The role of item fixation in haptic search

Enclosing objects in the hand is a common and efficient way of haptic exploration. Recently, the importance of grasping for more realistic haptic perception of virtual objects has been recognised in haptic interface design. While several studies on haptic perception have addressed haptic exploration of a single object, perception of several objects grasped together in the hand has received almost no attention yet. In this study we focus on the importance of freedom to manipulate the objects in the hand for three-dimensional shape perception. Furthermore, we investigate differences in detection speed for different positions in the grasping hand. Subjects were asked to search for a cube among spheres or for a sphere among cubes. Response times were measured for different locations of target shape in the hand. Also, the way in which the items were fixed was varied from allowing small displacements and rotation of the shapes to rigidly fixed. There were only differences in search times between the different positions in the hand, when the centre item was difficult to access because of the surrounding items. Finally, we show that search was faster when the items were rigidly fixed than when displacement and rotation was possible. This shows that more exploratory freedom does not necessarily make search for a three-dimensional shape faster.