Fixation Disparity

Fixation Disparity

Reading requires a three-dimensional eye motor control; it requires (1) saccades, an eye movement that moves the eyes from left to right, fixating word after word and (2) oblique saccades bring the eyes to the next line of the text.

Alignment of the two eyes to ensure they are aiming precisely at the same target during reading is paramount. Misalignment causes oculomotor control problems, often resulting in poor eye tracking ability, and consequently, difficulties with reading.

Research shows those who suffer specific learning disabilities, such as dyslexia, have a higher rate of problems with this binocular muscle control.

An examination has determined that you exhibit a slight misalignment of your visual axes. Put simply the two eyes are not fixating at exactly the same place, but rather there is a slight gap between the two eye's focus points. This is called a heterophoria. The heterophoria is specified depending on whether the gap is due to excessive convergence (esophoria), or inadequate convergence (exophoria).

The visual system can sometimes compensate for small misalignments through a process called fusion, in which the eye muscles employ extra effort to push or pull the eyes together.

In some instances, the eyes extra effort allows the gap to close, but not completely, resulting in a small misalignment of the eyes, but in which fusion is maintained. This has the potential to cause a significant amount of visual confusion, as each eye is fixating at a slightly different place, but both eyes are still sending a strong signal to the brain. This is called fixation disparity.

Sometimes the misalignment is too great for the eyes to compensate and fusion is not possible. The result would naturally then be double vision; however, it is very difficult for the brain to process a doubled image. Instead, the brain often temporarily 'switches off ' an eye, often the non-dominant eye. This is called suppression. Suppression occurs when fusion is not possible so double vision is not perceived. As fixation is continually changing (from the board, to book, to the window, to a friend sitting next door), the visual system is constantly calibrating and trying to maintain fusion. As fusion is only unachievable in certain situations, the visual system is often switching on and off the visual pathway of one eye as required.

Fixation disparity and suppression both cause an interruption to the sensory-motor feedback control system, which is critical to the complex vision-based learning process. Like a domino effect, other sensory systems can either underperform or overcompensate, resulting in multisensory symptoms.

The need to continually compensate and force fusion, fixation disparity and intermittent suppression, and the resulting need to compensate via sensory-motor fusion, all cause a number of symptoms:

- Poor reading comprehension
- Headaches, sore eyes and fatigue
- Getting lost visually; skipping words/lines whilst reading
- Inattention and poor concentration whilst reading


Decompensated heterophorias, fixation disparity and/or intermittent suppression require treatment, which often involves special prisms and powers in glasses to realign the eyes and to relax the focusing system. As the eye's alignment is different when looking far away compared to looking up close, often different powers and prisms are needed for different viewing distances. In this instance, a bifocal lens is prescribed. This treatment provides clear, single and comfortable vision with the aim to resolve vision-related symptoms and support the vision-based learning systems.