Thursday, 11 September 2014
You may have heard of the "rubber hand illusion" (video). By visibly stroking a rubber hand in time with stroking a participant's hidden real hand, you can induce for them the feeling of sensation in the rubber hand.
The rubber hand illusion is thought to occur because the brain tends to bind together information arising from different sensory modalities. The stroking sensation arrives at one's real hand, but the stroking is seen occurring at the same time at the rubber hand. The brain binds these two experiences and the visual modality wins, transferring the felt sensation to the rubber appendage.
Because the rubber hand illusion depends on the dominance of vision, Charles Michel and his colleagues wondered if a similar illusion would still occur for the tongue - one of our own body parts that we feel but rarely see. The researchers purchased a fake tongue from a magic shop (see pic), and for forty seconds they stroked this tongue with a cotton bud (Q-tip) at the same time as they stroked each participant's real tongue. The participants could see the stroking of the fake tongue, but the stroking of their own tongue was hidden from view.
Averaging responses from the 32 student participants, there was an overall sense among the students of being stroked on the rubber tongue, "thus demonstrating," the researchers said, "visual capture over the felt position of the tongue for the very first time." Further evidence for an illusory effect came from the fact that sensation in the rubber tongue was stronger when the stroking of the fake and real tongues was performed in synchrony as opposed to out of time. This synchronous stroking also led to more agreement from the students that they felt as though they could move the fake tongue, and that the fake tongue was their own.
Next, the researchers shone a laser pointer on the fake tongue as the participants watched. Twenty-two of the participants said that this triggered sensations in their real tongue - some said it felt cold, others warm, tactile and/or tingly. "I felt vibrations on my tongue moving in synchrony with the light movement," one student said.
The researchers say their results have shown that an illusion, similar to the rubber hand illusion, can be experienced with the tongue. We call this "the Butcher's Tongue Illusion," they said.
Michel, C., Velasco, C., Salgado-Montejo, A., & Spence, C. (2014). The Butcher’s Tongue Illusion Perception, 43 (8), 818-824 DOI: 10.1068/p7733
Psychologist magazine interview with cognitive neuroscientist and ‘master of illusion’ Henrik Ehrsson.
Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.