Abstract
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.
TopBackground
Testing stereopsis in young children is important for assisting in the identification and progression of conditions such as amblyopia, strabismus, and significant refractive error. In older children, it can be additive in detecting or monitoring other types of binocular vision disorders that can affect visual efficiency. Global stereopsis is a random dot stereogram (RDS) that is composed of computer-generated dots giving rise to stereoscopic forms if bifoveally fixated. RDS testing can quickly rule out most strabismus, including microtropias. There are no monocular cues to aid in detection. Local stereopsis on the other hand is a line stereogram using horizontal retinal disparity to elicit the perception of stereopsis. Local stereopsis can be detected through monocular cues, therefore making it less robust in detecting amblyopia and strabismus. It is important for the clinician to minimize the use of these cues by having the patient and stereo book remain still. Stereopsis norms increase with age as well as reliability in these tests (Pai, 2012; Tarczy-Hornoch, 2008). Table 1 outlines general norms for stereoacuity by age.
Table 1. Stereoacuity norms by age
| Age (years) | Norm (sec of arc) |
| 3-4 | 150 |
| 4-5 | 70 |
| 5-8 | 40 |
| 11+ | 20 |
(Schnell, 2020)
Clinical Pearl: Some children will need more time to perceive stereopsis targets (especially global); clinicians should be encouraging to garner the child’s interest in finding the hidden items.
Key Terms in this Chapter
Lang: A stereopsis test that uses kid friendly shapes and does not need to utilize stereo glasses.
Local: A type of stereopsis test that uses horizontal retinal image disparity to create the appearance of depth; monocular cues may be used to pass these tests.
Global: A type of stereopsis test using random dot stereograms which make it only achievable in a patient with bifoveal fusion and high-level cortical stereopsis.
Frisby: A stereopsis test using real depth due to transparent plates of varying thicknesses.
Titmus: A type of stereopsis test that is known for its large animal fly.
PASS: Preschool Assessment of Stereopsis with a Smile (PASS) is a type of stereopsis test that uses kid friendly smiley faces.
Randot: A registered trademark of Stereo Optical Company, Inc. This is a commonly used, shortened term for “Random Dot.”
Random Dot: Computer-generated dots giving rise to stereoscopic forms if bifoveally fixated.