Virtual Reality Version of the Rod and Frame Test

The rod and frame test estimates subjective visual verticality misperception. Specifically, the test measures how visual perception of the orientation of a central bar (rod) is influenced by the orientation of a peripheral visual reference frame around it. Implementation of the test was conducted in our lab using Unity software and C# scripting. The participants sat upright wearing a virtual reality (VR) head-mounted display (HMD) (HTC VIVE, HTC; New Taipei City, Taiwan) and were told not to move or tilt their head during the test. The VR environment consisted of a white frame (occupying ∼16°× 16° of the visual field) rotated at a trial-specific orientation and a white rod (11° long) inside the frame, its center coinciding with the frame’s center, but with its own independent orientation. Both were presented on a black background (screen resolution was 1,920 × 1,080). A sequence of 28 trials was presented during which the frame was initially at one of seven possible random positions: 0/±10/±20/±30° (0 was vertical, ++ was clockwise). Each of these initial frame positions was presented four times (Bagust, 2005). In addition, the initial angle of the rod was randomized (sampled from 0 to 180° range distribution). The participants’ task was to orient the rod perpendicular (i.e., vertical) to the true horizon, regardless of the surrounding frame’s orientation. This was achieved by rotating the rod around its center in a clockwise or counterclockwise direction using the VR system’s remote control. Importantly, the surrounding frame was unchanged by this manipulation. Once the participants perceived it as being vertical, they responded by pressing a button on the remote control, which led to the clearing of the display and the beginning of another trial.