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Fig. 6 Encoding of natural achromatic and chromatic contrast. (A) Computed responses of in vivo cones, the GB axis, and each respective log-opsin PC fit [all from Fig. 5 (I and N)] to each of the n = 30,000 individual natural spectra, plotted against each spectrum’s loadings onto PC1 (top row), PC2 (middle row), and PC3 (bottom row), as indicated. Responses were plotted on the y axis, and PC loadings were plotted on the x axis. In general, a column that shows a near-perfect correlation in one row, but no correlation in both other rows (e.g., column 1), can be seen as a tuning function that efficiently captures the respective PC (e.g., column 1 shows that red cones efficiently represent PC1 but not PC2 or PC3). (B) Corresponding summary statistics from (A), based on scene-wise Spearman correlations. (C) Spectral tuning functions of Drosophila R7/8 photoreceptors as measured in vivo at their synaptic output [data from (12)]. (D) Comparison of Drosophila tuning functions with the first three PCs that emerge from terrestrial natural scenes [data from (13)]. Here, PC3 is matched with a yyp8 axis as indicated [cf. fig. S6 (D to F)]. (E) Summary stats of Drosophila photoreceptor responses to each of the n = 4000 individual terrestrial natural spectra plotted against their respective PC loadings.