Haptic discrimination of virtual surface slope

We report the difference thresholds of the slope of a virtual surface rendered via a force–feedback haptic interface with the body frontal plane as a reference. The factors varied in experiments were the stiffness of a virtual plane, the lateral velocity with which the haptic probe scanned the plane, the length of a scanning interval, the movement direction of the probe to the body frontal plane (toward or away from the body), and lateral scanning direction (left-to-right or right-to-left). Measured slope thresholds ranged from 8.33° to 12.74° and were generally higher than or similar to previously published thresholds for haptic orientation or angle discrimination. The results suggested that haptic slope discriminability was independent of surface stiffness and lateral scanning velocity. Slope discrimination was largely affected by the lateral scan distance, indicating that the terminal difference of probe normal position can be an important sensory cue. In terms of scan direction, inward or rightward scans resulted in better slope discrimination than outward or leftward scans, respectively. These thresholds and findings have implications for haptics applications that involve geometric model modification or simplification of virtual objects while preserving their perceptual properties.