Radiation of pain: psychophysical evidence for a population coding mechanism in humans.

The spread of pain across body locations remains poorly understood but may provide important insights into the encoding of sensory features of noxious stimuli by populations of neurons. In this psychophysical experiment, we hypothesized that more intense noxious stimuli would lead to spread of pain, but more intense light stimuli would not produce perceptual radiation. Fifty healthy volunteers (27 females, 23 males, ages 14-44 years) participated in this study wherein noxious stimuli (43, 45, 47, and 49°C) were applied to glabrous (hand) and hairy skin (forearm) skin with 5-second and 10-second durations. Also, visual stimuli displayed on the target bodily area were used as a control. Participants provided pain (and light) spatial extent ratings as well as pain (and light) intensity ratings. In the extent rating procedure, participants adjusted the extent of the square displayed on the screen with the extent of pain (or light) that they experienced. Pain extent ratings showed statistically significant radiation of pain indicated by 12.42× greater spatial spread of pain (pain extent) than the area of the stimulation with 49°C (P < 0.001), in contrast to visual ratings, which closely approximated the size of the stimulus (1.22×). Pain radiation was more pronounced in hairy than glabrous skin (P < 0.05) and was more pronounced with longer stimulus duration (P < 0.001). Pain intensity explained only 14% of the pain radiation variability. The relative independence of the pain radiation from pain intensity indicates that distinct components of population coding mechanisms may be involved in the spatial representation of pain vs intensity coding.