Radiation Of Pain in Humans: Evidence for A Psychophysical Manifestation of a Neural Population Coding Mechanism?

How localized pain becomes widespread is unclear. Here, we hypothesize that more intense noxious stimuli lead to greater areas of perceived pain, but more intense light does not produce radiation. Three noxious (45, 47 and 49°C) and one innocuous (43°C) intensity stimuli were applied to glabrous (hand) and hairy skin (forearm) skin with 5s and 10s stimuli. Control stimuli comprised four different intensities of visual stimuli displayed on the target bodily areas. Healthy participants (N=50) provided pain (and light) intensity ratings as well as pain (and light) extent ratings. In the extent rating procedure, participants matched the size of the square displayed on the screen with the extent of pain (or light) which they experienced. We found the following: i) pain ratings revealed extensive radiation of pain indicated by 13.4× greater area of pain perceived compared to the actual area of the stimulation with 49°C (p < 0.001), which was not the case in the visual ratings (1.2×), ii) pain radiation was more pronounced in hairy skin (p < 0.05) and with longer stimulus duration (p < 0.001), iii) accuracy of extent ratings were exceptionally high: visual percept was not different from the size of visual stimuli displayed, iv) pain intensity explained only 14% of radiation. It can be suggested that efficient lateral inhibition present in vision prevented perceptual expansion, which is not the case in distributed nociceptive system. These observations likely echo the complex patterns of neuronal recruitment at the spinal cord level previously shown in mammals. Funding: CCHMC Research Innovation and Pilot (RIP) award, NIH NS39426 (R.C.) grant.