Tactile illusions

New Scientist put together a great collection of seven tactile illusions, and explanation of the neural processes that cause them. Optical illusions are a kick but it's even more fun when you get your whole body involved in the mindfuck. Here's one example:
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't hear the chalk squeaking across its surface, even though it is the same board and the same chalk.

This is an example of a "cross-modal interaction": what you feel is strongly affected by what you hear...

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 (Behavioural Brain Research, vol 196, p 145). One version discovered only recently is the parchment skin illusion, described by Veikko Jousmäki 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't have the technology to dampen different frequencies at home, try rubbing your hands together while wearing earplugs. They should feel smoother.
Tactile Illusions: Seven ways to fool your sense of touch


  1. Gah! Don’t make me even _think_ about the sound of chalk on a blackboard. Makes my hairs stand on end just thinking about those shrill squeaks.

  2. Is there a scientific name for the sensation you experience when a sound or feeling grates against you like fingernails on a chalkboard?

    I have experienced it extremely intensely recently with all kinds of things. At one point even the feeling of my teeth in my mouth sent me momentarily crazy. I am wondering if it is a sign of stress, and what the technical name is for it.

  3. Next time you’re walking up a steep hill, convince yourself that you are actually walking on a flat surface (adjust your line of vision and mental state to make it seem like the top of the hill is the horizon, or just lean back a little and make it seem like you are level)– a very weird feeling, like suddenly gravity has increased several degrees.

  4. Dave @3, I’ve always gotten that feeling from crunching in new-fallen snow or chomping down hard on a big piece of saltwater taffy. A whole-body shudder and instant transcendental revulsion.

    Odd how the nails-on-chalkboard is universal, but the others are idiosyncratic. I bet there’s figuring out why would make for interesting research. Has anyone done that before?

  5. Re: Church #2

    Synaesthesia is the pairing of sensory modalities that are not paired for most other people, e.g. sound and vision, sound and taste. These sensory illusions regard sensory modalities that are usually paired, such as chalk boards and squeaking. In this case these sensory modalities interact to inform us about the world as they have our entire lives (unless we are deficient in a sensory modality). When a perception occurs without its partner it sounds like our brains are a bit confused and therefore what we experience is different from normal.

    That’s a pretty basic explanation that could definitely be more eloquently put. Although not synaesthesic, these illusions sound similar. A person with synaesthesia may experience such illusions if one of their usual paired perceptions become dissociated. Perhaps it’s the closest that non-synaesthetes may get to experiencing an altered perceptual reality – apart from drugs of course.

  6. Nails on a chalkboard couldn’t bother me less, but focusing on the sound of a seatbelt being drawn gives me a similar sensation, as well as an itchy feeling in the nerves of my teeth. It was worse when I was younger, but it still happens if I focus on the noise.

  7. The April/May 2006 issue of Scientific American had a good article on this called “Illusions: Touching Illusions.” My favorite one was the chicken wire illusion. It doesn’t seem to work for everyone:

    Get some coarse chicken-cage mesh, preferably
    mounted in a wooden frame. Then hold the
    mesh between the palms of your hands. Nothing
    peculiar so far. Now start rubbing your palms
    against each other with the wire between them.
    Remarkably, your palms will feel like jelly or velvet.

  8. I attended magician Jay Alexander’s show with the San Francisco Simfonietta a few weeks ago, and one of his tricks was an amazing optical illusion.

    He talked about how the Maestro has a big head, then told the audience to look at this spiral-painted disc in his hand. You stared at it spinning for a count of 10, then looked at the Maestro’s head… and IT EXPANDED!! (or, more accurately, SEEMED to…)

    He then reversed the direction of the spinny disc thingie, told you to look at the Maestro’s head again, and his head shrank to a puny size!

    The effect was so pronounced that the entire audience gasped at first, then laughed out loud. Both times.

    It was an incredible illusion, and – unfortunately – I think the deeper meanings for society, perception, and the “wisdom” of groups, were lost within the context of a “magic show”.

  9. #8 That was the feeling I got after mowing the lawn. The constant vibration made everything feel softer.

  10. Here’s a tactile illusion from my childhood:

    Lay on your stomach, and have a friend lift you up off of the floor by your hands, about an inch or so, your arms stretched out straight above your head. After a minute have them slowly lower your arms, and tell them when you think your hands are just about to touch the floor. You will be wrong, every single time. Then have them lower your hands a little more, as you feel your hands “pass right into the floor”, until they actually touch, at which point everything will feel normal again in short order.

    It’s remarkably freaky :)

  11. Wierd… doesn’t even mention the hot/cold/warm water trick.

    Stick one hand in hot water, one in cold water and let them sit there fore a few minutes. Then stick them both, together, into room-temperature water.

  12. Gah! Don’t make me even _think_ about the sound of chalk on a blackboard. Makes my hairs stand on end just thinking about those shrill squeaks.


    You’d have hated me as your TA. I used to love silencing a class by running my nails through the blackboard (rather, greenboard).

  13. Other than burnt pot coffee, good coffee comes 90% from the aroma.
    I once smoked a pipe and the tobaccos that smelt the best usually burnt your tongue and weren’t very pleasing. On the other hand Prince Albert which smelled like a barn that badly needed to be aired out tasted pretty good.
    One summer out on the trail the only available water came from a sulfur spring. If you drank it while holding your nose it was fine.

  14. As far as I’m concerned, New Scientist has lost all credibility. Due to a reader complaint, they recently yanked a web article on how to spot a religious agenda in so-called science books that creationists like to peddle. Bravo.

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