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

Abstract

Reports of experiences of ownership of a fake hand following synchronous tactile and visual stimulation of fake and real hands have been attributed to multisensory integration mechanisms (the rubber hand ‘illusion’). However, it has been shown that the subjective reports expected, namely stronger experiences in the synchronous than asynchronous condition, and for ‘illusion’ rather than ‘control’ statements, are clear to subjects; thus, the reports may reflect response to demand characteristics. Subjective report is often accompanied by so-called ‘objective’ or ‘implicit’ measures. Like subjective report, an asynchronous control condition is typically employed for these measures. Here we report tests of expectancies for synchronous ‘illusion’ and asynchronous ‘control’ conditions for two indirect measures of rubber hand effects across two pre-registered studies (n = 140 and n = 45): proprioceptive drift (a change in perceived hand location) and skin conductance response (SCR; a measure of physiological arousal). Participants expectancies for the synchronous condition measures were greater than for asynchronous ‘control’ condition in both studies. Previous work has shown that subjects can voluntarily produce alterations in both subjective experience of body location and in SCR. Thus, indirect measures of rubber hand effects may reflect compliance, bias and phenomenological control in response to demand characteristics, just as for subjective measures, and the asynchronous control condition is in general invalid for controlling demand characteristics in rubber hand procedures. Valid control measures are required to support claims of a role of multi-sensory integration for both direct and indirect measures of rubber hand effects.

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

“…However, expectancies are not matched across these conditions. As we reported in the target article 1 and has since been shown elsewhere [3][4][5] , people expect to respond more strongly to the synchronous rather than asynchronous condition. Comparisons between these conditions are therefore confounded by hypothesis awareness (see Corneille & Lush 7 ).…”

“…hypnotisability) or by response bias (if they have low capacity for phenomenological control). This applies equally to 'implicit' measures of the RHI (e.g., skin conductance response and proprioceptive drift), as we have shown by measuring expectancies for these measures; as with subjective report, people expect the patterns of results that are typically obtained in RHI experiments 4 . Demand characteristics may also account for the results of the more fine-grained approaches described by Ehrsson et al, in which participants report which of two fake hands "felt the most like their own" 8 , or report "yes" when asked if the rubber hand felt like their own hand for synchronous but not asynchronous stroking 9 .…”

“…While they agree that this is the most common approach in the influential papers we reviewed, they assert (without evidence) that difference measures are more widely used for interpretation in less influential papers. Even if this were to be so, it would not undermine our claims because, as we have explained, difference measures are confounded by expectancies [3][4][5] . 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.…”

“…The second is whether or not the relationships are sufficiently substantial to pose a threat to existing accounts of the RHI. It is important to note that, throughout, Ehrsson et al's commentary fails to appreciate the implications of a critical issue: the asynchronous condition is not a valid control for demand characteristics 1,[3][4][5] The first disagreement regards our reported null relationship between hypnotisability and an RHI difference measure of subjective report (the mean agreement score for three statements describing either referred touch or 'ownership' experience). Ehrsson et al argue that this result contradicts our claims.…”

“…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.…”