Fusion is the ability to align two eyes and simultaneously process information from both equally and see space in a three dimensional way or commonly known as depth perception. Fusion can be further divided into 3 levels. First is simultaneous ability to for both eyes to recognize information from both eyes, 2nd is color fusion, the ability to process this information simultaneously or what is referred to as binocularity, and 3rd is 3-D, depth perception or stereopsis.
The most efficient way would be to use both eyes equally. A patient's response could be with only information from one eye, even with both eyes open. This condition would be called then complete suppression. The brain could receive information from one eye then mentally shut it down and alternate to the other eye. This is very common with presenting symptoms of learning difficulties. There is a constant mental confusion with information being processed by one eye then switching to the other. Adding to a less than efficient fixation pattern this person could constantly lose his place, have to use a finger/ruler to know where they are when reading or doing detailed tasks requiring vision. To see a typical example of a Binocularity graph with a person having learning difficulties.
This suppression is not something a patient has any control over just like breathing and your heart beating. It is a neurological adaptation so the stress of trying to use both eyes at the same place and at the same time. The patient learns a pattern of completely alternating the information processed from the two eyes, either completely or partial. This can cause massive confusion when trying to gain information ie: reading. This confusion can also lead to a condition of strabismus (turned eye,)where another adaptation is to turn one eye up, down, in or out to avoid using both eyes giving confusing information to the brain.
Amblyopia or lazy eye, can also develop from an inefficient visual system. In this case the brain effectively turns off the critical area of where we see, and creates an adaptation to use a less sharp area for its point of fixation, thereby reducing the stress of conflicting information and reducing how clear the eyesight is from one eye.
The brain can also not only shut one eye off, but it many times avoids the two eyes, by causing an intermittent drift of the positioning of the two eyes, so that it makes it easier to use just the one eye without interference from the other trying to process information at the same time. Many people adapt and begin to have a head tilt to avoid using both eyes when reading or some literally cover one eye under demanding visual tasks.
Clinics providing Vision Therapy using NeuroVisuopathy, have specialized programs to monitor this aspect of visual processing with each session of therapy. The campimeter or functional Visual Field measures the amount of information that is available, while measuring binocularity is indicating how it is actually being processed by the patient. While striving to achieve the ultimate goal of 100% simultaneous processing, many patients problems do not achieve this full capacity. Early research indicates the average entering patient is using about 19% binocularity initially that begin therapy without any serious brain injuries or dysfunctions, strabismus, or amblyopia. Their achievement is about 60% complete integration on average using the fast track methodology of therapy over 10 days completing two sessions a day.
When the Visual Field has expanded and there begins to be improvement in Binocularity integration, specific levels of treatment can begun to enhance awareness and integration of stereopsis(3-D). This then becomes the highest level of visual processing but can not begin till the foundation is fully established.