Depth Perception: What Is It and How Does It Work?

Depth perception is the ability to perceive the world in three dimensions (3D) and to judge the distance of objects. Your brain achieves it by processing different pictures from each eye and combining them to form a single 3D image.

Depth perception makes it possible for your eyes to determine distances between objects and to tell if something is near to you or far away.

Visual Cues and Depth Perception

Depth perception relies on visual cues. These cues are the physical signals and the brain's interpretation of them, which are responsible for your vision as the brain and your body work together.

In order to have depth perception, you must have binocular vision, also known as stereopsis. You also rely on monocular cues from each eye separately, as well as oculomotor cues that arise from the way your eyes move together to keep focus.

Monocular Cues

Monocular cues allow for some sense of depth perception even when you don't have two eyes working properly together. They're still needed even when they are, offering cues including:

• Motion parallax: This cue contributes to your sense of self-motion. It occurs when you move your head back and forth. Objects at different distances move at slightly different speeds. Closer objects move in the opposite direction of your head motion and faraway objects move with your head.
• Interposition: When objects overlap each other, it gives us monocular cues about which one is closer.
• Aerial perspective: Color and contrast cues offer clues as to how far away an object might be. As light travels, it scatters and causes blurred outlines, which your brain interprets as being farther away.

Binocular Cues

The most important aspect of binocular vision is having two eyes. People with vision from only one eye have to rely on other visual cues to gauge depth, and their depth perception is generally less accurate.

Not having accurate depth perception can have a major impact on your life, from early childhood learning to what jobs you're able to do. That's because your binocular cues are at work to create the depth perception needed for countless tasks.

Some of the cues provided through your binocular vision include:

• Changing disparity: These cues are a function of stereopsis, which allows your eyes to build depth perception on the basis of the distance between them. This sensitivity to the disparity, and how the brain processes the slight difference, contributes to an accurate 3D image.
• Velocity differences: Your binocular vision is responsible for processing differences in speed, or velocity, that contribute to depth perception as you and the world you interact with are in motion.

Oculomotor Cues

These cues are based on the way your eye muscles work, and include two key features known as convergence and accommodation. Here's what these oculomotor cues do:

• Convergence: This describes the way your eyes angle inward when you look at an object up close. This motion is at work when you're focusing on something that's only a short distance away.
• Accommodation: This cue also only works at short distances. Your eye muscles change the focus of the eye lens, helping you to see different objects at different distances with accurate depth perception.

These eye motions allow your eyes to work together to focus on an object with accurate depth perception.

Causes of Impaired Depth Perception

A lack of depth perception can be caused by numerous conditions. These include:

• Amblyopia: Also called "lazy eye," this is a condition in which one eye is weaker than the other. This typically happens because of abnormal vision development in childhood and features decreased vision in one or both eyes.
• Optic nerve hypoplasia: This occurs when the optic nerve, which sends visual signals from your eyes to your brain, has incomplete development before birth. It can result in partial or total vision loss in children.
• Strabismus: This occurs when the eyes point in different directions, such as one pointing straight ahead and the other pointing inward or down. In this case, the strabismus causes amblyopia.
• Blurry vision: Numerous conditions can cause the vision in one or both eyes to be blurry, as can trauma to an eye. Blurry vision causes amblyopia.
• Injury to one eye: Trauma can alter your vision, either temporarily or permanently.

Depth Perception Tests

Having a comprehensive eye examination is the first step in fully assessing your depth perception.

An optometrist or ophthalmologist will first test your vision by measuring your visual acuity or the quality of your vision. If one eye is very blurry and the other is not, your depth perception will be limited.

A cover test is used to check for strabismus, such as esotropia (in), exotropia (out), hypertropia (up) and hypotropia (down). This is an eye muscle problem where the eyes do not work well together, or when one eye is turned in, out, up, or down. If the eyes are not in perfect alignment with each other, double vision occurs, or more commonly, suppression.

If your eyes aren't lined up, your brain will suppress the image from the misaligned eye to keep you from having double vision. As a result, you're really only using one eye. When this happens, you will not have good depth perception.

Fine levels of depth perception are measured using tests, some of them relying on pictures or cartoons when designed for young children. These tests may include:

• Random dot stereograms, which measure 3D vision by using images made up of dots
• Contour stereotests, which check 3D vision with shapes and images designed to create contours

How Can I Improve Depth Perception?

It's possible to improve your depth perception, or that of your child, but the treatment will depend on the reason for their depth perception issues.

The lowest levels of intervention include simple strategies such as:

• Eye exercises designed to improve depth perception or boost athletic performance
• Limits on screen time to avoid overuse damage from the type of focus involved
• Improved lighting, often used to maximize depth perception ability in older people

Treatment for a specific condition such as strabismus or amblyopia, in which one eye is weaker than the other, will focus on making that eye stronger. Sometimes this can be done with eyeglasses, eye patches worn for part of the day, or eye drops. If the problem persists, some people may need surgery on their eye muscles to improve depth perception.

Researchers also are currently studying the use of virtual reality (VR) as a promising alternative in treating depth perception problems. One small study of 24 young people, divided into equal groups who played either 3D or 2D video games, found improved binocular vision among the 3D players.

More study is needed, but the use of VR may prove useful in treating amblyopia, strabismus, and other depth perception problems.