Abstract
Discrimination of different chromatic hues is a fundamental visual capability. Traditional measurements of color discrimination have used patches of a single homogeneous color. Everyday color vision however is based on natural objects which contain a distribution of different chromatic hues. Here we study chromatic discrimination using photographs of various natural fruit objects.
In a 4AFC experiment, four stimuli were briefly presented on a CRT monitor in a 2×2 arrangement. Three of the stimuli were identical (test stimuli) and the fourth one (comparison stimulus) differed. The stimuli were either homogeneous patches of light, or digital photographs of fruit objects (banana, orange, etc), and were displayed on top of a homogeneous background whose chromaticity was also systematically varied. The mean color of the comparison stimulus was varied along 8 different directions in color space from the test stimulus. Discrimination thresholds were measured along these 8 directions and ellipses were fitted to the resulting threshold contours.
In agreement with earlier studies, we found that discriminability was best when the test stimuli had the same average color as the adapting background. However, when fruit objects were used as stimuli, thresholds were elevated and threshold contours were elongated in a way that reflected the distribution of hues in the image. For test stimuli that had an average color different from the background, threshold contours for fruit objects and homogeneous patches were identical.
We conclude that the distribution of hues within natural objects can have a profound effect on color discrimination and needs to be taken into account when predicting discriminability.