Visual circuitry for distance estimation in Drosophila

Animals must infer the three-dimensional structure of the surrounding world from two-dimensional images on their retinas. For instance, animals use visual cues like motion parallax and binocular disparity to judge distances to objects as they navigate their environment. Although studies in several vision models have found neural activity that correlates with distance signals, the causal behavioral role of neurons and the range of possible properties that inform distance perception have remained poorly understood. Here, we developed a novel high-throughput behavioral assay to identify neurons in the fly visual system that are required for distance estimation during free locomotion. We found that silencing the primary motion detectors in the Drosophila visual system eliminated the ability to perceive distance in this paradigm, consistent with evidence that flies rely on motion parallax to judge distance. Through a targeted behavioral screen of visual neurons and in vivo two-photon microscopy, we identified a visual projection neuron in the lobula that encodes relative motion of foreground and background in a way that is tuned to motion parallax. Interestingly, it differs from previously identified parallax-tuned neurons in its lack of direction selectivity to moving bars or to moving backgrounds. This non-canonical parallax neuron’s tuning is interpretable in the context of parallax signals it would likely encounter during fly walking. Our results demonstrate how both direction selective and non-direction selective feature detecting neurons can contribute to distance estimation using parallax cues and provide a framework for considering broader classes of parallax-encoding neurons within visual systems in other organisms.