New evidence the neural rules of navigation are universal.
BY ADITHYA RAJAGOPALAN
It may seem absurd to compare a tiny fruit fly’s brain to that of a majestic elephant. Yet it is the dream of many neuroscientists to find deep rules that very different brains share. As Gilles Laurent, a neuroscientist at the Max Planck Institute for Brain Research in Frankfurt, Germany, who has studied a variety of animals, from locusts to turtles, has said, “Neural responses can be described by the same mathematical operation … in completely different systems.” Vivek Jayaraman, a researcher at the Howard Hughes Medical Institute’s Janelia Research Campus, and a former student of Laurent’s, believes that neuroscientists are on the verge of identifying some of these deep neural rules. Grasping them would advance another neuroscientific dream: to be able to predict animal behavior as easily as Newton could predict the behavior of a moving object.
Jayaraman and a small number of researchers studying the brain’s GPS have, in fact, already experienced the thrill of discovering one such rule. It governs something essential—the ability of an animal to keep track of where it’s headed. What’s more, recent experiments on flies hooked up to virtual-reality environments—one from Jayaraman’s group and another from Rachel Wilson (a former postdoc of Laurent’s) and colleagues at Harvard—show how a fruit fly’s visual cues ensure the stability of its heading. The findings offer insight into how mammals, like us, might build maps of their world.
When you’re moving freely, certain neurons are only active when you’re facing a particular way. It does not matter where you are—as long as you keep looking ahead, in that direction, the cell fires. These so-called head-direction cells were first identified in a region of the rat brain very close, and intimately linked to, the hippocampus, the home of place cells, and the basis for your map of the world. There are a large number of these cells, but any one cell is only active in a certain small region of the environment. They act like a map, telling you where you are. Such a map isn’t very useful without knowing where you’re headed. That’s why the brain also creates a mental compass, using head-direction, or compass, cells.
It is striking how closely these two systems work together. If you were to rotate the visual cues that a rat uses to define its heading, then the head-direction cells would remap themselves to this shifted world. At the same time, the place cells of the hippocampus would also rotate the mental map. These two groups of cells provide complementary information that, if correctly put together, are enough to help the rat navigate anywhere in its environment—even in the dark. Head-direction cells, in other words, don’t simply respond to visual cues. They maintain their activity even when a rat can’t see its surroundings (or when you cover your eyes). How does the brain set up this stable compass-like representation? More perplexingly, how does it maintain this stable heading without visual cues?
Scientists can predict whether a fruit fly thinks it is turning right or left.
Experimental evidence in rats and mice suggests that a doughnut-like structure of head-direction cells keeps the brain’s compass reliable. A 1995 paper, “A model of the neural basis of the rat’s sense of direction,” first proposed this doughnut or “ring-attractor” picture. In it, William Skaggs and his colleagues from the University of Arizona hypothesized that the head-direction cells—which did not seem to have any visible characteristic group structure—connect together to form an imaginary ring that creates a 360-degree map of two-dimensional space. “The aim of this effort is to develop the simplest possible architecture consistent with the available data,” the researchers wrote. “The reality is sure to be more complicated than this model.”
Each cell, a point on the ring, refers to a particular heading. Nearby cells on this ring refer to adjacent and similar headings, and activate each other, while cells that are far apart, and refer to opposing or near-opposing headings, deactivate each other. The researchers suggested a rule for how information from the brain’s visual areas enters the ring of head-direction cells, contributing to their characteristic activity pattern. They also incorporated the vestibular system of the brain, which detects head turns, to allow the ring to maintain its activity in the dark…