Visual Motion-Detection Circuits in Flies: Small-Field Retinotopic Elements Responding to Motion Are Evolutionarily Conserved across Taxa

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Abstract
The Hassenstein–Reichardt autocorrelation model for motion computation was derived originally from studies of optomotor turning reactions of beetles and further refined from studies of houseflies. Its application for explaining a variety of optokinetic behaviors in other insects assumes that neural correlates to the model are principally similar across taxa. This account examines whether this assumption is warranted. The results demonstrate that an evolutionarily conserved subset of neurons corresponds to small retinotopic neurons implicated in motion-detecting circuits that link the retina to motion-sensitive neuropils of the lobula plate. The occurrence of these neurons in basal groups suggests that they must have evolved at least 240 million years before the present time. Functional contiguity among the neurons is suggested by their having layer relationships that are independent of taxon-specific variations such as medulla stratification, the shape of terminals or dendrites, the presence of other taxon-specific neurons, or the absence of orientation-specific motion-sensitive levels in the lobula plate.