Fruit flies differ from us in many ways, including the fact that they cannot move their eyes relative to the rest of their head. However, this is not a problem, as new research shows that they move their retinas in their still eyes instead.

Fruit flies are not actually the first animals known to use the retinal movement strategy. Spiders have already been observed doing this, which prompted Lisa Fenk and Rockefeller University Assoc. Prof. Gaby Maimon to wonder at the flies. Dr. Fenk was a postdoctoral fellow in Maimon’s lab at the time and is now a group leader at Germany’s Max Planck Institute for Biological Intelligence.

Using a fluorescent molecule that binds to muscle fibers, scientists initially discovered that fruit flies do indeed have two muscles attached to each of their retinas—these muscles allow the retina to move both back and forth and up and down. When flies were subsequently placed in front of a panoramic LED screen with their heads still, they were found to move their retinas to follow moving patterns displayed on the screen.

Side view of a fruit fly eye, with the elastic band like retinal muscle highlighted in red

Igor Sivanovich

However, eye movements are not only used to track the action. When looking at stationary objects, our eyes still make small involuntary movements known as microsaccades. They prevent our visual neurons from adapting to visual stimuli, so our eyes continue to see what we’re looking at. Otherwise, our perceived image of the object will begin to fade until we make a conscious effort to move our eyes.

When flies were presented with still scenes, their retinas also made small microsaccade-like movements. It is thought that along with the purpose that microsaccades serve in humans and other animals, the retinal movements of flies may also help improve the resolution of their vision – while we have approximately 150 million photoreceptor neurons in each eye, fruit flies have only about 6000.

Also, insects’ movable retinas may allow them to do something we can’t.

When the flies were held on a treadmill-like apparatus with small gaps in the walking surface, their retinas moved toward each other in a criss-cross fashion as they crossed the gaps—which they could easily do. When the same test was performed on flies that were engineered to have slower-moving retinas, the insects had more difficulty moving through the gaps. This suggests that by sweeping their retinas together, fruit flies are able to judge the distance of the gaps they are approaching.

“It’s extremely interesting that fruit flies move their retinas, because it suggests that there may be a whole set of features yet to be discovered that the visual system uses to help gather and process information,” Fenk said. The scientists believe their findings may even lead to a better understanding of human cognitive disorders in which eye movements are impaired, such as autism and schizophrenia.

An article about the research was recently published in the journal Nature.

source: The Rockefeller University