Stereopsi

Stereopsi is the perception of depth and three-dimensional structure produced by the brain's interpretation of binocular disparity—the slight differences between the images projected onto the two retinas by the eyes' horizontal separation. When the brain fuses these two views, it computes relative disparities to infer distance, enabling a 3D representation of the world. Stereopsi relies on precise alignment of the eyes and intact binocular interaction, and can be influenced by luminance, contrast, and motion.

Developmentally, stereopsi emerges during infancy as binocular vision becomes coordinated; most people develop functional stereopsi in the first few years of life. The neural basis includes disparity-tuned neurons in the primary visual cortex (V1) and higher-order areas in the dorsal and ventral streams (e.g., V3A, MT, and surrounding areas) that extract depth from disparity and integrate it with other cues.

Measurement and clinical relevance: Stereoacuity is the smallest detectable binocular disparity, often tested with random-dot stereograms or specialized tests (e.g., Titmus, Randot). In healthy adults, stereoacuity can be around 20 to 40 arc seconds on standard tests; values degrade with age or visual impairment.

Disorders: Stereopsis can be reduced or absent in conditions such as strabismus (misaligned eyes) and amblyopia; this is termed stereoblindness when complete. Other conditions affecting the retina or optic pathways can impair depth perception. Treatments include correcting eye alignment, patching or penalization therapy for amblyopia, vision therapy, and, in some cases, surgery. People without stereopsi can still use monocular depth cues such as perspective, shading, motion, and occlusion.

History and applications: The phenomenon was demonstrated in the 19th century by Charles Wheatstone using the stereoscope. Today, stereopsi is central to three-dimensional displays in cinema and virtual reality, and is a fundamental consideration in ophthalmology and optometry.