Fig. 1. Schematic illustration of the experimental procedures. (A) Two vertical, sinusoidal gratings were projected on the front wall of a long, narrow tank. The projected gratings had opposing gradients of intensity and directions of motion (arrowheads). Upper panel: when both gratings drove the optomotor response with equal strength, the fish swam to the centre of the tank, where it experienced balanced motion stimuli from both sides. Lower panel: when one color was less efficient in driving the response (illustrated for the blue grating by making it darker), the fish moved away from the centre of the tank. The position of the fish indicated the point of perceived equi-luminance (asterisks) between the presented gratings. (B) Spectral compositions of the projected gratings measured at the output lens of the projector and the spectral locations of the rearing lights (broken vertical lines). (C) Intensity gradients at peak values of the projected sinusoidal gratings measured in the experimental tank at the white film used as a projection screen. The fishes were mainly active in bins –5 to +5 (see Fig. 2), where the intensity gradients were almost linear. (D) Diagram of the experimental schedule in blocks of 1 min. White arrowheads indicate that moving gratings were presented; black arrows show the flow of time. The order of the presented color combinations (I, II, III) was rotated after each fish (I II III, II III I, III I II, I II III, etc.) to avoid possible bias by chromatic adaptation.

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