All About Monocular Cues and How We Use Them

Monocular cues refer to the ways that each of your eyes takes in visual information that’s used to judge:

• distance
• depth
• three-dimensional space

Here’s how Jo Vrotsos, a doctor of optometry based in Aurora, Colorado, explained how monocular cues work:

“With monocular cues, images are interpreted as two-dimensional. Think of a painting that gives the illusion of depth by playing with the foreground and background. That is your eye and monocular cues in the real world — things that are closer are larger and move faster; things in the background are smaller and move slower.”

That’s the high-level idea behind monocular cues.

Vrotsos also said that “visual information, as seen with a single eye, can detect an object moving at a distance, but can’t necessarily decipher the entire scene.”

Now, let’s get into the six main subcategories of monocular cues that contribute to your vision.

Relative size

This monocular cue gives you the ability to measure how far away something is. It works by judging how big or small the object is and what that means in relation to other objects you’ve interacted with in the past.

Here’s an example: When you see a plane fly by in the sky above you, it looks really small. But you probably know that up close, a plane is huge.

What this means is that your vision (the plane you see in the sky) connects with your memory (a plane you’ve seen up close) to indicate to you that because the plane looks so small, it must be extremely far away.

Interposition

Interposition refers to what happens when two objects on a flat surface, like a drawing of two circles, look like they have some relation to each other in terms of distance, even when they’re not actually in 3-D space.

Here’s a longer explanation of the classic circles example: Let’s say you have two circles drawn next to each other on a piece of paper. In this case, both circles will appear to have the same depth.

But let’s say you then draw the circles so that they intersect with one another (kind of like a Venn diagram). If you color in one of the circles, it’ll look like it’s overlapping the other circle.

Your eye will then perceive that the overlapping circle is closer to or on top of the other circle. Now the circles will appear to have depth even though they’re still just 2-D drawings on a flat piece of paper.

Linear perspective

Linear perspective happens when the angles of two adjacent objects and the distance between them look smaller and smaller. This causes your eye to interpret those objects as increasingly farther away from you.

For example, imagine you’re drawing a road or train tracks extending into the distance. You might start drawing each side of the road or tracks at the bottom of your piece of paper.

As you continue to draw the road or tracks moving “away” from you, the lines might angle closer together toward the center of the paper. This will result in a triangular shape.

As you look at the triangle, the closer you get to its tip, the farther away your eye will interpret the road or tracks to be from your position. This is due to the angle of the lines and the fact that they’re closer together at the tip than where they start at the bottom of your piece of paper.

Aerial perspective

Aerial perspective is what makes far away objects look a bit blurrier, lighter in color, and less detailed than those closer to you.

Think about mountains off in the distance. They tend to be much lighter in shade and color than a mountain that’s much closer to you.

This happens because blue light scatters into the air when it interacts with the atmosphere — which often makes distant objects appear light blue.

Contrast of color also plays a role in aerial perspective.

Objects that are farther away tend to have rough, blurry edges because of the scattered light in the air, and colors tend to blur together. Closer objects, on the other hand, have more defined edges and a starker contrast of color.

Big objects, like mountains and skyscrapers, seem bigger and clearer when the air is clean because there are fewer particles to scatter the light.

Light and shade

The way that light hits an object creates shades of light and dark. This tells your eyes where an object sits in relation to the light and to objects nearby.

This cue can also tell you if something is upside down because the light source will hit the object differently, so that it visually contrasts with other parts of your environment.

Monocular motion parallax

This one’s a mindblower. The monocular motion parallax happens when you move your head and objects that are farther away appear to move at a different speed than those closer to you.

Try it out by looking at something far away. Then, slowly turn your head from left to right and back again.

You may notice that objects nearer to you appear to be moving in the opposite direction of the way your head is going. But objects farther away from you seem to follow the direction of your head.

Together, your two eyes combine to give you binocular cues. This refers to visual information you get from the overlapping of each eye’s monocular cues.

As Vrotsos explained, “With binocular cues, images are interpreted as three-dimensional. These cues are based on different images that our two separate eyes produce and put together to form a 3-D image.”

He continued, “You can now tell variations of distance. For example, the distance between me and a bike in front of me is smaller than the distance between me and the tree across my yard.”

Here are some mechanisms of binocular cues:

• Fusion. This is where your brain combines two separate eye images together to make a single image.
• Retinal disparity. The distance between retinas allows each eye to perceive slightly different information. This gives you stereoscopic vision, which you use to perceive depth, shape, and size.

Peripheral vision is what your eyes see on the sides when you’re looking straight ahead.

Some eye conditions can affect your perception of depth and your peripheral vision:

• Glaucoma. Glaucoma occurs when fluid in your eye doesn’t drain properly and builds up.
• Retinitis pigmentosa. With this condition, dark pigments deposit in your eye and can lead to tunnel vision.
• Scotoma. This refers to a blind spot in your field of vision.
• Stroke. Damage to blood vessels or nerves as a result of a stroke prevents your brain from processing visual information.
• Migraine. With migraine, you may experience temporary vision changes called auras.
• Detached retina. If your retina separates from the back of your eye, it’s known as a detached retina.

Here’s how these conditions can be treated:

• Glaucoma. This can be treated with specialized eye drops or surgery to correct fluid drainage.
• Retinitis pigmentosa. There’s no cure for this condition, but visual assistance devices or taking vitamin A may help reduce or slow the development of tunnel vision.
• Scotoma. You might try to brighten lights in your environment or use magnifiers to read print or digital text.
• Stroke. There’s often no treatment for vision changes associated with a stroke, but sometimes specialized glasses can help you see more clearly.
• Migraine. Pain medications and lifestyle changes, such as using dimmer lighting, may help reduce how often you experience migraine.
• Detached retina. Surgery is typically required to repair a detached retina. For minor detachments or tears, your eye doctor may be able to perform a procedure right in the office.

Both monocular and binocular cues give you essential visual information that helps you perceive the world around you.

Changes to your vision can make it more difficult to get around. Talk with your doctor or optometrist if you have any concerns about these changes.

Here’s what Vrotsos recommended: “Ask your optometrist if your medication is correct. Have them check your stereo vision and eye dominance.”

If your doctor suspects an issue, they might make a referral for vision therapy, Vrotsos said.