Discrimination of painful stimuli in human beings: influence of stimulation area.

Abstract

Briefly (100-400 ms) and in random order, painfully cold (-40.degree. C) and painfully hot (+60.degree. C) stimulators of various cross-sectional areas (1-100 mm2) and a needle were applied to the back of the human hand. The needle, punctate (1 mm2) painful heat, and punctate (1 mm2) painful cold all induced a pure pricking sensation with no thermal component. When asked which type of stimulator was applied naive subjects mistook the punctate hot stimulator (1 mm2) for the needle, but could readily identify the punctate cold stimulator (1 mm2) due to a more superficial and less intense pricking sensation. For greater areas the hot stimulators are the more distinguished from the needle as their area is increased. This discrimination is due to the thermal component of the pain sensation since for a given stimulus area, the frequency of correct recognition is directly proportional to the frequency of a thermal cue in the pain sensation. When the experiment of stimulator''s recognition is repeated, the subjects learn to distinguish the needle from the punctate hot stimulator (1 mm2); this is due to small differences in the intensity of the pricking sensation, and in the apparent area of the stimulation. The recognition of stimulators of greater area is also improved by repetition. In naive subjects pain induced by mechanical (needle) and punctate thermal stimuli has the same pricking quality. It may originate in nociceptors activated by thermal and mechanical nociceptive stimuli. The discriminative aspect of pain, whereby the nature of the stimulus is discerned, is apparently due to the simultaneous activation of nociceptors and mechanoor thermoreceptors.