The Café Wall illusion is a well-known optical illusion first observed by British psychologist Richard Gregory. It is named after a café in Bristol, UK, where this type of pattern was used on a tiled wall, drawing the attention of one of Gregory’s colleagues, Steve Simpson.

The illusion consists of a series of parallel horizontal lines separating rows of alternating light and dark colored tiles (usually black and white or grayscale). At first glance, the lines separating these rows appear to be slanted or misaligned, when in fact they are perfectly straight and parallel.

The underlying cause of this illusion is believed to be related to the contrast of the dark and light tiles and the intermediate color of the lines separating the rows. When the gray lines intersect with the black and white tiles, the gray appears lighter against a black tile and darker against a white tile due to a phenomenon known as simultaneous contrast illusion. This makes the lines appear offset.

This illusory perception is heightened by the offset placement of the tiles in each alternating row, amplifying the perceived misalignment. The contrasting colors stimulate different parts of the visual system, resulting in a discrepancy in how we perceive the image, thus creating the illusion of slanted lines.

In essence, the Café Wall illusion underscores the fascinating ways our brain interprets visual information, and demonstrates how easily our perception can be deceived by complex patterns and contrasting colors.

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