It isn’t just human eyes that can be duped by optical illusions. A new study has shown that the humble fly is also a sucker for illusions that meddle with motion perception. That might not necessarily be surprising given the puny size of a fly’s brain, but it is unexpected that humans appear to share many aspects of their perceiving motion with this vastly distinct species.

Reported in the journal Proceedings of the National Academy of Sciences, neuroscientists at Yale University closely watched the brain activity of fruit flies (Drosophila) as they were presented with optical illusions similar to the one below. As you can see, the image appears to slowly twist and turn, although it is a completely static image. 

Fruits flies have relatively simple brains, roughly the size of a poppy seed made of around 100,000 neurons (for perspective, humans have an estimated 100 billion neurons in our own brains). The brains of fruit flies have also been extensively studied and mapped by scientists, meaning it’s possible to closely track the activity of neurons in their visual system and understand how their brain is computing the visual information entering their eyes.

First of all, the researchers noted that the fly's head would instinctively move towards the image of the optical illusion, a fairly good indication that the fly was perceiving the static image as a moving image. More concretely, the fly’s brain activity reacted as if it was viewing a moving image when confronted with the optical illusion. This, the researchers say, is comparable to the way a human would perceive the optical illusion too. 

The researchers dug a little deeper into the research and started to play around with the fly’s brain cells using neurogenetics. By “turning off” the specific neuron types that deal with motion detection, the illusion of motion disappeared, thus confirming their hypothesis. 

“It was exciting to find that flies perceive motion in static images the same way we do,” Damon Clark, study author and associate professor of molecular, cellular, and developmental biology, physics, and neuroscience at Yale University, said in a statement. 

“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 said. “Understanding these shared strategies can help us more fully understand the human visual system.”