A line, not a space, represents visual distinctness of borders formed by different colors.

When observers are asked to rate the visual distinctness of borders formed by the junction of two photic stimuli, normal trichromatic subjects behave in a manner similar to that of tritanopes in a color mixture experiment. All stimuli that look the same to the tritanope produce the same border distinctness with any other stimulus. Sets of such stimuli, whose members do not form borders with each other, map as single points along a curved line, where the Euclidean distance between pairs of points representing the two stimuli is nearly proportional to the rated distinctness of the border formed between them. In the absence of luminance differences, the perception of contour apparently depends on the stimulation of only two cone types.