The Science of Optical Illusions

You recognize it. It is something familiar, but you cannot quite identify it. An image should be decipherable and easy to understand, yet you sometimes cannot see it for what it truly is. The brain is responsible for conveying reality to us, allowing us to process things that we see, hear, taste, touch, and smell. That intersection is where optical illusions occur. Optical illusions are visual images or animations that our eyes see but that our brains cannot process.

At the moment, scientists have not concluded exactly how optical illusions work, but there is consensus that optical illusions are caused by the contrast between what the eyes perceive and how the brain interprets such information. Since the brain cannot understand what the eyes are seeing, the brain uses past experiences to draw connections and make sense of what is visualized. 

Normally, light entering our eyes activates cells in the retina, sending electrical signals along the optic nerve to the brain. Optical illusions take advantage of the fact that these sensory systems are slow. More specifically, our sensory systems are 50 milliseconds behind, and our visual sensory systems are hundreds of milliseconds behind. Although that might not seem like long, these milliseconds are essential to properly assessing reality.

However, our brains don’t solely rely on visual and sensory information. If they did, we would not be able to swing baseball bats to hit a ball or have fast enough reflexes to dodge a dodgeball. That is why when processing information, our brain creates a story for itself from multiple factors, trying to determine where an object will go or what will happen so that we can be prepared. This story becomes our reality and all that we know. Thus, what we perceive in an optical illusion is different from what is reality.

Optical illusions make use of six primary elements: color, context, brightness, shadows, blinking dots, and selection. The retina has three light-sensing cells, which are also known as cones: a red cone, green cone, and blue cone. For this reason, when you gaze at one color for a prolonged period of time, the retina is desensitized. Without a fully active retina, you may perceive color differently because one of the cones is overworked. 

Additionally, context is extremely important. Consider height—when something is farther away, it appears smaller, and when a larger object is placed with extremely large and extremely small objects, said object will appear larger next to the smaller one. Optical illusions also use different brightnesses to confuse the brain. If two objects of the same color are placed on surfaces with different colors, the colors of the original objects may appear different. Shadows affect how we see light, meaning that some images can appear lighter or darker—or closer or farther away—depending on their shadows. 

There are a total of three types of optical illusions: physiological illusions, cognitive illusions, and literal illusions. Physiological illusions occur when the brain is overstimulated by what the eyes are seeing. These illusions use so much light, movement, color, dimension, and size that confuse the brain. Interestingly, while these illusions are 2D and static, they often appear 3D and dynamic. A famous example of a physiological illusion is the Waterfall Illusion. This illusion uses color and shape to create an aftereffect that makes a stationary object appear to be moving. On the other hand, cognitive illusions occur when the brain attempts to understand what the eyes are seeing based on other images and the subconscious. This optical illusion reveals what the brain infers and understands about something that has not been explained, and it is observable both in images and conceptual reality. The Gambler’s Fallacy is an example of a cognitive illusion where the individual assumes that independent past events influence future ones; if a coin lands heads up multiple times in a row, an individual is likely to assume that the next flip will be tails even though both outcomes are equally probable. Lastly, literal illusions are when the perceived image is different from the images that are present. Often, this means that two images are interpreted as one. When the brain decides to focus on one part of an image, it uses the “filling-in” phenomenon to understand the empty space. A classic example of a literal illusion is Rubin’s Vase, where an individual either sees two faces or a vase.

Optical illusions can also use blinking dots to confuse the light receptors in our eyes, creating dark spots in our vision. Other literal optical illusions use the technique of selection, where two images form a single image. In this case, it is difficult for the brain to focus on more than one image at once.

Optical illusions are fascinating reminders that what we perceive is not always reality. While our brains are responsible for conveying the most factual information to us, they fail to properly interpret what may truly be present. Our brains draw on past experiences to make sense of things, often causing misconceptions. Whether through color, context, brightness, or other visual tricks, optical illusions take advantage of how our sensory systems and brains process information. They reveal the complex and sometimes flawed ways our minds construct reality, showing that our understanding of the world is not just about what we see but how we see it.