People have two eyes and they have to work together. The brain has to merge the image of each eye into a single (sensory fusion) and then calculates how distant it is (stereo.). In order to achieve this, eye muscles must be coordinated to point exactly on what is being watched (motor fusion).
Sometimes, sensory fusion fails, because an eye has a blurred image, or motor fusion, because one or several hypo/hyperacting muscles, paralysis, ... and then there is a strabismus (misaligned eyes). Then, the brain usually chooses the image of an eye and ignores the other, because double vision is very disagreeable and frustrating. The eye, its central vision is suppressed, can be become amblyop (lazy eye) due to the lack of stimulation. The lazy eyes, although compensated with the necessary graduation, can not see 100% visual acuity.
There are people who have a normal sensory and motor fusion, with no strabismus, but they are not comfortable with their vision, they often find it difficult to describe. They do not usually have any refractive defect, that is, they do not have to wear glasses to see 100% in distant vision.
When this situation occurs, the following characteristics must be studied: the vergences (it is the ability to direct the eyes of distant vision close or upside down), the accommodation (the focus in close vision) and the motility . The problem is often the fact that one or more of these abilities are not effective enough