Tactile size constancy mechanisms are different for objects and points on the skin surface

Several studies have shown the presence of large anisotropies for tactile distance perception across several parts of the body. Tactile distance between two touches on the dorsum of the hand is perceived as larger when they are oriented mediolaterally than proximodistally. This effect can be partially explained by a process of tactile size constancy. It is unknown whether the same kind of compensation is taking place also when objects size has to be estimated. We investigated whether tactile anisotropy that typically emerges when participants have to estimate the distance between two touches is also present when a continuous object touch the skin and participants have to estimate its size. In separate blocks, participants judged which of two tactile distances or objects on the dorsum of their hand felt larger. One stimulation was aligned with the proximodistal axis, the other with the mediolateral axis. Results showed that across distances were consistently perceived as larger than along ones, whereas the size of the objects was not. These results suggest that a processing of tactile size constancy is more effective when the tactile size of an object has to be estimated compared to when the distance between two touches has to be judged.