Dynamic trajectory cues drive sequenced integration in approach detectors

Detecting approaching objects is essential for survival, and studies across animals have identified neurons and circuits selective for objects that grow in size over time, a hallmark of visual approach. However, approach as a physical process generates a rich ensemble of correlated cues governed by geometry and physics, of which size expansion is just one, and how the brain incorporates this broader cue structure to detect approach remains unknown. Here, we show that a visual stimulus without size expansion but with modulated luminance, a cue intrinsic to approach, elicits compelling percepts of approach and retreat in both humans and the fruit fly Drosophila, revealing an unsuspected, cross-species percept of approach based on luminance alone. Using targeted genetic silencing and two-photon calcium imaging in flies, we identify neurons that mediate evasive responses to both expansion-based and luminance-based approach, establishing them as general approach detectors. Guided by the geometry of approach, we show that luminance cues should precede detectable expansion signals during naturalistic approach. Consistent with this, these neurons combine luminance and expansion cues synergistically, but only when luminance changes precede expansion, matching their natural temporal order, thus performing sequenced cue integration. Together, these findings anchor a cross-species perceptual phenomenon to a defined circuit computation and reveal how neural circuits are tuned to the dynamic cue structure of natural events.