Glossary

Vision Therapy Terms, in Plain Language

If a doctor or report uses a word you don't recognize, look it up here. We'll keep this growing as we add content.

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Accommodation: The ability of the eye to change focus from far to near and back again. Driven by the ciliary muscle inside the eye. Accommodative problems are one of the most common reasons children struggle with reading even when they "have 20/20 vision."
Related: Accommodative Insufficiency , Accommodative Infacility , BCC
Accommodative Infacility: Difficulty changing focus quickly between far and near. The child may complain that words go blurry when looking up from a book and back down, or that the board "takes a second" to clear when they look up.
Related: Accommodation , NRA / PRA
Accommodative Insufficiency: Reduced ability to focus the eyes at near. Symptoms include blur, eye strain, and headaches with reading. One of the most common conditions treated with vision therapy.
Amblyopia · Lazy eye: Reduced vision in one eye that is not correctable by glasses alone, due to abnormal visual development early in life. Often associated with strabismus or unequal refractive error.

BCC · Binocular Cross Cylinder: A clinical test that measures how much the eyes over- or under-focus at near while using both eyes together. Helps determine if a "near add" prescription is appropriate.
Binocular Vision: The ability of the two eyes to work together as a single coordinated system. Most reading and learning problems traceable to vision are binocular problems, not eye-health problems.

Convergence: Both eyes turning inward to look at a close object. The opposite of divergence. Most near-vision symptoms involve convergence in some way.
Convergence Excess · CE: Tendency for the eyes to over-converge at near. Often causes blur, suppression, or eye strain with reading.
Convergence Insufficiency · CI: The most common binocular vision problem. The eyes don't team well at near, causing headaches with reading, eye strain, double vision, avoidance of close work, and trouble focusing. Highly responsive to vision therapy.
Cover Test: A clinical test where the doctor alternately covers each eye and watches how the uncovered eye moves. Detects eye-alignment problems (phorias and tropias).

Diplopia · Double vision: Seeing two images of a single object. Can be horizontal, vertical, or oblique. A red-flag symptom that warrants a binocular vision evaluation.
Divergence: Both eyes turning outward, used to look at a distant object. The opposite of convergence.
Divergence Insufficiency · DI: The eyes have trouble diverging for distance viewing. May cause distance blur, intermittent double vision, or asthenopia.

EOMs · Extraocular Muscles / Motility: The six muscles attached to each eye that control eye movement. Tested during a comprehensive evaluation by checking pursuits (smooth following) and saccades (rapid eye jumps).
Esophoria / Esotropia: A tendency (esophoria) or constant turn (esotropia) of one or both eyes inward. May cause double vision, eye strain, or suppression. Esotropia in childhood is associated with amblyopia if untreated.
Exophoria / Exotropia: A tendency (exophoria) or constant turn (exotropia) of one or both eyes outward. Exophoria at near is one of the hallmarks of convergence insufficiency.

Fixation Disparity: A small misalignment of the eyes within the limits of fusion. Both eyes still see one image, but with effort and often symptoms.
Fusion: The brain's ability to combine the slightly different images from each eye into a single, three-dimensional perception.

NPC · Near Point of Convergence: The closest distance the eyes can converge before one drifts outward. Measured in centimeters from the nose. A 'normal' NPC is roughly 5–7 cm; >10 cm is considered receded and suggestive of convergence insufficiency.
Related: Convergence Insufficiency
NRA / PRA · Negative / Positive Relative Accommodation: Tests that measure how much the eyes can relax accommodation (NRA) or stimulate accommodation (PRA) while keeping fusion. Reduced ranges suggest accommodative-vergence imbalance.

Optometric Vision Therapy · OVT: A doctor-supervised program of in-office and home activities prescribed to develop or rehabilitate visual skills. Distinct from "eye exercises" sold online, which are not individualized or supervised.

Pediatric Optometrist: An optometrist with additional training in children's vision. Many are also residency-trained in vision therapy and developmental vision (Pediatric / Vision Therapy / FCOVD).
Phoria: A latent (hidden) tendency of the eyes to drift out of alignment when binocular vision is broken, typically only seen on testing. Measured in prism diopters (Δ). 'Exophoria' means the eye drifts out, 'esophoria' means it drifts in.
Prism Diopter · Δ: The unit used to measure eye-misalignment and the strength of prism lenses. A 1Δ prism deflects light 1 cm at 1 meter.
Pursuit: A smooth eye movement used to follow a moving object, like tracking a tennis ball or following text along a line.

Saccade: A rapid, ballistic eye movement used to jump from one point to another, like reading word-to-word across a line.
Stereopsis · Depth perception: The ability to perceive depth and three dimensions based on the slightly different views from each eye. Measured in seconds of arc; 40" is considered excellent stereopsis.
Strabismus · Eye turn: A condition where the eyes do not point in the same direction. May be constant or intermittent, and can occur in any direction (eso-, exo-, hyper-, hypo-).
Suppression: The brain ignoring the image from one eye to avoid double vision. Common in strabismus and amblyopia.

Vergence: The simultaneous movement of both eyes in opposite directions (toward or away from the nose) to maintain single binocular vision.
Vergence Range: A measurement of how much extra convergence (base-out, BO) or divergence (base-in, BI) the eyes can sustain before fusion breaks. Reduced ranges are characteristic of binocular vision disorders.
Visual-Motor Integration · VMI: The coordination of visual perception with fine-motor output, most commonly tested by copying shapes or writing on paper.
Visual Processing: How the brain interprets the information coming from the eyes, distinct from eyesight (acuity). Includes visual memory, visual closure, visual spatial skills, and visualization. Often part of why a child 'has 20/20 vision' but still struggles in school.

Worth 4 Dot: A red/green test using a flashlight with four illuminated dots and red-green glasses. Detects whether a patient is using both eyes (fusion), suppressing one eye, or seeing double.