Tactile diplopia (diplesthesia) on the human fingers.

The third and fourth fingertips of five subjects were pressed against each other so as to produce a skin displacement. A single spherical stimulus was then applied simultaneously to the two fingertips in order to test perceptual experiences with different amounts of skin displacement. The results show that the probability of detecting one stimulus when a single stimulus was applied to the skin in the resting position was 0.90. This probability decreased with increments in skin displacement. At the maximum skin displacement tested the probability of detecting one stimulus when a single stimulus was applied to the two displaced fingertip surfaces was only 0.04: this means that the single stimulus was perceived to be double with a probability of 0.96. The occurrence of this doubling, similar to Aristotle's crossed-finger illusion, shows that a diplopia-like phenomenon is present in the somesthetic system. How this 'tactile diplopia' could represent an interesting approach to the study of tactile perception is discussed.