At first sight, it looks like a regular color palette. At a closer investigation, you may notice false shadows appearing where the sides of rectangles meet. This is called the Mach bands illusion, named after an Austrian physicist, Ernst Mach, who discovered it.

A popular theory links the Mach bands illusion to lateral inhibition. This is a phenomenon that occurs between retinal neurons when one neuron or sensory cell activates a neighboring neuron. Lateral inhibition makes us perceive the darker areas even darker, and the lighter areas appear much lighter.

Like other visual and perceptual illusions, Mach bands illusion helps scientists study the way the eye and brain process visual information.