Fig. 2: Measurement of chromatic integration.

a Schematic representation of the chromatic-integration stimulus. 500-ms presentations of opposing contrast of UV and green were interleaved with 2 s of background illumination (gray). b Schematic depictions of all contrast combinations used for assessing chromatic integration. The contrast combinations are arranged into two sets, corresponding to green-On-UV-Off (magenta box, top) and to green-Off-UV-On (blue box), with pairs of contrast-reversed combinations vertically aligned and assigned to a stimulus index (1–11). c Response measurements used for constructing the chromatic-integration curves. Spike counts (s) were obtained over a 200-ms window during stimulation (50–250 ms after stimulus onset), and the background activity (b), measured as the spike count during the 200 ms prior to the stimulus, was subtracted (s − b). The yellow bar shows the stimulus period. d PSTHs for a sample On cell responding to the contrast combinations of the chromatic-integration stimulus (below: stimulus indices 1–11). e Chromatic-integration curves for the same cell as in d, surrounded by exemplary raster plots for some of the contrast combinations. At the balance point, activity is canceled out, which indicates linear chromatic integration. f Same as e, but for a sample Off cell with sizeable responses at the balance point, which indicates nonlinear chromatic integration. Shaded regions around the data curves mark the range of mean ± SEM.