Psychology professor Kohske Takahashi of Chukyo University in Japan presents a new optical illusion called the Curvature Blindness Illusion. The first thing you should know is that all the lines across the page are exactly the same shape. You can see that in the corners with the light and dark contrasting backgrounds. But in the gray background, some seem to be curvier, while others seem to be more angular. That's because of how the colors (actually shades of gray) of the lines are segmented. When the colors break at the top and bottom of the curve (in the line perceived as a zig zag) we see a corner where they meet. The lines with colors that continue over the upper and lower curve makes us perceive a gentler curve when the background contrast is low.
This illusion has two implications. First, the percepts of a gentle curve (wavy line) are impaired by discontinuity of contrast polarity at the turning point, while the percepts of an obtuse corner are not; this implies that the mechanisms of the gentle curve and obtuse corner detection are separable in terms of contrast polarity. Second, the illusory percepts of an obtuse corner (zigzag line) appears and replaces the gentle curve, which indicates the imbalanced competition between these two percepts.
Cool, huh? You can read about the experiments conducted with this illusion in Takahashi's paper in the journal i-Perception. -via Digg