Effects of Anxiety and Morphine on Discrimination of Intensities of Painful Stimuli

The testing of analgesics, especially morphine, has yielded a great deal of conflicting evidence concerning the effects of drugs on pain perception. The recent reviews by Wikler (1) and Edwards (2), and the monograph by Wolff and Wolf (3), cover thoroughly the studies that have attempted to measure changes in the pain perception threshold. Although many investigators have reported rises in the pain perception threshold of some of their subjects following the administration of morphine, the majority have obtained variable results. Wolff, Hardy, and Goodell (4) found consistent rises following the administration of morphine under certain conditions. Using the same technique, Andrews (5), Isbell (6), Denton and Beecher (7), and Chapman and Jones (8) found, following morphine, that the pain perception threshold might be elevated, lowered or unchanged. Similar results were obtained by Isbell and Frank (9) in studies on the effect of analgesics on tooth pain perception thresholds. It would seem that the discrepancies were not due to faulty apparatus or lack of objectivity in handling the data. Rather, the reasons appear to lie in conditions that were not held constant or that were beyond the control of the experimenters. One such variable that has been mentioned in several studies as possibly contributing to unpredictability of results is the emotional status of the subject at the time the experiment is performed. Included would be such factors as response of the subject to the experimental room, to the complex apparatus, to the attitude of the experimenter, and to the expected painful stimuli. The uncontrolled variable in the subject's behavior would then seem to comprise that class of responses which is termed anticipatory, i.e., especially effectively toned responses that are anticipatory of pain. This, of course, is one type of fear or anxiety. Isbell and Frank (9) also studied the effect of morphine on the ability of subjects to estimate intensities of painful stimuli. In their investigation, the pulp nerve of a tooth containing a silver amalgam filling was stimulated electrically at intensities three times the absolute perception threshold value. These stimuli, which served as standards, were definitely painful. After morphine the subjects were required to manipulate rheostat knobs on the stimulating apparatus until the test stimuli were reported as being equal to the standard. (The apparatus was so adjusted that turns of the knob did not correspond with the intensity of stimuli or with dial scale readings.) No change in the accuracy of estimation of painful stimuli was observed following the administration of morphine. However, analysis of the conditions under which the studies were conducted indicates that considerable effort was expended to reduce anxiety in the subjects; they were given considerable control of the experimental situation since they manipulated the stimulator knob themselves and applied the electrode to their own tooth fillings. In addition they were trained in estimating intensities of painful stimuli for approximately one week prior to testing. In view of conditions and results discussed! in published studies on pain perception thresholds, it might be expected that if conditions had been favorable for enhancement of anxiety (anticipatory responses to expected unpleasant stimuli), Isbell and Frank would have obtained different results. The present investigation is an attempt to determine the extent to which anxiety and morphine alter pain intensity estimation. The experiment was designed to investigate the effect of controlled variations in the experimenter's treatment of the subject on estimation of pain intensities, while the number, the order, and the intensity of the stimuli were kept constant for all groups. If it could be