Mechanisms underlying modulation of repeated thermal stimulation

The perception of pain is a dynamic process involving both psychological and physiological variables that is continually modulated by facilitatory and inhibitory mechanisms. Painful stimuli with a prolonged temporal profile provide an opportunity to evaluate properties of pain modulation in contrast to shorter stimuli that provide a limited picture. The current study used a series of prolonged thermal stimuli to evaluate the presence of modulatory mechanisms as a function of time delay between trials (interstimulus interval, ISI: 3, 10 min). During each trial, the experimental stimulus was administered to the subject's palm for 30 seconds. Pain intensity was continuously recorded with an electronic version of the visual analog scale (VAS). Stimulation temperatures were individualized to produce a peak rating of 35-50 on a scale of 0 (no pain) to 100 (most intense pain) in order to standardize pain ratings. Repeated trials of prolonged stimulation demonstrated that subjects were able to continually rate the intensity of a painful stimulus. However, ratings displayed a period of adaptation within a single trial and between trials. Repeated measures ANOVA revealed a significant main effect of trial in which subjects consistently rated the painful stimulus lower during the second trial. Pain ratings during the third trial returned toward ratings of the first trial. A small but non-significant difference was observed between different ISI. Based on these observations, stimulus parameters were modified to determine potential mechanisms. These parameters will be discussed in detail. This suggests that pain intensity is continuously modulated by internal mechanisms and influenced by temporal characteristics of the noxious stimulus. Supported by the UF College of Dentistry Seed Grant and the Comprehensive Center for Pain Research.