A new thermal stimulation method for human psychophysical studies: pain intensity clamping.

A method for testing changes in pain sensitivity of human subjects over the course of prolonged thermal stimulation is introduced. It uses a Peltier-device-based thermode to generate a thermal contact stimulus, an electronic visual analog scale to continuously record the pain intensity and a system that controls selected stimulus parameters (temperature or pulse timing) as a function of the pain intensity rating. The stimulus parameter that is modulated to clamp pain intensity near a desired setpoint serves as the response variable and is used to infer pain sensitivity. Advantages of the method are that it automatically finds the stimulus magnitude that elicits predetermined pain intensity, regardless of how sensitive or insensitive the subject is, and it allows prolonged stimulation, because it does not allow pain intensity to escalate to unacceptable levels due to progressive sensitization. The subject is blinded regarding experimental effects because average pain intensity remains constant regardless of sensitization or pharmacological interventions.