It turns out that flies are fooled by optical illusions as easily as humans are, which may help explain why we both are.

Flies have small brains, so it is easy to track the activity of neurons in their visual system and determine why they (and maybe we) sometimes perceive motion in static images.

And that is exactly was Damon Clark and colleagues from Yale University, US, did in their lab. They presented flies with optical illusions similar to the one at right then monitored their behaviour.

In this sort of image, humans see the circles rotating in different directions. The effect is particularly pronounced when their eyes move or blink.

Flies instinctively turn their bodies toward any perceived motion, and when presented with the illusion those in the study turned in the same direction as the motion that humans perceive in the pattern.

At the same time, the researchers examined specific neuron types that govern motion detection in flies and found a pattern of responses created by the static pattern.

By turning off two types of motion-detecting neurons, they eliminated the illusion entirely. By turning off just one of the two types, they created flies that perceived illusory motion in the opposite direction than they did with both neurons active.

Based on this data, they theorised that the optical illusion results from small imbalances in how the different types of motion detectors contribute to how flies respond, or don’t respond, to illusions.

Since there are similarities between fly and human visual processing, they designed experiments to test whether the theory might also apply in humans.

They asked 11 participants to tell them about the motion they saw in the visual illusion. Those experiments suggested that while human visual systems are more complicated, a similar mechanism underlies this illusion of motion in humans.

“The last common ancestor of flies and humans lived a half billion years ago, but the two species have evolved similar strategies for perceiving motion,” Clark says. “Understanding these shared strategies can help us more fully understand the human visual system.”

The findings are published in the journal Proceedings of the National Academy of Sciences.