Simultaneous contrast in goldfish

Two groups of goldfish were trained to strike an illuminated target for food reward. The experimental group was reinforced with 10 worms for response to one color and with 1 worm for response to a different color. The control group was reinforced with 1 worm for response to each color. The experimental group gave clear evidence of discrimination of reward magnitude and also of simultaneous negative contrast. These results bear on the relation of the mechanisms of simultaneous and successive contrast.