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Placebo effects on verbal and nonverbal expression of pain

University of British Columbia

This investigation examined the impact of a potent source of social influence—the placebo—on verbal and nonverbal pain expression. Placebos exert a powerful influence on pain in both clinical and laboratory settings; nevertheless, the placebo remains a poorly understood phenomenon. In the present study the placebo was employed as a social influence designed to alter subjects' expectations or beliefs concerning their response to a noxious event. Furthermore, an attempt was made to classically condition a placebo response in accordance with the conditioning theory of placebo. The pain stimulus consisted of 500-millisecond electrical shocks. The placebo was an inert white cream described as an effective and quick-acting local anaesthetic. To gain a broad assessment of verbal and nonverbal pain expression, two self-report measures and an objective behavioural measure—facial expression—were used. Self-report comprised the "Sensory" and "Unpleasantness" ratio scales (derived from Gracely, McGrath, & Dubner, 1978). Facial expressiveness was coded using the Facial Action Coding System (FACS) developed by Ekman and Friesen (1978). Psychological interventions have been said to primarily alter patients' affective response to pain, thus it was expected that the greatest impact of the placebo would be reflected by the Unpleasantness Scale. The Sensory ratings and facial activity were expected to reflect less of a change. Sex differences were analyzed but were not expected because the pain stimulus intensity was individualized. Anxiety ratings taken before the trials with placebo were expected to be lower than anxiety ratings before the no-placebo trials; this was expected to be positively related to a placebo response (defined as lower pain expression during placebo trials than during no-placebo trials). In the baseline phase, subjects were tested with and without the placebo. They received the expectation manipulation that the analgesic cream would act as a local anaesthetic. Next, to condition a placebo response, in the positive conditioning group shocks administered with placebo cream were surreptitiously decreased; in the negative conditioning group shocks were increased; and in the baseline (control) group shocks remained at baseline levels. A final return-to-baseline phase tested the conditioned placebo response. A significant placebo response was found to be most clearly represented by the self-report measures and less by the facial activity. Control group subjects, who were exposed only to the verbal directive that they were receiving an analgesic cream, demonstrated a clear placebo response on both the Unpleasantness and Sensory self-report scales. Thus both self-report measures reflected a substantial placebo effect. Nonverbal expression reflected an effect only with half of the control group—the males—who exhibited significantly less facial activity during the placebo trials compared to the no-placebo trials. However, neither of the two groups exposed to manipulated shock levels demonstrated a significant conditioned placebo effect on any dependent measure. There was a modest relationship between facial activity and self-report. Self-reported anxiety was somewhat predictive of the degree to which subjects rated placebo trials as less painful than no-placebo trials, but the direction of this relationship was opposite to that hypothesized. In brief, subjects led to expect an analgesic cream, but not exposed to the conditioning manipulation, reliably exhibited a placebo response on both self-report scales; the males in this group also exhibited a placebo response as measured by facial activity. No support was found for the conditioning theory of placebo effects.

Text

2010-07-26

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http://hdl.handle.net/2429/26929

Psychology

University of British Columbia

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