A side view of a fly eye with the rubber-band like retinal muscle in red. The muscle is innervated by a retinal motor neuron (green) that controls retinal movements. Credit: Igor Siwanowicz
Although we don’t feel it, our eyes are constantly making tiny involuntary jitters, called microsaccades, to prevent our visual neurons from adapting to the object in focus. This ensures we continue to see what we’re looking at, rather than the image fading once our eyes become accustomed to the sight.
But what if you physically couldn’t move your eyes? Insects can’t and they make due, right? A new study from Rockefeller University shows insects have evolved to solve this problem. Rather than adjusting their vision by moving their heads, they actually move their retinas inside their eyes.
“It is super interesting that fruit flies move their retinas because it suggests there could be a whole other set of features yet to be discovered that the visual system uses to help gather and process information,” said author Lisa Fenk, who was a postdoctoral scholar at Rockefeller University when she conducted the study.
In addition to new information on the biology of animal eye movements, Fenk says this research could ultimately lead to a better understanding of cognitive disorders like autism and schizophrenia, in which eye movements are impaired.
For the study published in Nature, Fenk and team first had to determine whether fruit flies even have muscles attached to their retinas. Using a fluorescent molecule that binds to muscle fibers, microscopic images taken by the team revealed two glowing muscles for each retina.
Next, the researchers held the flies’ heads steady in front of an LED screen displaying moving patterns and used cameras to track the position of the flies’ retinas. They found that the flies’ retinas moved along with the patterns—similar to how human eyes dart back-and-forth to track the ball during a game of tennis.
Surprisingly, the researchers discovered the flies’ retinas weren’t just tracking patterns. Rather, they were making intentional side-to-side and up-and-down movements when the insects were viewing a stationary scene—similar to the microsaccades of humans and other vertebrates.
After detecting correspondence between the retinal jitters and neural activity of the flies, the researchers hypothesize the tiny shifts have big purposes. They think the shifts, which move the retina by only a degree or less in visual angle, improve both resolution and depth perception.
“Retinal movements may thus help to explain how fruit flies, which have only about 6,000 photoreceptors per eye—a trifle compared with the hundreds of millions of receptors in a human eye—can still see surprisingly well,” said the research team.
To test depth perception, the Fenk and team had the flies “walk” on a treadmill-like contraption featuring small gaps. All the flies made convergent or cross-eyed retinal movements just when they were about to cross the gaps. On the other hand, flies engineered to move their retinas more sluggishly had trouble getting across the gaps.
The researchers suspect that flies sweep the viewpoints of their two retinas as a way of judging distance. It’s a specific trick humans don’t have—if we did, we could judge distance by simply crossing our eyes.
Fenk and associate professor Gaby Maimon will now test if flies perform retinal movements when they sleep, or when they shift their attention to different parts of the visual field.
“We’ve only the scratched the surface of understanding what functions these retinal movements serve,” said Maimon.