Color discrimination and color categories

The subjective boundary in colour space between reddish and greenish hues is not aligned with either of the chromatic channels that have been found by electrophysiological recordings from the retina and the lateral geniculate nucleus of primates (Derrington, Krauskopf & Lennie, 1984). Yet we found that chromatic discrimination is enhanced in the vicinity of the perceptual red-green boundary. Thresholds were measured for brief (100-msec) foveal stimuli using a spatial forced-choice procedure: observers were required to judge whether the left or the right half of a bipartite field was the greener. In interleaved runs we also obtained empirical estimates of the subjective boundary between reddish and greenish colours. Both types of measurements (performance measurements of thresholds and subjective judgements of hue) were made in the presence of a steady background that was metameric to CIE Illuminant D65. To exclude the possibility that larger thresholds in more distant areas from the blue-yellow line arise because of uncertainty in correctly ordering colours that lay within the same category, we ran a control experiment. In this experiment, the task was still a criterion-free forced choice, but the observer had to judge only whether a difference of chromaticity was present. A minimum of the subjects correctly completed this task. Byzov classically proposed the general rule that sensory neural channels are maximally sensitive to perturbations around the activity level set by the background. In the present case, the chromaticities that give the lowest threshold are chromaticities that lie along the subjective red-green hue boundary, implying a neural channel that is aligned with the red-green axis of colour space.

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