, 2009), or that L4 is specialized to detect front-to-back motion (Rister et al., 2007, Takemura et al., 2011 and Takemura et al., 2008). We have shown that L4 must get functionally significant inputs from cells other than L2 (Figure 3). Such inputs could be provided directly by photoreceptors or via the interneuron

amc (Meinertzhagen and O’Neil, 1991 and Rivera-Alba et al., 2011) but require a sign inverting synapse between photoreceptors and L4. Although predictions of connectivity based on anatomy will be tremendously helpful, our analysis of the L2-L4 link sounds a cautionary note regarding the importance of functional validation for these connections. Using genetic reagents restricted to L4, we saw no effect of silencing L4 on behavioral responses to translational motion or rotational Selleckchem SP600125 motion cues (Figures 5, 6, 8, and 9). Finally, we detected a role for L4 in the startle response caused by the appearance of static contrast patterns (Figure 8). Thus, our results argue that L4 does not have a specific role in motion

detection, though it is possible that L4 provides input to motion detecting circuits under stimulus Selleckchem Ferroptosis inhibitor conditions outside the range we have explored. These results contrast with a previous behavioral study that proposed a central role for L4 in motion vision based on a driver line that was expressed strongly in L3 and L4, as well as weakly in L2 and L5 (Zhu et al., 2009). Given that L3 functions in motion detection, it is likely that the phenotypes observed in this previous work can be attributed to the effects of inactivating L3, in combination with other lamina neurons. Finally, we note that the pattern of connections made by L4 is also consistent many with a role for L4 in spatial summation (Rister et al., 2007 and Takemura et al., 2011). In this view, L4 serves to pool information about local contrast changes. Two very different mechanisms by which motion detecting pathways could be made selective for light or dark edges have been proposed. In one view, the L1 and L2 inputs into motion detectors are independently half-wave rectified such that each pathway predominantly transmits information about only contrast increments

or contrast decrements, as well as a weaker signal proportional to the average intensity of light (Eichner et al., 2011, Joesch et al., 2010, Reiff et al., 2010 and Joesch et al., 2013). Alternatively, edge contrast selectivity can also be achieved through the incorporation of differential weighting of computations that detect specific correlations in the stimulus (Clark et al., 2011). Here, the motion detectors downstream of both L1 and L2 must receive information about both contrast increments and decrements. While calcium-imaging experiments using large contrast steps argued that L2 is half-wave rectified (Reiff et al., 2010), a subsequent study using dynamic stimuli demonstrated that L2 is sensitive to both contrast increments and decrements (Clark et al., 2011).