How Depth Perception Works

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 us or far away.

In order to have depth perception, you must have binocular vision, also known as stereopsis.

The most important aspect of binocular vision is having two eyes; people relying on 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.

Visual Cues and Depth Perception

Monocular cues allow for some sense of depth perception even when you don't have two eyes working properly together, such as:

• Motion Parallax: This occurs when you move your head back and forth. Objects at different distances move at slightly different speeds, closer objects moving in the opposite direction of your head movement and far-away objects moving 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 tell give us clues 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.

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.
• Blurry vision: Numerous conditions can cause the vision in one or both eyes to be blurry, as can trauma to an eye.
• Injury to one eye: Trauma can alter your vision, either temporarily or permanently.

Testing Depth Perception

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

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

When an optometrist checks your eye muscles, they'll perform a cover test. A cover test measures how well your eyes work together and will check for the presence of strabismus.

Strabismus, such as esotropia or exotropia, 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 out of the turned 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, such as random dot stereograms or contour stereotests. Some of these tests are designed for young children and use cartoon characters so that the doctor can tell if the child is using binocular vision.