New Scientist has a feature on "seven ways to fool your sense of touch": Tactile Illusions.
Some tactile illusions have been known for a long time - one is named after Aristotle - but in general they have been harder to discover and demonstrate than visual illusions. "Optical effects are easily probed. You can study vision with a piece of paper or make fundamental discoveries using a slide projector," says Hayward. "Tactile effects are not so easy."
Now that researchers have started to develop new ways to probe the sense of touch, however, tactile illusions are enjoying a golden age. "There has been a surge in the past few years as it has become easier to manipulate and present stimuli," 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.
One of the oldest tactile illusions is the Aristotle illusion. It is easy to perform. Cross your fingers, then touch a small spherical object such as a dried pea, and it feels like you are touching two peas. This also works if you touch your nose.
This is an example of what is called "perceptual disjunction". It arises because your brain has failed to take into account that you have crossed your fingers. Because the pea (or nose) touches the outside of both fingers at the same time - something that rarely happens - your brain interprets it as two separate objects.
A variation on the Aristotle illusion is to cross your fingers, close your eyes and then touch two different objects simultaneously - a piece of Blu Tack and a dried pea, say - one with each fingertip. You will need someone to guide your fingers onto the objects, and the illusion doesn't always work, but if you're lucky your sense of touch will tell you that the objects are the opposite way round from where they actually are. This is because your brain fails to correct for the fact that your fingers are crossed over." (Continued via Touch Usability, New Scientist) [Usability Resources]