Eye Teaming (Binocularity)
Eye teaming, or binocular vision, is a visual efficiency skill that allows both eyes to work together in a precise and coordinated way. Good eye teaming allows sustained, single, and comfortable vision, and is the basis for depth perception.
We are continuously attempting to aim both eyes at objects during all visual activities. Each eye sends a separate, slightly different image to our brain’s visual cortex, where the images are combined (or fused) into a single image. If we can do this easily, we will have a stable, 3D view of the world. If we cannot do this easily, objects can appear double or move, creating a confusing uncomfortable view of the world. Eye teaming problems typically cause double vision, headaches, blurred vision and eyestrain, especially during reading and close work.
The two most common eye teaming problems that affect reading and close work are convergence insufficiency and convergence excess. Convergence insufficiency is when the eyes have a strong tendency to turn out during reading and close work. Excessive effort must be used to keep the eyes from drifting out, and the effort involved causes double vision, headaches and other symptoms. Convergence excess is when the eyes have a strong tendency to turn in during reading and close work.
Neither of these problems are detectable when just looking at a child. More dramatic forms of eye teaming problems are called strabismus, when an eye can actually be seen drifting or wandering at times.
This is what reading looks like with an eye teaming problem:
Most people are not aware that we have to refocus our eyes when we look from one place to another. This is because the focusing system usually operates so well that objects always appear in focus. However, in reality a focusing adjustment is made every time we look from one place to another. This adjustment is made with the help of a muscle called the ciliary muscle, or focusing muscle, which is located inside the eye. Accommodation is the term used for this focusing process.
For example, when a child looks from the board to their desk, they must contract or tighten this ciliary muscle. This causes the lens inside the eye to change shape and allows the child to see the print clearly. When the child looks back up from their desk to the board, they must now relax the focusing muscle to once again achieve clear distance vision.
A focusing problem occurs when an individual is unable to quickly and accurately relax or contract the focusing muscle, or if this muscle contraction cannot be maintained for adequate periods of time such as during reading or desk work.
Tracking (Saccades & Pursuits)
In order to use our vision efficiently, the eyes must move accurately, smoothly, and quickly from place to place. Eye movements allow accurate scanning of the visual environment for information. For example, every time a child looks from the board to their desk, the eyes must accurately jump from one target to another. The same is true for reading as the eyes jump from one word to another while scanning a line of print. Tracking is also important for following moving objects in sports, and for directing our eyes to move our hands towards a target. Eye-hand coordination in any activity starts with accurate eye movements.
Tracking skills are considered the fine motor aspect of vision. When a person has a tracking problem, eye movements are slow, inaccurate, or require head or finger movement to help the eyes track. This can interfere with reading fluency and comprehension, copying, handwriting, and sports performance.
People with tracking problems may show the following behaviors or signs: frequent loss of place during reading, skipping lines, omitting and substituting words during reading, head movement or finger pointing during reading, slow reading speed, poor copying, and poor eye-hand coordination.