, 2011, Joesch et al., 2010 and Rister et al., 2007). Silencing L4 neurons also decreased full-field optomotor responses at low contrasts and very fast stimulus speeds and impaired the ability of flies to track rapidly oscillating patterns (Figure S7A). In contrast to L2 and L4, we found that the columnar, centrifugal neurons C2 and C3 play an important role in shaping behavioral responses to regressive motion stimuli. C2 and C3 are GABAergic neurons (Fei et al., 2010 and Kolodziejczyk et al., 2008) that arborize in multiple layers of the proximal and distal

medulla and send axons into the lamina, where they are primarily presynaptic on several neuron types, including L1, L2, and

Lai neurons (Meinertzhagen and O’Neil, INK1197 clinical trial 1991 and Rivera-Alba et al., 2011). In the distal medulla, C2 and C3 both receive presynaptic input from L1 and form synapses on L2; C2 is also presynaptic to L1 (Takemura et al., 2008). In addition to the distal medulla, C3 neurons arborize in the proximal medulla, primarily in layer M9 (Figures 1C and 2H). Examination of the C3 terminals in the medulla revealed that putative dendritic arbors in layer M9 showed a stereotyped orientation, with processes extending posteriorly from the branch Cisplatin cell line point off the main axon (Figures 5C and 5D). This directionality was highly stereotyped (33/33 neurons

from 3 brains). Closer examination revealed that these arbors extend into neighboring columns (Figure 5E), reminiscent of the the multicolumnar projections of L4 in lamina (Figure 5B; Strausfeld and Campos-Ortega, 1973) and medulla (Takemura et al., 2011). This organization suggests that C3 neurons receive synaptic input from posterior medulla columns and provide output to more anterior lamina and medulla columns. Such an asymmetric circuit could enhance the detection of regressive motion by amplifying signals translating from posterior to anterior across the eye. Consistent with this hypothesis, we found that silencing C3 neurons abolished steering responses to regressive motion stimuli moving at high speeds (Figure 5K, bottom row) but did not affect responses to progressive motion (Figure 5K, top row) or basic optomotor stimuli (Figure S7C). C2 neurons also had multicolumnar, presumably dendritic, arborizations in the medulla (Figures 5F–5H). Most of the C2 arbors in layer M10, while variable in their detailed shapes, were strongly asymmetric (18/20 neurons from 19 brains), extending preferentially in a dorsal direction relative to the main neurite (Figures 5G, 5H, and S3D). This multicolumnar profile of C2 neurons suggests that they may also be involved in integrating signals from neighboring columns.