The letters “HHMI” might look like this to a fly after being filtered through its brain.
image courtesy of Igor Siwanowicz, Johannes Seelig, and Vivek Jayaraman

Fly Brain Filters

A cluster of neurons help flies make sense of a visual scene.

What goes on in a fly’s brain as it buzzes around a room? Does it recognize every object, or does it take cues from general lines and shapes? Using two-photon laser scanning microscopy and calcium imaging, Vivek Jayaraman, a lab head at Janelia Farm Research Campus, probed the brains of fruit flies to find out.

An area deep in the fly brain, called the central complex, lets flies recognize visual landmarks while they’re moving and use that information to orient themselves, locate safe places, and avoid not-so-safe ones. Until recently, however, scientists didn’t know how the fly central complex takes in and processes visual information. Fly brains are very tiny, and the only way to study them was by immobilizing the flies, which prevents any sort of mobility study. A few years ago, however, Vivek’s team figured out how to immobilize a fly’s head in a two-photon microscope, while its wings and legs move freely.

Vivek’s postdoctoral researcher Johannes Seelig used the technique to look at a cluster of neurons in the central complex called ring neurons. When flies were placed in a small virtual-reality arena and presented with simple patterns of light, their ring neurons responded more strongly to vertical bars than horizontal bars projected on the walls. This made sense, since flies have an innate tendency to walk or fly toward vertically oriented stimuli. The neurons were, in effect, extracting, or filtering out, visual information.

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Fly responses to visual stimuli in a virtual reality arena. Clockwise from top left: Changes in ring neuron activity (tracked with fluorescent markers); the visual stimulus presented to the fly in the arena; the fly’s behavior; changes in calcium levels in the ring neurons. When the fly is not flying, its responses to the moving bar are more pronounced. Video courtesy of the Jayaraman Lab.

“These input neurons seem to help break down the visual scene around the fly into particular features that flies care about,” Vivek says. “Later, neurons in the central complex presumably use these features to decide what to do in their surroundings.” As the duo reported in Nature on November 14, 2013, this orientation preference mirrors what scientists have found in mammals—that certain neurons in the visual cortex tune in to an object’s orientation.

Next, Vivek plans to look deeper into the central complex. “By marching through these networks, we hope to begin to understand how sensory information is integrated to make motor decisions,” he says.