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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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