tenotomy

A surgical procedure used to weaken the superior oblique muscle by dividing its tendon between the trochlea and scleral insertion. Cutting the tendon reduces the muscle’s actions of depressing, intorting, and abducting the eye. It is commonly performed to treat superior oblique overaction and vertical strabismus.

tertiary position

A tertiary position is any oblique position of the eyes that involves a combination of both vertical and horizontal deviation from the primary straight-ahead gaze. Tertiary positions are achieved through combined actions of the rectus and oblique extraocular muscles.

third-degree fusion

Also known as stereopsis, third-degree fusion is the binocular visual perception of depth and three-dimensional space based on the ability to integrate the horizontal retinal disparity between the two eyes’ slightly different viewpoints. The brain processes these disparate retinal images to extract precise depth information, allowing for accurate judgment of relative distances and fine depth discrimination.

three-step test

The Parks-Bielschowsky three-step test is a diagnostic procedure used to isolate which cyclovertical extraocular muscle underacting in cases of vertical strabismus.  Visit the Eye Care Calculators page to use the three-step tool.

thyroid myopathy

A mechanically restrictive, incomitant deviation of the eyes associated with thyroid dysfunction. It results from inflammation and enlargement of the extraocular muscles, especially the inferior and medial rectus muscles, leading to limited eye movements and a misalignment that varies in different gaze positions.

tonic convergence

The baseline convergence maintained by the tonic contraction of the extraocular muscles, primarily the medial rectus muscles, which contributes to the distance phoria.

torsion

Also known as cyclodeviation, torsion refers to the rotation or wheel-like movement of the eye around its anteroposterior axis, which corresponds approximately to the line of sight when looking straight ahead. It allows the eyes to rotate in a clockwise or counterclockwise direction and plays a role in maintaining proper binocular alignment across different gaze positions and head tilts.

tranaglyphs

From transparent anaglyph, tranaglyphs are a variety of training and test targets consisting of transparencies with various red and green details or images. They are viewed while wearing anaglyph glasses with complementary red and green filters over each eye. This dissociates the images so the brain must combine them into a single stereoscopic 3D percept through sensory fusion.

trochlear nerve

Also known as cranial nerve IV, the trochlear nerve is a purely motor nerve that innervates the superior oblique muscle of the eye. The “Trochlea” refers to the pulley-like structure the superior oblique tendon passes through, giving the nerve its name.

Anatomically, it emerges from the dorsal aspect of the brainstem, decussates to the opposite side, and innervates the contralateral superior oblique muscle—no other cranial nerves innervate a muscle contralateral of their nucleus.

TV trainer

A vision therapy device consisting of a plastic filter attached to a television screen. The patient views the screen through red-green or polarized filters, dissociating the images seen by each eye. It is used to treat suppression and develop binocular vision skills by presenting separate targets to each eye.