School achievement is very dependent on reading. No matter how far your child goes in the educational system, the importance of reading and the amount of reading increases every year.
- Reading requires both eyes to be directed at the same point on the page. An ability to turn both eyes a bit inward (called convergence) is needed, and the convergence movement should be easy to sustain.
- Eye movements are made by using six muscles that wrap around the eyeball. Every complete vision exam includes an evaluation of the proper function of these muscles.
- For reading, convergence is only part of the story. Because the muscles that move the eyes are seldom attached for perfect alignment, some effort is required to keep both eyes directed at the same point. If you keep both eyes open and block the view of one eye by holding your hand in front of it, the eye will drift to a position of rest. If you then move your hand to block the other eye, you will notice that whatever you are looking at seems to jump (usually sideways, but occasionally uphill or downhill). If you then move your hand back to block the first eye, you will see whatever you are looking at jump back in the opposite direction. The movement you see actually represents the corrective effort that you must make whenever you use both eyes together.
- Reading requires constant muscular effort. The muscles that aim the eyes work constantly so that the eyes team together for reading. To read a line of print the eyes must accurately converge at the beginning of the line, they next jump to a point further along the line, and then jump again to a further point along the line. These jumps are called saccadic movements. A good reader may make 3 or four jumps to get to the end of a line of print. A poor reader may have to stop at every word or perhaps even at every letter. The difference in effort that a good reader needs to complete a line of print compared to the effort a poor reader must make is huge.
- Vision shuts down when we change our view from one point to another! Just look at your own right eye in the mirror. Now look at your left eye. Did you see your eyes move? Was the movement too fast to follow? Ask someone else to do this experiment and watch their eyes. The movement is not lightning fast. It’s obvious that vision shuts down when you change your direction of gaze. Children who have problems keeping both eyes aimed at the same point, must put greater effort into making the necessary jumps across a line of print when reading if they must constantly re-aim their eyes between jumps.
- The other muscle effort needed for reading is supplied by the eye’s focusing muscle (the ciliary body) located behind the iris. Even if both eyes can be easily and consistently aimed, the focusing muscle has to contract properly and consistently to keep the eye in focus. The medical term for this focusing is called accommodation.