Demand Characteristics Confound the Rubber Hand Illusion

Peter Lush,Simine Vazire,Alex Holcombe

doi:10.1525/collabra.325

Abstract

Reports of experiences of ownership over a fake hand following simple multisensory stimulation (the ‘rubber hand illusion’) have generated an expansive literature. Because such reports might reflect suggestion effects, demand characteristics are routinely controlled for by contrasting agreement ratings for ‘illusion’ and ‘control’ conditions. However, these methods have never been validated, and recent evidence that response to imaginative suggestion (‘phenomenological control’) predicts illusion report prompts reconsideration of their efficacy. A crucial assumption of the standard approach is that demand characteristics are matched across conditions. Here, a quasi-experiment design was employed to test demand characteristics in rubber hand illusion reports. Participants were provided with information about the rubber hand illusion procedure (text description and video demonstration) and recorded expectancies for standard ‘illusion’ and ‘control’ statements. Expectancies for ‘control’ and ‘illusion’ statements in synchronous and asynchronous conditions were found to differ similarly to published illusion reports. Therefore, rubber hand illusion control methods which have been in use for 22 years are not fit for purpose. Because demand characteristics have not been controlled in illusion report in existing studies, the illusion may be, partially or entirely, a suggestion effect. Methods to develop robust controls are proposed. That confounding demand characteristics have been overlooked for decades may be attributable to a lack of awareness that demand characteristics can drive experience in psychological science.

Highlights

In the rubber hand illusion (RHI; Botvinick & Cohen, 1998), synchronous brush strokes on a participant’s concealed hand and a visible fake hand prompt reports of illusory sensations of touch and of ownership of the fake hand
Demand characteristics in the RHI may act as implicit imaginative suggestions, generating expectancies which are met by the voluntary top-down control of phenomenology, just as in direct imaginative suggestion within the context of ‘hypnosis’
Illusory experience in the RHI is likely to reflect the top-down control of phenomenology to meet expectancies (‘phenomenological control’) rather than, or in addition to multi-sensory integration or top-down processes which are not driven by demand characteristics (Lush et al, 2019; Dienes et al, 2019)

Summary

Introduction

In the rubber hand illusion (RHI; Botvinick & Cohen, 1998), synchronous brush strokes on a participant’s concealed hand and a visible fake hand prompt reports of illusory sensations of touch and of ownership of the fake hand. The RHI is thought to reflect the role of multimodal integration in embodiment and to demonstrate that a fundamental aspect of conscious selfhood can be disrupted by a surprisingly simple intervention (for reviews see Braun et al, 2018; Riemer, Trojan, Beauchamp & Fuchs 2019) The validity of such claims rests upon the efficacy of methods to control for demand characteristics (“the totality of cues which convey an experimental hypothesis to the subject”, Orne, 1962). In addition to subjective report, indirect measures are often claimed to reflect changes in embodiment mechanisms, e.g., perceived hand location (Botvinick & Cohen, 1998), skin conductance response (SCR; Armel & Ramachandran, 2003) and brain imaging (Ehrsson, Holmes & Passingham, 2005) Because it is subjective report which links indirect measures to changes in experience, these claims depend on the validity of controls for demand characteristics in subjective reports. These three ‘illusion’ statements and six ‘control’ statements (or modifications and subsets of them) have since appeared in the majority of RHI research (see Riemer et al, 2019 for a thorough review of RHI methodology), though it is worth noting that a small minority of researchers consider the ‘control’ statements not as controls for suggestion but as part of the illusion (e.g. Haans et al 2012)

Methods

Results

Conclusion