Felt hole size depends on force and on the pliability of the effector

When small holes are felt with the tongue they are perceived to be larger as compared to when felt with the index finger. It is an open question why the size perceived with the tongue differs from size perceived with the finger. We suggest that differences in perceived size are due to differences in the effector's deformation at the edge of the explored hole, which correlate with the effector's pliability. The more pliable the effector is, the more it will deform and the hole will be perceived to be larger. In two experiments we tested this hypothesis. In Experiment I we manipulated the force applied by the tongue, and thus the tongue's deformation. As predicted, the tongue perceived holes to be larger when higher forces were applied. Experiment II investigated how the toe perceives size as compared to the tongue and finger. The toe is less pliable than the finger and the finger is less pliable than the tongue. Also as predicted, holes at the toe were perceived to be smaller than holes at the finger and considerably smaller than holes at the tongue. The findings corroborate our pliability-deformation hypothesis of haptic size perception.