This page and the next two explain convergence insufficiency and alternating suppression, two visual problems that your child might be experiencing if he's having reading problems.
Convergence insufficiency is an inability to focus both eyes simultaneously on a single point, such as a letter or word, for a sustained period of time. It is a common vision disorder often found in children struggling with reading. The inability to sustain the effort is the reason that symptoms of convergence insufficiency are often missed in routine eye examinations. Many struggling readers can converge their eyes to focus on a single point long enough to pass the test in a routine eye examination but can't do so for the sustained period typical when reading. This is why symptoms often show up on the second or third page of a story when a child reading aloud begins to falter, becomes hesitant and increasingly inaccurate, and shows other signs of visual stress like rubbing his eyes or even using his hand to cover one eye.
Convergence Insufficiency Indicates a Poorly-Developed Visual Skill
The ability to converge your eyes is a skill that develops in childhood. A baby can’t do it, but by age 2 or 3, most children can. Divergence is the opposite of convergence. When your eyes diverge you end up focusing at a distance. During convergence your eye muscles move your eyes closer together so that you can focus on a near point. Obviously, reading requires development of this skill, since most reading is done at a relatively close distance.
Children experiencing convergence insufficiency have failed to adequately develop the convergence skill. If they can’t efficiently converge to a near point, they could experience considerable visual stress. If they can converge to a near point, but can’t sustain the effort, they might start out reading well, then gradually start falling apart, making more and more mistakes until they get a chance to take a short break. This is why it's referred to as convergence insufficiency rather than convergence inability.
Developmental optometrists are trained to assess convergence problems and recommend treatment. Sometimes the treatment can be as simple as a set of glasses with an appropriate prism in the lenses, but in my experience the treatment usually ends up being vision therapy.
Normal Development of Convergence Skill
Some of the literature on convergence insufficiency indicates that convergence skills are developed during the crawling phase of childhood when a child is required to focus on his hands to see where he is placing them. When he looks up to see where he's going, his eyes diverge; when he looks down to see where he's placing his hand next, he is training his eye muscles to converge both eyes on his hand. As the process unfolds, the child is essentially practicing his eye convergence skill each time he raises and then lowers his head.
Similarly, the old “monkey bars” that young children played on built convergence skill because with poor convergence, depth perception is lacking and with poor depth perception, a child will miss the next bar he's reaching for. This is also true of the suspended horizontal ladder seen on many playgrounds today. Miss the next rung and you fall or dangle helplessly, so there’s an incentive to concentrate your visual efforts on a precise point.
But some children's visual skills fail to develop on schedule. If your child skipped crawling and went right to walking, he might have done so because he was so visually uncomfortable trying to look at the floor in front of him that he struggled mightily to get to his feet where the view was easier on the eyes (because he didn’t need to converge his eyes to see where he was placing his hands.)
The Brain’s Workaround to Fix Convergence Insufficiency
As I’ve said elsewhere on this site, vision problems usually run in families. And, due to the relative scarcity of developmental optometrists in the past, most adults who had vision problems like convergence insufficiency as children have never had them addressed, though they might eventually have overcome them one way or another.
Interestingly, if you tested those adults today you would probably find that many of them didn’t actually correct convergence insufficiency, but instead developed compensating mechanisms. One of those mechanisms is to use only one eye when reading, so that the brain isn’t confused by two conflicting images. I had a parent in my office one day whose child I was certain had an untreated vision problem and I was trying to demonstrate something to her. When she put on a pair of polarized glasses, instead of seeing what she was supposed to see, she said that one side of the target was completely black. Without going into detail, that is what would logically happen if she was only using one eye for near work.
However, she had both eyes open, so what was happening? Well, it turns out that the brain is inventive when presented with a problem like poorly developed convergence skill. As described on the next page, Alternating Suppression, vision therapists can use red/green glasses to spot a child (or an adult) who is suppressing the visual input from one eye. In the adult’s case, that suppression (not blindness, but suppression) is very likely permanent and affects the same eye consistently. However, in the case of a young child, vision therapists often observe a phenomenon called alternating suppression, which I believe is the result of the brain attempting to train normal convergence ability before taking the more extreme route of shutting down the input from one eye when doing near work.
Correcting Convergence Insufficiency
Because it is a skill, proper convergence ability can be developed. In fact, most children ordinarily do develop proper convergence skill. Those who don't, often the very children who struggle with reading, will usually benefit from vision therapy. If you think of vision therapy as physical therapy for the eyes, you won't be too far off, at least when it comes to correcting convergence insufficiency.
I should point out, though, that vision therapy doesn't actually teach a child to read. Instead it enables a child to view print more comfortably. However, if he's already learned to read earlier and during vision therapy puts that learning to good use, it could easily appear to the parent that vision therapy has taught him to read. Conversely, if he now can read comfortably, but has never learned how, the parents could end up being disappointed with the results of vision therapy. That is, the parents might not realize that their child still has to learn how to read before he can put his recently-developed visual skills to good use.
The next page discusses a Convergence Insufficiency Study that demonstrated the usefulness of vision therapy in correcting a quite common vision problem among struggling readers.