From line to dots: an improved computerised rod and frame system for testing subjective visual vertical and horizontal

Perception of subjective visual vertical (SVV) and horizontal (SVH) has traditionally been measured by rotating a mechanical rod either with or without a frame present. The computerised rod and frame (CRAF) system has previously only been used to measure SVV. We have expanded the use of this system by testing its feasibility to measure SVH. This was done by comparing two groups of subjects (n = 103) randomly assigned to be tested for SVV or SVH.

Preliminary results showed a higher than expected percentage of individuals with SVH errors < 0.5 degrees . This was attributed to additional visual cues provided by the changing appearance of the rod as it approached the horizontal. A solution to this problem was sought by replacing the rod by two dots to mark its ends. In a second investigation 30 subjects were tested using both the "rod as line" and "rod as dots" presentation. Bland and Altman analysis showed no difference between the rod and dots presentations in the measurement of SVV, but confirmed a fixed error of -0.93 degrees between rods and dots for SVH. Changing the rod from a line to dots in the computer system resulted in errors for both SVV and SVH that were comparable to previous studies using manual systems.

The computerized rod and frame system may be improved by replacement of the line with two dots. This reduces clues provided to the subject by the appearance of the rod on the screen.