This project studies the ability of rhesus monkeys and humans to discriminate small differences in innocuous warm pulses and noxious heat pulses applied to the face. It also evalutes the influence of attention on thermal discriminative ability. Both monkeys and humans are better able to discriminate small differences in noxious heat pulses (47 C) than innocuous warm pulses (39 C). Since primary afferent warm fibers provide as much differential temperature information as do thermal nociceptors, this difference in performance must be centrally mediated. Attentional state appears to influence the ability to detect small temperature changes on the face. A signal correctly indicating the location of a subsequent thermal change improves detection performance while an incorrect signal worsens performance. Consequently, attentional factors may influence the perception of and response to oral-facial pain.