To do this, we presented flies with a full-field, random intensity stimulus with a standard deviation of 35% contrast about a mean luminance and a 200 ms correlation

time. The relatively fast intensity changes in this stimulus effectively prevent strong adaptation from taking place on timescales longer than 3MA 200 ms. As expected, intense periods of illumination prompted a reduction in intracellular calcium levels in both cell types. Periods of decreased illumination induced an increase in calcium levels (Figure 5A). In this stimulus regime the maximum correlation between contrast and calcium signal occurred with a delay of 80–130 ms (data not shown), consistent with the indicator CP 690550 kinetics, the imaging frame rate, and our observations of the flash responses. To examine whether responses to contrast increases were equal and opposite to contrast decreases, we plotted the calcium-indicator

ratio against the contrast presented 100 ms earlier for all three axon terminals (Figure 5B). The output of all three terminals varied linearly with the delayed input contrast. A purely linear function accounted for 97% and 89% of the mean delayed response variance of the L1 signals in M1 and M5; a quadratic term accounted for less than 1% of additional variance in each case. Similarly, a purely linear function accounted for 99.6% of the variance in L2 responses, while adding a quadratic term accounted for less than 0.1% of additional variance. As a second approach to measuring response linearity, we fit a linear-nonlinear (LN) model to the calcium response of these cells as a function

of contrast history by using methods frequently used to characterize responses in vertebrate retina (Figures S5A and S5B; Baccus and Meister, 2002, Chichilnisky, 2001 and Sakai et al., 1988). These linear kernels were strongly predictive of the average responses of L1 and L2 to these stimuli (Figures S5A and S5B). Furthermore, plots of the actual responses versus those predicted by these filters were highly linear (Figure S5C). Thus, we found no evidence that edge selectivity could emerge simply through the directed transmission of contrast increases Thalidomide through L1 and contrast decreases through L2. A biologically plausible model for the HRC has been proposed to include four independent computations of the multiplication events that underlie responses to sequential presentation of two bright, two dark, bright then dark, and dark then bright bar pairs (Hassenstein and Reichardt, 1956). However, it is unknown whether these four putative computations are actually independently implemented and whether fruit fly behavior can be elicited by each of the unit computations.