Grasping Weber's illusion: the effect of receptor density differences on grasping and matching

Weber found that distances between tactile stimuli on a high-receptor-density area are perceived as being larger than identical distances on a low-receptor-density area (Weber's illusion). Previous studies of visual illusions suggest that illusion effects vary with the type of response given. Here we tested a modified version of Weber's illusion in which a solid object was placed on the forearm or hand. Blindfolded participants were required either to give a size estimation or to grasp the object. The results showed that size estimation of solid objects was consistent with Weber's illusion, whereas grasping responses showed an opposite pattern (e.g., larger hand opening for objects on the forearm). A second experiment showed that this pattern is not due to biomechanical differences induced by the difference in spatial position of the target objects on the hand and arm. We suggest that the larger grip aperture when grasping objects on the arm were due to an increase in safety margin as a response to greater uncertainty about the object dimensions due to reduced receptor density.