Passive tactile spatiotemporal illusions
Several tactile illusions are caused by dynamic stimulus sequences that press against the stationary skin surface.
- One of the best known passive tactile spatiotemporal 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. The rabbit illusion, also called sensory saltation, occurs in vision and audition as well as in touch.
- The tau effect or perceptual length contraction is an illusion in which equally spaced taps to the skin are perceived as unequally spaced, depending on the timing between the taps. Specifically, a shorter temporal interval between two taps causes the illusion that the taps are closer together spatially. This illusion occurs also in vision and audition.
- The kappa effect or perceptual time dilation 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. Specifically, a longer spatial interval between taps causes the illusion that the taps are separated more in time. This illusion occurs also in vision and audition.
- 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.
- One of the least known passive tactile spatiotemporal illusions is the twisted lip illusion, in which a vertical edge touched to the two lips during a grimace is perceived to be tilted in the direction opposite to the skewed lips.
Tactile adaptation illusions
Many illusions in vision are caused by adaptation, the prolonged exposure to a previous stimulus. In such cases, the perception of a subsequent stimulus is altered. This phenomenon is sometimes referred to as a contingent after-effect. Similarly, adaptation can cause such illusions in the sense of touch.
- 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.
- If a person is lying on their stomach with arms stretched in front and another person raises their arms about 2 feet off the ground and holds them there for approximately one minute, with the person on the ground having their 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.
- Focal adaptation evoked by prolonged stimulation to a skin area causes the illusion that two subsequently presented stimulus points straddling that area are farther apart than they actually are. This perceptual repulsion illusion is analogous to various visual repulsion illusions such as visual tilt effects.