V 527
Stereo acuity: Psychometric assessment of threshold
C. Schmitt, M. Ball, M. Bach, G. Kommerell
Background: Conventional methods for measuring stereo acuity suffer from one or more of the following shortcomings: The number of presentations at each disparity step is small, the threshold is not systematically approached, and stereo acuity cannot be strictly distinguished from position hyperacuity (e.g. "rings" in the Titmus-Test). To avoid these shortcomings, a computerised test has been developed.
Methods: A stereo configuration is displayed on a computer screen using time multiplexing (phase-difference haploscopy). The configuration consists of a vertical bar that can be presented "in front of" or "behind" a frame. The subject has to choose between these two possibilities by pressing one of two buttons. The stereo disparity is varied on a logarithmic scale, and the threshold is estimated by the "best Pest" algorithm, which determines the most likely disparity based on all previous responses. The size of the bar and the frame is kept at a constant factor relative to the stereo disparity presented at any instant. Anti-aliasing allows for fine disparities independent of the pixel raster. To avoid monocular cues the position of the bar is randomly displaced.
Result and conclusion: The procedure avoids the shortcomings of conventional stereo tests. It is largely independent of the examiner and therefore suitable for the evaluation of treatment methods that are thought to improve stereo acuity.
Abt. Neuroophthalmologie und Schielbehandlung, Universitäts-Augenklinik, Killianstr. 5, D-79106 Freiburg