How do optical illusions work?

Optical illusions are more than just a trick of the eye; they include the brain’s processing of visual information.
Cultural influences may shape how we interpret optical illusions.
Optical illusions are also useful tools for researchers seeking to better understand visual perception and brain function.

Take a look at this photo. What are you seeing? A grid of squares? Great. Take another look at the white space at the “street crossings.” Although this image, known as the Hermann Grid, is just a black-and-white grid of squares, it looks to contain something more: small gray discs, or blobs of darkness, at the intersections of the white lines. This grid is a typical example of an optical illusion, in which your mind is misled into seeing something that isn’t there. You can see the dark blobs in the white areas, but when you look directly at the point where the blob should be, it vanishes because it was never there in the first place.

This is only one of many examples of the eyes playing tricks on the brain. Illusions mislead us for a variety of reasons. Adjacent items can alter the way you perceive things. Playing with perspective can alter your perception of an object. Sometimes illusions work because of flaws in the regular anatomy of our eyes. But let’s not only blame those “windows to the soul.” The brain is also responsible for making us believe what we see. It is sometimes too easy to form assumptions about how the world should be rather than how it actually is, leading us to see things inaccurately.

How Do Optical Illusions Work? - YouTube

You have undoubtedly seen and been deceived by countless of optical illusions, and you are not alone. Rewind history to the ancient Greeks. Even Aristotle acknowledged how easily the mind may be misled by what it perceives. He observed that when you stared at a waterfall and then turned your eyes to static rocks nearby, the rocks appeared to be flowing in the opposite direction of the cascade.

Nature, too, is playing a joke on us. We don’t entirely understand what happens in our brains when we witness optical illusions, but since the nineteenth century, scientists and artists have been learning more about the mismatch between reality and perception and what it tells us about the brain.
The perception of optical illusions is regulated by our brains. For example, the brain may easily switch between two different viewpoints of an object, transforming something two-dimensional on a sheet of paper into an entity that we perceive as three-dimensional. But how?

It’s complicated. David Hubel and Torsten Wiesel shared the 1981 Nobel Prize in Physiology or Medicine for their findings on how the brain understands the coded communications supplied to it by the eyes. (That year, the honor went to numerous recipients.) They discovered that the brain uses a step-by-step mechanism to interpret what the eye sees. Each nerve cell or neuron in the brain controls a specific detail in the retinal image pattern. Despite Hubel and Wiesel’s discoveries, and our understanding of the many areas of the brain that deal with color, form, motion, and texture, scientists still don’t fully understand how all of the messages combine to make our overall perception of an item.

Scientists can use MRI scans to study what happens in our brains when we see illusions. They discovered that neurons can compete with one another to detect light and dark patches. The winning neurons determine the message sent to your brain and, as a result, what you perceive [source: Hogenboom].

According to one theory proposed by researchers, some illusions deceive us because they take advantage of the brain’s constant attempt to predict what will happen next in order to compensate for the small lag time between when an event occurs and our perception of it. Sometimes the prediction does not correspond to the reality that the illusion depicts.

Optical Illusions | Optics for Kids

Another idea attempts to explain “apparent motion” illusions, such as the so-called snake illusion, in which things appear to move across the page. Scientists believe that the small — nearly imperceptible — quick movements our eyes make (known as saccades), which are ordinarily smoothed out by the brain to give us a single picture, are responsible for us perceiving motion when there is none. Others argue that the illusion works because it sends so much information to our retina at once, which causes confusion in our visual cortex.

Obviously, not all illusions function in the same manner, and some ideas do not always stand up when minor changes are made to the illusion. In summary, we still don’t understand why our brains are so muddled!

Optical illusions are everywhere. Aristotle saw optical illusions in waterfalls. In the film “Indiana Jones and the Last Crusade,” Indiana Jones saw one in the rocks while making his leap of faith across the vast abyss. And we see them everywhere, from M.C. Escher drawings to popular online memes (was that outfit blue or gold?).

In fact, once we’ve seen the “trick” in an illusion, it’s almost impossible to undo it. We simply cannot move our consciousness back to a moment when we were unaware of what we had just learned. Once the past knowledge is available, our brain swiftly retrieves it and integrates it with the visual cues obtained from actually viewing the illusion. The “can’t un-see” sensation that many individuals experience when viewing illusions is a prime example of the brain doing more than merely translating what our eyes perceive.

Furthermore, optical illusions are not just the result of our eyes and brains; societal variables may also have a significant impact on our perception. While the scientific basis for how optical illusions work is common to all humans, when some illusions are displayed to people from different cultures, not everyone sees the same thing or misses the same visual clues [sources: Schultz, Alter].

For example, consider the Müller-Lyer illusion. Of research, most European South Africans believed the lines were different lengths, yet bushmen of several South African tribes accurately identified them as the same lengths. Scientists have theorized that people in Western societies are used to seeing straight lines and geometric shapes, and people with other cultural experiences aren’t exposed to the same geometric configurations, so their brains don’t jump to the same conclusions when exposed to illusions built on geometric trickery.

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