The rising sun illusion fools humans and mice
Ren et al.
The optical illusions that affect our brightness perception also appear to work in rats, suggesting that the relationship between perception and pupil size evolved early in mammals.
The rising sun illusion consists of a series of petal-like shapes surrounding a white center. The petals are yellow in the center and black on the edges, and this gradation, combined with the shape of the petals, makes the white part in the center appear brighter than it actually is.
Nelson Totter of the University of Helsinki, Finland, says most people are fooled by this illusion, to the extent that their pupils constrict because their brains believe they are seeing something so bright. This response is known as the pupillary reflex and helps protect the retina from bright light.
“Higher-order regions of the brain can control these lower-order 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 rising sun illusion while tracking their pupils and found that they contracted in a similar way to humans.
The findings suggest that mental processes in rats may affect pupil size, says Totah. “This gives us a new window into the animal mind-body connection,” he says.
But Totah said it’s important to note that, like humans, we don’t really know if rats perceive images as brighter than they actually are. “You can’t ask a rat what he thinks,” says Tota. “All we can say is that this illusion, which appears subjectively bright to humans and causes pupillary constriction in humans, is also true in rats.”
It is reasonable to postulate that the brain mechanisms caused by this illusion in humans also exist in rats, and this suggests that the brain’s ability to control the pupillary reflex was much more advanced in mammalian evolution than previously thought. “But I don’t know why this ability evolved,” he says.
Nilli Lavie of University College London says it’s not surprising that the illusion of brightness affects pupil size in rats as well. “This makes sense because animals need to use cues about brightness and contrast to navigate their environment,” she says.
But the advantage of discovering this ability in rats is that researchers can more easily study the neurons involved in this illusion and determine how exactly it effects changes in bodily function. This is what happened.
“By studying the different types of mind-body connections, I hope we can begin to understand some general principles that the mind uses to influence the body,” says Totah.
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