Fig. 11. Large-field expansion may serve as a trigger for saccades. (A) The outputs of the horizontal motion detectors were spatially summed over each half of the two quadrants making up the frontal 180° of the fly’s field of view (regions 2 and 3 in Fig. 9). The difference between these two spatial sums represents the gross horizontal expansion within the region experienced by the fly (see Appendix for details). The dashed red lines indicate the focus of expansion, while the red arrows schematically represent large-field expansion. (B) Horizontal expansion (H_Exp,Ips and H_Exp,Cont) cannot alone serve as a saccade trigger. Gray lines indicate individual expansion traces, aligned at the initiation of the saccade; red lines indicate the mean value, and blue lines represent ±S.D., as in Fig. 10. The y-axis scaling is the same as in Fig. 10B, and the calculation of H_Exp,Ips and H_Exp,Cont is as described in the Appendix; there has been no normalization. During flight within a textured background, the fly experiences significant horizontal expansion on its ipsilateral side, but not on its contralateral side (upper). However, this horizontal expansion is absent during flight within the uniform background (lower). The traces come from the same set of saccades as those in Fig. 10. (C) Calculation of the vertical expansion (V_Exp,Ips and V_Exp,Cont) from the output of the local motion detectors. To determine vertical expansion, the outputs of the elementary motion detectors sensitive to vertical motion were summed over the top and bottom halves of each frontal quadrant (see Appendix). The difference between these two spatial sums represents the gross vertical expansion experienced by the fly. (D) Prominent vertical expansion preceded saccades during flight within both the textured (upper traces) and uniform (lower traces) backgrounds and was greater on the ipsilateral side. The y-axis scaling is the same as in B; there has been no normalization.