Ventral motion parallax enhances fruit fly steering to visual sideslip

Flies and other insects use incoherent motion (parallax) to the front and sides to measure distances and identify obstacles during translation. Although additional depth information could be drawn from below, there is no experimental proof that they use it. The finding that blowflies encode motion disparities in their ventral visual fields suggests this may be an important region for depth information. We used a virtual flight arena to measure fruit fly responses to optic flow. The stimuli appeared below (n=51) or above the fly (n=44), at different speeds, with or without motion parallax cues. Dorsal parallax does not affect responses, and similar motion disparities in rotation have no effect anywhere in the visual field. But responses to strong ventral sideslip (206 deg/s) change drastically depending on the presence or absence of parallax. Ventral parallax could help resolve ambiguities in cluttered motion fields, and enhance corrective responses to nearby objects.
Methods
We recorded the steering responses of Drosophila melanogaster when presented with ventral and dorsal visual stimuli in a rigid tether virtual arena. Tethered flies were illuminated from above with an infrared light, while photodiodes below measured the shadow produced by each wing beat. Since flies steer by changing the relative amplitudes of left and right wing beats, attempts to turn produce a differential voltage by the sensor pair, and is reported as the voltage difference in wing beat amplitude, ΔWBA. This information is saved in a binary file per experiment and per fly. The data stored here has not been processed. We provide a Python code that can be used to read it, analyze it and reproduce the figures we present in our paper.