Trajectory of contact region on the fingerpad gives the illusion of haptic shape

When one explores a solid object with a fingertip, a contact region is usually defined. When the trajectory of this region on the fingerpad is artificially controlled so as to resemble the trajectory that is normally present while exploring a real object, the experience of shape is created. In order to generate appropriate local deformation trajectories, we built a servo-controlled mechanism that rolled a flat plate on the fingerpad during the manual exploration of virtual surfaces so that the plate was kept tangent to a virtual shape at the point of virtual contact. An experiment was then designed to test which mode of exploration maximized the shape information gain: active versus semi-active exploration, where semi-active exploration is when one hand touches passively and the other moves the target object, and the use of single versus multiple points of contact. We found that subjects were able to perform curvature discrimination at levels comparable to those achieved when using direct manual contact with real objects, and that the highly simplified stimulus provided by the device was a sufficient cue to give the illusion of touching three-dimensional surfaces.