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James Hillis, David Brainard; A shadowy dissociation between discriminability and identity. Journal of Vision 2004;4(11):56. doi: https://doi.org/10.1167/4.11.56.
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Color provides information for discriminating and identifying surfaces. To preserve information key to accomplishing these two tasks, visual sensitivity is adjusted for prevailing viewing conditions. These adjustments have different requirements for the two tasks. To preserve discriminability, response differences to light reflected from different surfaces must be maintained. To preserve identity, response to light from the same surface under different conditions must be fixed. We recently established that mechanisms controlling sensitivity for simple changes in background color and contrast are the same for color discriminability and identity. Given the distinct requirements for the two tasks, these results are quite surprising. We have now examined conditions where the background scene indicates compensation to achieve identity that is not required to preserve discriminability. Specifically we measured discriminability and identity for achromatic spots in and out of an apparent shadow. The background scene was based on Adelson's checkerboard illusion and the local surround of our test stimuli was the same in and out of the shadow. We inferred the shape of response curves for spots in and out of the shadow using a pedestal + test paradigm. We also had observers match the apparent brightness of spots presented in and out of the shadow. Contrary to our previous results, observers' matches were inconsistent with inferred shape of the response curves: There was a clear dissociation between effects of context on identity and discriminability. We conclude that mechanisms compensating for light variations created by cast shadows are independent of mechanisms underlying basic sensitivity adjustments.
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