"I felt as if the rubber hand were my hand." That's what many people have said when they have participated in the rubber-hand experiment, which is designed to test the conditions for the sense of bodily ownership.
In the first report of the rubber-hand illusion, Botvinick and Cohen (1998) describe two experiments. In the first one, ten subjects were seated with their left arms on a table and a screen beside the left arm to hide it from the view of the subject. A life-sized rubber hand and arm was placed in the middle of the table in full view of the subject. While the subject looked at the rubber hand, the experimenters used two small paintbrushes to gently stroke the hidden hand and the rubber hand, timing the brushing so that it was synchronous.
After ten minutes, subjects answered a questionnaire about their experience. Many of them reported that during the experiment there were times when they seemed to feel that the rubber hand had sensed the touch of the brush. "It seemed as though the touch I felt was caused by the paintbrush touching the rubber hand." "I found myself looking at the dummy hand thinking it was actually my own."
To provide a control condition, some subjects experienced asynchronous brushing of the real hand and the rubber hand--the brushing was not done simultaneously. These subjects did not report the illusion of feeling the brush stroke in the rubber hand.
The experimenters inferred that the rubber-hand illusion here shows that bodily self-identification arises from the interaction of at least three sensory modalities--vision, touch, and proprioception. Vision monitors where the limbs are in space. Touch monitors body conformation and its contact with external objects. Proprioception is the sensation of the body's position and movement, relying on mechanosensory neurons distributed throughout the body, which are called proprioceptors (Tuthill and Azim 2018). The illusion arises because the brain's attempt to reconcile the visual inputs from the rubber hand and the tactile inputs from the hidden hand distorts the proprioceptive sense so that in the brain's representation of the body, the rubber hand takes the place of the real hand.
The experimenters tested this hypothesis of proprioceptive distortion in their second experiment. Subjects were put into the conditions of the prior experiment for a prolonged period (30 minutes). Then, with their eyes closed, the subjects were asked to move the right index finger under the table until it was in alignment with the left index finger, which was still on top of the left side of the table. The subjects' reaches were displaced toward the rubber hand, and the magnitude of the displacement was proportional to the reported duration of the hidden-hand illusion. Thus, their proprioceptive sense of the location of their left hand had been moved in the direction of the rubber hand.
Something similar to the rubber-hand illusion can be induced in experiments with phantom limb patients. Some patients who had had their left arm amputated were put into an experiment in which a mirror was placed in front of them, so that when they looked at the stump of their left arm, they saw the reflection of their intact right arm. When they saw their right arm being touched, they reported feeling that their phantom left arm was touched (Ramachandran and Rogers-Ramachandran 1995).
A similar illusion can occur when an experimenter simultaneously strokes an amputee's stump, which is out of view, and a visible rubber hand located near the stump: the patient starts to feel that the rubber hand is part of her body, so that the rubber hand becomes her phantom hand (Ehrsson et al. 2008). This is an important finding for neuroprosthetics, where it's important to develop artificial limbs that feel like real parts of the body.
This experimental study of the rubber-hand illusion has been replicated many times (Blanke et al. 2015; Longo et al. 2008; Moseley et al. 2012; Tsakiris 2010, 2017). The later studies have been able to use improved neurophysiological technology--such as functional neuroimaging--to identify the neural correlates of the sense of body ownership. These studies have identified a network of areas in the brain--including premotor, temporopariental, and occipital areas, as well as the insula. The insula is known as the hub of the interoceptive system of the brain, but it is also engaged in the experience of bodily ownership during the rubber-hand illusion.
Interoception arises from sensations coming from inside the body and the visceral organs that signal physiological states such as thirst, hunger, need for air, heartbeat, coolness, warmth, distension of the bladder, sexual arousal, and other internal states. Interoceptive signals arise from the cardiovascular, respiratory, gastrointestinal, and urogenital systems.
This indicates that our sense of our bodily self depends on the multisensory integration in the brain of the exteroceptive body (the body as viewed from the outside) and the interoceptive body (the body as viewed from the inside) (Tsakiris 2017).
Over the past year, the research on the rubber-hand illusion has been challenged by psychologists studying hypnosis who say that these reports of illusory body ownership might be the product mostly of hypnotic suggestibility (Florio et al. 2020; Lush 2020; Lush et al. 2020; Makin 2020; Seth et al. 2020). Peter Lush and his colleagues at the University of Sussex have said that the experiments with the rubber-hand illusion have been confounded by what psychologists call "demand characteristics" or "suggestibility effects." That is to say, when people are recruited to be subjects for experiments like this--and it's often college students--they can many times figure out what is expected by the experimenters, and if so, many of the subjects will tend to say whatever they think the experimenters want them to say. They will think: Oh, if they want me to say that I feel as though this rubber hand is really my hand, then that's what I will say.
This propensity can be measured by hypnotic suggestibility: some people are more easily hypnotized than others, and the researchers who study this can rate people for their suggestibility. Lush and his colleagues conducted experiments in which they found that those subjects who were more suggestible were more likely to report an intense feeling of the rubber-hand illusion. This seemed to be a lot like the placebo effect: people can show a beneficial effect from a drug or therapy, even though they were not given the real drug or therapy, because they believed they had.
I have had an exchange of emails with Manos Tsakiris, a neuropsychologist at the Royal Holloway University of London, who is one of the leading researchers on the rubber-hand illusion. When I questioned him about the work of Lush and colleagues, he said that if you look at their data, you will see that it does not deny the reality of the rubber-hand illusion. Their data shows that both those who are highly suggestible and those who are low in suggestibility report feeling the rubber-hand illusion. There is only a difference of degree, in that the highly suggestible subjects feel the illusion more intensely. There is nothing surprising in this, he observes, because there will always be individual variability in psychological experiments like this due to individual differences in personality.
I accept this response. But I still think Tsakiris and others in RHI research need to answer the argument of Lush and colleagues that even if the RHI is a real subjective experience, it is very weak, and so it does not show that our ownership of our body is easily malleable. When subjects have undergone the RHI experiment, they do not leave with a permanent or enduring illusion that the rubber hand is really their hand. Even if they have had a fleeting feeling of the illusion--a few seconds or minutes--they have no trouble realizing that this has been only a momentary feeling, and that the rubber hand is not their real hand.
After all, if the RHI experiment really did induce an enduring feeling that the rubber hand had replaced their real hand, this would be an unethical experiment that would never be approved by any Institutional Review Board for Protection of Human Subjects! I assume that whenever Tsakiris submits a proposal for a RHI experiment, he tells his university review board that the experiment is harmless--that it will not induce any enduring state of confusion about body identity.
In contrast to the transient feeling of the RHI, people with certain kinds of brain damage can completely lose their sense of body ownership in a limb, and they can even believe quite strongly that one of their limbs actually belongs to someone else (Vallar and Ronchi 2008). Some people with xenomelia ("foreign limb") even have an obsessive desire to amputate a healthy limb because it seems alien to them (Hilti et al. 2013). The RHI does not show this kind of serious disruption in the sense of bodily self-ownership.
There is another problem here that runs through all of this research on the psychology and neuroscience of subjective experience: the researchers cannot observe directly the inherently private experience of subjective feelings and thoughts that are invisible to the researchers; they have only an indirect access to this invisible world through the introspective self-reporting of their subjects.
Can this invisible world of subjective experience be reduced to the visible material world of the body and the brain? Or is this subjective world of conscious self-awareness the activity of an immaterial soul?
TRICKING THE IMMATERIAL SOUL?
If one's highly suggestible mind is easily tricked into believing that a rubber hand is one's real hand, even if only momentarily, does that manifest the power of the immaterial soul to change the material body and brain? That's the conclusion drawn by Denyse O'Leary (2020) in her assessment of the debate over the RHI.
O'Leary has often argued that natural science supports both the dualistic reality of an immaterial soul and material body as opposed to materialist reductionism and the truth of Intelligent Design Theory as opposed to Darwinian evolution. Not surprisingly, then, she has criticized some of my arguments; and I have responded to her.
She sees the RHI as similar to the placebo effect in that it shows how our subjective mental expectations have effects on the body and the mind. This is the kind of evidence she cites in her book The Spiritual Brain (2007)--coauthored with neuroscientist Mario Beauregaard--arguing that neuroscience provides evidence for the existence of the soul as an immaterial and immortal spirit.
She is correct in pointing to the mystery of consciousness and to the failure of neuroscience to resolve that mystery in materialist terms by explaining exactly how the brain creates the mind through purely material causes. Nevertheless, she herself has not resolved this mystery by asserting that the immaterial soul has been created supernaturally by God, but without explaining exactly how God does this. This is the fallacy of explaining a mystery with a mystery.
I will say more about this in a future post on Thomas Willis and the neuroscience of dualism.
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