Tactile illusions are illusions that exploit the sense of touch. Some touch illusions require active touch (e.g., movement of the fingers or hands), whereas others can be evoked passively (e.g., with external stimuli that press against the skin).
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.
There's also the reverse Aristotle illusion: cross your fingers and touch the inside of a corner of a room or a box. This time, because the wall is contacting the insides of your fingertips, you should feel one surface, not two. Some people even experience three.
A similar effect can be achieved by holding your hands in front of you with palms down. Close your eyes and get somebody to lightly tap the back of both hands once, one after the other, with as short an interval as possible between the taps. Open your eyes and wave the hand that was tapped first. You'll get it right every time. Now do it again with crossed arms. If the taps are sufficiently close together - less than 300 milliseconds or so - you'll get it wrong a lot of the time.
This clearly has something to do with a failure to "remap" your body schema to take your crossed hands into account, but that can't be the whole story as single taps are easy to get right even with crossed hands. Neuroscientists think it happens because your brain is trying to do too many things at once: remap your body schema and also work out the order of the taps. The second task sometimes interferes with the remapping and causes it to fail (Nature Neuroscience, vol 4, p 759).
Amazingly, the illusion can also be made to work with sticks. Hold two wooden spoons out in front of you, one in each hand, with arms uncrossed, and get somebody to tap the ends of the spoons in quick succession. Again, you automatically know which stick was tapped first. But cross the spoons (not your arms) over and you'll get it wrong. Even more weirdly, if you cross your arms and the spoons, the two crossings-over cancel each other out and it again becomes obvious which one was tapped first (Journal of Neurophysiology, vol 93, p 2856).
Last year, Marc Egeth of the Children's Hospital of Philadelphia, Pennsylvania, reported a variant on this illusion. Stick your tongue out and turn it upside down (not everyone can do this), then run a finger along its top and bottom. It will feel as if your finger is touching a numb part of your tongue, while your tongue will register the touch on the opposite side from where it is being touched. Again, this is down to a failure to remap your body schema to take your unfamiliar tongue position into account (Perception, vol 37, p 1305).
Another simple tactile illusion relies on fooling your perception of distance. Take a paper clip, straighten it out, then bend it so that the tips are about a centimetre apart. Now close your eyes and run it from the tip of your index finger to your forearm via your palm and wrist. As you move the tips of the paper clip from an area of high acuity - your fingertip - to lower-acuity areas, it feels as if they are getting closer together, or even that only one end of the paper clip is touching your skin. This is because your forearms are less well set up for discerning fine structure than the tips of your fingers.
And have you ever felt that?:
If a person wears a baseball cap for a long period of time and then takes it off, it may still be felt.
If a person turns their tongue upside down, and runs their finger along the front, it will feel like the finger is moving in the opposite direction.
If a person pushes outwards with their hands against something for a while, then stops, it will feel as if there is something stopping the person's hands from closing together. Similarly, if a person pulls outwards with their arms, for example pulling their pants outwards, then stops, it will feel as if something is keeping their hands from staying at their sides.
If a person is lying on his/her stomach with arms stretched in front and another person raises his/her arms about 2 feet off the ground and holds them there for approximately one minute, with the person on the ground having his/her eyes closed and head hanging, then slowly lowers the arms to the ground, it will feel as if the arms are going below the level of the rest of the body.
After exercising on a treadmill or walking on a moving sidewalk for extended periods, a person will often feel 'pulled forward' when they step off onto stationary ground.
If two people join their opposite hands and one slides his index and thumb over two joined fingers he will feel the other finger like it was one of his.
If a person has been in the sea for a long time, they may afterwards still feel the ocean current pushing and pulling them.