paradigm, in which a cold water bath was applied as the conditioning stimulus and tonic heat as a test stimulus. Subjects were instructed to change the temperature of the thermode in response to variations in perception to tonic heat in order to maintain their initial rating over a two-minute period. Two additional test stimuli (i.e., lower limb noxious withdrawal reflex and pressure pain threshold) were included as positive controls for conditioned pain modulation effects.

Participant-controlled temperature revealed conditioned pain modulation effects on temporal summation of pain (P = 0.01). Increased noxious withdrawal reflex thresholds (P = 0.004) and pressure pain thresholds (P < 0.001) in response to conditioning also confirmed inhibitory conditioned pain modulation effects.

The measured interaction between conditioned pain modulation and temporal summation of pain supports the participant-controlled temperature approach as a promising method to explore dynamic inhibitory and facilitatory pain processes previously undetected by rating-based approaches.