Depth perception, also known as stereopsis, is the visual ability to perceive the world in three dimensions and accurately judge the distance between objects. This function occurs when the brain combines the slightly different images received by each eye into a single, cohesive three-dimensional view. When this ability is compromised, specialized testing is necessary to diagnose the lack of stereopsis and quantify the degree of impairment.
The Role of Binocular Vision in Depth Perception
The foundation of true depth perception lies in binocular vision, which is the coordinated use of both eyes to focus on a single point. Because the eyes are horizontally separated, each eye captures a slightly different perspective of the same scene. This minute difference in the retinal images is called binocular disparity, which the brain’s visual cortex processes to calculate distance and create the sensation of depth. While binocular input provides the highest quality depth information, people can also perceive distance using monocular cues, such as relative size, overlapping objects, and motion parallax.
Standard Clinical Tests for Stereopsis
Eye care professionals use specific tests to measure a person’s stereoacuity, which is the finest degree of depth that can be perceived, measured in seconds of arc. The Titmus Fly Test is a widely used screening tool, particularly for children, because it evaluates both gross and fine depth perception. It uses a large image of a house fly to test for gross stereopsis by asking the patient to “pinch” the fly’s wings. The test also includes graded series of animals and circles to measure finer stereoacuity, typically ranging from 800 down to 40 seconds of arc, using polarized glasses.
A more rigorous method is the Random Dot Stereotest, such as the Randot Test, which is considered a more sensitive measure of true stereopsis because it eliminates all monocular cues. These tests present two seemingly identical patterns of random dots, and the three-dimensional image is only visible when the two images are fused by the brain using polarized glasses. The Randot test often includes a graded circle portion that measures stereoacuity down to 20 seconds of arc, with forms invisible to an eye viewing the pattern alone. The patient is asked to identify which shape or circle appears to float forward from the background.
The Frisby Stereotest is a unique alternative that does not require the patient to wear any special glasses. Instead, it uses transparent plates of varying thicknesses, with a hidden target printed on one side and the surrounding random pattern on the other. The actual physical displacement of the target relative to the background creates a real-depth stimulus. The clinician measures stereoacuity by observing the distance at which the patient can reliably identify the target across the different plate thicknesses.
Understanding the Causes of Impaired Depth Perception
A failure on stereopsis tests indicates a problem with the visual system’s ability to fuse the images from both eyes. One common cause is strabismus, a condition where the eyes are misaligned (turning inward, outward, upward, or downward), which prevents the brain from accurately combining the two visual inputs. Another frequent cause is amblyopia, or “lazy eye,” where the brain suppresses the image from one eye to avoid double vision, thus preventing the necessary binocular input. Additionally, any condition that severely limits vision in one eye, such as a major eye injury or disease, results in monocularity and a lack of fine depth discrimination.