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Small may be mightier than we think when it comes to brains. This is what neuroscientist Marcella Noorman is learning from her neuroscientific research into tiny animals like fruit flies, whose brains hold around 140,000 neurons each, compared to the roughly 86 billion in the human brain.
In work published earlier this month in Nature Neuroscience, Noorman and colleagues showed that a small network of cells in the fruit fly brain was capable of completing a highly complex task with impressive accuracy: maintaining a consistent sense of direction. Smaller networks were thought to be capable of only discrete internal mental representations, not continuous ones. These networks can “perform more complex computations than we previously thought,” says Noorman, an associate at the Howard Hughes Medical Institute.
You know which way you’re facing even if you close your eyes and stand still.
The scientists monitored the brains of fruit flies as they walked on tiny rotating foam balls in the dark, and recorded the activity of a network of cells responsible for keeping track of head direction. This kind of brain network is called a ring attractor network, and it is present in both insects and in humans. Ring attractor networks maintain variables like orientation or angular velocity—the rate at which an object rotates—over time as we navigate, integrating new information from the senses and making sure we don’t lose track of the original signal, even when there are no updates. You know which way you’re facing even if you close your eyes and stand still, for example.
After finding that this small circuit in fruit fly brains—which contains only about 50 neurons in the core of the network—could accurately represent head direction, Noorman and her colleagues built models to identify the minimum size of a network that could still theoretically perform this task. Smaller networks, they found, required more precise signaling between neurons. But hundreds or thousands of cells weren’t necessary for this basic task. As few as four cells could form a ring attractor, they found.
“Attractors are these beautiful things,” says Mark Brandon of McGill University, who was not involved in the study. Ring attractor networks are a type of “continuous” attractor network, used not just to navigate, but also for memory, motor control, and many other tasks. “The analysis they did of the model is very thorough,” says Brandon, of the study. If the findings extend to humans, it hints that a large brain circuit could be capable of more than researchers thought.
Noorman says a lot of neuroscience research focuses on large neural networks, but she was inspired by the tiny brain of the fruit fly. “The fly’s brain is capable of performing complex computations underlying complex behaviors,” she says. The findings may have implications for artificial intelligence, she says. “Certain kinds of computations might only require a small network,” she says. “And I think it’s important that we keep our minds open to that perspective.” 
Lead image: Yuri Hoyda / Shutterstock
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