Contact Surface Curvature Length Specifies Graspability

Although prehensile activity is part of everyday life, it is still not clear what is the information that specifies graspability. In Experiment 1 we manipulated size and shape and asked observers to perceive if objects are graspable. The ratio of object diameter and handspan was proposed as the invariant, however, the results showed that it did not map onto perception in a 1:1 fashion. We recomputed the ratio using surface curvature length of the object instead of the diameter as the relevant object property. The thinking was that the hand forms an arc when making contact with the surface of the object, thus, the size of the area over which the hand makes contact with the surface of the object should be a better candidate variable. 25 3D-printed objects varying in size and shape (from cube to sphere) were shown to 22 participants. The results revealed that the ratio of contact surface curvature length and hand span map onto perceived graspability in a 1:1 fashion irrespective of shape. This result demonstrated that the new ratio is the invariant that specifies perceived graspability. In Experiment 2 we tested if the curvature/handspan ratio generalizes to nonsymmetric objects. Perception was not specified by the ratio, but accuracy (that incorporates actual actions) was. This result demonstrates the need to consider perception and action together when investigating the nature of information that specifies performance in affordance tasks.