Video: How the fly brain chooses between two escape takeoffs

HHMI has released a video summarising research from the Card lab at Janelia Research Campus on how the brain of Drosophila melanogaster recombines separately processed visual features to choose an escape manoeuvre. The fly brain detects features of the visual world — shape, orientation, colour, motion — in parallel, but that information must then be recombined to guide behaviour; this study asks how that recombination determines which of two escape takeoffs a fly performs when a predator looms into view.

A fly’s long takeoff — extending its wings before jumping and beginning to flap — is more stable, while its short takeoff is a bare jump with folded wings that leaves the fly tumbling and needing to right itself in the air; both happen faster than an eye blink. Earlier work from the same lab had shown that a short takeoff occurs when a single pair of large descending neurons, the giant fibers, fire before the other descending neurons that drive a long takeoff. Here, the authors recorded giant fiber activity intracellularly while presenting looming stimuli that simulate an approaching predator, and used genetic tools to silence candidate feature-encoding neurons.

Rather than a single type of looming-sensitive neuron feeding the giant fibers, they found two: one encoding the looming stimulus’s angular velocity, and another encoding its angular size. Their model is that these two signals are summed in the giant fibers — a slow-looming predator mostly drives the size-encoding channel, which excites the giant fibers but not always to spiking threshold, favouring a long takeoff; a fast-looming predator drives both channels together, pushing the giant fibers over threshold earlier and biasing the fly towards the faster, short takeoff.