IMAGINE you are lying in the bath with your toes poking out of the water. A drip starts to form on the tap; you watch as it grows, then drops onto your big toe. Ooh! Not pleasant 鈥� but was the drip boiling hot or icy cold? It鈥檚 impossible to tell.
What you just experienced was a tactile illusion 鈥� something psychologists are increasingly interested in. For at least 200 years, they have used visual and auditory illusions to uncover the inner workings of sensory perception. Now it is the turn of touch.
鈥淭here鈥檚 a lot of interest in tactile illusions,鈥� says Vincent Hayward, an electrical engineer and haptics researcher at the Pierre and Marie Curie University in Paris, France. 鈥淢any people think of perception as vision, but clearly perception is a lot more. You have a very diverse sensory machine.鈥�
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Some tactile illusions have been known for a long time 鈥� one is named after Aristotle 鈥� but in general they have been harder to discover and demonstrate than visual illusions. 鈥淥ptical effects are easily probed. You can study vision with a piece of paper or make fundamental discoveries using a slide projector,鈥� says Hayward. 鈥淭actile effects are not so easy.鈥�
Now that researchers have started to develop new ways to probe the sense of touch, however, tactile illusions are enjoying a golden age. 鈥淭here has been a surge in the past few years as it has become easier to manipulate and present stimuli,鈥� says Charles Spence, an experimental psychologist at the University of Oxford. As a result, in recent years we have seen the discovery of numerous tactile illusions that are no less mind-boggling than their visual counterparts.
Another reason for the interest is the drive to add tactile or 鈥渉aptic鈥� interfaces to phones and other consumer devices. This has already been done to some extent. When you switch your phone to vibrate or play a video game with a rumbling controller, you are using haptic technology. The plan is to go beyond those elementary applications: for example, adding interfaces so that you can feel who is calling without taking your phone out of your pocket, or an MP3 player you can search by touch alone. There is also interest in designing interfaces that make it easy for blind people to access information.
To make haptics work you need to understand tactile illusions, says Hayward. For one thing, if you want to deliver information by touch, you need to understand the limitations of the system. There鈥檚 also the possibility that tactile illusions can be exploited to make haptics more effective. 鈥淭here鈥檚 a lot of interest in how much tactile information you can deliver via touch screens or vibrating interfaces,鈥� says Spence. 鈥淥ne way is to use tactile illusions.鈥�
Tactile illusions are often harder to experience that visual ones, but there are many that can be achieved with a little bit of care, perseverance and a few ordinary household items (). 鈥淚f you push perception into a corner it behaves in interesting ways,鈥� says Hayward. 鈥淭here are many more to discover.鈥�
