Differential Effects of Thermal Stimuli in Eliciting Temporal Contrast Enhancement: A Psychophysical Study.

Offset analgesia (OA) is observed when pain relief is disproportional to the reduction of noxious input and is based on temporal contrast enhancement (TCE). This phenomenon is believed to reflect the function of the inhibitory pain modulatory system. However, the mechanisms contributing to this phenomenon remain poorly understood, with previous research focusing primarily on painful stimuli and not generalizing to non-painful stimuli. Therefore, the aim of this study was to investigate whether TCE can be induced by noxious as well as innocuous heat and cold stimuli. Asymptomatic subjects (n=50) were recruited to participate in two consecutive experiments. In a first pilot study (n=17), the parameters of noxious and innocuous heat and cold stimuli were investigated in order to implement them in the main study. In the second (main) experiment, subjects (n=33) participated in TCE paradigms consisting of four different modalities, including noxious heat, innocuous heat, noxious cold, and innocuous cold. The intensity of the sensations of each thermal modality was assessed using an electronic visual analog scale. Temporal contrast enhancement was confirmed for noxious heat (p < 0.001), noxious cold (p = 0.034) and innocuous cold (p = 0.002). Conversely, TCE could not be shown for innocuous heat (p = 1.00). No significant correlation between TCE modalities was found (r < 0.3, p > 0.05). The results suggest that TCE can be induced by both painful and non-painful thermal stimulation, but not by innocuous warm temperature. The exact underlying mechanisms need to be clarified. However, among other potential mechanisms, this may be explained by a thermo-specific activation of C-fiber afferents by innocuous heat and of A-fiber afferents by innocuous cold, suggesting involvement of A-fibers rather than C-fibers in TCE. More research is needed to confirm a peripheral influence. PERSPECTIVE: This psychophysical study presents the observation of temporal contrast enhancement during noxious heat, noxious cold, and innocuous cold stimuli, but not during stimulation with innocuous warm temperature in healthy volunteers. A better understanding of endogenous pain modulation mechanisms might be helpful in explaining the underlying aspects of pain disorders.