Internal compass guides fruit fly navigation

Scientists have shown why fruit flies don’t get lost. Their brains contain cells that act like a compass, marking the direction of flight.

It may seem like a small matter, but all animals — even Siri-dependent humans — have some kind of internal navigation system. It’s so vital to survival that it is probably linked to many brain functions, including thought, memory and mood.

“Everyone can recall a moment of panic when they took a wrong turn and lost their sense of direction,” says Sung Soo Kim of the Howard Hughes Medical Institute’s Janelia Research Campus in Ashburn, Va. “This sense is central to our lives.” But it’s a complex system that is still not well understood. Human nerve cells involved in the process are spread throughout the brain. In fruit flies, the circuitry is much more straightforward.

Two years ago, Janelia researchers reported that the flies appear to have a group of about 50 cells connected in a sort of ring in the center of their brains that serve as an internal compass. But the scientists could only theorize how the system worked. In a series of experiments published online May 4 in Science, Kim and his Janelia colleagues describe how nerve cell activity in the circle changes when the insects fly.

The scientists tethered Drosophila melanogaster flies to tiny metal rods that kept them from wriggling under a microscope. Each fly was then surrounded with virtual reality cues — like a passing landscape — that made it think it was moving. As a fly flapped its wings, the scientists recorded which nerve cells, or neurons, were active, and when. The experiments revealed that clusters of about four to five neurons would fire on the side of the ring corresponding to the direction of flight: one part of the ring for forward, another next to it for left, and so on.

The researchers then tested whether artificially activating cells at different points along the ring, essentially moving the compass needle, would disrupt the flies’ sense of direction. If the insects thought they were heading north, the scientists used a laser to stimulate neurons on the south side of the ring. When that happened, the flies attempted to make random turns. “It seemed very confusing for the fly,” Kim says.

When this same experiment was conducted in darkness, which by itself is disorienting, the results were difficult to interpret, the scientists say, because it was impossible to know how much the inability to see was responsible for the haphazard flight.

Mammals don’t have the same convenient ring of navigation cells. Instead, neurons known as “head direction” cells are found in many brain regions. Those cells turn on as you move, noting the direction you’re facing, helping you find your way around even without a GPS device announcing the next turn. “We know that head direction cells exist in mammals, but we don’t know the architecture of the system,” says Hervé Rouault, a coauthor of the new study.

While internal navigation is more complicated in mammals, it’s important to understand a basic system like the one found in fruit flies, says Adrien Peyrache of McGill University in Montreal, who studies the neural basis of direction in rodents. In mammals, head direction cells don’t form a ring, but they do act in concert, he says. “One of the core questions is how the systems are wired.”