Merlin D. Tuttle, Bat Conservation International

or just making your way to a familiar bathroom, the neurons inside your skull are frantically juggling and crunching all kinds of complex calculations whenever you navigate. It's a formidable task, and just how the heck brain cells do it is still largely a mystery.

Today, in a groundbreaking study in the journal Nature, as a team of Israeli neuroscientists report found that acrobatic bats have an intricate kind of mental compass that works in three dimensions. By studying the brains of bats mid-flight, the scientists saw for the first time how this mental compass tracked its own up-down, left-right, and rotating motion.

"This is the first study that's shown any neural correlate to 3D navigation," says Arseny Finkelstein, the neuroscientist at the Weizmann Institute of Science in Rehovot, Israel who led the experiment. "What we found is that there are basically three types of brain cells—albeit with some overlap—that are sensitive to each one of these dimensions."

To make this discovery, the scientists needed astoundingly small recorders weighing just a few grams that could fit in a bat's brain and record its brain cells firing without interfering with the animal's flight. By synchronizing this information with data detailing the direction of the bats' heads mid-flight (obtained through high-speed video cameras), the researchers put together a picture of the 3D mental compass in action.

Merlin D. Tuttle, Bat Conservation International

The team found that specific neurons fired when the bat faced in a certain direction on a horizontal plane—orienting the bat like a hand-held compass. That much had been observed in previous studies of rats. But bats, who must navigate in three dimensions, have a much more complex kind of compass. For example, when the flying bat was twisted (think a barrel roll) or when the bat faced downward or upward, separate groups of brain cells either fell silent or started firing.

"There are even some cells that are sensitive to a certain combination of dimensions," Finkelstein says. "For example, they'd only fire if the bat was pointing its head at a specific direction and pitching its head upwards."

Strangely enough, the scientists also found that when trying to understand how the bat's compass translates to a mental-coordinate system, only a strange geometrical shape fit their data. While it's much easier to picture the bat's mental-coordinate system as a sphere—in simple terms, the compass always points toward a specific direction like a directional arrow piercing through a ball—a more accurate coordinate shape is actually a doughnut, explain the scientists.

Why? Because a donut-shaped coordinate system allows the bat, (whose direction is regularly, and rapidly inverted when it flips upside down to rest or take-off) to have opposing brain cells fire in conjunction with each other. This is thought to help stabilize it during vertigo-inducing acrobatics. In other words, the researchers sometimes saw east-facing cells and west-facing cells fire together when the bat flipped, which is confusing on a sphere, but is possible when the coordinate-grid is overlaid on a donut.

If you find this concept baffling, you're not alone.

"Frankly, at first I found the concept pretty hard to grasp," says Dave Rowland, a neuroscientist at the Centre for Neural Computation, in Trondheim, Norway who was not involved in the study. "It's not that bats think of their world as donut-shaped, or there's anything donut-shaped in their brain. This is just the topology that best fits for us to conceptualize what's going on."

Rowland says this research makes fascinating headway toward understanding how other animals, such as dolphins or humans, conceptualize traversing through three dimensions. "And this is a great example of how we can take advantage of the wildly diverse natural behaviors of animals to probe these basic mysteries in neuroscience," he says.

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