Visual detection of paradoxical motion in flies

Summary From psychophysics it is known that humans easily perceive motion in Fourier-stimuli in which dots are displaced coherently into one direction. Furthermore, motion can be extracted from Drift-balanced stimuli in which the dots on average have no distinct direction of motion, or even in paradox -motion stimuli where the dots are displaced opposite to the perceived direction of motion. Whereas Fourier-motion can be explained by very basic motion detectors and nonlinear preprocessing of the input can account for the detection of Drift-balanced motion, a hierarchical model with two layers of motion detectors was proposed to explain the perception of -motion. The well described visual system of the fly allows to investigate whether these complex motion stimuli can be detected in a comparatively simple brain.The detection of such motion stimuli was analyzed for various random-dot cinematograms with extracellular recordings from the motion-sensitive Hl-neuron in the third visual ganglion of the blowfly Calliphora erythrocephala. The results were compared to computer-simulations of a hierarchical model of motion detector networks.For Fourier- and Drift-balanced motion stimuli, the Hl-neuron responds directionally selective to the moving object, whereas for -motion stimuli, the preferred direction is given by the dot displacement. Assuming nonlinear preprocessing of the detector input, such as a half-wave rectification, elementary motion detectors of the correlation type can account for these results.Abbreviations EMD elementary motion detector