Did you know that like most standard eye tests, the Snellen test does not check one’s depth perception? Read on to learn more about the tests that specifically check your depth perception.
When it comes to seeing the world in three dimensions (3D), depth perception plays a crucial role. Without it, there’s no way we can recognize distances between people and/or objects in all directions. Many experts explain that animals are able to sense the distance of objects (depth sensation) within their environments. However, the term perception is reserved for humans, since we are able to tell each other about the experiences of distance.
By definition, depth is looking into a hole or tube and estimating forward distances. To do this accurately, one must have binocular stereoscopic vision, or stereopsis. If someone lacks stereopsis, perceiving depth may be more difficult and less accurate, and they must rely on visual cues other than stereopsis.
Our eyes use three methods to determine distance:
- The size a known object has on your retina - Knowing the size of an object due to previous experience helps our brains calculate the distance based on the size of the object on the retina.
- Moving parallax - A great example of this is standing face to face with someone and moving your head side to side. The person in front of you moves quickly across your retina, but the objects that are further away don’t move very much at all. This helps your brain calculate how far or close something is from you.
- Stereo vision - Since our eyes are about two inches apart, each eye receives a different image of an object on its retina, especially when an object is close up. When the object is far away, this method doesn’t work as well since objects appear more identical when further away from your eyes.
There are different types of depth perception, known as depth cues. These cues are classified into binocular (both eyes), monocular (one eye), and inferred (combined binocular and monocular cues). All require input from the eye(s) to the brain in order to perceive depth, but it depends on which cue a person has to determine how well they perceive distances and sizes.
The term stereopsis means that a person sees clearly with two good eyes, and he/she 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, they converge. The convergence then stretches the extraocular muscles, and kinesthetic sensations from the extraocular muscles help with depth/distance perception. Other binocular cues include:
- Retinal disparity - Disparity means different. So retinal disparity simply means that each eye receives a slightly different image due to the angle from which each eye is viewing an object.
- Fusion – When the brain brings the retinal images from the two eyes to form one object, its called fusion. Fusion takes place when the objects appear the same.
Monocular cues allow a person to perceive depth and sizes of objects with one eye. Relative size is when two objects are known to be the same size, but their actual size is unknown. Relative size then allows someone to estimate and perceive relative depth of the two objects. Other monocular cues include:
- Interposition – Interposition cues occur when there is an overlapping of objects
- Linear perspective – When objects of known distance appear smaller and smaller, it’s interpreted as these objects being further away.
- Aerial perspective - The relative color and contrast of objects gives us clues to their distance. When scattering light blurs the outlines of objects, the object is perceived as distant.
- Light and shade - Shadows and highlights can provide information about an object’s depth and dimensions.
- Monocular movement parallax - When our heads move side to side, objects at different distances move at different speeds, or relative velocity. Closer objects move in the opposite directions of the head movement, and farther objects move with our heads.
Depth Perception Tests
There are two main types of tests given to determine how well one can perceive depth: 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 horizontal disparate stimuli. An example of a contour stereotest is the Titmus Fly Stereotest.
Treating Problems with 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 image 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 and block out unclear images of the bad eye, so they don’t interfere with the images from the good eye.
- 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