Infamous rubber hand illusion shows altered body perception in Tourette Syndrome

Posted on 23 October 2018

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You might have seen the rubber hand illusion on YouTube, or even BBC’s QI. The illusion is a trick-of-the-mind: Watching a fake rubber hand being stroked with paintbrushes, while feeling you own hand being stroked in the same way (but out of sight), can make you feel that this rubber hand is part of your own body. In the ‘entertainment version’ if the fake hand is then threatened by a spike or hammer it will get a strong reaction, showing that you really feel the rubber hand is part of you. People also judge their real hand to be nearer to the rubber hand than it actually is – a less dramatic way of measuring the strength of the illusion

We used this rubber hand illusion (without the threat) in a scientific study of people with Tourette Syndrome (TS) to see if they have altered experience of their body. If so, we might understand more about the feelings behind tics and the compulsion or urge to move. We thought that differences in the way people with TS perceive their body may help explain what generates feelings of an urge to move and tic.

We ran the rubber hand illusion with adults with and without TS: Both groups reported that they felt the illusion strongly, but unlike controls people with TS did not show the same ‘drift’ in space of their own hand towards the fake one. This suggests that people with TS are less susceptible to changes in perception of bodily position from these ‘multisensory’ experiences, perhaps because they have a stronger feeling of their own body.

We also looked at whether the experience of the illusion was related to differences in symptoms.  We found that the more people with TS felt that the rubber hand was part of their own body, the worse their premonitory sensations (feelings of pressure or itch before tics). Also, the more people with TS felt a sense of ‘agency’ – feeling like they controlled and could have moved the rubber hand if they wanted – the worse their tics were overall.

We think these two symptoms of TS probably map on to slightly different parts of the brain in TS, hence why they also relate to two different aspects of feelings about the rubber hand. This study is one of several papers that will be published in scientific journals from our research team over the next few months, which will delve in greater depth into the body and brain basis of TS symptoms, so keep an eye on the Tourettes Action website for updates.

We would like to thank the participants who contributed their time (and hands) to this study, and Tourettes Action for helping us advertise for participants.

Rae CL, Larsson DEO, Eccles JA, Ward J, Critchley HD. (2018) Subjective embodiment during the rubber hand illusion predicts severity of premonitory sensations and tics in Tourette Syndrome. Consciousness & Cognition

https://authors.elsevier.com/a/1XvlW_NzVjBXz-

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