Tropical Fish Can Be Tricked by Visual Illusions

A team of researchers has found that fish can be tricked by visual illusions. Triggerfish perceived a complex illusion the same way a human’s brain interprets it. The details are in a paper that was just published in the journal Scientific Reports.

A visual illusion occurs when the brain incorrectly perceives the properties of an object or scene. Scientists still aren’t sure what causes certain illusions but they’re useful for studying the perception abilities and visual systems of animals. Researchers from the University of Queensland in Australia decided to test a species of marine fish.

The team trained lagoon triggerfish (Rhinecanthus aculeatus), a predatory tropical fish found in reefs, to peck at colored squares for food rewards. Once the fish were consistently hitting the correct targets, the researchers exposed them to a visual illusion called Purves and Lotto’s lightness cube illusion. The illusion, which fools humans, makes two identical squares on a cube appear different colors—brown and orange. The illusion works by tricking the brain into thinking that the cube is shaded. This makes the brain assume that one square is getting a different amount of light and it adjusts the colors based on how “shaded” a square is. This confusing effect is still poorly understood.

The triggerfish were prompted to peck the correct colored square on a cube but this time, they were given a cube with the lightness illusion. Like humans, the triggerfish fell for the trick and picked the wrong colored square. This shows that reef fish are capable of perceiving certain visual illusions the same way as humans.

The team’s findings provide new insights into visual systems and how nonhuman animals, such as tropical fish, perceive illusions. Since fish have simpler neural processes compared to humans, the mechanisms that contribute to visual illusions may be earlier in the visual pathway.


Simpson et al. Coral reef fish perceive lightness illusions, Scientific Reports (2016).

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