Fly brains filter out visual information caused by their own movements, like humans

Our brains are constantly barraged with sensory information, but have an amazing ability to filter out just what they need to understand what’s going on around us. For instance, if you stand perfectly still in a room, and that room rotates around you, it’s terrifying. But stand still in a room and turn your eyes, and the same visual input feels perfectly normal. That’s thanks to a complex process in our brain that tell us when and how to pay attention to sensory input. Specifically, we ignore visual input caused by our own eye movements.

A visual filter: The image above shows a well-studied set of visual neurons within the fly brain. The Maimon lab has shown for the first time that certain visual neurons, including some within the set shown, are silenced as a fly rapidly turns right or left during flight.

Now, researchers at The Rockefeller University have identified a similar process in flies, whose brains ignore visual input caused by their own flight turns. This advance will allow researchers to better understand how ongoing behavior influences visual perception.

“Fly brains are small, so discovering that flies can ‘silence’ visual inputs means that we can aim for a comprehensive understanding of how this silencing process is implemented,” says study author Gaby Maimon, head of the Laboratory of Integrative Brain Function at Rockefeller. Postdoctoral fellows Anmo J. Kim and Jamie K. Fitzgerald, alongside Maimon, report their findings in the August issue of Nature Neuroscience.

By studying the details of this silencing mechanism in flies, we will gain a better understanding of how what we do influences what we see, Maimon says. These mechanisms are disrupted in people with schizophrenia, who have trouble interpreting sensory input properly.

In 1950, researchers Erich von Holst and Horst Mittelstaedt performed a simple experiment: They rotated the environment surrounding flies, and saw they walked in circles in response. But when flies perform a normal turn while walking, they also experience a similarly rotating stimulus on their retina, which they happily ignore. The researchers postulated that the brain must contain a mechanism that tells organisms to ignore what they see when they rotate their eyes or bodies. But the scientists didn’t have the technology to prove it.

To determine if silencing does, in fact, happen in flies, Kim, Fitzgerald, and Maimon attached flies to a tiny harness that allows the animals to flap their wings in tethered flight— the flying equivalent of walking on a treadmill — while the experimenters measured electrical activity from neurons. When these flies attempted to turn left or right, the researchers saw electrical signals that indicated the motor part of the brain was briefly silencing the visual part, says Maimon says. “In simple terms, the motor part of the brain is telling the visual part ‘don’t pay attention to this information, this is not relevant to anything that’s happening in the world.’”

Scientists have seen signatures of the same process in primate brains, but because humans and other primates have billions of neurons, it’s difficult to understand the precise mechanisms involved, Maimon says. “Since fly brains contain 100,000 times fewer neurons than human brains, we should be able to develop a much deeper understanding of this process, and build a blueprint for how to think about sensory silencing across all animals, and, ultimately, even humans.”

Nature Neuroscience online: August 3, 2015 Cellular evidence for efference copy in Drosophila visuomotor processing Anmo J Kim, Jamie K Fitzgerald, and Gaby Maimon