Biologically Inspired Small Target Detection Mechanisms

A challenging engineering problem is the ability to detect and discriminate small moving objects against complex moving backgrounds. An evolutionary priority has tuned this capability within fly vision systems, so that they may detect, track and chase other flies, either for territorial or mating purposes. This is achieved within the confines of a light weight, low power system of less than a million neurons. Using in-vivo, intracellular, electrophysiological techniques, our lab has recorded from neurons within the fly brain that respond to a visual stimulus of small moving targets, even when included within a complex surround. Intracellular responses are still present when the velocities of the target and background are matched, ruling out relative motion cues as the definitive factor in this target discrimination task. Intracellular recordings at an earlier stage of the visual pathway show neurons with rectified, transient responses that have independent adaptation to stimulation with contrast increments versus contrast decrements. These rectifying transient cells (RTC) have an ´on´ and an ´off channel that we hypothesise would serve well as an intermediate stage in the detection of small moving targets. To test the feasibility of this hypothesis, we have developed a model that includes the filtering characteristics of early visual processing, followed by the rectification and independent adaptation properties of the RTC and finally a temporal nonlinear facilitation between a delayed ´off channel, and an undelayed ´on´ channel. The model output shows an enhancement in target discrimination, without the need for relative motion cues, as seen in physiological experimentation. Characteristic properties of neuronal responses from small target detecting cells are well matched by the outputs of this model.