The Impact of Visual Information on the Haptic Perception of Stiffness in Virtual Environments

Abstract To study the impact of visually presented spatial cues on the human perception of mechanical stiffness in virtual environments, a three degree of freedom, force-reflecting haptic interface, the Planar Grasper, was utilized. In a series of psychophysical experiments on the discrimination of stiffness of two virtual springs, subjects pressed the springs and felt the corresponding displacements and forces through their hands, in addition to seeing the deformation of the springs displayed graphically on a computer monitor. Unknown to the subjects, the relationship between the visually presented deformation of each spring and actual deformation was systematically varied between experimental trials. This relationship ranged from fully registered (visual deformation was equal to the actual deformation of each spring) to completely interchanged (visual deformation of the softer spring was equal to the deformation of the harder spring for that force and vice versa). The results demonstrated a clear visual dominance over the kinesthetic sense of hand position. The subjects essentially ignored all kinesthetic hand position information regarding spring deformation, and based their judgment on the relationship between the visual position information and the indentation force sensed factually. This caused an increasing misperception of stiffness with increasing mismatch between the visual and haptic position information, culminating in totally erroneous judgments when the two were interchanged. These results indicate such haptic illusions can be exploited to overcome some of the limitations of haptic interfaces and to enhance the range of haptic experience.