The term depth perception refers to our ability to determine distances between objects and see the world in three dimensions. To do this accurately, one must have binocular stereoscopic vision, or stereopsis. If someone lacks stereopsis, they are forced to rely on other visual cues to gauge depth, and their depth perception will be less accurate.
Our eyes use three methods to determine distance:
- The known size of an object on your retina: Knowing the size of an object from previous experience helps our brains calculate distance based on the size of the object on the retina.
- Moving parallax: This is what happens when you stand face to face with someone and move your head side to side. The person in front of you moves quickly across your retina, while objects that are farther away do not move very much at all. This helps your brain calculate how far an object is from you.
- Stereo vision: Since our eyes are about two inches apart, each eye receives a different image of an object, especially when the object is close up. When the object is far away this method does not work as well, since these dual images of objects appear more identical when the object is farther from our eyes.
We use a number of different tools for depth perception; these tools are known as depth cues. These cues are classified into binocular (both eyes), monocular (one eye), and inferred (combined binocular and monocular cues). A person’s ability to perceive distances and sizes depends on which cues are available to them.
The term stereopsis means that a person sees clearly with two good eyes, and sees images with stereoscopic vision. Someone who only sees with one eye lacks this tool and must rely on other cues to determine depth. When someone uses both eyes to focus on the same object, their eyes converge. The convergence then stretches the extraocular muscles, and kinesthetic sensations from the extraocular muscles help with depth and distance perception. Other binocular cues include:
- Retinal disparity: Retinal disparity simply means that each eye receives a slightly different image due to the different angle from which each eye views an object.
- Fusion: When the brain uses the retinal images from the two eyes to form one object, it is called fusion. Fusion takes place when the objects appear the same.
Monocular cues allow a person to judge depth and the sizes of objects with one eye. Other monocular cues include:
- Interposition: Interposition cues occur when there is an overlapping of objects
- Linear perspective: When objects of known distance appear to grow smaller and smaller, the perception is that these objects are moving farther away.
- Aerial perspective: The relative color and contrast of objects gives us clues to their distance. When scattering light blurs the outlines of an object, the object is perceived as distant.
- Light and shade: Shadows and highlights can provide clues to an object’s depth and dimensions.
- Monocular movement parallax: When our heads move from side to side, objects at different distances move at different speeds, or relative velocity. Closer objects move in the opposite direction of the head movement, and farther objects move with our heads.
Depth Perception Tests
There are two types of tests that are conducted to determine depth perception: the contour stereotests and the random-dot stereotest.
- Random-dot stereograms are used to eliminate monocular cues. Examples include the Randot Stereotest, the Random-dot E Stereotest, and the Lang Stereotest.
- Contour stereotests are used to evaluate two horizontally disparate stimuli. An example of a contour stereotest is the Titmus Fly Stereotest.
Depth Perception Problems
Problems with depth perception may develop when another condition is present. Common conditions that cause depth perception problems include:
- Blurred vision: typically in one eye
- Cranial nerve palsy: partial or full paralysis of the third, fourth, or sixth cranial nerves
- Strabismus: misalignment of the eyes
- Anophthalmos: absence of one or both eyes; may be congenital or due to trauma, infection, or other causes
Consequently, problems with depth perception may cause additional problems such as:
- Inability to perform normal tasks such as driving or reading
- Children may have difficulty learning
- Athletes may have difficulty performing in sports
Treating Problems of Depth Perception
If you have a hard time perceiving depth, you have options. Vision therapy is the preferred way to treat depth perception issues. Vision therapists can train a person’s brain to fuse the images from each eye, or in the worst-case scenario, to ignore the image from the bad eye. Eye doctors can also prescribe contact lenses or eyeglass lenses to hinder or block unclear images from the bad eye so they do not interfere with images from the good eye.
Depth perception exercises may also be useful. There are several different depth-perception exercises to choose from:
- Eye Rolling: This exercise benefits those who may have weak eye muscles. Eye-rolling exercises help to strengthen nerve impulses that give us the ability to perceive depth properly. When beginning this exercise, slowly roll your eyes clockwise for a few minutes, then switch and roll them counterclockwise for a few minutes.
- Shifting the Gaze: This exercise is usually used in conjunction with eye rolling. While eye rolling, shift your gaze slowly, especially when first beginning the exercise.
- Resting the Dominant Eye: When one eye is dominant over the other, the weaker eye will inevitably strain. This exercise involves covering the dominant eye for several minutes to allow the weaker eye to take over the vision responsibilities. This is usually done with an eye patch.
- Low Light: Resting the eyes from light may improve poor depth perception. This does not mean go to bed earlier; it is more like a regrouping for the eyes. Low-light situations ease pressure on the dominant eye without causing strain in the weaker eye.
Talking to Your Eye Doctor
If you or your child are struggling with depth perception, feel free to use these questions to begin a conversation with your eye doctor:
- Which tests can we perform to determine whether I have a problem with depth perception?
- Which eye exercises should I do at home? How often should I do them?
- How else can we go about improving my depth perception?
- Will a lack of depth perception interfere with my daily activities?
- Will my depth perception gradually decline as I age?
- What can I do to prevent my depth perception from diminishing?
- R. Atkins, MD “The Eye Care Revolution” (Kensington Books, 2004) 7
- J. Di Girolamo “The Big Book of Family Eye Care” (Basic Health Publications, Inc., 2011) 96-97; 111
- J. Weizer, MD; J. Stein, MD, MS “Reader’s Digest Guide to Eye Care” (Quantum Publishing, 2009) 72-73