Classic Optical Illusion Solved after More Than 100 Years

level of light emitted by images is at the root of optical illusion

By Jonny Lupsha, Wondrium Staff Writer

Scientists now know why a color looks brighter or darker against gradient backgrounds, Science Alert reported. The common optical illusion has to do with the actual light emitted from the image rather than our analysis of gradients. Our brains deceive us surprisingly often.

Brain and brain wave concept
Optical illusions can trick your brain into perceiving a visual effect first in one way and then second in another way. Photo by Illustration Forest / Shutterstock

A new experiment solved a classic optical illusion: Why do two equally bright shapes look brighter or darker when placed against different spots along a gradient background? “If a shape is surrounded by something dark, we perceive it as brighter than if they’re amidst lighter shades,” the Science Alert article said. This concept is known as simultaneous brightness contrast. But when and why, in terms of thought process, does it happen?

The answer is luminance—the actual light levels emitted by an image. “It turns out that luminance, even though we’re not always conscious of it, does contribute to our brightness estimates, suggesting high-level thought processes are not required to make this judgment between contrasts.”

Our brains trick us all the time. Understanding optical illusions can help teach us why.

Bad Optics

The quickest way to find the root of why our brains get tricked is to define phenomena like optical illusions.

“All types of illusions, by definition, are times when the brain constructs sensory perception in an incorrect way—it’s a misperception of reality,” said Dr. Steven Novella, Assistant Professor of Neurology at the Yale School of Medicine. “Generally, objects in our world appear to be stable and they appear to be accurate with respect to reality. That property, psychologists refer to as constancy—and optical illusions, by definition, represent an exception to constancy, a time when objects are either not stable or they do not accord with reality.”

Dr. Novella said the reason this happens is because our brains have to make assumptions about the world we sense around us. Most of those assumptions are correct and they help us make up our perception of reality. However, those perceptions that are incorrect are optical illusions. In fact, constructed optical illusions, like magic tricks, exploit known misperceptions of reality.

Types of Optical Illusions

There are several kinds of optical illusions that play on different ways we perceive the world.

“There are perspective illusions, which all exploit the ways in which our brains can construct three-dimensional images out of two-dimensional input,” Dr. Novella said. “Visual input is all two-dimensional. Our retinas are flat-curved surfaces and the brain has to then infer from that two-dimensional input a three-dimensional world with relative size and distance and movement.”

The brain does this mostly with stereoscopic vision, Dr. Novella said, meaning it instantly and constantly compares the information both eyes give it. This can be tested by covering up one eye and seeing how your brain adapts.

“There are optical illusions that are also based upon relative shade and size of objects,” Dr. Novella said. “There are illusions based upon ambiguous stimuli where the brain can construct the image in more than one way and your brain will tend to switch back and forth between different constructions. There are afterimage optical illusions that the visual system will adapt to light and color, for example, and then when that is changed the adaptation causes an afterimage that’s not really there.”

Knowing that this is happening in our brains is the first step in unraveling the mysteries of optical illusions.

Dr. Steven Novella

Dr. Steven Novella contributed to this article. Dr. Novella is Assistant Professor of Neurology at the Yale School of Medicine. He earned his MD from Georgetown University and completed his residency training in neurology at Yale University.