Influence of shape elements on performance during haptic rotation

In this work, we investigate how local shape elements of a grasped object affect performance of haptic rotation. Blindfolded participants were asked to grasp a rotary knob using thumb and index finger and to rotate it 90° counterclockwise around its own axis. The knobs exhibited a suitably distributed “grasp conform” combination of local shape elements (edged, flat or round). We tested all possible scenarios where both fingers had to grasp one of the three shape elements, resulting in a total of nine experimental conditions. Based on the rotation angle, determined with a novel apparatus named Twister, we evaluated the variable and signed errors. The results imply that a round rotary knob is the hardest to rotate by a correct goal angle, while other local shape elements can benefit the performance. Independent of the experimental condition, we found a bias towards rotating too far.