Humans have been intrigued by optical illusions for thousands of years. Desert mirages, the image of water shimmering in hot desert sand, or the “waterfall illusion” where objects seem to be moving upward after viewing the downward cascade of a waterfall, are examples of natural optical illusions. Modern optical illusions are usually intentionally man-made. They can be as simple as the black and white drawing that shifts between and old crone and a beautiful woman, or as complex as Stereograms, where a picture of a seemingly blurred image becomes a 3-D Lion leaping out of a jungle scape.
What is really going on in the brain to make a person see something that isn’t really there? While there is no one answer – optical illusions are a bit of a mystery even in the scientific realm – there are a series of answers depending on the type of visual illusion one is experiencing. It’s a process of how the eyes and the neural circuitry in the brain work together to make sense of the world around us. The right geometric pattern, proportions, lighting, etc. trick our brain into seeing things that aren’t there, or not seeing things that are, because our brains have deeply ingrained patterns of “what is”.
Common Types of Optical Illusions
Over time, optical illusions have been categorised in terms of what kind of visual and mental processes form the image in the human mind. Common types of illusions are:
Geometric or Angular
In this type of illusion, spatial symmetry and color variation trick the eyes. Eyes vibrate slightly when they are taking in an image. These vibrations are called “micro-saccades”. They move in tiny increments to take in the variations in an image and it is their own movement that creates an illusion of motion within a still image.
Geometric or Angular Illusions
The Zollner illusion and Fraser’s Spiral are good examples of this common optical illusion. The eyes are challenged by so many parallel lines, spirals, or tilted elements that parallel lines can begin to appear as angled towards each other, or the eye will begin to perceive a series of concentric circles as forming a spiral. The eyes begin to overestimate the angles and create a new image for the brain.
These can be some of the most fun to figure out. The “Devil’s Fork” is a common example, as are M.C. Escher’s famous art pieces. This is a realm where art and science collide. Our eyes have been trained to see a beginning, a middle, and an end because it is a common theme in the world. Trees have roots, a trunk and branches. Staircases begin at a lower level and lead to a higher one. In the “Devil’s Fork” the brain perceives the towers to be connected to the U-stems at the bottom even though the actual lines do not follow appropriately.
In this type of an illusion, our eyes use what they have been conditioned to deduce regarding size and distance to form proportion. But in a 3-D image, such as Shepard’s “Turning the Tables,” identically shaped parallelograms appear as two completely different sizes. This can actually agitate an observer once the discrepancy is highlighted because the eyes still can’t perceive the “sameness” without physically tracing or cutting out one and placing it over the other.
Some people struggle to see optical illusions, which is why science cannot give concrete reasons for interpretations. Regardless, for most people, optical illusions provide an interesting insight, and evidence, to how fragile our perceptions of reality really are. They thrill us with their unsettling aura of mystery.