The alien awakened by a rubber hand

What happens if you administer a tactile illusion to a brain-damaged patient whose hand is out of their control? A team of researchers has done just that, figuring that illusions could offer new insights into complex neuropsychological disorders.

The patient in question was a 69-year-old lady whose left-sided stroke had left her with alien hand syndrome*. Most of the time her right hand was held in a clenched position that she couldn't open. Occasionally, accompanied by a mild electric sensation, it moved involuntarily, jerking, or even slapping her in the face.

Michael Schaefer and his colleagues at Otto-von-Guericke University Magdeburg tested the lady on two sensorimotor illusions – the traditional rubber hand illusion and the lesser-known somatic rubber hand illusion. The first involved the patient placing one of her arms on the table-top, with the other underneath. A rubber arm was placed alongside her real arm on the table. The researcher then stroked the patient's hidden arm and the rubber arm in synchrony. When the illusion works it creates the sensation of feeling in the rubber arm, as if it's a part of the person's body. In fact the patient experienced no feeling in the rubber arm at all, regardless of whether it was her healthy arm or alien arm that was being stroked under the table. The rubber hand illusion doesn't work for everyone so this null finding is not particularly surprising.

Things got more interesting when the researchers tested their patient with the somatic rubber hand illusion (see picture, above). This procedure involved the rubber arm being placed between the patient's two real arms on a table-top. This time, the patient was blindfolded and the researcher (wearing plastic surgical gloves) picked up one of the patient's hands and used it to tap the rubber hand. At the same time, and in synchrony, the researcher tapped the patient's other hand. This procedure creates the strong illusion for the participant that they are touching their own hand rather than the rubber hand – a feeling that the patient said she experienced.

But something surprising also happened when the researchers tried out this illusion. Within moments, the patient's alien hand leapt up off the table and was grabbed by her healthy hand. She said she felt an electric sensation in her alien hand prior to it rousing. The illusory experience seemed to have awakened her alien hand. This effect occurred every time the procedure was repeated. But crucially it only happened when it was the patient's healthy hand that was used to tap the rubber hand, whilst the patient's alien hand was simultaneously tapped by the researcher (and not when the illusion was done the other way around). The awakening effect also disappeared when the procedure was repeated with the patient's blindfold removed, which is known to destroy the illusion.

All this suggests that it wasn't touching the alien hand per se that roused it, but rather it was the experience of the body illusion. Schaefer and his colleagues think that their patient has a disconnect between the anterior supplementary motor area (SMA) at the front of her brain (involved in inhibitory control) and other brain regions involved in movement. They reckon this impaired motor integration somehow interacted with the illusory feelings of body ownership triggered by the rubber hand trick. Perhaps, they said, the illusion further weakened the SMA's already compromised control of the alien hand.

"Although our results should be confirmed by further studies, we believe that the examination of experimental-induced illusions in patients with disorders of self-embodiment is promising and might help us to develop treatments for these diseases in the future."

Further reading

Michael Schaefer, Hans-Jochen Heinze, and Imke Galazky (2012). Waking up the alien hand: rubber hand illusion interacts with alien hand syndrome. Neurocase: The Neural Basis of Cognition DOI: 10.1080/13554794.2012.667132