2.2. Measurement of SVV: The Computerized Rod and Frame Test (CRFT)

We have utilized a computerized version of the rod and frame test (CRFT) to assess verticality perception. This test is a modified version of the RFT [50, 51], in which a virtual line marked by two white dots at its ends was used instead of a continuous line to reduce clues to verticality, which might be provided by the stepped appearance of a displayed solid line. There is no difference between the rod and dots presentations in the measurement of SVV [51]. The test was performed while sitting in a comfortable position with no head restraint; however, participants were instructed to keep their trunks and heads fixed and maintain their feet in a flat position. The virtual rod was viewed in a two-dimensional (2D) display through head mounted video eyeglasses (VUSIX iWEAR, VR920 Video Eyewear) (Figure 1(a)) restricting the field of vision and providing an image that spanned an angle of 30 × 23 degrees of the visual field (the equivalent of viewing a 1.42 m screen from a distance of 2 m). Where necessary, the video eyeglasses were used over spectacles.

Presentation on the computer screen of the three frame conditions of the computerized rod and frame test (CRFT). (a) No-frame (q, r combinations). (b) Frame^−18°(u, v combinations). (c) Frame^+18° (w, x combinations).

Participants rotated the dots around their virtual center in 0.5° increments in either clockwise (CW) or counterclockwise (CCW) directions using the mouse buttons until the “rod” was considered vertical. The space bar of the computer keyboard was then pressed to record the rod alignment relative to vertical and move the program to the next presentation. Recording of rod alignment tilt on the RFT test was conducted on 14 presentations in total. The first two presentations were for demonstration of the test and used to confirm that the participant understood the task. These measurements were not included in the analysis.

For the remaining 12 presentations, visual vertical was measured in three visual contexts (Figure 1) with four trials for each visual context: no visual reference\frame (dark SVV); the frame tilted counterclockwise (−18°, Frame⁻¹⁸) and tilted clockwise (+18°, Frame⁺¹⁸) with respect to the vertical. To eliminate possible tilt and learning effects, the order of display presentations was randomly selected by the computer at the beginning of each trial from a bank of four sequences for each frame condition. Participants were informed of the importance of spatial accuracy, and there was no time restriction for completing the task.