Oct 26, 2018 - ample of the classic rubber-hand illusion ... rience the illusion in both conditions, when ..... brains, situated agents, and the future of cog-.
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sounds were implemented into the FBI experimental setup we would not expect them to have an influence on the illusions. Nevertheless, this point seems important to us since the experiment results would be more robust if there were an implementation of head-related movement analysis in a genuinely multi-sensorial context. We thought about this supplementary condition control while reading §11, where the authors emphasize that the head-related signals encompass visual input of the whole scene and visuovestibular coherence. The experimental setup being innovative on this point, we were wondering whether it could be interesting to investigate more thoroughly the differences between the two stimuli. This led us to ponder whether someone would be able to overcome the obstacles inherent to the 1PP FBI setup. Indeed, to our knowledge the cognitive field is regularly visioncentred. In the investigation of the body schema and body image, in feeling and agency and ownership of one’s body, the visual cues are predominant. We acknowledge that in 1PP FBI it is difficult to add the auditory dimension in the experimental setup. However, this should not prevent us wondering about the alternatives. Experimental difficulties should not restrain us to inquire about alternative mechanisms at work in one’s feeling of ownership/agency over one’s body. Vision is central, but it is not the only sense at play. Are blind people deprived of this feeling? Of course not, but their frame of reference is generally not used in experimental conditions. Designing an FBI setup for blind people could be a challenge. To our knowledge, there is one interesting example of the classic rubber-hand illusion (Petkova, Zetterberg & Ehrsson 2012). In this setup, blind people are never subject to the illusion, however, sighted people experience the illusion in both conditions, when seeing the rubber hand and when being blindfolded, respectively. Does this mean that blind people overperform at proprioceptive tasks? Or does this mean that people using vision use vision-related mechanisms even when blindfolded? (Q4) The question we would like to put to the authors, as a future perspective of research, is how could we create an authentic multi-sensorial context in order to further our understanding of the different senses at play in our body
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image/schema transformation? And thus, how could we go beyond experimental limitations and, in an enactivist perspective, bring experimental knowledge and one’s experience closer? Could we imagine a setup unifying vision, audition, touch and proprioception that could enable us to overcome design obstacles and refine our knowledge of the body schema and the body image from multiple perspectives and not only a vision-centred one?
References Kokkinara E., Kilteni K., Blom K. J. & Slater M. (2016) First person perspective of seated participants over a walking virtual body leads to illusory agency over the walking. Scientific Reports 6: 28879. https://doi. org/10.1038/srep28879 Kokkinara E., Slater M. & López-Moliner J. (2015) The effects of visuomotor calibration to the perceived space and body, through embodiment in immersive virtual reality. ACM Transactions on Applied Perception 13(1): 3. Petkova V. I., Zetterberg H. & Ehrsson H. H. (2012) Rubber hands feel touch, but not in blind individuals. PloS ONE 7(4): E35912. https://doi.org/10.1371/journal. pone.0035912 Alexandre Foncelle is a reasearch engineer in neuroscience and cognitive science at the ImpAct team, INSERM U1028, Université Lyon 1. Alexandre earned a PhD in neuroinformatics and works now as a data scientist. Jean-Félix Gross is a doctoral researcher in philosophy at the Université Jean Moulin Lyon 3; his work lies in the intersection of medicine and philosophy. Jean-Félix is working on the placebo effect from an enactivist perspective. He is affiliated to the IRPHIL and S2HEP laboratory in Lyon. Received: 23 October 2018 Accepted: 26 October 2018
The Illusion Illusion Sebastian Dieguez
Université de Fribourg, Switzerland sebastian.dieguez/at/unifr.ch
> Abstract • Conflicting and puzzling results in bodily illusions research might arise because researchers have been operating under the false premise that they were studying illusions. I suggest that the rubber-hand and full-body illusions are not illusions after all, but something else entirely, and believing otherwise is succumbing to the illusion illusion. « 1 » In the last paragraph of the target article (§31), Marte Roel Lesur et al. highlight “a seemingly paradoxical idea regarding bodily illusions.” The crux of this paradox, they write, is that “while these illusions are a result of the experimental procedure, they are also a cause of their own resistance to sensorimotor incoherence,” so that “the resistance to the illusion partly depends on parameters of the illusion itself.” Such a conclusion takes stock of new experimental data, a review of previous conflicting results and a theoretical proposal on the interactions between body schema and body image. « 2 » It has been 20 years since Matthew Botvinick and Jonathan Cohen’s (1998) seminal Nature publication on the rubber-hand illusion, which successfully launched a whole industry of rubber-hand and related virtualbodies research. In this commentary, I would like to suggest that Roel Lesur et al.’s “paradoxical idea” might reflect a much deeper problem within this entire area of research. « 3 » As a starting point, let us consider this idea of a “resistance to the illusion.” What do the authors mean? If I understood the suggestion correctly, this refers to the observation that under some circumstances, illusory ownership of a body part or whole body would still be experienced during control conditions that should, in principle, diminish or annihilate illusory ownership. In particular, the authors suggest that when a virtual visual world is fully contingent on one’s own head movements, then the virtual self that is seen would be felt to belong to oneself even when other virtual inputs are not synchronous with their counterpart physical inputs. It is as if head movement/
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The Illusion Illusion Sebastian Dieguez
virtual world synchrony “takes all” and thus allows minor inconsistencies at limb level to be neglected. Thus, once a central and global illusion is present, it cannot be broken down by peripheral procedures usually aimed at not inducing illusory ownership: “Sure,” the bodily self might say, “I feel touches on my skin that I do not see at the expected moment or place, but, so what? Everything else seems so genuine!” I must apologize for such a schematic summary, but if this is not too far off the mark, then it raises an intriguing question: are there any bodily illusions at all? « 4 » Here is the issue with Roel Lesur et al.’s “paradoxical idea”: in the classical rubber-hand illusion, participants do enjoy full-fledged head movements/visual world congruency. After all, the appearance of a perceptual world and a self at its center is itself the product of optimally calibrated multisensory inputs that produce the feeling of a unitary and coherent point of view; our very brain is such a perfect head-mounted display. Why, then, should this natural and powerful “user’s illusion” (Dennett 1991) allow for minor, local and obvious multisensory transgressions to produce illusory ownership over transparently fake body parts? Why should not our normal experience of the bodily self display, at all times, “resistance” to bodily illusions, similar to the “resistance” offered, according to the authors, by sophisticated technical devices tracking head and/or eye movements? « 5 » It seems to me that there are two ways to answer this question. Either illusory limb identification, such as during the classical rubber-hand illusion, is not illusory after all, or it is illusory, but we sometimes fail to measure this illusion properly. Let us address the second option first. The usual demonstration of the rubber-hand illusion involves comparing a “synchronous” and an “asynchronous” condition. As far as I can tell, the “asynchronous” condition was initially meant as a control condition aimed at ruling out “placebo” responses. Let us put it this way: it is widely assumed that the rubberhand illusion should arise due to the highly implausible coincidental perception of tactile and visual stimulations seemingly referring to the same information input, namely being touched on one hand. This would presumably be very unlikely in a natural setting, and so the human brain somehow integrates
these inputs in an economical manner by inferring that the artificial hand is “one’s own.” Thus, this should not happen during asynchronous visuo-tactile inputs: decorrelating the inputs would simply convey tactile inputs on one’s hand, and unrelated inputs in the visual field, something that is not puzzling to our brain and needs no resolving. If this is correct, then the very procedure to induce the rubber-hand illusion also provides its explanation, and the asynchronous control condition, because it does not “work,” not only suggests that this explanation is correct, but also that participants are not reporting make-believe sensations during the synchronous condition: they do have an illusion of ownership in, and thanks to, the synchronous condition. Of course, if the rubber-hand illusion also “worked” in the asynchronous condition, then the finding would be theoretically puzzling (why do we not have illusory ownership over other bodies all the time?), or conceptually trivial (merely looking at a rubber hand sort of makes it “seem” like one’s own hand) or indicative of participants’ suggestibility (the whole situation is weird, so I guess those scientists want me to answer that this rubber thing could be my hand; Orne 1962). As such, it would be uninterpretable, and rather uninspiring. But this is, apparently, the very situation faced by some studies investigating the first-person perspective full-body illusion: illusory ownership arises even during the asynchronous – control – condition. This could be interpreted, as the authors do, as some sort of “resistance”: the illusion “survives” the control condition. But it could also mean that the experiment completely failed, or that participants are subject to experimental demands, or that they make up their responses, or that there is no illusion in the asynchronous condition, but still one in the synchronous condition, which is then improperly compared to an unsatisfactory control condition. « 6 » But these interpretations all seem strangely ad hoc and would need additional investigations to disentangle them. In my opinion, for the time being, we should prefer our first, even if quite disheartening, hypothesis: whatever it is that we call “illusory limb identification” in an experimental paradigm is no illusion after all. To my knowledge, this claim has never been taken seriously before, or even merely considered, in the
20 years that we have been saying that the rubber-hand illusion is an illusion. Yet this proposal, it seems to me, might help clarify the “paradox” highlighted in the target article’s conclusion, and make better sense of the conflicting results reviewed in the paper than the authors’ hypothesis of the primacy of head movements contingency. « 7 » Although widely used since the very beginnings of psychological science, the term “illusion” has been notoriously difficult to define. Richard Gregory (1997: 1123) tried to circumscribe the notion to “systematic visual and other sensed discrepancies from simple measurements with rulers, photometers, clocks and so on.” But as Barbara Gillam (1998: 95) put it, “a discrepancy between perceived reality and objective or physical reality” would also cover simple pictures, namely flat images fostering “an impression of three-dimensional (3-D) space,” yet “this 3-D impression is not considered an illusion.” Focusing on tactile illusions, Vincent Hayward (2008: 743) suggests that “an illusion is a percept arising from a specific stimulus delivered under specific conditions that gives a different conscious experience when the conditions are changed,” and adds that such a change in perception “should be surprising, unexpected, even amusing when the perceiver becomes aware of it.” But then, many authors also feel quite comfortable with the idea that all our perceptual and conscious processes are to some extent “illusory,” an idea often backed by the kind of simple, stripped-down, geometric-optical stimuli that strongly suggest that vision is a sometimes unreliable constructive process (you think this line is longer than this one? Think again!). Is the rubber-hand illusion an illusion in the same sense that classical visual illusion such as the Müller-Lyer or the checkerboard illusion are illusions? « 8 » I would claim that it is not. Experimental illusions such as illusory limb identification and full-body illusions never quite manage to deceive the mind as simple visual illusions do. Their effect seems to be rather variable across individuals, and the manner in which the illusion is elicited seems to preclude a strong sense of surprise at the moment of its discovery. As far as I know, no experimental participant ever exclaimed: “Wait, you’re telling me this isn’t my hand/ body?!” Of course, at all times during such
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experiments the participants are fully aware of what is going on, that a fake hand is present, that they are surrounded by and wearing a complex set of tracking, recording and displaying technological devices, and so on. As such, they can reasonably expect the kind of experiences they are supposed, and expected, to experience, and what is revealed to them can then hardly be said to be “counter-intuitive” or frankly “surprising” (or “surprisal-ing,” Clark 2013: 196). This is not to say that such “illusions” cannot be, in some weaker sense, surprising and amusing, but compared to the sheer simplicity and utter strength of the classical optical illusions, it should make one wonder what exactly is going on during bodily illusions, and what their measurements do or do not reflect. « 9 » So, I suggest that bodily-self researchers might have been operating under an “illusion illusion,” that is the illusion that they were studying an illusion all along. But if not an illusion, then what does happen during the experiments described in the target article? It might be, I propose, something akin to what Tamar Gendler (2008) calls “alief,” that is, a consciously perceived discordant belief, or mismatch between belief and behavior. Here are some examples: you have very good reasons to believe, and you do believe, that the bridge you are walking on is perfectly safe, and yet you feel insecure; you know that the horror film you are watching is a nutty fiction full of special effects projected on a two-dimensional screen, and yet you are scared, and even shriek at the protagonist to not “just check” the basement; you watch a rerun of a football game of which you know the result, and yet you feel excitement at the delayed action; you bump your little toe on a nearby item of furniture, and you get angry at the inanimate object. In all these cases, despite your belief and knowledge that something is the case, you “alieve” something else entirely. Note that Gendler never uses the term “illusion” to describe such cases, and indeed, it would not seem quite right to label them as such. « 10 » I suggest that something of this kind happens during the rubber-hand and associated bodily illusions. You know that you are looking at a fake hand, and yet the concordant sensory inputs seem to suggest that you are “feeling” it. But you do not “believe” that you “own” the fake hand or that
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you feel touch “in” it, and you do not “have” the “illusion” that you do either. You merely alieve these things, because the whole situation simply “looks” like the rubber hand is “yours,” just like being high above the ground “looks” scary no matter how safe you are, just like a horror movie “is” unsettling even if you know it is all fake, just like a football game can “be” exciting even when you know the outcome, and just like it can “feel” appropriate to verbally abuse an inanimate object. As it happens, this idea was unwittingly suggested to me by a participant during a rubber-hand demonstration: asked whether he felt that the rubber hand felt like his own hand, he said “yeah, sort of, but that is only because you are touching my hand at the same time”… Would anyone experiencing, say, the Ebbinghaus illusion for the first time, spontaneously claim: “yeah sure, the circle kind of looks smaller, but that is only because the surrounding circles are larger”? « 11 » Just how much “alief ” one experiences during bodily “illusions” would then depend on a host of relevant factors, such as the willingness to participate, the ability to suspend ongoing belief, the expectations one has about the procedure and what the researchers expect, in short, the global evaluation of the situation, and not just the experimental procedure per se. This would still allow for differences between conditions, and even associated implicit responses such as skin conductivity, proprioceptive pointing or specific neural correlates, but without any need for the concept of illusion or even invoking the “plasticity of the bodily self.” In more sophisticated settings involving technological apparatus, alief would also adapt to the ongoing expectations, experiences and requirements of the situation, and sometimes dispel the effects of operant variables observed in other contexts. « 12 » If this, or something like this, is even remotely correct, then the “paradoxical idea” and conflicting results highlighted in the target article dissolve, and there is no need to postulate a “resistance” to such and such “illusion,” or specific constraints to bodyschema/body-image interactions, because the findings can be explained by an entirely different mechanism that does only indirectly involve the bodily self, but mostly requires a mismatch between what the agent thinks her attitude and feelings should be in light of her
beliefs and knowledge of the situation, and what seems to be produced by an elaborate display explicitly aimed at deceiving her senses. Note that this is very much unlike classical visual illusions where two lines, for instance, seem and will continue to seem of different lengths, although they can be shown not to differ in length when measured with a ruler. There is no such independent comparison that could convince the subject in a bodily illusion experiment: she knows all along what is going on. As such, a fake hand or a virtual body is not different from a horror movie or a bridge: they are aliefs, not illusions. Only the experimenters, here, are victims of an illusion, the illusion illusion.
References Botvinick M. & Cohen J. (1998) Rubber hands “feel” touch that eyes see. Nature 391(6669): 756. Clark A. (2013) Whatever next? Predictive brains, situated agents, and the future of cognitive science. Behavioral and Brain Sciences 36: 181–253. Dennett D. C. (1991) Consciousness explained. Little, Brown & Co., New York. Gendler T. S. (2008) Alief and belief. Journal of Philosophy 105(10): 634–663. Gillam B. (1998) Illusions at century’s end. In: Hochberg J. (ed.) Perception and cognition at century’s end. Academic Press, New York: 95–136. Gregory R. L. (1997) Knowledge in perception and illusion. Philosophical Transactions of the Royal Society of London B 352: 1121–1128. Hayward V. (2008) A brief taxonomy of tactile illusions and demonstrations that can be done in a hardware store. Brain Research Bulletin 75: 742–752. Orne M. T. (1962) On the social psychology of the psychological experiment. American Psychologist 17: 776–783. Sebastian Dieguez is a neuroscientist at the Laboratory for Cognitive and Neurological Sciences, University of Fribourg, Switzerland. He works on neuropsychological disorders and irrational beliefs such as beliefs in conspiracy theories, and also writes for the general public in Cerveau & Psycho and Vigousse. His latest book is Total Bullshit! Au cœur de la post-vérité (2018). Received: 19 October 2018 Accepted: 23 October 2018
