Read about all the tactile illusions in our special feature
One of the newly discovered tactile illusions is called 鈥渢actile rivalry鈥�, a haptic version of a wider class of illusion called perceptual rivalry.
The best example of perceptual rivalry is the Necker cube, a simple line drawing of a 3D cube that can be interpreted as being viewed from slightly above or slightly below (see diagram). Stare at a Necker cube and the two interpretations spontaneously swap around in your mind鈥檚 eye. This is visual rivalry, and it is thought to exist to allow your brain to make sense of ambiguous inputs: when there is no correct answer to the question 鈥渨hat am I looking at?鈥� your brain tries one interpretation, then switches to the other.
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Visual rivalry has been used to study vision for more than 200 years, but only recently has it become apparent that other senses do similar things. In 2006 French researchers discovered auditory rivalry, where a stream of electronic beeps can be interpreted in two different ways (listen at ).
Now a touch equivalent has been discovered too. There is no tactile equivalent of the Necker cube, but there is for a similar illusion called visual apparent motion.
Imagine four dots arranged in a square, in which the dots in opposite corners flash on and off alternately (see diagram, or at ). This creates the illusion of two dots moving either horizontally or vertically, or perhaps rotating around the corners of the square. As with the Necker cube, if you stare at it for long enough the motion spontaneously flips from one interpretation to another.
A team led by Christopher Moore at the McGovern Institute for Brain Research at the Massachusetts Institute of Technology recently discovered that the same thing happens with touch. Using a grid of tactile stimulators, they delivered a tactile equivalent of the four flashing dots onto volunteers鈥� fingertips. Sure enough, the volunteers perceived the dots as moving either horizontally or vertically, and their perceptions spontaneously switched after a minute or so. In touch too, if the brain isn鈥檛 sure what it is sensing, it avoids committing you to one interpretation or another ().
