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.