Functional Specialization of Parallel Motion Detection Circuits in the Fly

In the fly Drosophila melanogaster , photoreceptor input to motion vision is split into two parallel pathways as represented by first-order interneurons L1 and L2 ([Rister et al., 2007][1]; [Joesch et al., 2010][2]). However, how these pathways are functionally specialized remains controversial. One study ([Eichner et al., 2011][3]) proposed that the L1-pathway evaluates only sequences of brightness increments (ON-ON), while the L2-pathway processes exclusively brightness decrements (OFF-OFF). Another study ([Clark et al., 2011][4]) proposed that each of the two pathways evaluates both ON-ON and OFF-OFF sequences. To decide between these alternatives, we recorded from motion-sensitive neurons in flies in which the output from either L1 or L2 was genetically blocked. We found that blocking L1 abolishes ON-ON responses but leaves OFF-OFF responses intact. The opposite was true, when the output from L2 was blocked. We conclude that the L1 and L2 pathways are functionally specialized to detect ON-ON and OFF-OFF sequences, respectively. [1]: #ref-17 [2]: #ref-11 [3]: #ref-6 [4]: #ref-4