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Christoph Witzel, Thorsten Hansen, Karl R. Gegenfurtner; Categorical discrimination of colour. Journal of Vision 2008;8(6):577. doi: 10.1167/8.6.577.
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Colour vision leads to the conception of linguistically delimited colour categories that enable us to communicate the reflectance properties of objects. The question arises whether this demarcation of the continuous colour signal implies an actual stepwise variation in colour perception, a phenomenon that has been called categorical perception. In this case the categorization of colour might be inherent in the low-level perceptual processes that allow the discrimination of different wavelength combinations. To answer this question we investigated the relationship between discrimination thresholds, reaction times to color differences, and colour categories for different hues of approximately equal saturation along a color circle in the Derrington-Krauskopf-Lennie (DKL) colour space. If there is categorical perception on this level of visual processing one would expect a reduction of discrimination thresholds at the category borders. In the first experiment we determined the boundaries and transitions between the colour categories green and blue individually for each observer. We measured the differential decision boundary between these two categories and the probabilities of colours to be included or excluded in either of the two categories. In the second experiment we used a staircase technique to measure discrimination thresholds along the transition between the two categories. These discrimination thresholds can be considered as a local metric of colour space that is independent of the particular colour space in which they are measured. Therefore we used the discrimination thresholds to determine equally distant stimuli above threshold. We measured reaction times for discriminating these stimuli within and across the borders of green and blue. We found no reduction of discrimination thresholds at the category boundaries. However, reaction times seem to be sensitive to the transition of colour categories.
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