Tactile distance anisotropy on the tongue.

A large literature has described illusions of tactile distance perception. Across many body parts, there is an anisotropic bias for tactile distances to be perceived as larger when oriented across body part width than when oriented along body part length. This study investigated whether there is a similar bias on the tongue. A forced-choice judgment task was used in which participants judged which of two tactile distances felt larger either on the tongue or on the hand dorsum, a region for which anisotropy is well established. Anisotropy was measured using the method of constant stimuli. Clear anisotropy was found on both body parts, with distances oriented with body part width overestimated compared to those oriented with body part length. These results provide further evidence that tactile distance anisotropy is widespread across the body.