Temporal Summation of Pain Comprises Dissociable Probabilistic and Magnitude Components Revealed by Laser Stimulation

Temporal summation of pain (TSP) is a candidate phenotype marker of central pain facilitation, but variable test-retest reliability limits its clinical utility. This variability may arise from peripheral confounds in contact-based stimulation and from conflating facilitation probability with its magnitude. We evaluated whether contactless laser stimulation, which preferentially engages nociceptive afferents, enables reliable measurement of TSP by disaggregating these behavioral components. Twenty-seven healthy adults completed two identical visits separated by 1 to 2 weeks. Noxious laser stimulation was delivered in 30 triplet trials to the left dorsal hand. TSP was quantified using the following three complementary measures: (1) induction rate, defined as the proportion of triplets exhibiting facilitation; (2) conditional magnitude, defined as the mean proportional increase in pain when facilitation occurred; and (3) unconditional magnitude, defined as the mean proportional change across all triplets, regardless of whether facilitation occurred. Laser stimulation reliably induced temporal summation at the group level. No significant between-visit differences were detected in these measures. Single-measure and average-measure intraclass correlation coefficients were 0.49 and 0.65 for induction rate, 0.77 and 0.87 for conditional magnitude, and 0.65 and 0.79 for unconditional magnitude, respectively. Across blocks, overall pain intensity decreased, consistent with habituation, while induction rate showed no significant block effect and conditional magnitude showed block-related increases. These findings dissociate pain facilitation from global habituation and frame temporal summation as a composite behavioral process; isolating facilitation probability from magnitude may improve interpretability and support future mechanistic pain phenotyping. PERSPECTIVE: Contactless laser stimulation enables reliable measurement of temporal summation of pain by reducing contact-related peripheral confounds and dissociating facilitation occurrence from magnitude. These findings indicate that, at the behavioral level, temporal summation reflects separable probabilistic and amplification components, supporting its use for mechanistic pain phenotyping.