Categories of placebo response in the absence of site-specific expectation of analgesia.

Experimental placebo analgesia is induced by building an expectation of reduced pain in a specific body part, usually using an inert cream in the guise of a local anaesthetic in conjunction with conditioning. We investigated non-site-specific placebo analgesia by conditioning subjects to expect the anaesthetic cream on one arm, without specifying if they will definitely receive the cream, or to which arm it might be applied. Painful heat pulses (150 ms) from a CO2 laser were delivered randomly to both arms. A treatment group (n=24) underwent three experimental blocks (pre-cream, conditioning after cream, and post-conditioning). During the conditioning block, the intensity of the stimulus was reduced on one arm only. In the post-conditioning block it was returned to the painful level. We evaluated the change of intensity rating post-conditioning compared to the pre-cream block. In contrast to a control group (n=16), the treatment group reported a significant reduction in intensity ratings (F(1,38)=12.1; p=0.001). In the treatment group, we observed a range of placebo responses: unilateral responders (33.3%), subjects with a placebo response in the conditioned arm only; bilateral responders (33.3%), subjects reporting reduction in the intensity ratings in both arms, and non-responders, whose intensity ratings were not influenced by conditioning. We discuss these responses in terms of different levels of expected analgesia, facilitated by the absence of a site-specific focus for the treatment. We suggest this allowed the individuals suggestibility to influence their assessment of the pain experience by combining different levels of expectation with the information from the actual pain stimulus.