Because optical illusions are, as the name suggests, illusions warping the way the human brain processes visual stimuli, the best place to start when trying to understand the mechanism of these illusions is with how the brain interprets optical information in the first place. For this blog, specifically how the brain perceives distance. Now if you have functioning eyes, you’re likely already familiar with the concept of depth perception; in fact, you use it every day. From judging how far away a bin is when you’re tossing your trash to stepping around objects to ensure you don’t become intimately acquainted with the floor, you (or rather your eyes and brain) are constantly using depth perception to measure the location of objects relative to yourself.
Binocular Vision
For accurate depth perception, functional use of both eyes (or binocular vision) is ideal. Since our eyes are not positioned at exactly the same spot on our face, the images our brain receives from each eye is slightly different by a few millimeters. This is known as stereoscopic vision, or our eyes’ ability to view objects in minutely different ways. If you want to test out this phenomenon, try looking at an object first with your left eye closed, then with your right eye closed, and finally with both eyes open. You’ll notice that the object appears to move depending on which eyes are or aren’t closed.
Then, in a process known as convergence, the brain combines the two images into a single image (as shown in the image above). 2D information is changed into 3D information that the brain can then use to judge distance. However, this begs the question of how do we perceive depth with one eye?
Monocular Vision
Let’s talk about cyclopes. Knowing how depth perception works, our first instinct is to think cyclopes are unable to measure depth. Yet if we consider our own vision, we’re still clearly able to approximate the distance between objects even with one eye closed. However, relying on only one eye (or monocular vision) does not give entirely accurate information about the exact location of an object, which means if you’re ever in the unlikely scenario of being hunted down by a cyclops, worry more about it catching up to you than any long distance weapons it may be using. While we can’t use monocular vision to craft a 3D image to determine depth, our brain uses other methods as a substitute.
Motion Parallax
If you move your head back and forth, you’ll notice that objects close to you seem to move backwards at different speeds. This is known as motion parallax. Since objects relatively farther away from us will appear to move at different speeds compared to objects relatively closer to us, our brain uses this information to perceive depth.
Interposition
Interposition is when objects overlap each other. If one object is covering another object, we know that the object being covered is farther away from us than the object doing the covering.
Relative Size
Another simple one, the size of certain objects relative to one another cues the brain as to how far away something is. Large objects will get smaller the farther away they are from the viewer, so if you know that one object is larger than another (say a car versus a person) but see that the larger object appears smaller, then you can reasonably assume that the larger object is farther away. This makes for some rather interesting optical illusions.
Aerial Perspective
Aerial perspective uses color, contrast, and texture of objects to determine their relative location. We are able to see objects that are closer to us in sharp contrast, with clear color and texture. Similarly, objects that are far away from us may appear fuzzy or distorted at the edges, and small details may be hard to detect.
Accommodation
Though this works with both eyes, accommodation is still considered a monocular cue. When we need to focus our vision on something, the muscles in the eye distort the lens in order to focus on the object. This is why you feel strain when you view an object close up as opposed to farther away. Depending on the distance of the object, our brain is able to detect the differences in muscle feedback and judge how far away an object is. However, this method is only accurate for very short distances.
Conclusion
Many optical illusions that play on depth perception, such as ones where people appear to walk through fences or make water flow against gravity, manipulate one or more of the above methods that our brain uses to process visual cues from binocular and monocular vision. So next time you see something baffling and impossible, remember that either your brain is being tricked using its own tricks or you’re looking at the results of some serious photo-editing magic.
The Art of Optical Illusions 