Four experiments were performed. Two used the track-ball method in which subjects continuously adjust a visual display to assess the intensity of pain sensations evoked by 3-sec thermal stimuli of varying intensity. In the first, 27 subjects rated the intensity of 42 stimuli varying between 43-49 degrees. Both peak response and response duration were associated with stimulus intensity, but the highest association was with the area under the curve which integrates information from both intensity and duration. The second track-ball study presented trains of six 49 degrees C stimuli to assess the effects of the substance P blocker, CP 99,9941, and fentanyl on pain mediated by A delta and C-fiber nociceptors. Fentanyl significantly reduced track-ball ratings of both A delta-mediated and C-fiber-mediated pain sensations evoked at the ankle and arm. Fentanyl had no effect on pain thresholds, and neither CP 99,994 nor placebo altered any measure. The significant efficacy of fentanyl demonstrated the sensitivity of the method, and the negative effect of CP 99,9941 indicates that this substance P blocker had no effect with a method known to produce temporal summation of C-fiber-mediated pain sensation. In a third study, 20 subjects used a simple rating scale and a reaction time button to rate the intensity and latency of pain sensations produced by two thermal stimuli (45 & 47 degrees C) delivered in a preset protocol both to the same skin location and to varying skin locations. Reaction times to sensations evoked at the same location increased significantly, indicating that the sensation evoked by the first stimuli is mediated by A delta nociceptors and the later responses are to C-fiber mediated heat pain sensations. Repeated stimulation of the same location significantly reduced ratings of sensory intensity (P< 0.0001). This simple paradigm provides separate measures of A delta and C-fiber mediated pain sensation useful for clinical evaluation. The fourth study evaluated the influence of pain expectations in the rating and recall of acute pain produced either by venipuncture or dental surgery. Subjects predicted the affective dimension of venipuncture pain but not oral surgery pain. Prediction of the intensity of either pain condition was poor. Affective ratings of venipuncture pain, while predicted accurately, were poorly recalled in comparison to the other ratings. These results suggest that pain recall depends both on the type of pain and the pain dimension that is evaluated.
