Rats fooled by optical illusion may shed light on evolution of the eye

An optical illusion that plays with our perception of brightness also seems to work on rats, suggesting that the connection between perception and pupil size evolved early on in mammals.

The Asahi illusion is made up of a series of petal-like shapes surrounding a white centre. The petals are yellow close to the centre, transitioning to black at the edges, and this gradient, combined with the shape of the petals, makes it seem as if the white centre is brighter than it actually is.

Most people are tricked by this illusion, says Nelson Totah at the University of Helsinki in Finland, to the extent that our pupils constrict as our brains believe we are looking at something very bright. This reaction is known as the pupillary reflex and it helps protect our retinas from bright light.

“Higher-level brain regions can control these lower-level reflexes and we wanted to see if this ability was unique to humans,” says Totah.

To do this, Totah and his colleagues showed 14 rats the Asahi illusion while tracking their pupils, and found that they constrict just like in humans.

The findings suggest that the rats’ mental processes can affect pupil size, says Totah. “It gives us a new window into the mind-body connection in animals,” he says.

But Totah says it is important to note that we don’t actually know if the rat perceives the image as brighter than it actually is, in the same way humans do. “We can’t ask rats what they think,” says Totah. “All we can say is that this illusion, which appears subjectively bright to humans and causes pupil constriction in humans, also does so in rats.”

It is reasonable to assume that the brain mechanisms triggered by this illusion in humans are also present in rats, says Totah, which suggests that the brain’s ability to control the pupillary reflex occurred far earlier in mammalian evolution than previously thought. “But I have no idea why this ability evolved,” he says.

Nilli Lavie at University College London says she isn’t particularly surprised that the brightness illusion’s effects on pupil size are present in rats also. “It makes sense as animals also need to use cues about brightness and contrast to navigate their environments,” she says.

However, she notes that the advantage of discovering this ability in rats is that researchers can now more easily study the neurons involved in this illusion and determine how exactly it results in a change in bodily function.

“Hopefully by studying different types of mind-body connections we can start to understand some general principles that the mind uses to affect the body,” says Totah.

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