Edge detection depends on achromatic channel in Drosophila melanogaster

Edges represent important information in object recognition, and thus edge detection is crucial for animal survival. Various types of edges result from visual contrast, such as luminance contrast and color contrast. So far, the molecular and neural mechanisms underlying edge detection and the relationship between different edge information-processing pathways have been largely undemonstrated. In the present study, using a color light-emitting-diode-based Buridan's paradigm, we demonstrated that a blue/green demarcation is able to generate edge-orientation behavior in the adult fly. There is a blue/green intensity ratio, the so-called point of equal luminance, at which wild-type flies did not show obvious orientation behavior towards edges. This suggests that orientation behavior towards edges is dependent on luminance contrast in Drosophila. The results of mutants ninaE(17) and sev(LY3); rh5(2); rh6(1) demonstrated that achromatic R1-R6 photoreceptor cells, but not chromatic R7/R8 photoreceptor cells, were necessary for orientation behavior towards edges. Moreover, ectopic expression of rhodopsin 4 (Rh4), Rh5 or Rh6 could efficiently restore the edge-orientation defect in the ninaE(17) mutant. Altogether, our results show that R1-R6 photoreceptor cells are both necessary and sufficient for orientation behavior towards edges in Drosophila.