The Influence of Expectations, Attention and Test Paradigm on the Efficacy of the Pain Processing System

Despite the widespread use of CPM by researchers, there is still no gold standard for conducting this test. Both a sequential and a parallel application of the test stimulus and the conditioning stimulus are often used interchangeably and side by side. In both paradigms, the pain intensity or pain threshold of the test stimulus is first determined. Subsequently, in the parallel protocol, the test stimulus is reapplied while simultaneously applying the conditioning stimulus. In the sequential protocol, the conditioning stimulus is applied first, and after removing the latter stimulus, the test stimulus is reapplied (the CPM effect is visible up to 5 minutes after removal of the conditioning stimulus). The difference between the initial and repeated test stimulus (during or after conditioning) is quantified to express the CPM effect.

Even the publication of an expert opinion, which decided in favor of the sequential paradigm, has so far changed little in the use of the two paradigms. In that same publication, international CPM experts considered the sequential order 'purer' than the parallel order, as the results of the latter can be attributed to a shift of attention to the conditioning stimulus. However, this claim has little support from the literature. Some researchers even argue that cognitive mechanisms such as attentional shift might be part of the underlying mechanism of the CPM effect. Existing research also shows that CPM effectiveness is influenced by expectations. For this, previous studies conditioned test subjects by telling them in advance that the conditioning stimulus would reduce or increase pain. However, when the experimental CPM paradigm is used to investigate the effectiveness of the pain-inhibiting system, no such suggestions are made. However, this does not preclude that when the test protocol is explained to the subject, he or she does not create intrinsic expectations regarding the pain that is provoked. However, few studies have investigated this and the results of these studies are inconsistent and therefore do not allow conclusions to be drawn.

The aim of this study is to compare a sequential protocol with a parallel protocol on the one hand, and to measure the influence of expectations, attention and distraction on the CPM effect on the other.

In this study, the test stimulus of the CPM measurement will consist of a mechanical stimulus applied with an analog algometer (Force Dial model FDK 10 and 40 Push Pull Force Gage, Wagner instruments, Greenwich, Connecticut) on the Brachioradialis and Quadriceps muscle of the dominant side. The pressure pain threshold will be determined and used as an outcome measure. The conditioning stimulus consists of immersing the hand (up to 10 cm above the wrist) of the non-dominant side in warm water at a temperature of 45,5°C for 6 minutes. This temperature has been proven to be a reliable stimulus, eliciting solid effects.

At the time of testing, the CPM measurement will therefore be performed four times, each time with a small variation. The order of the variations is determined at random. There is a break of 10 minutes between each performance, so that the effect of the previous performance has worn off. The four variations of the CPM paradigm are the following: