Parallel descending neurons differentially encode target and looming information in the hoverfly visual system

Detecting small, fast-moving objects is crucial for behaviors such as pursuit, territorial defence, and predator avoidance. At long range, however, potential mates, rivals, and predators produce remarkably similar visual signals, potentially making them difficult to distinguish. Nevertheless, many flying insects, including the hoverfly Eristalis tenax, can respond rapidly and appropriately to these ambiguous visual cues, implying specialised neural pathways that extract behaviorally relevant information. Descending neurons link visual processing to motor output, thus transforming visual signals into behavioral actions. In E. tenax, both target selective descending neurons (TSDNs) and looming-sensitive descending neurons respond to small targets, yet their respective contributions to target processing remain unclear. Using electrophysiological recordings, we compared receptive field organization and sensitivity to target size and motion direction across these neuronal populations. Although TSDNs and looming-sensitive descending neurons differed systematically in their target coding, a subset of looming-sensitive descending neurons closely resembled TSDNs, sharing small receptive fields and sharp target tuning despite retaining sensitivity to looming stimuli. These findings reveal previously unrecognized diversity among descending neurons tuned to small targets and suggest that parallel descending pathways encode distinct information from similar visual cues. Such an organization may enable common visual inputs to support both target pursuit and the detection of approaching threats.