Dictionary of terms relating to
eyesight and vision

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Pertaining to the sense of touch.


Slow, smooth tension. (See "Fusional Vergence")

Tonic Vergence

Convergence due to the basic tonicity (tension) of the extraocular muscles, which are responsible, in part, for the distance phoria. Deficient tonic vergence would result in exophoria and excessive tonic vergence results in esophoria. (See "Vergence")


Tonometry is a procedure that measures the pressure inside the eyes. The test is used to screen for glaucoma, a disease in which pressure inside the eyes increases to the point that it impairs vision and if left untreated, may cause blindness.


Red/green targets used with red/green glasses to develop eye teaming skills.

Tunnel Vision

A constriction of the visual field that is commonly caused by chronic glaucoma, retinal degeneration, a tumor, or a brain disorder that interferes with the fibers that connect the optic nerve to the brain. (Please note that visual stress, emotional stress, or emotional trauma can also cause a constriction of the visual field.) (See "Streff Syndrome".)