About Stereopsis
Stereopsis is the perception of depth arising from differences between the images received by the two eyes. Because the eyes have slightly different locations on the face, each eye receives a little bit different information and perspective from the visual world. The difference in images between eyes is known as binocular disparity. During stereopsis, the eyes and brain are able to process the binocular disparity so that the two images appear as one single 3D image, not two separate images. When stereopsis is working properly, it contributes to a person’s ability to have depth perception and properly judge distances and motion.
Depth Perception
Depth perception is the ability of the visual system to see images in 3D and judge depth, even though images are projected onto the back of the eye (the retina) in a 2D format. While stereopsis is an important component of depth perception, it is not the only factor. Monocular vision, or vision through just one eye, also provides cues to depth. These cues include relative object size, overlap and motion parallax, which can contribute to depth perception even when stereopsis is reduced.
Importance of Stereopsis
Depth perception through stereopsis touches on many everyday tasks, such as, driving a car, threading a needle, or hitting a fast-moving object like in tennis or baseball. The ability to perceive depth varies greatly across the population. A number of tests have been developed to test these variances, including to determine who has defective stereopsis and who has high-quality depth perception. One commonly used clinical test is the Stereo Fly.
Stereopsis can be relevant to professional tasks involving fine depth judgments, including some surgical procedures. Occupational vision requirements vary by role and applicable standards; a stereopsis test alone does not establish overall fitness for a particular job.
Stereopsis Testing
Stereopsis testing assesses whether a person can detect binocular depth and, where the test permits, measures the smallest disparity they can distinguish. Many tests use polarized or red-green glasses to present different images to each eye. Random-dot tests use patterns in which a target becomes visible through binocular disparity. Contour tests use recognizable shapes, such as circles, presented at different disparities to assess stereoacuity. The Stereo Fly is a coarse stereopsis target that may appear to stand out from the page. Its result should be interpreted alongside the test instructions and other clinical findings; a response to the fly alone does not establish the full extent of a patient’s stereoscopic ability.
How it's Developed
Stereopsis develops during infancy as the visual system learns to use information from both eyes. Conditions that disrupt clear, coordinated binocular input can affect its development, including a congenital cataract, substantial differences in refractive error between the eyes, or strabismus (eye misalignment). These conditions can also be associated with amblyopia, which is reduced vision resulting from abnormal visual development. Amblyopia and strabismus are related but distinct conditions; amblyopia is not simply a problem with the eye muscles.
