| Summary: | Narrowly-tuned, selective noise masking of chromatic detection has been taken as evidence for the existence of "higher-order" color mechanisms. Experiment 1 replicates earlier observations of selective masking of tests in the (L, M) plane of cone space when the noise is placed near the corners of the detection contour. We used unipolar Gaussian blob tests, with three different noise color directions, and show that there are substantial asymmetries in the detection contours,
asymmetries that would have been missed with bipolar tests such as Gabor patches. A new chromatic detection model, which is based upon probability summation of linear cone combinations, incorporates a linear contrast energy vs. noise power relationship that predicts how the sensitivity of these mechanisms changes with noise contrast and chromaticity is presented. With only six unipolar color mechanisms - the same number as the cardinal model - the new model accounts for the threshold
contours across the different noise conditions, including the asymmetries and the selective effects of the noises. Our model is different from the cardinal model, in at least one important way. Four of the six detection mechanisms here have opposed L and M cone inputs and that is the feature that allows it to predict selective masking across noise conditions without a large number of mechanisms.
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