Double Vision

Double Vision

Keeping an image single is another factor in visual input.

This is controlled by the vergence system. Vergence is how you move your eyes in or out to look at objects at different distances. If your eyes are not coordinated together well and pointing in the same place, you can experience double vision. This can not only cause confusion, but could lead to misalignment of the eyes (phorias), noticeable eye turns in or out (strabismus), having your brain shut down an image (suppression) and can lead to your brain disregarding most of the information coming from one or both eyes (amblyopia).

Any of these conditions can cause symptoms such as eyestrain, fatigue, avoidance and headaches. If you notice a child having these symptoms, you should refer them to a behavioral optometrist for a comprehensive developmental vision exam.


You can actually watch someone use their vergence system by having a friend look over your shoulder at an object in the distance. If they do not have any eye turns (strabismus), both of their eyes should be looking straight ahead. Now ask your friend to look at the end of a pencil held approximately 1″-2″ from their nose. Did you notice how both of their eyes turned in? This cross-eyed appearance is called convergence. Now have them look back out at the distant object. Did you notice how their eyes went back out again? This outward movement of your eyes is called divergence. This ability can be measured by your eyecare provider.

Accommodation and Vergence Working Together

These two systems (along with your pupil that changes size to allow the proper light to come in and helps with depth of focus) ideally work together to keep images you are attending to clear and single.

You can think of accommodation as finding the distance at which to focus (defining the plane focus), while vergence defines the point of focus within the plane. The closer we want to focus and point our eyes, the more “power” or effort we need to do so.

It is important to have effortless control over these systems so when the going gets tough and you need to work a little harder, you can!  It is also important this process is automatic.

Accommodation and vergence should work hand in hand with liberal degrees of freedom between the two to help bring you clear and single images. If an individual exhibits extra effort to maintain single and clear vision, symptoms usually worsen over time. Unfortunately, some people (especially children!) can develop adaptation strategies to cope with these problems, like avoidance or amblyopia, so they may end up not having a variety of vision symptoms.

If these two systems are not working smoothly or efficiently together and one system is working too hard or not hard enough, the result is a visual system that fatigues and basically breaks down after short periods of visual stress. Usually this results in the brain fighting between either keeping the image clear, but double or keeping the image fuzzy and clear.

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