Missing Depth Cues in Virtual Reality Decrease Performance of Three-Dimensional Reaching Movements

Abstract

Goal-directed reaching movements in three-dimensions are important for our interaction with the environment. Instrumented setups displaying virtual targets for rehabilitation training of reaching movements often provide limited depth cues, which may affect movements. This work aims to quantify effects of limited depth cues on reaching movements. We developed a virtual environment for assessing three-dimensional reaching movements that allows different depth cues to be enabled or disabled. By imposing a fixed spatial tolerance around targets for speed-accuracy trade-off, completion time normalized with the straight-line distance to the target was used to measure reaching performance. In the present study, 8 (control) subjects using a typical monitor setup applied in rehabilitation were compared to 7 subjects using a head-mounted display and receiving additional depth cues, namely, hard-referenced objects of known size, motion parallax due to tracked head-movements, and stereopsis. Control subjects required on average 9.88 s/m straight-line distance, while subjects using the head-mounted display required only 3.15 s/m straight-line distance. Additionally, movement trajectories of control subjects showed a different pattern, indicating a lack of reliable depth information. Thus, state-of-the-art rehabilitation setups are challenging already for healthy subjects. This challenge can be reduced by improving the provided visual depth cues.