Exploratory movements in unconstrained tactile search with virtual surfaces

The objective of this study was to determine how humans use tactile information to guide exploratory movement of the hand. A virtual environment made it possible to adjust the tactile input parameters online, based on subject behavior, something which is impossible with conventional techniques. We employed a haptic interface which applied distributed differential traction to the skin of the fingertip and investigated the effect of reducing tactile contrast on the ability to locate a virtually rendered ridge on a textured background. The difficulty of the task increased as the stimulus amplitude used to render the ridge decreased and the stimulus amplitude used to render the textured background increased. Difficulty depended more strongly on the amplitude of the textured background than on the amplitude of the ridge. The time taken to locate the ridge increased with difficulty. Most frequently, subjects crossed the ridge either once or twice before definitively establishing its location. The ability to locate the ridge did not appear to be sensitive to the velocity at which the ridge was crossed unless the difficulty was high. Our results suggest that haptic interfaces based on tactile stimulation to guide exploration could be used for both diagnostics and rehabilitation of sensorimotor hand function.