In order for you to look at an object close to your face, your eyes must turn inward (converge) toward the object. When looking at far objects, they move the other direction or diverge. Convergence and divergence are unique as these are the only eye movements that are not conjugate (meaning the eyes move in the same direction) but are instead termed disconjugate eye movements.
Convergence is the ability to turn the two eyes inward toward each other to look at a close object. We depend on this visual skill for near-work activities such as desk work at school, working on a smartphone type device, or even in sports when catching a ball.
Convergence occurs by stimulation of the medial rectus muscle of both eyes while simultaneously relaxing the lateral recti. This action requires stimulus by the third cranial nerve (oculomotor nerve) and inhibition of the sixth cranial nerve (abducens nerve).
Convergence is more complex than just moving the eyes inwards. When the eyes converge, the focusing system is engaged, and the pupils get slightly smaller. This set of three processes - technically termed convergence, accommodation, and miosis - are known as the near triad.
Divergence is the opposite of convergence and is the ability to turn the two eyes outwards to look at a distant object. We depend on this skill for distance activities such as reading the board at school, driving and watching TV.
To diverge, the opposite of the near triad must occur. Now the eyes diverge, accommodation is inhibited, and the pupils slightly dilate.
Your optometrist may perform a simple eye test. This test is called NPC or Near Point Convergence. This test measures the distance from your eyes to where both eyes can focus without double vision.
A condition having trouble maintaining an image single at near is called Convergence Insufficiency. The inability to keep an image single at distance is referred to as Convergence Excess.
Convergence Insufficiency affects approximately 5% of children in the United States and may have a serious impact on an individual's performance in school, choice of jobs, and quality of life.
These conditions create an onset of symptoms such as seeing double at near or far while viewing an object. Most common symptoms include:
Barnyard Bounce is a game designed to treat convergence insufficiency. Patients adjust the horizontal position of a chicken visible only in the right eye to match an egg shown only in the left eye (video of the task here).
Example of two eyes converging on the chicken (shown only the right right eye) and the egg (shown only to the left eye). In the left, the patient has normal convergence and places the chicken in the same place in the world as the egg. In the right, the patient cannot converge far enough, and although they perceive the chicken to be lined up with the egg, they are separated in the world.
The American Optometric Association recommends seeing your eye care professional for routine eye examines. If the symptoms mentioned above are persistent, your eye care professional may suggest seeing a behavioral optometrist that specializes in the treatment of binocular disorders. A vision therapy program may be introduced that will include in-office and/or at-home therapy activities.