This study examines the ability of monkeys to detect small temperature changes superimposed upon noxious levels of thermal stimulation. Past research suggests that the pain sensing system is a relatively crude, impercise system whose primary function is to signal the presence of potentially tissue damaging stimuli. However, the results of the present study show that monkeys can detect extremely small temperature changes in the pain sensitivity range applied on either the lip or the volar surface of the forearm. These data also suggest that the detection latencies to small incremental temperature shifts in the noxious range provide a measure of the perceived intensity of the sensation. Comparison of the stimulus-response functions obtained from the face with those obtained from the forearm indicate that both areas are equally sensitive to small temperature changes. Furthermore, the sensitivity of the monkey to small temperature changes is not immutable, but rather can be systematically altered by attentional factors.
