Optical illusions explained in the eyes of a fly

The reason why people perceive movement in certain static images has mystified not only those who see these optical illusions, but also the neuroscientists who have tried to explain the phenomenon. Now Yale neuroscientists have found answers in the eyes of flies, they report on August 24 in the newspaper Proceedings of the National Academy of Sciences.

It turns out that flies are fooled by optical illusions as easily as humans.

“It was exciting to see that flies perceive movement in static images the same way we do,” said Damon Clark, associate professor of molecular, cellular and developmental biology and physics and neuroscience at Yale.

The small brain of flies makes it easy to track the activity of neurons in their visual system. Two members of Clark’s lab, Margarida Agrochao and Ryosuke Tanaka, presented flies with optical illusions similar to the one above. They then measured the behavior of the flies to check if the insects perceive movement in this optical illusion the same way humans do. Flies instinctively turn their bodies towards any perceived movement; When presented with the optical illusion, the flies would rotate in the same direction as the movement humans perceive in the pattern.

At the same time, the researchers looked at specific types of neurons that govern motion detection in flies and found a pattern of responses created by the static pattern. By turning these same neurons on and off, the researchers were able to change flies’ perception of illusory movement. By disabling two types of motion sensing neurons, they have completely eliminated the illusion. By disabling only one of the two types, they created flies that perceived an illusory movement in the opposite direction of the two active neurons. Based on this data, the researchers hypothesized that the optical illusion results from small imbalances in the way that different types of motion detectors contribute to how flies respond or do not respond to illusions. .

Since there are similarities between visual processing in flies and humans, the researchers designed experiments to test whether the theory they developed for flies could also apply to humans. They asked 11 participants to tell them about the movement they saw in the visual illusion. These experiments suggest – unsurprisingly – that human visual systems are more complicated than flies, but the results suggest that a similar mechanism underlies this illusion of movement in humans.

“The last common ancestor of flies and humans lived half a billion years ago, but both species developed similar strategies for perceiving movement,” Clark said. “Understanding these shared strategies can help us better understand the human visual system.”