Tactile illusion

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).

Examples

• One of the best known passive tactile illusions is the cutaneous rabbit illusion, in which a sequence of taps at two separated skin locations results in the perception that intervening skin regions were also tapped.^[1]

• The tau effect is an illusion in which equally spaced taps to the skin are perceived as unequally spaced, depending on the timing between the taps.

• The kappa effect is a complementary illusion to the tau effect: taps separated by equal temporal intervals are perceived to be separated by unequal temporal interval, depending on the spatial intervals between the taps.

• If a person exposes their forearm and closes their eyes or turns their head in the opposite direction while a second person slowly traces a finger from the wrist upward to the crook of the elbow, many people are unable to say when the crease of their elbow is being touched.

• When eating, if a person holds food with one texture and another texture is presented to the mouth, many people perceive the perceived freshness and crispiness of the food to be between the two textures.^[2]

• If one hand is immersed in cold water and the other in hot for a minute or so, and then both hands are placed in lukewarm water, the lukewarm water will feel hot to the hand previously immersed in cold water, and cold to the hand previously immersed in hot water.

• An example of an active touch illusion is the contingent after-effect. When the thumb and forefinger are slid repeatedly along the edge of a wedge, a rectangular block then handled in the same manner will feel deformed.

• Moving the crossed index and middle finger along an edge evokes the perception of two parallel edges. Similarly, if a person crosses their index and middle finger and then rolls a marble between the tips of the fingers, two marbles are perceived.

• 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.

• When touching paradoxical objects,^[3] you can feel a hole when actually you are touching a bump.^[4] These "illusory" objects can be used to create tactile "virtual objects".^[5]

• 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.

References

Further reading

• Flanagan, J.R., Lederman, S.J. (2001). "Neurobiology: Feeling bumps and holes, News and Views" (PDF). Nature 412 (6845): 389–91. doi:10.1038/35086674. PMID 11473294. 
• Hayward V, Astley OR, Cruz-Hernandez M, Grant D, Robles-De-La-Torre G (2004). "Haptic interfaces and devices" (PDF). Sensor Review 24 (1): 16–29. doi:10.1108/02602280410515770. 
• Robles-De-La-Torre G. (2006). "The Importance of the Sense of Touch in Virtual and Real Environments" (PDF). IEEE Multimedia 13 (3, Special issue on Haptic User Interfaces for Multimedia Systems): 24–30. doi:10.1109/MMUL.2006.69.