Peripheral drift illusion

The illusion is easily seen when fixating off to the side of it, and then blinking as fast as possible. Most observers can see the illusion easily when reading text with the illusion figure in the periphery. The motion of such illusions is consistently perceived in a dark-to-light direction.

Two papers have been published examining the neural mechanisms involved in seeing the PDI (Backus & Oruç, 2005; Conway et al., 2005). Faubert and Herbert (1999) suggested the illusion was based on temporal differences in luminance processing producing a signal that tricks the motion system. Both of the articles from 2005 are broadly consistent with those ideas, although contrast appears to be an important factor (Backus & Oruç, 2005).

Rotating snakes

Rotating snakes is an optical illusion developed by Professor Akiyoshi Kitaoka in 2003. A type of peripheral drift illusion, the "snakes" consist of several bands of color which resemble coiled serpents. Although the image is static, the snakes appear to be moving in circles. The speed of perceived motion depends on the frequency of microsaccadic eye movements (Alexander & Martinez-Conde, 2019).