In the article Eye Convergence and Reading Ability you will find an explanation of the visual skill called convergence and how it typically develops in young children. If it fails to develop, the brain eventually adapts in various ways. One of those adaptations that shows up when a child is learning to read is alternating suppression. What happens is that the child's brain starts alternately suppressing the vision of one eye, then the other, in an effort to develop convergence ability. As the article states, at some point the brain might give up and use just one eye for reading, and other near work as well, but first it will try alternating suppression.
Detecting Suppression
Let me describe a common method of demonstrating that someone is suppressing the input from one eye, either all the time, or intermittently as in the case with alternating suppression. First, a clear plastic overlay with transparent alternating red and green stripes is placed over a page of print.
The overlay also has a clear stripe between each red and green stripe. The person being tested then puts on a pair of glasses with red/green lenses. The glasses have one red lens and one green lens. The transparent sheet is laid with the stripes oriented vertically on a page of print and the glasses are worn when reading the page.
The test uses the fact that red and green filters used together will filter out all light that has to pass through both filters simultaneously. Given that fact, here’s what each eye sees in the above situation:
Right eye behind a red lens: Sees the print under the clear stripe and the red stripe. Both appear red, but the light coming through the clear stripe will appear brighter than the light having to go through both the red stripe and the red lens. But the print behind the green stripe will not be visible. The entire stripe will appear black because the red lens and green stripe acting together filter out all transmitted light.
Left eye behind a green lens: Sees the print under the clear stripe and the green stripe, again with the clear stripe somewhat brighter. In this case the print behind the red stripe will be blacked out, again because the combination of a green lens and a red stripe filters out all the transmitted light.
When you put both eyes together, assuming no suppression is going on, the person being tested can read all of the words on the page. The right eye can see words covered with the red stripes, the left eye cans see the ones covered by the green stripes, and both eyes can see the words covered by clear stripes.
Permanent Suppression is Possible
If the brain of an adult, or even an older child, finally gives up on the visual system ever developing appropriate convergence ability, sometimes it will adapt by consistently ignoring (suppressing) the input from one eye when reading. If that has happened, a person viewing print through the red/green arrangement described above will, depending on which eye the brain chose to suppress, not be able to see any of the words behind either the red or green stripe on the plastic overlay.
Let's say that the input from the right eye (behind the red lens) is being suppressed by the brain. Then the red stripes on the overlay will appear black because the brain is only accepting input from the left eye behind the green lens and the combination of a red stripe and a green lens cancel out all the light waves. Note that the right eye still sees; it's just that the brain has adapted to ignore the visual input from that eye during near work like reading. However, if the input from the left eye is manually suppressed, say with an eye patch, then the input from the right eye will again register and be processed by the brain. In effect, the brain downgrades visual input from that eye unless that's all it's getting. If that's the only visual input it's getting, it accepts and processes it normally. Granted, this sounds like a pretty radical adaptation, and it is. First, the brain uses alternating suppression
Alternating Suppression: The Brain's Attempt to Train Convergence Ability
It turns out that when you put these red/green glasses on children with convergence problems, you often find that their eyes are alternately suppressing. A vision therapist can tell this because the blacked out line slowly changes. The therapist will pause when the child can't see a word and in a short time he can finally see it and read it aloud. What this means is that for short periods of time, the brain is refusing input from one eye. Then, a bit later, it refuses input from the other eye. In between, input from both eyes is being received and processed.
Now let's return to normal reading without the red/green glasses and overlay. If a child has poorly developed convergence ability, then during the time that both eyes are functional the brain is receiving conflicting information because both eyes are not centered on the same spot on the page. In other words, the child sees double, especially if he’s been at it for a bit. Then, the brain shuts down one eye, say the left one, for a second or two. All of a sudden the page makes sense, because the brain is finally seeing a single image. Then the suppression stops and doubled vision reappears.
Next, the right eye suppresses and all is well again, maybe. Or maybe not, since the child might well have been following the lead of the right eye for the past several seconds, even during the period of doubled vision, and all of a sudden that eye has suppressed. But the left eye is focused two words over, or even one line higher or lower in rare cases.
Or if not two words over, maybe it’s just a letter or two over, in which case the world just became verery confusing. No, that’s not a typo. It’s what very would look like if your vision suddenly shifted back two letterers or even threhree. Do you see why learning phonics might be a bit of a challenge? And don’t forget, when the eyes switch back the other way, your child mit suddenly miss two letters or posly even three, or just jump a word or two.
I have personally observed a child reading whose eyes were not even tracking on the same line. Every few seconds, he would suddenly be reading a word one line lower or one line higher on the page. While this is a less-common phenomenon, it can occur if the child is undergoing alternating suppression of his vision and has very poor convergence ability. Incidentally, he wasn’t doing well in school and had become something of a behavior problem besides. Should this be surprising?
Alternating suppression is a real phenomenon. It seems reasonable to assume that this is a mechanism that the brain uses in an attempt to train a child’s convergence ability. It does so by continually demonstrating clarity (when one eye is suppressed) alternated with confusion and resultant discomfort (both eyes being used simultaneously) while keeping both eyes in the game for the time being (the alternating process.) In time, if a child occasionally achieves clarity with both eyes simultaneously, he will find that to be the more comfortable situation and subconsciously work toward achieving that situation more frequently. If all goes well, the brain have trained his ability to converge his eyes to a single near point consistently and accurately.
Compensating Behaviors
If, however, the brain fails in this endeavor, a compensating behavior can occur. The brain eventually gives up and picks one eye to do the near work. The result is that the child, by then possibly even near adulthood, becomes functionally blind in the other eye when reading. The vision is still there when examined, but when both eyes are open the brain is only accepting the input from one eye at near point. This is why some adults who continue to experience convergence problems can finally enjoy reading. They are using only one eye to read, usually with no idea they are doing so.
Incidentally, another obvious compensating mechanism is far less subtle, though usually still a subconscious event. If your child covers one eye consistently with his hand, or turns his head sideways to read so that his nose is blocking the print from one eye, then he’s figured out that he is more comfortable seeing print with only one eye at a time. He probably isn’t even aware that he’s doing it, although I do know one child who flat out stated that he takes his written tests with one eye closed because he does better that way.
Of course, neither the clumsy compensating techniques of the young child nor the subtle compensating done by the brain of a young adult are desirable outcomes. It is far more preferable to fix the issue by getting it properly diagnosed and treated. Along the way, you are likely to find that other higher-level visual skills also failed to develop appropriately and these can be addressed as well. And, of course, you will also hopefully find that other near efforts, such as practicing handwriting, will come more easily.
If you're seeing this sort of behavior in your child, consider the information in this article on Vision Therapy.
