What's up with the Rubber Hand Illusion?

Abstract

The rubber hand illusion (RHI) is a cornerstone of the scientific literature on embodiment. We have recently published a series of studies investigating the RHI, in particular its relationship to hypnotic (imaginative) suggestibility, and the validity of commonly used control conditions. These studies have generated substantial discussion regarding our claims, how they should be interpreted, and what all this means for past and future experimental studies of embodiment experiences [see, for example, (Makin, Scientific American, 2020)]. To clarify these issues, here we first summarise our main points (there is of course much more in the papers) and then offer responses to some frequently asked questions.

Hypothesis awareness confounds asynchronous control conditions in indirect measures of the rubber hand illusion.

“…Therefore, it is possible that RH cross-modal congruency effects may also be accounted for by interpreting demand characteristics as requiring imagining the fake hand is your own hand in the synchronous condition, but not in the asynchronous condition. Note that other methods (e.g., two-alternative forced choice designs; Chancel & Ehrsson, 2020) may also be confounded by demand characteristics (see (Seth et al, 2021)).…”

Reply to: No specific relationship between hypnotic suggestibility and the rubber hand illusion - Rubber hand effects remain confounded by hypothesis awareness and substantially related to trait phenomenological control

“…1 It transpired that the strong implicit demand characteristics inherent in the RHI could not be overcome by our instructions. 1,15 This, along with all other points raised regarding pre-registered analyses, is clearly stated in the target paper 1 . We note that Ehrsson et al's arguments are also not based on preregistered analyses and are therefore also exploratory and post-hoc.…”

“…Furthermore, even if these papers happened to be the only papers which employed the methods discussed, given how influential they are, the implications for understanding of rubber hand effects would still be great. We disagree with the claim that (by focusing on the synchronous condition for exploratory regression analyses) we ignored the control measures, which we in fact describe in detail in the target paper 1 and elsewhere 2,3,12,14,15 .…”

“…A simple explanation for the similar correlations between RHI reports and phenomenological control for both synchronous and asynchronous conditions is that both reflect suggestion effects. Lower mean scores for visual hallucination than, e.g., ownership may reflect the lower expectancies for visual hallucination experience [3][4][5] or the relatively high difficulty of generating visual hallucination experience" 1,3,12,15 Ehrsson et al repeatedly refer to our transparently declared deviation from preregistration for correlational analyses. We could not proceed with preregistered investigation of expectancy-related differences between instruction conditions because instructions did not change expectancies.…”

“…Other RHI measures may also be confounded, but this has not been tested. 1,3,12,15 Future experiments, with adequate control conditions, are needed to establish whether mechanisms beyond hypothesis awareness effects (e.g., multisensory integration) play a significant role in RHI measures. Beyond addressing potential confounds, such experiments may open substantial new opportunities for psychological research by shedding new light on the complex interactions between bottomup and top-down influences that shape all perceptual experiences, within and beyond the laboratory.…”