Binocular Vision: What It Is and How It Is Tested

You look at a cup of coffee and see one cup, not two. This sensation is provided by binocular vision — the ability to see an object clearly with both eyes simultaneously and perceive it as a single image. Below is how this system works and what exactly the doctor checks.

What binocular vision is and how it differs from monocular vision

Binocular vision (from the Latin bīnus — "double" and oculus — "eye") is provided in the cortical part of the visual analyzer through fusion — the merging of two separate images that arise in each eye into a single combined perception. If such a system does not develop, a person sees only with the right or only with the left eye — this is monocular vision. There is also an alternating variant: sometimes with the right, sometimes with the left eye, monocular alternating. When different images are presented to each eye, the phenomenon of binocular rivalry occurs: a person perceives first one, then the other picture, and they periodically replace each other.

How stereoscopic vision works

Binocular vision is also called stereoscopic. The point is that the eyes are located at a distance from each other and receive slightly different projections of the same object. The brain compares them and adds a sense of depth. With strabismus, depth perception may be impaired.

Binocular vision and strabismus: why it is impaired

Strabismus is a condition in which the eyes look in different directions: one forward, the other inward, outward, up, or down. Each eye is surrounded by six muscles that work as a team. With strabismus, this team is uncoordinated, and two different images are sent to the brain. The child's brain learns to ignore (suppress) the picture from the weaker eye. If strabismus is not treated, the ignored eye will never begin to see well — this is amblyopia, "lazy eye." Sometimes amblyopia occurs first and itself leads to strabismus. The absence of binocular vision with both eyes open externally manifests as gradually developing strabismus. About one third of children with strabismus develop amblyopia.

How binocular vision is tested by an ophthalmologist

Testing binocular vision is a set of simple tests that the doctor performs during an appointment. Among them:

• examination of the corneal light reflex (reflection of light on the cornea);
• alternate cover/uncover test;
• visual acuity testing and retinal examination;
• standard ophthalmological examination and neurological examination.

The degree to which the eyes deviate from the correct position is also assessed.

When to see a doctor

A consultation is needed if a child has constant strabismus or if the child is older than 3 months and the strabismus appears and disappears — in children younger than this age, intermittent strabismus is usually not an alarming sign. Any doubts about a child's vision are also a reason: the child regularly tilts the head to one side or keeps one eye closed when looking at objects. An adult should seek medical attention if strabismus or double vision has appeared in adulthood. Strabismus requires timely assessment.

Strabismus by itself is not a reason to call an ambulance; 103 or 112 — if double vision occurs suddenly together with signs of a stroke.

This material is for reference: it helps to understand what the doctor is talking about, but does not replace a consultation with an ophthalmologist.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer