The Freiburg Stereoacuity Test: automatic measurement of stereo threshold

Background: There is a need for a stereotest with the following properties: (1) Natural viewing conditions, i.e. stimulus contours visible for each eye alone, but no or hardly any cue for monocular detection, and (2) suitability for threshold determination over a wide range of disparities. To comply with these requirements, we developed the Freiburg Stereoacuity Test. Method: The stimulus configuration is shown on a visual display unit (VDU) using phase-difference haploscopy with ferromagnetic liquid crystal shutters. The stereo target consists of a vertical bar that can be presented "in front of" or "behind" a frame. The sizes of the bar and the frame are kept constant relative to the stereo disparity. Anti-aliasing allows for disparities finer than the pixel raster. To mask monocular cues the bar is displaced randomly to the right or left. The stereo threshold was determined in two observers with normal eyes, using first the method of constant stimuli and then the best PEST. Both procedures were repeated with observers wearing scatter transparencies that reduced their visual acuity to about 1/10. In addition, the two observers with insight into the test design and two strabismic patients performed the best PEST procedure with one eye only. Results: With constant stimuli both observers achieved a stereoacuity of 2.6 arcsec and 3.1 arcsec, respectively, taking a hit rate of 75% as the threshold. The best PEST revealed a stereoacuity of 2.5 arcsec and 3.0 arcsec, respectively. The scatter transparencies raised the threshold to 261 and 257, respectively. With one eye only, the two observers with insight into the test design exploited the subtle position cue and reached a coarse pseudostereopsis. The two strabismic patients did not utilise the position cue. Conclusion: The Freiburg Stereoacuity Test allows determination of stereoacuity over a wide range of disparities (1-1000 arcsec). Although the stimuli can be seen with each eye alone, monocular depth cues are sufficiently masked. The Freiburg Stereoacuity Test is available at http://www.ukl.uni-frei-burg.de/aug/bach/fst/.