A fruit fly the size of a sesame seed can keep a straight course across a room, hold a bearing in the dark, and remember which way is “home”. It does this with a structure in its brain called the central complex, where a ring of neurons in the ellipsoid body — the E-PG or “compass” cells — carries a single localized bump of activity. The position of that bump around the ring is the fly’s internal sense of heading. As the fly turns, partner “shifter” neurons move the bump around the ring at exactly the right speed, integrating angular velocity into a running estimate of direction. This wiring is not a guess: it was traced cell by cell in the Drosophila connectome (the hemibrain in 2020 and the whole-brain FlyWire reconstruction in 2023–24).
This exhibit runs that circuit as a real ring attractor — thirty-six rate neurons whose local excitation and global inhibition force the population to hold exactly one bump, never two and never none. Turn the fly with the dial or the arrow keys and watch the bump follow. The point lands when you switch the lights off: with no landmark to correct it, the bump keeps tracking from self-motion alone, but a small, honest gain error makes it slowly drift — the fly’s internal compass and its true heading pull apart, just as a real fly’s does in the dark. Turn the lights back on and the landmark snaps the bump back to truth. That is the difference between dead reckoning and a fixed reference, made visible.
A ring attractor is one of the most beautiful ideas in neuroscience: a network that represents a continuous variable — an angle — as a stable pattern of activity that can hold its value without any input, and update it smoothly when the world moves. The same motif appears in your own head-direction cells, deep in the mammalian brain. From a wasp’s reflex to a mouse’s map to a fly’s compass, the thread is the same one that runs through every DigiSim lab: computation is what patterns of activity do.
Specifications
- Circuit
- Ellipsoid-body E-PG ring + P-EN shifter cells (central complex)
- Model
- 36-unit ring attractor — local excitation, global inhibition, one bump
- Behaviour
- Tracks heading, persists in the dark, drifts, vision re-anchors
- Source
- Wiring confirmed by the Drosophila connectome (hemibrain, FlyWire)
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