Picture yourself exiting a subway car. You step onto the platform and for a moment, you’re completely disoriented. It feels weird, because when you entered the car at another station, you knew north from south, but now you’re turned around. According to researchers at Weizmann Institute of Science in Israel, that feeling is likely caused by a temporary failure in what they think is the mammalian brain’s 3D compass — a compass that doesn't rely on a magnetic field, but that orients our brains relative to our surroundings. And as it turns out, the best way to represent this 3D compass is by modeling it after a donut.
the best way to represent this 3D compass is by modeling it after a donut
To understand how animals orient themselves in three dimensions, the researchers studied bats. These mammals are an ideal choice, because they need to be able to locate themselves in space even when they are flying with their bellies pointing toward the sky. To figure out how bats achieve this navigational feat, the researchers used a video-monitoring system and microelectrodes that they inserted in the bats’ brains. These electrodes transmitted brain signals during flight and helped the researchers track the animals’ head rotation. The resulting readings allowed the researchers to identify neurons within the bat's brain that are tuned to specific 3D angles, sort of like a 3D vector. Moreover, the researchers realized that those neurons are located in a different region of the brain than those used to compute head directions in 2D — a finding that indicates that these two parameters are computed independently from each other.
"Basically what we found is that if you want to direct your head at a tree branch that’s at a certain elevation and angle from you," says Arseny Finkelstein, a neuroscientist at the Weizmann Institute and a co-author of the bat navigation study published in Nature today, "you [will] want to compute this [in a] 3D direction. This ‘3D head direction cell’ can do that."
entirely possible that the human brain possesses the same type of 3D compass
Humans are obviously very different from bats, but Finkelstein thinks that it’s entirely possible that the human brain possesses the same type of 3D compass. Even though we can't fly without the help of technology, we live in multi-layered environments — buildings with multiple floors are one example — and that means that being able to understand and process elevation, as well as direction in 3D is crucial.
That's why discovering these "3D head direction cells" is so important. Until recently, our understanding of navigation was limited to cells that can help us form a mental map of our surroundings, as well as cells that help our brains navigate on a horizontal plane. But little was known about which cells work in combination with our mental maps to allow navigation in 3D. "This was never identified in any species, not just bats, but in mammalians species," Finkelstein says. "Part of that was just because it used to be hard to track those angles — there was a technological gap." But advancements in recording technology allowed this study to go forward. And the information that the researchers gathered is what lead to them to model the mammalian mental compass in the shape of a donut.
Finkelstein, et al. / Nature
"It’s not an anatomical structure, but a functional representation," Finkelstein says. "It’s the structure that we think these neurons follow. It’s how they work together." The donut model is ideal, Finkelstein says, because it gives animals a lot of information about when their heads are inverted. A sphere wouldn’t be as detailed. "That’s a mathematical argument," Finkelstein says. But it’s also based on biological necessity. "The animal cares about this information."
The donut isn't an anatomical structure. It's a functional representation
According to David Rowland and May-Britt Moser, two neuroscientists at the Norwegian University of Science and Technology who didn't participate in this study, the researchers' work "demonstrates the immeasurable value" of using an animal behavior approach to study neuroscience. "By studying an animal that behaves in 3D," they wrote in a news story for Nature published today, "[Finkelstein and colleagues] have discovered one way in which the mammalian brain orients in 3D."
To underscore the importance of his findings, Finkelstein points to vertigo — a condition that's often described as a sensation of spinning while remaining stationary. He says that this might be caused by a misalignment between our brain's mental map and our 3D compass. Vertigo can be an extremely dangerous when it happens to pilots, because they lose the ability to tell which way is up, and which is down. "But we don’t know what triggers the realignment of the map and the compass," he says, "or how we integrate the map with the compass to move around." If we could understand that, we might be able to help pilots avoid major accidents.
But applications like that are still a long way off. In the meantime, Finkelstein and his colleagues plan to see if they can demonstrate that the same general mechanisms exist in rats and humans. "Animals need to navigate in 3D space — there are very few animals that only move on planar surfaces," Finkelstein says. "And identifying these cells that we didn't know about before; that's a big advancement," he says. But it's also "something that we need to understand better."