It appears some demonstrated experimental effects depend more heavily on personality than previously thought: Serious Problems With Interpreting Rubber Hand “Illusion” Experiments

Serious Problems With Interpreting Rubber Hand “Illusion” Experiments. Warrick Roseboom; Peter Lush. Collabra: Psychology (2022) 8 (1): 32274. https://doi.org/10.1525/collabra.32274

The rubber hand “illusion” (RHI), in which participants report experiences of ownership over a fake hand, appears to demonstrate that subjective ownership over one’s body can be easily disrupted. It was recently shown that existing methods of controlling for suggestion effects in RHI responding are invalid. It was also shown that propensity to agree with RHI ownership statements is correlated with trait phenomenological control (response to imaginative suggestion). There is currently disagreement regarding the extent to which this relationship may cofound interpretation of RHI measures. Here we present the results of simulated experiments to demonstrate that a relationship between trait phenomenological control and RHI responding of the size reported would fundamentally change the way existing RHI results must be interpreted. Using real participant data, each simulated experiment used a sample biased in selection for trait phenomenological control. We find that using experiment samples comprised only of participants higher in trait phenomenological control almost guarantees that an experiment provides evidence consistent with RHI. By contrast, samples comprised of only participants lower in trait phenomenological control find evidence for RHI only around half the time – and of greater concern, evidence specifically for “ownership” experience just 4% of the time. These findings clearly contradict claims that the magnitude of relationship between phenomenological control and RHI responding is a minor concern, demonstrating that the presence of participants higher in trait phenomenological control in a given RHI experiment sample is critical for finding evidence consistent with RHI. Further study and theorising regarding RHI (and related effects) must take into account the role that trait phenomenological control plays in participant experience and responses during RHI experiments.

Keywords: embodiment, phenomenological control, hypnosis, imaginative suggestion, rubber hand illusion

Discussion

Quantitative study of subjective body ownership in the RHI is accomplished using participant ratings of (dis)agreement with a series of subjective statements about referred touch (S1 and S2) and (putatively) ownership (S3). Propensity to agree with these statements has previously been shown to be related to trait phenomenological control – the domain general ability to meet expectancies arising from direct or implicit imaginative suggestion (including demand characteristics; Dienes, Lush, et al., 2020; Lush et al., 2020). It has been claimed that this reported relationship is too small to be of concern (Ehrsson et al., 2021; Fan et al., 2021). To concretely demonstrate the potential influence of this relationship, here we simulated a series of experiments with different degrees of sampling bias, selecting a disproportionate number of participants with higher trait phenomenological control. Given the relationship between subjective agreement and phenomenological control, this induced sampling bias may be analogous to the deliberate exclusion of participants (e.g. Chancel & Ehrsson, 2020; Ehrsson et al., 2005) who don’t report a strong experience of the RHI (up to ~30% of people; Riemer et al., 2019). We show that the frequency with which a RHI experiment will provide evidence consistent with subjective agreement for a RHI is directly related to the proportion of participants in the experiment sample that are higher in phenomenological control. Put more concretely - only experiments run with a large proportion of participants higher in phenomenological control will provide evidence for the RHI. This finding directly contradicts the statement that the reported relationship between RHI responding and trait phenomenological control is too small to be of concern.

Why does selecting participants based on trait phenomenological control affect whether a RHI study will conclude in favour of evidence for RHI or not? It can only be because they are substantially related – reiterating the result reported by Lush et al. (2020). Why is it a problem if the same people who are likely to report agreement with RHI questions also happen to be higher in phenomenological control? To interpret the clearly substantial relationship, we must first consider the nature of the constructs these measures are supposed to represent – body ownership for RHI responding; and propensity to respond to suggestion, implicit or explicit, with imagination for phenomenological control. Phenomenological control is a domain general ability (Dienes, Lush, et al., 2020; Lush et al., 2020), relating to sensory and decision processes over and above those specifically related to experiences of body ownership or multisensory experiences of bodily sensation. This is evident both in the measure of phenomenological control used in Lush et al. (2020; SWASH), which interrogates a variety of possible experiences, not just those related to bodily experience or ownership (e.g. abilities related to amnesia or musical and visual hallucination, in response to suggestion) and in several recent findings: Relationships between trait phenomenological control and anomalous experiences when demand characteristics (and therefore participant hypothesis awareness) have not been adequately controlled have been reported (Lush et al., 2020) for other body-related effects including vicarious pain (reports of pain in response to seeing people in apparently pain-inducing situations) and mirror-touch synaesthesia (reports of felt touch in response to seeing people touched), and also for non-body related experiences like the visually-evoked auditory response (Lush, Dienes, Seth, et al., 2021; reports of sound experienced when watching silent videos). The ability to control experience in response to suggestion is evident across perceptual and decision domains and there is much evidence that imaginative suggestion effects can be experienced as subjectively ‘real’ (see Dienes, Palfi, et al., 2020; Lynn et al., 2020; McConkey, 2008), and that hypnotisability is distinct from social compliance (e.g. see Moore, 1964; Tasso et al., 2020; but see Polczyk & Pasek, 2006). In an explanatory framework, a domain general process subsumes domain specific demonstrations of its operation. Therefore, we are left with limited options for interpretation of the result from Lush et al. (2020):

1. participants who can control their experience across sensory and decision domains through phenomenological control are also controlling their experience in RHI experiments (as suggested by Lush et al., 2020)

2. participants who can control their experience in other domains through phenomenological control happen to be the same participants who can experience the RHI, but for distinct reasons (e.g. pure multisensory integration account)

3. the same participants who can control their experience in other domains don’t control their experience in RHI but respond as though they do for other unspecified reasons – perhaps a response bias induced by social context or pure confabulation just for RHI case

The first interpretation is perhaps the most parsimonious in that it requires only that we reconsider the RHI as an act of phenomenological control in response to suggestion and that it allows for group level report of RHI ownership experience to reflect genuine experience. It is important to note that this position does not rule out a multisensory contribution to the RHI, but rather that another cognitive process is in play. This is an important distinction. If we were to say that there is no problem treating an act of phenomenological control (which is necessarily creative and interpretative) as evidence for a sense of ownership, we might also study visual perception through phenomenological control in response to imaginative suggestion. If people report the experience of “seeing” a unicorn in response to a direct or indirect imaginative suggestion, we would be obliged to take this as evidence for the existence of unicorns on the same basis that RHI researchers claim that reports of the RHI are evidence for a sense of body ownership. Certainly, visual imagery is constrained by the properties of visual processing and previous perceptual experience, as is the RHI likely constrained by the properties of (multi)sensory processing and previous experience. The existence of such constraints is not in contention. But the interpretation of imaginatively suggested experience is not the same as studying the properties of multisensory perception, as is the common interpretation of RHI studies (e.g. Ehrsson et al., 2005). Saying that you are “studying the rubber hand instantiation of the influence of phenomenological control on perception and decision making” is perfectly reasonable. But this statement is not consistent with the broader claims made in most papers regarding RHI where it is stated that the RHI represents a fundamental case study in understanding how subjective body ownership is determined in humans.

The second interpretation, while less parsimonious than the first may still be possible and is not explicitly ruled out by the results presented here or previously. However, given that hypothesis awareness is not controlled for in the existing RHI literature (Lush, 2020; Lush, Seth, et al., 2021; Reader, 2021), evidence for this interpretation cannot depend on that existing literature until the possible influences of phenomenological control are ruled out. There are methods by which phenomenological control effects may be controlled; control conditions which are not confounded by demand characteristics could be developed by a two-step procedure which takes into account both expectancies and differences in the relative difficulty of suggestion effects (see Lush, 2020 for details). This requires a difficult process of matching expectancies and direct imaginative suggestion response across candidate illusion and control measures. Note that while this makes the study of embodiment illusions more challenging than has been the tradition, the combined evidence from many small sample studies in which demand characteristics were uncontrolled has only generated illusory evidence which is of no value to theories of embodiment (except perhaps as a cautionary tale). Similar issues have been dealt with successfully in other fields. For example, the suggested approach has much in common with the use of placebo controls in clinical trials, which, although it adds considerable cost to experiments, has been standard practice for decades (Beecher, 1955).

The third interpretation should be considered in light of evidence that phenomenological control may confound interpretation of other effects. The RHI is one of four distinct effects for which such relationships have now been shown (Lush, Dienes, Seth, et al., 2021; Lush et al., 2020) which involve a range of modalities. Correlation is not causation, of course, and relationships between the RHI and phenomenological control may be attributable to some unknown cause (perhaps some difference in multi-sensory integration mechanisms common to RHI response and phenomenological control - even for cases of phenomenological control that require no multisensory percept). However, given the simplicity of the proposal that demand characteristics can act as implicit imaginative suggestions in measures of experience, and the growing evidence that this occurs in a range of effects, it is a more parsimonious explanation that all of these effects are confounded by phenomenological control than that some other explanation is in play for each individual case.

Note that, whenever expectancies are consistent with experimental aims (e.g. participants are hypothesis aware), any given result may be attributable to hypothesis awareness effects other than phenomenological control (e.g., faking or imagination; see Corneille & Lush, 2021). However, for both the RHI and response to imaginative suggestion, there is extensive evidence that at least some reports reflect genuine experience (see Dienes, Palfi, et al., 2020 for a discussion of this evidence as it relates to phenomenological control). Of course, if phenomenological control scales were to reflect trait differences in faking (for example), this would be an important consideration for interpretation of the RHI (and other reported experiences which correlate with trait phenomenological control).

Researchers familiar with the RHI will be likely to note that these results (and those of Lush et al., 2020) only relate to the subjective statements, while there is extensive evidence from ‘implicit’ measures such as proprioceptive drift, and convergent evidence from neuroimaging and animal studies. As mentioned previously, there are only two aspects linking RHI to phenomenological change – extended report and subjective ratings. All other findings, such as proprioceptive drift or changes in measured blood flow (fMRI) or scalp potential (EEG), are linked to changes in subjective ownership only through broad correspondence or co-occurrence with these subjective reports. If human participants’ ratings on the subjective statements are predominantly related to their trait phenomenological control, then proprioceptive drift (or neuroimaging) results are correlating with phenomenological control, not specifically rubber hand subjective ownership. Again, this is because phenomenological control is a domain general ability (Dienes, Lush, et al., 2020; Lush et al., 2020), relating to sensory and decision processes over and above those specifically related to experiences of body ownership or multisensory experiences of bodily sensation. Note that the asynchronous control measure is also employed for “implicit” RHI measures (e.g., proprioceptive drift), and participants show hypothesis awareness for these effects (Lush, Seth, et al., 2021).

Regarding animal studies (e.g. Fang et al., 2019; Wada et al., 2016), while it may be completely reasonable to posit that, for instance, a rat may have a sense of body ownership, the reverse inference on which such claims rely when referencing RHI is problematic. Because it is not possible to simply ask an animal about their subjective experience, to conclude that an animal has a change in subjective experience of body ownership during, for example, a “rubber tail illusion” paradigm (Wada et al., 2016) we start with a potential relationship in human participants between a behavioural observation and subjective reports (e.g. correlate proprioceptive drift measure in hand with magnitude or time course of change in subjective ownership reports). We then attempt to obtain observations of a similar behaviour in rodents (e.g. something like proprioceptive drift of tail). We must then make the conceptual leap that the observed behavioural phenomena (proprioceptive drift in humans and something like it in rodents) are equivalent. Through this combination of directly observing one relationship related to subjective experience of ownership (subjective ownership responding and proprioceptive drift are related in humans), then observing this single phenomenon in rodents (behaviour with similar properties to proprioceptive drift), that we have assumed to be equivalent to a similar observation in humans (hand proprioceptive drift), we can end up with the reverse inference that the rodent also has change in subjective experience of ownership. Importantly, there is, and can be, no direct evidence to support this claim. It is important to also note that the same issue of reverse inference is present in any study using implicit measures that doesn’t provide evidence for concurrent changes in subjective experience (e.g. Tsakiris & Haggard, 2005). If the foundation statement about human subjective body ownership on which this set of inferences is based is instead reflecting some other ability (e.g. to control experience in response to suggestion – phenomenological control), then it is clear that all subsequent inferences are questionable. This is not to say that nothing can be discovered about multisensory perception of the body using neuroimaging or animal models. Our claim in no way invalidates behaviourist approaches that seek to map body perception to neural function through observed behaviour. However, any instances wherein these findings are related to subjective ownership through their connection with RHI-like paradigms are clearly problematic if the original basis for claims of changes in subjective body ownership during rubber hand exposure are not being correctly interpreted.

It is sometimes claimed (e.g. Ehrsson et al., 2021; Tsakiris, 2010) that RHI effects reflect a combination of top-down and bottom-up processes. However, these top-down effects are believed to be distinct from demand characteristic effects and RHI response is considered to be “resistant to suggestions, thoughts, and high level conceptual knowledge” (Ehrsson et al., 2021). Only a few researchers have stated theoretical positions consistent with the theory that demand characteristics play an important role in these effects (Alsmith, 2015; Dieguez, 2018). An influential theory proposes that top-down effects arise when internal representations of body image are matched against incoming sensory signals (Tsakiris, 2010; though see Chancel & Ehrsson, 2020 for a counter-argument that apparently top-down effects are attributable to multisensory congruency). However, given that demand characteristics have not been controlled in existing RHI studies, it is premature to consider theoretical proposals of top-down effects which are not related to demand characteristics.

In sum, agreement with three different subjective statements is the standard basis for linking all aspects of RHI phenomena with body ownership. Propensity to agree with these reports is strongly related to the domain general ability for phenomenological control. Experiments wishing to link RHI experience specifically with subjective body ownership must take this relationship into account or their results cannot be taken to indicate general properties of human body ownership.