When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate towards each other (convergence), while for an object farther away they rotate away from each other (divergence). Exaggerated convergence is called cross eyed viewing (focusing on the nose for example) . When looking into the distance, the eyes diverge until parallel, effectively fixating the same point at infinity (or very far away).
Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation, sometimes known as the accommodation-convergence reflex.
As opposed to the 500°/s velocity of saccade movements, vergence movements are far slower, around 25°/s. The extraocular muscles may have two types of fiber each with its own nerve supply, hence a dual mechanism.
In ophthalmology, convergence is the simultaneous inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision when viewing an object. This is the only eye movement that is not conjugate, but instead adducts the eye. Convergence is one of three processes an eye does to properly focus an image on the retina. In each eye, the visual axis will point towards the object of interest in order to focus it on the fovea. This action is mediated by the medial rectus muscle, which is innervated by Cranial nerve III. It is a type of vergence eye movement and is done by extrinsic muscles. Diplopia, commonly referred to as double vision, can result if one of the eye's extrinsic muscles are weaker than the other. This results because the object being seen gets projected to different parts of the eye's retina, causing the brain to see two images.
Near point of convergence (NPC) is measured by bringing an object to the nose and observing when the patient sees double, or one eye deviates out. Normal NPC values are up to 10 cm. Any NPC value greater than 10 cm is remote, and usually due to high exophoria at near.
In ophthalmology, divergence is the simultaneous outward movement of both eyes away from each other, usually in an effort to maintain single binocular vision when viewing an object. It is a type of vergence eye movement.
- Basic exophoria
- Convergence insufficiency
- Convergence micropsia
- Divergence excess
- Basic esophoria
- Convergence excess
- Divergence insufficiency
- Fusional vergence dysfunction
- Cassin, B (1990). Dictionary of Eye Terminology. Solomon S. Gainesville, Fl: Triad Publishing Company. ISBN 0-937404-68-3.
- Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company, 1990.
- Reeves & Swenson, "Disorders of the Nervous System" Dartmouth Medical School http://www.dartmouth.edu/~dons/part_1/chapter_4.html
- Saladin, "Anatomy and Physiology The Unity of Form and Function, 6th Ed. McGraw-Hill"
- "FOR PARENTS, STUDENTS, FARSIGHTED CHILDREN: What is Convergence Insufficiency Disorder? Eyestrain with reading or close work, blurred vision, blurry eyesight, exophoria, double vision, problems with near vision or seeing up close, headaches, exophoric". Convergenceinsufficiency.org. Retrieved 2012-03-05.
- American Optometric Association. Optometric Clinical Practice Guideline: Care of the Patient with Accommodative and Vergence Dysfunction. 1998.
- Duane A. "A new classification of the motor anomalies of the eyes based upon physiological principles, together with their symptoms, diagnosis and treatment." Ann Ophthalmol. Otolaryngol. 5:969.1869;6:94 and 247.1867.