What is PASS

Stereopsis develops very early in life and is thought to be present in a normally developing child by six months of age. In order to develop stereopsis, multiple components of visual development must be intact including visual acuity and bifoveal fixation. Stereopsis is the most sensitive way to assess sensory fusion but can be unreliable in very young age groups due to difficulty understanding the test or instructions. It is best to choose an option with global stereopsis (high level cortical stereo), as local stereopsis may overestimate ability due to available monocular cues. Global is created using random dot stereograms (RDS) – computer-generated patterns to create a stereoscopic form, while local contains line stereograms which create horizontal retinal image disparity giving the perception of depth. Stereopsis can be affected by strabismus, amblyopia, and other binocular vision dysfunctions that interfere with visual efficiency (especially in school-age children). The chapter discusses the most commonly used clinical tests of global and local stereopsis.