Form vision is traditionally regarded as processing primarily achromatic information. Previous investigations into the statistics of color and luminance in natural scenes have claimed that luminance and chromatic edges are not independent of each other and that any chromatic edge most likely occurs together with a luminance edge of similar strength. Here we computed the joint statistics of luminance and chromatic edges in over 700 calibrated color images from natural scenes. We found that isoluminant edges exist in natural scenes and were not rarer than pure luminance edges. Most edges combined luminance and chromatic information but to varying degrees such that luminance and chromatic edges were statistically independent of each other. Independence increased along successive stages of visual processing from cones via postreceptoral color-opponent channels to edges. The results show that chromatic edge contrast is an independent source of information that can be linearly combined with other cues for the proper segmentation of objects in natural and artificial vision systems. Color vision may have evolved in response to the natural scene statistics to gain access to this independent information.

Goldfish (Carassius auratus) were trained to discriminate triangles and squares using a two choice procedure. In the first experiment, three goldfish were trained with food reward on a black outline triangle on a white background, while a black outline square was shown for comparison. In transfer tests, a Kanizsa triangle and a Kanizsa square were presented, perceived by humans as an illusory triangle- or square-shaped surface of slightly higher brightness than the background. The choice behavior in this situation indicates that goldfish are able to discriminate between both figures in almost the same way as in the training situation. In control experiments goldfish did not discriminate between shapes in which humans do not perceive the illusion. A series of generalization experiments was performed indicating the similarity between the tested shapes and the training triangle. From all these findings we conclude that goldfish are able to perceive an illusory triangle or square within the Kanizsa figures. In a second experiment, four goldfish were trained on a white outline triangle versus a white outline square, both on black background with white diagonal lines. In transfer tests in which the shapes were replaced by gaps within the white diagonal lines, goldfish were clearly able to discriminate between the two patterns based on the illusory contours. This was not the case in tranfer tests with phase shifted abutting lines.