Disturbed Fixation disparity  

Each eye receives an image. The two images are sent to the brain via the optic nerves. There, one image is made from those two images. We call this sensory fusion (cooperation between the eyes and the brain).
If those two images are shifted relative to each other because the eyes are not looking at exactly the same point or the images are not quite the same, the brain corrects this. That's called Fixation Disparity.


Three-dimensional and stereo vision

Fixation Disparity in itself is not an abnormality but part of the normal cooperation between the eyes. Fixation Disparation ensures that we see three dimensions (3D or stereo vision).


The eyes do not stand still in the eye sockets, but constantly move. For example, if you look from left to right, the eyes fluctuate a little. This means that the images also move. The brain then controls the eye muscles in such a way that the eyes can make small corrections so that the images fall into the right place in both eyes. We call this motor fusion (cooperation between the eyes themselves).


Development up to ten years

Shortly after birth, the eyes still have to develop. A baby cannot yet see well and a newborn's eyes are also not yet able to work together. The eyes and brain still have to learn this. These developments are completed around the age of ten.

The Fixation Disparity may not develop properly during this period. An impaired Fixation Disparity may occur. There is nothing that can be done to change this (as far as is currently known). Such a disruption can be determined by creating a Fixation Disparity curve.


After brain injury

After brain injury, the visual system may be disrupted.
For example, when problems arise with focusing (accommodation) and motor fusion (cooperation between the eyes themselves), it also becomes more difficult to bring and maintain the two images together. The sensory fusion (Fixation Disparation) works less well and therefore the images are corrected less well. If the images are too far apart, double images can occur.


Complaints may include: headache above the eyes that increases during the day. The complaints can also increase after working or reading on a computer. Complaints of blurred vision and inability to focus are also mentioned. Busy moving images (such as traffic situations) can also cause difficulty seeing, as can difficulty seeing at dusk. Balance and equilibrium problems may occur. Fatigue and difficulty reading are also known complaints.


Prism glasses or practice

Prism glasses move the image slightly so that the collaboration can correct better.
You can also choose to practice the cooperation between the eyes and fixation (visual training or orthoptic practice), so that the Fixation Disparity can be better corrected. Of course, a combination treatment is also possible.
If the Fixation Disparity is disturbed or abnormal, one should be careful with prisms. That is why expert advice is required from, for example, an orthoptist.



Our Dutch page, on which this page is based, was written by orthoptist Anne Marie of: Zicht en Zien practice. She is a member of the Dutch Association of Orthoptists.