, 1989, Hassenstein and Reichardt, 1956 and Reiff et al., 2010). Our results define this computational architecture with respect to its input pathways (Figure 8B). As the calcium signals detected in L1 and L2 are not themselves rectified and as both pathways contribute to behavioral responses that involve both lightening and darkening,
these pathways must each feed into Vandetanib mouse partial HRCs, which perform computations with information about both intensity increases and decreases. The HRC downstream of L1 computes responses to both sequential brightening and darkening. However, it also computes responses to the sequential dark-bright stimulus combination, the reverse-phi percept that is specifically associated with light edges. Incorporating this additional computation tunes behavioral response to moving light edges. The HRC downstream of L2 also responds to sequential darkening and lightening, as well as to the bright-dark combination, and thus can use the appropriate reverse-phi signal to become selective for behavioral responses to moving dark edges. Thus, each pathway computes a subset Ruxolitinib of HRC operations to assemble a filter tuned to a specific type of moving edge. This architecture provides a computational
mechanism for the specificity of these two input channels (Joesch many et al., 2010), while the intact circuit would still respond appropriately to all four paired correlations that mediate turning responses in wild-type flies. This model uses the intrinsic correlations present in light and dark edges to create selective filters by differentially weighting the four unit computations of the HRC. Because the four pairwise contrast combinations are present in different proportions in light edges and dark edges and particularly because the two reverse-phi combinations are each associated with a single edge type, a circuit can respond selectively to an edge
by appropriately weighting reverse-phi signals. We have defined the four computations here by the contrasts of the first and second bars in the bar-pair experiments to which they correspond. However, we note that because the four multiplications act on filtered versions of the contrast input for quickly varying inputs they will not always correspond to the instantaneous contrasts. Importantly, if the nondelayed filter were to transmit too much of the DC component of the intensity, the result would be that the two pathways would individually promote turning responses to static spatial gradients because the static gradient would be interpreted as a reverse phi in one direction.