A series of paired lines passes through areas of white, Grey, and black. The lines remain the same throughout. They have a consistent wavy (sinusoidal) shape.
The difference between the sets is the placement of light and dark segments: one Set has the tone change at the bottom of the curve.
As the sets of lines pass through the grey area, some of them seem to take on an angular, zig-zag quality. The effect is extremely compelling.
Psychologist Kohske Takahashi of Chukyo University of Japan discovered the Illusion. He suggests that when the brain's visual system is faced with ambiguous cues about whether it's seeing curved or straight-segmented lines, it favors the angular cues:
"The underlying mechanisms for the gentle curve perception and those of obtuse corner perception are competing with each other in an imbalanced way and the percepts of corner might be dominant in the visual system."-----
For a high level discussion, read the comments after the Discover Magazine blog post.
Read Mr. Takahashi's scientific paper.
Thanks to several of you who let me know about this.