excerpts from an article on Vision, Learning and Nutrition
by Donald J. Getz, OD, FCOVD, FAAO
(This is technical but very interesting. I have broken it up a bit and added emphasis.)
Briefly -convergence problems and tracking problems seem often to be associated and either or both can cause a child to read reading difficult, exhausting and unsatisfying.
Adequate Convergence: During the act of reading, the demand is for the two eyes to turn inward so that they are aimed at the reading task. If the eyes have a tendency to deviate outward, the child must use excess effort and energy to maintain fixation on the reading task.
Most studies have shown that the greater the amount of effort involved in reading, the lower will be the comprehension and the lower will be the performance.
When reading, the eyes do not move smoothly over a line of print. Rather, they make a series of fixations looking from word to word. When an exophoria exists, each time fixation is broken and moved to the next word, the eyes will tend to deviate outwards and they must be brought back in to regain fixation.
Human nature being what it is, the child generally has an avoidance reaction to the reading task. This is compounded by the fact that anything the child doesn’t do well, he would rather not do.
This is the child who looks out the window rather than paying visual attention. He is commonly given labels. He is often accused of having a short attention span and not trying. He is told that he would do better if he tried harder, but he has tried harder to no avail.
He is often labeled as having dyslexia, minimal brain dysfunction, learning disability, etc. Commonly, he loses his place while reading and/or uses his finger or a marker to maintain his place. While making the eye movements during the act of reading, he might not land on the next word, but rather land a few words further on. Consequently, he commonly omits small words or confuses small words. Often, he just adds a word or two to make the sentence make sense.
If the two eyes are pointing at the same point in space, a person will see the fixated object as being single. Double vision or overlapping vision results if the two eyes are not exactly pointing at the same point.
Don’t expect a child to tell you that his vision isn’t clear. He has no yardstick of comparison to inform him that his vision differs from the vision of anyone else.