Visual Target Detection

We study neural mechanisms of visual target detection using invertebrate physiological models. In recent years, insect visual systems have become an important model for studying how the brain can detect and analyze complex moving patterns. Despite their simplicity, insects seem to analyze the visual world in much the same way as humans and other vertebrates. Our main research animals are dragonflies and dipteran (two winged ) flies. Both of are superbly adapted to this task and engage in aerial pursuit of other insects, either as prey (dragonflies) or as part of sex behaviour. More recently, we have also begun to investigate visual physiology in jumping spiders (see below).

The hoverfly Eristalis tenax is a bee mimic. Unlike bees they do not sting and they also have larger eyes - both factors which make them appealing subjects for physiological experiments! Like many insects, they participate in high-speed manoeuvres during courtship (see photo). These are non-trivial tasks that involve the ability to distinguish objects that move relative to the remaining visual surround. To do this insects utilize specialized neurons in higher-order visual ganglia of the brain. Eristalis has proven valuable as a stable physiological model for studying higher-order vision.

To understand how hoverflies detect small targets moving across their visual field of view we use sophisticated intracellular electrophysiological recording techniques and state-of-the-art visual display software (www.visionegg.org). This gives us the capacity to deduce what happens within individual neurons in response to visual stimuli that we have complete control over. We have recently been able to show the exquisite ability of some of these target neurons to give a robust response to target motion, even in the presence of confounding moving background clutter. We are now carrying out experiments aimed at understanding how this remarkable task is achieved.

Sex specific neurons: We have also found that female hoverflies, like males, have target neurons despite the fact that this has earlier been described as a male-specific feature. As hoverflies are non-predatory, we hypothesize that female flies utilize these neurons during courtship to enable them to detect fit males for possible mating. We study this by comparing sex differences of the neurons themselves, and also by analyzing courtship in the field.

Visual Physiology in jumping spiders: Spiders are another group of visual predators with superb target vision. They are among the only group of terrestrial invertebrates that have simple lens eyes (like those of humans) as their main seeing organs. Among spiders, a group of tiny (and harmless) hunters, the jumping spiders (Salticidae) just 5 mm in length are remarkable in having eyes of such high quality that they rival those of primates. They use their eyes in tasks to navigate and to catch prey (they do not make a web). Some of their visual behaviour shows surprising complexity, including planning complex detour routes across novel terrain towards goals of interest, even though they lose sight of their goal en route. Similar behaviour has rarely been shown in any animals other than humans. These spiders have featured in numerous TV specials, including Sir David Attenborough’s award winning documentary for the BBC “Spiders from Mars”. Despite their remarkable eyesight, very little is yet known about the brain behind the eye. We have recently established collaboration with a Dr Ximena Nelson (University of Canterbury, New Zealand & Macquarie University, Sydney) to analyse responses of visual neurons to moving patterns and behavioural techniques to isolate the stimuli that the animals respond to.