Scientists at Brighton and Sussex Medical School (BSMS) have used an eerie illusion of having an extra hand to understand Tourette syndrome. People with the condition reacted differently to others when undergoing this ‘rubber hand illusion’, suggesting that symptoms of Tourette syndrome may arise, in part, because the way in which the brain represents the body is harder to adjust.
Dr Charlotte Rae, Research Fellow at BSMS, who conducted the study, said: “We wanted to understand why people with Tourette syndrome experience symptoms such as feelings of urges to move their limbs, which can be a causal factor in their involuntary movements known as tics. The results showed that their body perception was considerably less malleable than that of participants who do not have Tourette syndrome. Not only that, but there was a direct correlation between their awareness of their hand’s location and their impulse to tic.”
If you watch the fingers of a fake rubber hand being stroked in front of you, while feeling your own fingers being stroked (out of sight) in the same way, you may well get the uncanny feeling that the rubber hand is part of your own body. Some people even pull away their own hand when they see a hammer approaching the now ‘embodied’ fake hand. This rubber hand illusion demonstrates we continually use information from our vision and touch to update where, and even what, we think is our body.
During the rubber hand illusion, people without tics and people with Tourette syndrome both reported feeling that after stroking, the rubber hand felt like part of their own body. However, there were differences in where they thought their own (hidden) hand was located, an objective measure of body perception. Usually when people experience the rubber hand illusion they feel their own hand has ‘drifted’ towards the fake one. This did not happen in the people with Tourette syndrome, who were in fact were more accurate when judging the position of their own hand. The combination of feeling ownership of the fake hand, and having an accurate, more rigid perception of one’s own hand, predicted the severity of Tourette symptoms (tics and the urge to tic).
Dr Rae added: “These findings may help with new ways of treating Tourette syndrome, perhaps by using medicines or physical exercises that loosen the more rigid unconscious perceptions of body position.”
The study has been published here >