Monday, December 20, 2021

Flies Navigate Using Complex Mental Math

Amazing stuff!

"... Both teams showed that two types of brain cells (called PFNd and PFNv cells) track a fly’s heading (which way it’s facing) and its velocity (speed) as it walks. Since a fly has a nearly 360-degree field of vision ... to watch flies navigate in a 360-degree virtual reality environment. She improved on existing spherical treadmills for flies by adding an immersive virtual reality environment and placing the entire contraption in a clear plastic bubble, so that a camera underneath could record fly movement. By tracking neural activity as the fly navigated, the Wilson team found that both cell types are involved in making geometric calculations.

The neurons keep track of sideways and backward movements and register the compass direction in which the fly is moving. An animal could be facing north and walking forward, or facing west and walking to the right, Wilson says, and the brain would know that in both cases, the animal is actually moving in the same direction: northward.

That ability to adjust one’s orientation relative to both the body and the outside world is crucial for dead reckoning ...
discovered that the fly’s brain uses PFNd and PFNv cells to do this vector arithmetic. As the scientists report, a neural circuit in the fly brain can rotate, scale, and add the four vectors represented by different PFNd and PFNv populations to track the fly’s direction of travel relative to its body orientation.

To visualize this computation during flight, the researchers tethered flies to a platform in the center of a miniature arena and imaged their brains. Starlike dots displayed on the arena’s walls made the flies feel like they were traveling in various directions.

Inside the fly brain, activity patterns from PFNd and PFNv cell groups formed a sine wave, a type of regular, repeating wave that shows up often in physics and mathematics. The height and position of the waves vary depending on which way the fly was traveling, the team discovered. By adding the PFNd and PFNv waves, the fly was able to mathematically calculate if it was moving northward, eastward, etc. ..."

From the abstract of the 2nd paper:
"Many behavioural tasks require the manipulation of mathematical vectors, but, outside of computational models it is not known how brains perform vector operations. Here we show how the Drosophila central complex, a region implicated in goal-directed navigation, performs vector arithmetic. First, we describe a neural signal in the fan-shaped body that explicitly tracks the allocentric travelling angle of a fly, that is, the travelling angle in reference to external cues. Past work has identified neurons in Drosophila and mammals that track the heading angle of an animal referenced to external cues (for example, head direction cells), but this new signal illuminates how the sense of space is properly updated when travelling and heading angles differ (for example, when walking sideways). We then characterize a neuronal circuit that performs an egocentric-to-allocentric (that is, body-centred to world-centred) coordinate transformation and vector addition to compute the allocentric travelling direction. This circuit operates by mapping two-dimensional vectors onto sinusoidal patterns of activity across distinct neuronal populations, with the amplitude of the sinusoid representing the length of the vector and its phase representing the angle of the vector. ..."


Flies Navigate Using Complex Mental Math | HHMI For the first time, scientists have shown exactly how a fly brain creates a mental map of the body’s movement through the world.

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