Heat pain increases the perceived magnitude of an innocuous electrical stimulus: Evidence for a peripheral mechanism

Painful heat produced an increase in the perceived magnitude of an innocuous electrical stimulus applied either to the sural nerve or to the skin of the dorsum of the foot. The increased sensitivity was observed when the painful heat was spatially coincident with the electrical stimulus, and when it was not coincident but adjacent within the same dermatome. Painful heat had no effect when it was applied to the contralateral foot, which makes it unlikely that attention or arousal played any role in the increased electrical sensitivity produced by ipsilateral heat. The painful heat also produced an increase in the amplitude of the sural nerve compound action potential (CAP). The heat-pain-related changes in the CAP and subjective magnitude ratings were in the same direction, which suggests that the latter were due at least in part to a temperature-dependent change in the electrical sensitivity of the peripheral afferents.