Investigating endogenous pain modulation through pupillometry: can pupil size reflect offset analgesia?

Painful stimuli trigger autonomic nervous system responses, including changes in pupil size, which provide a physiological measure of pain processing and modulation. The endogenous pain modulatory system plays a key role in regulating pain, and its dysfunction contributes to a variety of chronic pain conditions. Offset analgesia (OA) offers a behavioral measure of this system but relies on subjective ratings.

This study aimed to determine whether pupil size can serve as a physiological proxy of OA in healthy individuals, complementing traditional pain ratings.

Twenty-nine healthy participants completed an OA paradigm applied on the left volar forearm with 3 thermal phases: T1, 47°C for 5 seconds; T2, 48°C for 5 seconds; and T3, 47°C for 20 seconds. Subjective pain intensity was rated verbally (0-10), and pupil size was recorded continuously via infrared pupillometry. Analgesic effects for pain ratings and pupil sizes were quantified by comparing T2 maxima with T3 minima and analyzed using linear mixed-effects models.

Participants showed a strong OA effect, with pain ratings decreasing by 52% (P < 0.001, Cohen d = 2.09). Pupil sizes decreased by 20% (P < 0.001, Cohen d = 1.52), exceeding natural resting-state variability. Pain sensitivity measures were not associated with the amount of pupil size changes during OA.

Pupil size decreased significantly during OA, consistent with the expected analgesic effect. Whether this truly reflects the endogenous pain modulatory system remains to be confirmed in future studies that include a constant-temperature control condition to establish specificity of the observed pupil response.