Investigating placebo mechanisms: could a PDP system exist?

Abstract

Placebo mechanisms are a neglected area of research. Kirsch’s (1985) response expectancy theory described the stimulus-placebo response process as direct and unmediated. However, Hyland (2011) argued that the placebo effect is mediated by a parallel distributed processing (PDP) system, whereby all cognitive input is processed simultaneously, resulting in perseveration of the placebo effect. The present study examined this theory by utilising a TENS device as a placebo ‘effecting’ reaction times to a computer task, with an additional cognitive load task to further test the PDP system’s existence. Although a general placebo effect was found that increased in the cognitive load conditions, no evidence of a perseveration effect was obtained. Possible reasons for this and implications for future work are discussed