BACKGROUND & OBJECTIVE
Placebo effects are a topic of great interest in the healthcare world generally, and with respect to pain specifically. In the context of physiotherapy practice, part of the interest is probably driven by the numerous studies indicating that various common interventions don’t appear to work via their long-held theoretical mechanisms. There is also appeal in the idea that placebos offer something for nothing, a sort of ‘free-kick’ added to the effect size of any treatment, regardless of whether that treatment is otherwise useful or not. This study looked at whether providing visual stimuli (lights) that were paired with painful stimuli (hot element on the skin) influenced peoples’ report of pain in response to the hot element. At the same time the researchers looked at whether performing resisted elbow flexion also influenced pain reporting.
46 healthy volunteers were recruited via advertising at a university campus. The experiment involved being strapped into a chair and receiving painful heat stimulation on the inside of the forearm. Participants underwent a training phase, where they were shown 3 different coloured lights immediately followed by receiving 3 different intensities of heat: low heat (paired with a green light), medium heat (yellow light) and high heat (red light). The idea was that they would learn to associate a certain stimulus intensity with a certain colour. They then went through a series of trials where they got the different coloured lights, but always the same medium heat stimulus. Throughout the trials they received the heat while they flexed their arm against a robotic arm with a set resistance, and also when at rest.
Immediately before each heat stimulus (but after they had seen the coloured light) participants were asked to rate their expected pain level, and then asked to rate their actual pain score after receiving the heat stimulus. The researchers compared pain scores when the lights and heat were matching, and when they were not matching; and with the resisted elbow flexion task, and without.
When continually receiving the medium heat stimulus, participants rated their pain as higher when they were given the red light, and lower when given the green light. This was interpreted as measuring placebo effects (green) - approximately 12 points on