What Is Binocular Fusion?


Binocular fusion, or just “fusion” as it’s also known, is a term used to describe the brain’s ability to gather and process information received by each eye from a single image.

If your eyes are not perfectly aligned, you might be experiencing blur, discomfort, or double vision.

It’s a common issue that affects young and old people alike.

However, younger patients have a more flexible visual system that can adapt to the problem and prevent double vision by ignoring or at least suppressing the second image that shouldn’t be there.

The process is known as cortical suppression. Eye misalignment can be turned inward, outward, up, or down, leading to poorer vision clarity, known as amblyopia or lazy eye.

Fusion is sometimes experienced only when looking at objects from specific distances. For example, a patient could have perfect vision at a distance, but they might struggle to read from up close.

Some people experience blurry vision, and others see double images when focusing on something near.

Fusion reflex and its development

The fusional reflex is maintained by cerebral activity at all times. It can be conditioned or acquired, but both types result from the experience we gain through outside stimuli.

It takes time to form these reflexes through continuous reinforcement, but once they are formed, they become unconditional reflexes.

When it comes to single binocular vision, it includes all activities from the retina to the brain. Together, they work to maintain the images your eyes receive.

The fusion mechanism involves a few elements that work together. They are:

  • Fixation reflex,
  • Refixation reflex
  • Conjugate of fusional reflexes
  • Disconjugative reflexes

When children are born, they don’t control these reflexes. As their eyes are still not developed completely, babies can’t control their fixation reflex, so they keep moving their eyes without the ability to focus on something specific.

It takes weeks for the eyes to start developing and up to 6 months until the child can build eye muscles needed to focus on something specific. Here’s a quick overview of the postnatal development of the optomotor reflex:

  1. A newborn baby can follow a light binocularly after 2 to 3 weeks.
  2. After six weeks to six months, it can follow a light with both eyes or binocularly.
  3. The convergence reflex is present at birth, but it develops only after the first month. The process takes about five months to complete, or when the child reaches six months.
  4. Since the ciliary muscle develops a few months after birth, the accommodation is slowed down due to the development of convergence which also takes up to 6 months.

Many studies performed on infants proved that stereoacuity can be detected in babies aged between 2 to 5 months.

However, once they turn six months, children spend the next two and a half years developing stereo acuity. Other studies concluded that the process goes on until they reach the age of 9.

When this information is put together, we can say that binocular vision is developed between four months and nine years.

The peak of the transformation is at about two years old, and the system is mostly developed by the age of 4. In other words, the first two years are essential for developing single binocular vision.

Any problem that occurs during this time can harm the reflex pathway and badly affect binocular vision development. The most common obstacles in this process are the following:

  • Sensory obstacles
  • Central obstacles
  • Motor obstacles

These issues lead to improper adaptations to binocular dysfunction during this sensitive period. That can lead to other problems such as amblyopia, suppression, and anomalous retinal correspondence.


In its definition, fusion is the unification of different visual expectations. For example, images seen by both retinas are practically fused to form a single visual perception. There are two types of fusion, sensory and motor fusion. Let’s see more about them below.

Sensory Fusion

Sensory fusion is our brain’s ability to connect two similar images, one seen with each eye, into a single image. It’s the peak of retinal correspondence, and it tells us what the world around us looks like. For sensory fusion to work, both eyes must be looking at the same area, and they must produce an image of a similar size, sharpness, and brightness. If the images are not identical, sensory fusion can’t occur.

Motor Fusion

Motor fusion is all about eye muscles. Its role is to ensure that the eyes are aligned to maintain sensory fusion. It’s an exclusive function of the extrafoveal retinal periphery. Both types of binocular fusion take place in the visual cortex.