Multisensory Integration Dominates Hypnotisability and Expectations in the Rubber Hand Illusion

Abstract

Some recent papers by P. Lush and colleagues have argued that the rubber hand illusion (RHI), where participants can feel a rubber hand as their own under appropriate multisensory stimulation, may be caused mainly by hypnotic suggestibility and expectations (demand characteristics). These papers rely primarily on a study with 353 participants who took part in a RHI experiment carried out in a classical way with brush stroking. Participants experienced a synchronous condition where the rubber hand was seen to be touched in synchrony with touch felt on their corresponding hidden real hand, or the touches were applied asynchronously as a control. Each participant had a related measure of their hypnotisability on a scale known as the Sussex-Waterloo Scale of Hypnotisability (SWASH). The authors found a correlation between the questionnaire ratings of the RHI in the synchronous condition and the SWASH score. From this, they concluded that the RHI is largely driven by suggestibility and further proposed that suggestibility and expectations may even entirely explain the RHI. Here we examine their claims in a series of extensive new analyses of their data. We find that at every level of SWASH, the synchronous stimulation results in greater levels of the illusion than the asynchronous condition; moreover, proprioceptive drift is greater in the synchronous case at every level of SWASH. Thus, while the level of hypnotisability does modestly influence the subjective reports (higher SWASH is associated with somewhat higher illusion ratings), the major difference between the synchronous and asynchronous stimulation is always present. Furthermore, by including in the model the participants’ expectancy ratings of how strongly they initially believed they would experience the RHI in the two conditions, we show that expectations had a very small effect on the illusion ratings; model comparisons further demonstrate that the multisensory condition is two-to-three-times as dominant as the other factors, with hypnotisability contributing modestly and expectations negligibly. Thus, although the results indicate that trait suggestibility may modulate the RHI, presumably through intersubject variations in top-down factors, the findings also suggest that the primary explanation for the RHI is as a multisensory bodily illusion.

Uncertainty-based inference of a common cause for body ownership

“…Thus, more broadly, our results advance our understanding of the multisensory processes that support the perception of one’s own body, as they serve as the first conclusive empirical demonstration of BCI in a bodily illusion. Such successful modeling of the multisensory information processing in body ownership is relevant for future computational work into bodily illusions and bodily self-awareness, for example, more extended frameworks that also include contributions of interoception ( Azzalini et al, 2019 ; Park and Blanke, 2019 ), motor processes ( Burin et al, 2015 ; Burin et al, 2017 ), pre-existing stored representations about what kind of objects that may or may not be part of one’s body ( Tsakiris et al, 2010 ), expectations ( Chancel et al, 2021 ; Guterstam et al, 2019b Ferri et al, 2013 ), and high-level cognition ( Lush et al, 2020 ; Slater and Ehrsson, 2022 ). Future quantitative computational studies like the present one are needed to formally compare these different theories of body ownership and advance the corresponding theoretical framework.…”

“…This observation is also consistent with previous studies reporting the induction of the rubber hand illusion for visuotactile asynchronies of as long as 300 ms ( Shimada et al, 2009 ), which are perceptually noted. While it seems plausible that p same reflects the real-world prior probability of a common cause of the visual and somatosensory signals, it could also be influenced by experimental properties of the task, demand characteristics (participants forming beliefs based on cues present in a testing situation, Weber and Cook, 1972 ; Corneille and Lush, 2022 ; Slater and Ehrsson, 2022 ), and other cognitive biases.…”

Quantifying body ownership information processing and perceptual bias in the rubber hand illusion

“…Collectively, these findings are conceptually important because they show that illusory changes in body ownership in the RHI are related to visuo-tactile information processing and spatial-perceptual bias towards the fake hand. They provide a validation of a critical assumption in multisensory theories of body ownership 35,37,38,41,77,78 , bodily selfconsciousness 38,[79][80][81][82] , prosthetic embodiment 21 , embodiment in virtual reality 22 , and teleoperated humanoid robots 83 , self-recognition 84 , as well as certain theories of embodied cognition 85 and the sense of self [86][87][88] , by providing bias-free evidence (the sensitivity measure) that links subjective changes in body ownership to sensory signal processing; the sensitivity findings also rule out the possibility that body ownership and the RHI might be entirely explained by cognitive processes, as has been suggested in some recent theories [89][90][91][92][93][94][95] . In addition, our study provides a proof-of-concept that SDT and psychophysics can be used to investigate bodily illusions, and adds to the relatively few studies that have tried to analyse multisensory and visual perceptual illusions using this approach.…”

No reduction in corticospinal excitability during the rubber hand illusion

“…Furthermore, it is noteworthy that less than half of our participants showed a reduced MEP amplitude in the rightSync condition compared to rightAsync. This occurred despite the behavioural results showing clear and significant differences in the RHI measures between the key synchronous and asynchronous conditions at the group level, with all participants affirming that they experienced the illusion (although subjective report from a single subject on a questionnaire cannot be taken as conclusive evidence that the person actually perceived the illusion, since questionnaire ratings may not be well protected against compliance, cognitive bias, suggestibility, or differences in decision criteria (Chancel & Ehrsson, 2020;Chancel, Ehrsson, & Ma, 2021;Lush, 2020;Lush et al, 2020;Reader, 2022;Slater & Ehrsson, 2022). This indicates that a reduction in corticospinal excitability may not be a reliable outcome of the RHI.…”