Haptically perceived orientation of a planar surface is altered by tangential forces

We describe an experiment showing that addition of tangential forces can alter the haptically perceived orientation of a flat surface in a systematic fashion. Using a stylus mounted on a Phantom controlled by the GHOST software package, subjects performed single horizontal strokes along vertical surfaces at a number of orientations around the mid-sagittal plane. After each trial, subjects (who could not view their hand) judged whether the surface was rotated around the vertical axis in the clockwise or counter-clockwise direction from the sagittal plane. Psychometric curves for perceived surface orientation were measured in this way for various magnitudes of tangential forces along the stroke direction or in opposition to it. Psychometric curves shifted systematically with varying force magnitude, indicating that resistive forces tilted the perceived surface into the path of motion, while assistive forces tilted it away. More generally, the results show that perception of global surface orientation is not exclusively based on the location of the surface in space, but also on the forces encountered along the surface