Facilitation of neural responses to targets moving against optic flow

Abstract

<jats:title>Abstract</jats:title><jats:p>For the human observer, it is difficult to follow the motion of small objects, especially when these are displayed against background motion. However, insects efficiently do this, as evidenced by their ability to capture prey, pursue conspecifics or defend territories, often in foliage-rich areas. This ability has been attributed to insect optic lobe neurons sharply tuned to target motion, as these respond robustly to targets even against moving backgrounds. However, their presumed post-synaptic counterparts, the target selective descending neurons (TSDNs), do not, suggesting inhibition by widefield motion. Importantly, though, previous experiments used panoramic background images, which not only lacked 3D cues, but also high-contrast features, experienced in natural flight. To mitigate this, we here use widefield motion consisting of many targets moving coherently to simulate the type of 3D optic flow that would be generated by the animal’s own motion through the world. We show that TSDNs are inhibited by syn-directional optic flow, consistent with previous findings. However, and more surprisingly, we show that TSDNs are facilitated by optic flow in the opposite direction to target motion. Our results thereby emphasize that the responses of descending neurons cannot simply be inferred from their presumed pre-synaptic counterparts.</jats:p>