Discrimination of innocuous and noxious thermal stimuli applied to the face in human and monkey.

Four humans and one monkey performed two-choice discriminations between simultaneous 3-sec heat stimuli applied to the face. All subjects produced more accurate discriminations in the noxious thermal range (approximately 47 degrees C) than in the innocuous thermal range (approximately 39 degrees C). The difference threshold, defined as the smallest temperature difference detected on 75% of the trials, was smaller for every subject at 47 degrees C than at 39 degrees C. The monkey's discriminative performance was comparable to that of humans in the noxious range, but inferior to that of humans in the innocuous range. Subjects' superior discrimination at noxious temperatures cannot be easily accounted for by differences in primary afferent activity of warm fibers and heat-nociceptive fibers. However, differences in central processing or attentional modulation could contribute to superior discrimination in the noxious range. These findings indicate that heat-sensitive nociceptors are capable of transmitting precise information about noxious thermal input to the skin.