The Pinna-Brelstaff illusion is a classic example of an illusory motion effect, which involves static images appearing to move due to certain visual triggers. In this case, the illusion consists of two concentric circles composed of radial lines or sectors, usually alternating in color, with a black dot at the center.
The illusion is interactive and requires the viewer’s active participation to work. When you focus on the black dot and move your head back and forth towards and away from the illusion, the circles seem to rotate or move in a direction opposite to your head’s movement. Interestingly, the rotation appears to be faster when you move your head closer and slower as you move away.
This illusory motion is believed to be caused by differences in processing speeds for different types of visual stimuli in our brains. As you move your head, the information your brain processes changes — the radial lines closer to the center of your field of vision (closer to the black dot) move across your retina faster than those at the edge. This differential motion creates a perception of rotation, even though the image is static.
Essentially, the Pinna-Brelstaff illusion underlines our visual system’s fascinating complexity and shows how our perception can be manipulated to see motion where there is none.