Stereopsis occurs when the 2 retinal images of an object in front of or behind the plane of fixation—which have small disparities due to the horizontal separation of the eyes—are cortically integrated, resulting in a perception of relative depth. Both contour stereopsis and random-dot stereopsistests present horizontally displaced copies of the same stimulus to each eye separately (usually by having the patient wear polarized or red-green glasses). Contour stereopsis tests present horizontally displaced figures (one to each eye) that are recognizable to each eye individually. For contour stereoscopic figures with larger disparities, monocular cues in the form of decentration of the image seen by 1 eye are present, which could enable some patients to falsely pass. Random-dot stereopsis tests avoid such artifacts by embedding the stereoscopic figure in a background of similarly random dots; the dots in the area of the figure but not those in the background are shifted between the eyes, such that there is a stereoscopic percept, but neither eye alone can perceive the figure.
In the Titmus test, contour stereopsis is tested at near using polarized glasses. The ability to detect elevation of the fly’s wings above the plane of the card indicates gross stereopsis (3000 seconds of arc). Finer levels of stereoacuity can also be demonstrated using stereoscopic images employing less horizontal disparity; at each level, the patient must identify the one stereoscopically presented figure out of a group of otherwise similar figures.
Clinically useful random-dot near stereopsis tests include the Randot test, which requires polarized glasses and measures stereoacuity down to 20 seconds of arc; the TNO test, which requires red-green glasses and measures stereoacuity down to 15 seconds of arc; the Random-Dot E test, a forced-choice test also requiring polarized glasses and employed mainly in pediatric vision-screening programs; and the Lang stereopsis tests, which do not require glasses to produce a random-dot stereoscopic effect and therefore may be useful in children who are not willing to put on glasses for testing.
Stereopsis can also be measured at distance using a chart projector with a vectographic slide, the Smart System PC-Plus (M&S Technologies, Inc.; Niles, IL), or the Frisby Davis Distance Stereotest (Stereotest Ltd, Sheffield, United Kingdom). Distance stereoacuity tests may be helpful in monitoring control of intermittent exotropia.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.