Do fish perceive illusory motion?

Motion illusion refers to a perception of motion that is absent or different in the physical stimulus. These illusions are a powerful non-invasive tool for understanding the neurobiology of vision because they tell us, indirectly, how we process motion. There is general agreement in ascribing motion illusion to higher-level processing in the visual cortex, but debate remains about the exact role of eye movements and cortical networks in triggering it. Surprisingly, there have been no studies investigating global illusory motion evoked by static patterns in animal species other than humans. Herein, we show that fish perceive one of the most studied motion illusions, the Rotating Snakes. Fish responded similarly to real and illusory motion. The demonstration that complex global illusory motion is not restricted to humans and can be found even in species that do not have a cortex paves the way to develop animal models to study the neurobiological bases of motion perception.