The activation of CT fibers or affective touch does not affect pain sensitization by secondary hyperalgesia

Interpersonal affective touch, that preferentially engages C-tactile (CT) afferents, has been shown to produce analgesic effects, yet its role in pain sensitization remains poorly understood. This study explores whether touch delivered at CT-optimal parameters--either artificial or interpersonal--modulates secondary hyperalgesia (SH) induced by high-frequency stimulation (HFS). Two experimental studies were conducted. In Study 1, 46 participants underwent SH induction during two conditions: robot stroking at CT-optimal velocities and vibrotactile stimulation. In Study 2, 64 participants (32 couples) experienced SH in two separate sessions: alone or accompanied by their partner delivering affective touch. Pain reports, electroencephalographic activity (N-P complex and time-frequency activity), electrocardiographic (ECG) and electrodermal activity data (EDA) were collected. HFS successfully established SH in both studies; however, no significant differences were found in the SH area between CT-stimulation and control conditions. In Study 2, partner-delivered affective touch significantly reduced reported acute pain during HFS compared to the alone condition, whereas no such effect was observed in Study 1 (robotic vs. vibrotactile). At the neural level, no condition effects were observed in the N-P complex, EEG time-frequency data and ECG indexes in either study. Tonic EDA after HFS was higher when participants received stroking from their romantic partner compared to being alone. No sex differences were observed. Overall, while affective touch from a romantic partner may reduce acute pain during nociceptive stimulation -an effect not observed with robotic CT-stimulation- neither robotic nor interpersonal affective touch appeared to modulate the development of secondary hyperalgesia.