Recalibrating Color Categories Using World Knowledge

Abstract

ABSTRACT-When the perceptual system uses color to facilitate object recognition, it must solve the color-constancy problem: The light an object reflects to an observer's eyes confounds properties of the source of the illumination with the surface reflectance of the object. Information from the visual scene (bottom-up information) is insufficient to solve this problem. We show that observers use world knowledge about objects and their prototypical colors as a source of top-down information to improve color constancy. Specifically, observers use world knowledge to recalibrate their color categories. Our results also suggest that similar effects previously observed in language perception are the consequence of a general perceptual process.Observers can use color to identify objects (Brainard, 2004;Oliva & Schyns, 2000;Tanaka, Weiskopf, & Williams, 2001). Here, we show that the reverse is also true: Observers can use objects to identify color. Knowledge about an object's prototypical color influences perception of instances of that object. The same hue is categorized as yellow when viewed on a banana, but as orange when viewed on a carrot. More important, this categorization bias also affects the perception of other, colorneutral objects (i.e., artificial objects that can have different colors, such as socks and cars). Observers who see a banana with an ambiguous hue between yellow and orange later categorize this hue on a color-neutral object as yellow; observers who see a carrot with the same ambiguous hue later categorize the hue as orange.This top-down flow of world knowledge influencing perception is one way in which observers might achieve color constancy, the ability to see an object as having the same color despite differing illumination conditions. Achieving color constancy is a perceptual challenge because the light that reaches the eye confounds illumination with the surface reflectance of an object.To use color information for object recognition, observers need to compensate for the influence of illumination and see the color of an object as invariant over illumination conditions. Such compensation is made possible, in part, by using bottom-up information available from the image itself, such as global and local spectral means of the light, as well as the maximal intensities (Kraft & Brainard, 1999) and brightness-hue correlations in the image (Golz & MacLeod, 2002). Generally, color constancy is less than perfect because these sources of information are not always available (Brainard, 2004;Kraft & Brainard, 1999). The degree of color constancy achieved also depends on whether observers are explicitly instructed to focus on color constancy (Arend & Reeves, 1986;Troost & de Weert, 1991).Color constancy could, in principle, be improved by a top-down flow of world knowledge. In the experiments we report here, we found evidence for such a mechanism: Knowledge about objects' natural colors is used to recalibrate color categories. That is, objects are used to identify colors.Observers use world knowledge when ...

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