Figure-Ground Discrimination in the Visual System of Drosophila melanogaster

Abstract. Drosophila inelanog~lster is able to detect a small visual object hidden in a background of identical texture, as long as there is relative motion between their retinal images. The properties of figurc-ground discrimination in the walking fly are studied under experimental conditions where the positions of figure and ground oscillate sinusoidally with similar fre-quency and similar amplitude but with different phase. The following points have becn established. (a) The average turning reaction of the stationarily walking Drosophila depends on phase; contrary to results obtained with the flying Musca (Reichardt and Poggio, 1979), antiphasic oscillation of figure and ground does not suppress the attractiveness of the figure. (b) A translatory response has been found which also de-pends on the phase difference of the oscillatory move-ments of figure and ground. (c) The time course of the responses and its intra- and inter-individual variability do not seem to fit into a rigid model of figure-ground discrimination. 1