Insect Detection of Small Targets Moving in Visual Clutter

Karin Nordström,Paul D Barnett,David C O'Carroll,Tom Collett

doi:10.1371/journal.pbio.0040054

Abstract

Detection of targets that move within visual clutter is a common task for animals searching for prey or conspecifics, a task made even more difficult when a moving pursuer needs to analyze targets against the motion of background texture (clutter). Despite the limited optical acuity of the compound eye of insects, this challenging task seems to have been solved by their tiny visual system. Here we describe neurons found in the male hoverfly,Eristalis tenax, that respond selectively to small moving targets. Although many of these target neurons are inhibited by the motion of a background pattern, others respond to target motion within the receptive field under a surprisingly large range of background motion stimuli. Some neurons respond whether or not there is a speed differential between target and background. Analysis of responses to very small targets (smaller than the size of the visual field of single photoreceptors) or those targets with reduced contrast shows that these neurons have extraordinarily high contrast sensitivity. Our data suggest that rejection of background motion may result from extreme selectivity for small targets contrasting against local patches of the background, combined with this high sensitivity, such that background patterns rarely contain features that satisfactorily drive the neuron.

Highlights

Visual detection of targets that move within visual clutter is a common task in animals searching for prey or conspecifics
Despite the limited optical acuity of a compound eye, the challenging task of target detection in visual clutter seems to have been solved by the tiny visual system of insects, making them excellent models for studying higher order visual properties [2]
Neuron Classification We recorded from neurons in the lobula complex of 74 male hoverflies that were strongly excited by the motion of small, black targets (0.88 square) within a fronto-dorsal receptive field (Figure 1)

Summary

Introduction

Visual detection of targets that move within visual clutter is a common task in animals searching for prey or conspecifics. In birds of prey such as kestrels, for example, astonishing optical acuity permits visual detection of prey at distances of up to 275 m from a perch [1] This task becomes even more challenging if the pursuer is itself in motion so that targets need to be analyzed against the motion of background texture (i.e., against moving clutter). We describe here a class of neurons in the hoverfly, Eristalis tenax, which are tuned to the detection of small moving targets We show that these neurons continue to respond selectively to the motion of small targets under a variety of confounding background motion stimuli. This is a property that has not previously been described for any insect neuron

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Conclusion