Salient features in 3-D haptic shape perception

Shape is an important cue for recognising an object by touch. Several features like edges, curvature, surface area and aspect ratio are associated with three-dimensional shape. To investigate saliency of three-dimensional shape features we developed a haptic search task. The target and distractor items consisted of shapes (cube, sphere, tetrahedron, cylinder, ellipsoid) which differed in several of these features. Exploratory movements were left as unconstrained as possible. Our results show that this type of haptic search task can be performed very efficiently (25 ms/item) and that edges and vertices were the most salient features. Furthermore, very salient local features, like edges, can also be perceived through enclosure, an exploratory procedure usually associated with global shape. Since subjects had to answer as fast as possible, this suggests that speed may be a factor in selecting the appropriate exploratory procedure.