The horizontal stripes in the image above appear less contrasting than the vertical stripes when viewed from a distance. In reality, however, they are identical, as can be seen when viewed up close. When you turn your head 45° to the left or right, this illusion disappears. When you turn your head 90° to the left or right, the horizontal stripes become vertical, and therefore they appear more contrasted. I noticed this illusion in the image of the Fibonacci spiral. This square image looks as if it were made up of four triangles—two filled with horizontal stripes and two filled with vertical stripes. I believe this illusion works on a similar principle as the vertical-horizontal illusion. I also believe that this is related to the response of certain groups of neurons to various stimuli. Neurons react differently to various stimuli such as vertical stripes, diagonal stripes, horizontal stripes, etc. (see the book Gödel, Escher, Bach, Chapter 11). It is remarkable that this illusion affects everyone with varying intensity—some people see it very clearly, while for others it does not work at all. This illusion is even more evident in the image below.
