Thermonociceptive interaction: interchannel pain modulation occurs before intrachannel convergence of warmth

Non-noxious warmth reduces both perceived pain intensity, and the amplitude of EEG markers of pain. However, the spatial properties of thermo-nociceptive interaction, and the level of sensory processing at which it occurs remain unclear. Here, we investigated whether inter-channel warmth-pain interactions occur before or after intra-channel spatial summation of warmth. Warm stimuli were applied to the fingers of the right hand. Their number and location were manipulated in different conditions. A concomitant noxious test pulse was delivered to the middle finger using a CO2 laser. We replicated the classical suppressive effect of warmth on both pain perceived intensity and EEG markers. Importantly, inhibition of pain was not affected by the location and the number of thermal stimuli, even though they increased the perceived intensity of warmth. Our results therefore suggest that the inhibitory effect of warmth on pain is not somatotopically organized. They also rule out the possibility that warmth affects nociceptive processing after intra-channel warmth summation.