Read about all the tactile illusions in our special feature
If you happen to have a chalkboard and some earplugs handy, try this out. Write something on the board, then rub it out and write it again wearing earplugs (or, better still, noise-cancelling headphones). The board will feel much smoother when you can鈥檛 hear the chalk squeaking across its surface, even though it is the same board and the same chalk.
This is an example of a 鈥渃ross-modal interaction鈥�: what you feel is strongly affected by what you hear. Another way of demonstrating this is to tap somebody鈥檚 skin once while at the same time playing two or three closely spaced electronic beeps. They will feel two or three taps.
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Other sensory domains also interact to create illusions. One of the best known is the McGurk effect, where listening to a string of identical syllables such as 鈥渂a ba ba ba鈥� while watching someone mouth 鈥渂a da la va鈥� makes you hear sounds that are not there (). Visual-auditory interaction works the other way too. If you show somebody a single flash on a computer screen accompanied by two beeps, they will usually report seeing two flashes (see ).
In recent years, psychologists have discovered that the cross-modal interaction is particularly powerful between hearing and touch, perhaps because both senses perceive mechanical energy (). One version discovered only recently is the parchment skin illusion, described by Veikko Jousm盲ki of Helsinki University of Technology in Finland.
He rigged up a microphone and got volunteers to rub the palms of their hands together next to it while feeding the sound of their rubbing into their ears via headphones. If he accentuated the high frequencies, people reported that their hands felt smooth and dry, like parchment. Damping down the high frequencies led to them feeling rougher and more moist. If you don鈥檛 have the technology to dampen different frequencies at home, try rubbing your hands together while wearing earplugs. They should feel smoother ().
Something very similar was reported in 2005, when Charles Spence of the University of Oxford found that the perceived crispness of potato chips depends on how they sound when you bite them. Damp down the sound of crunching, or muffle the higher frequencies, and people report that the chips feel stale. You can experience this illusion with a bag of potato chips and some earplugs ().
Spence also found that the same rule applies to sparkling water and electric toothbrushes. Place a microphone in front of a glass of sparkling water, then pump up the volume or accentuate the high frequencies, and the water feels fizzier on your tongue. Electric toothbrushes can be made to feel smoother by damping down high-frequency sounds ().
