Multisensory impact of visual stimuli on detection thresholds of thermo-nociceptive inputs conveyed by Aδ and C fibers

Nociception is seen as an alarm system that allows detecting and reacting to potential physical threats. It is hypothesized that such alarming function is optimized through interactions with other sensory systems such as vision that allows identifying surrounding objects that might have an immediate impact on the body. Here, we test the hypothesis according to which visual stimuli could modulate the response threshold to heat stimuli. Using an adaptive psychophysical procedure allowing, among other things, to take into account differences in conduction distance and velocity within visual and thermo-nociceptive pathways, laser-induced thermal stimuli of different temperatures were applied on one hand dorsum, and detection thresholds of thermal sensations conveyed by C- and Aδ-fibers were measured, respectively. Measures were taken while visual stimuli were presented either near the stimulated hand, near the opposite hand or at a neutral position in front of the participant. Results showed that the detection threshold of Aδ fibers was decreased when the visual stimuli occurred near the stimulated hand as compared to when they occurred near the opposite hand. Such modulation of nociceptive thresholds by non-somatic stimuli, at least that of Aδ fibers, could reflect a defensive mechanism to facilitate detection and response to external threats. Conversely, due to their slow conduction velocity, sensory inputs conveyed through C fibers could be less sensitive to multisensory interactions because they would be less involved in immediate defensive reactions and more involved in monitoring the general state of the body.