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K. Wolf, G. Davis, L.T. Sharpe, A. Hurlbert; How Do Colour-anomalous Observers Perceive Simultaneous Chromatic Contrast? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1915.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The colour appearance of a surface is, under certain conditions, largely determined by its chromatic contrast with its immediate background. This phenomenon, known as simultaneous chromatic contrast (SCC), has been implicated in many aspects of colour perception, such as colour constancy. We investigate whether observers with anomalous colour vision perceive SCC in the same way as colour-normal observers. Methods: Our stimulus consists of a 1° target square set centrally against a 30x20° coloured background and displayed on a calibrated computer monitor. In each experimental trial, a neutrally coloured reference square is presented against a neutral background, and the observer is asked to memorise its colour. After 500 ms, we shift the background colour to one that stimulates the L, M or S-cones more strongly, or that introduces an equiluminant colour shift along the L-M axis. The chromaticity of the central square is also changed to one of ten test values taken from an axis that intercepts the chromaticities of both the neutral reference and the inducing background. To avoid using colour terms, the observer simply states whether the new square appears different or identical to the first, and from his responses we determine the perceptually neutral point. The distance between this and the neutral reference is taken as a measure of contrast strength. Results: We investigated both uniformly coloured stimuli and textured stimuli, for which we have previously shown that the simultaneous contrast effect is greatly reduced for colour-normal observers (Wolf and Hurlbert, ARVO 2002, VSS 2002). We collected data on ten colour-anomalous observers, comprising both protanomals and deutanomals with varying degrees of impairment, assessed by Nagel-anomaloscopy and the Farnsworth-Munsell 100 hue test. SCC is as strong in colour-anomalous observers as in normals for uniformly coloured stimuli. Yet, contrary to our results for colour-normal observers, texture differences between the square and background do not weaken chromatic contrast. Conclusions: We provide evidence that colour-anomalous observers see SCC effects differently from colour-normals. This implies qualitative changes beyond a simple decrease in discrimination ability, both in their post-receptoral colour-processing and in their colour perceptions of scenes with higher-order statistics.
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