Our group has done a few rubber hand illusion experiments, not that we are really serious players in this area – if you want to take part in a rubber hand illusion experiment, start loitering around UCL – it is almost certain someone there will be doing an such an experiment. The brain mechanisms that underpin the rubber hand illusion are almost certainly dependent on multisensory integration. That is, it is the congruent input from two or more senses that seems to induce the mistaken feeling that one can feel the touch that one can see. We have used the rubber hand illusion as a model of what happens to some people with chronic pain, for example replicating the blood flow and tactile processing changes that occur in CRPS,[1] but there are obvious limitations to that because the rubber hand illusion does not hurt and CRPS obviously does, and we don’t really show that participants ‘disown’ their real arm, as they do in severe cases of CRPS. Regardless of all that, a recent study[2] suggests that we might be able to engage similar mechanisms to those engaged in the rubber hand illusion, without multisensory input but, instead, with only visual input and mental imagery.
A bunch of Swiss researchers sat participants in front of a monitor and showed them a film of slow movements of two virtual arms. In one condition, participants were advised to simple watch the film. In the other condition, participants were advised to both watch the film but also imagine that the arms they were viewing were in fact, their own arms. Then, without warning, one of the virtual arms was stabbed by a knife and started bleeding. Sound like a fun experiment? Fun for the experimenters perhaps, but less so for the participants. Not surprisinging, both groups had a spike in skin conductance on their palms – the stabbing caused a sympathetic nervous system response. The ‘watch plus imagine it is you’ group had a bigger response. This finding is pretty predictable I reckon, until perhaps you see the virtual arms and knife and blood. The authors explain their result by implicating the mirror neurone system. They provocatively ask whether, to induce this sense of ownership over a virtual or rubber arm, do we really need multisensory input and the whole ‘bother’ of using a rubber hand? They seem to conclude that we do not. I am unconvinced though. What would happen if one was to see a virtual cat on the screen and told to watch the cat and then the cat got stabbed? I would not be that surprised if I got a quick spike of SNS activation. I would not be surprised if my spike in SNS activation was greater if I had also been imagining that I was indeed the cat. “Be the cat.” Would this mean that I had embodied the cat? I think an alternative explanation relates to attention. Indeed, the authors report that subjects’ reported feelings of ownership of the virtual arms was not different between groups. So, I reckon this study is not compelling enough to conclude, as the authors do, that embodiment of virtual limbs can just be imagined. But what it if it can? Does this have clinical implications? Might there be times when we actually want DISembodiment? Could I be a cheetah? I always wanted to be a cheetah – any cat? a lion?….
Right now, however, I am interested in whether this finding can be applied to ones own body. That is, we often get people to observe their own body and we think that visual feedback is good. I refer the interested reader to the latest noinotes on the virtual vagina – time to get out the mirror? Well, if we do, this study suggests that we would be well served to get people working harder to actually connect with the body part they can see. I know that there are big proponents of this sort of somatocognitive connectedness – the mindfulness stuff comes to mind here – and one might suggest that by truly connecting with the body part you are watching, you would actually augment multisensory integration. As David Butler would say, perhaps it is all just ‘homoncular refreshment’ and multiple sensory inputs refresh better than unimodal sensory inputs.
Anyway, the paper is freely available and it includes some interesting reflections on the effect and its applications to rehabilitation settings. It has not, however, convinced me to chuck out our rubber hands.
About Lorimer Moseley
Lorimer is NHMRC Senior Research Fellow with twenty years clinical experience working with people in pain. After spending some time as a Nuffield Medical Research Fellow at Oxford University he returned to Australia in 2009 to take up an NHMRC Senior Research Fellowship at Neuroscience Research Australia (NeuRA). In 2011, he was appointed Professor of Clinical Neurosciences & the Inaugural Chair in Physiotherapy at the University of South Australia, Adelaide. He runs the Body in Mind research groups. He is the only Clinical Scientist to have knocked over a water tank tower in Outback Australia.
Link to Lorimer’s published research here. Downloadable PDFs here. And last, but by no means least, here is Lorimer talking about what we are doing here at Body in Mind.
References
[1] Moseley, G., Olthof, N., Venema, A., Don, S., Wijers, M., Gallace, A., & Spence, C. (2008). Psychologically induced cooling of a specific body part caused by the illusory ownership of an artificial counterpart Proceedings of the National Academy of Sciences, 105 (35), 13169-13173 DOI: 10.1073/pnas.0803768105
[2] Hägni K, Eng K, Hepp-Reymond M-C, Holper L, Keisker B, et al. 2008 Observing Virtual Arms that You Imagine Are Yours Increases the Galvanic Skin Response to an Unexpected Threat. PLoS ONE 3(8): e3082.