Placebo effects: From pain to motor performance

Abstract

The placebo effect can be elicited by two main mechanisms: via classical conditioning and cognitive expectations. These two mechanisms have been traditionally investigated in one single domain. The aim of the present study is to investigate how a placebo effect obtained in one modality (pain tolerance) can be transferred to another modality (motor endurance) and which mechanisms (i.e. reinforced expectation/conditioning or expectations alone) are more responsible for this placebo transferability. Participants were tested in a pain tolerance task and in a motor task in two different experiments: after a conditioning procedure on pain tolerance in which they were made to believe in the effectiveness of an analgesic treatment procedure (experiment 1) or without a previous conditioning procedure (experiment 2). In both experiments, objective (pain tolerance and number of finger flexions) and subjective (pain and fatigue perception) measures were recorded. In experiment 1 subjects experienced an increase of pain tolerance and a reduction of pain perception as well as an increased number of flexions and a reduction of reported fatigue. Conversely, in experiment 2 subjects experienced only a reduction of pain and fatigue perception with no significant increase of objective measures. Our results point out that it is possible to transfer the positive effects observed on pain to a motor task but only if verbally induced expectations are reinforced by previous experiences. This result could represent an important step to apply placebo procedures in pathological conditions such as chronic fatigue or Parkinson's disease.

Keywords: Expectation; Fatigue; Pain tolerance; Placebo; Previous experiences.