Stereoscopic vision, also known as depth perception, refers to the ability to perceive the three-dimensional shape of objects and the relative distances between them. Stereoscopic vision is based on binocular single vision. When external objects form images on corresponding points of the two retinas, these images are fused by the visual center in the occipital lobe of the brain into a single, complete, and three-dimensional perception. This function is referred to as binocular single vision. Binocular single vision is divided into three levels: Level I is simultaneous perception, Level II is fusion, and Level III is stereopsis. Methods for examining stereoscopic vision include the barrier reading method, the Worth 4-dot test, the synoptophore, random-dot stereograms, and the Bagolini striated glass.
The synoptophore is used to assess distance stereoscopic vision. It examines the three levels of binocular vision using specific image pairs:
Simultaneous Perception Slides
These are used to measure the subjective and objective angles of strabismus. The subjective angle of strabismus is measured when both eyes are simultaneously fixating, while the objective angle of strabismus is measured under monocular fixation. Normal retinal correspondence is present when the subjective angle equals the objective angle. A difference greater than 5° suggests abnormal retinal correspondence.
Fusion Slides
These consist of a pair of identical images, each containing a distinct feature as a control point. The individual aligns the two images so that the control points overlap. The two arms of the synoptophore are then moved inward (to test convergence) and outward (to test divergence) until the images no longer overlap or the control points are lost. The inward movement range corresponds to convergence, the outward movement range corresponds to divergence, and the sum of these two values represents the fusion range. Normal fusion ranges are as follows: convergence, 25°–30°; divergence, 4°–6°; vertical divergence, 2△–4△.
Stereoscopic Vision Slides
These consist of slightly differing images for each eye, which, when viewed through the synoptophore, create a perception of depth.
Random-Dot Stereogram
When prepared as synoptophore slides, random-dot stereograms can be used to examine distance stereoscopic vision. When produced as printed images, they can assess near stereoscopic vision. Commonly used tools include the Titmus stereogram and Yan Shaoming's stereoscopic vision chart. The Titmus stereogram uses polarized glasses, while Yan Shaoming's stereoscopic vision chart uses red-green glasses. Both methods provide quantitative measurements of stereoscopic vision. Normal stereoscopic acuity is ≤60 arc seconds.