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

Detecting small, fast-moving objects is essential for behaviors like pursuit, territorial defense, and predator avoidance. Small targets, however, can produce visual signals that closely resemble those of potential mates, rivals, or predators, making them challenging to differentiate. Despite this complexity, many flying insects, such as the hoverfly Eristalis tenax, demonstrate rapid and appropriate responses to these ambiguous visual cues.

This suggests specialized neural pathways that extract behaviorally relevant information from visual signals. In E. tenax, descending neurons serve as the link between visual processing and motor output, converting visual cues into appropriate behavioral responses. Within this species, both target-selective descending neurons (TSDNs) and looming-sensitive descending neurons respond to small targets; however, their individual contributions to target processing remain unclear.

By employing electrophysiological recordings, researchers compared the receptive field organization and sensitivity to target size and motion direction across these two neuronal populations. While TSDNs and looming-sensitive descending neurons exhibited distinct patterns in their target coding, a subset of neurons sensitive to looming stimuli shared similarities with TSDNs.

These neurons possessed small receptive fields and demonstrated sharp tuning to target stimuli, even though they retained sensitivity to looming stimuli. This study reveals a previously unrecognized diversity among descending neurons tuned to small targets, indicating that there are parallel descending pathways responsible for encoding distinct information from similar visual cues.

This organization may allow common visual inputs to support both target pursuit and the detection of approaching threats, highlighting the remarkable adaptability of the hoverfly's visual system.