![]() The illumination discrimination task (IDT) that we employ was introduced by Pearce, Crichton, Mackiewicz, Finlayson, and Hurlbert ( 2014). In this study, we approach color constancy by probing the discriminability of global illumination changes on a scene with unchanging surface reflectances. Results from such studies demonstrate that the overall degree of color constancy may depend on the spectral reflectance of the surface under view (Burnham et al., 1957 Helson & Michels, 1948 Ling & Hurlbert, 2008), the spectral character of the change in illumination (Brainard & Wandell, 1992 Daugirdiene, Kulikowski, Murray, & Kelly, 2016 Delahunt & Brainard, 2004 Worthey, 1985), the ensemble of surfaces in the scene (Bäuml, 1994, 1995), and manipulation of cues in the image that might mediate constancy (Kraft & Brainard, 1999 Yang & Maloney, 2001). illumination change Craven & Foster, 1992). Other paradigms do not directly assess surface color appearance but instead measure constancy through-for example, categorical color naming (e.g., Olkkonen, Witzel, Hansen, & Gegenfurtner, 2010 Troost & de Weert, 1991), object selection tasks (Radonjić, Cottaris, & Brainard, 2015, 2016), or classification of the physical origin of image changes (e.g., material vs. These generally involve an assessment of the color appearance of individual surfaces across a change in illumination-for example, by asymmetric surface color matching (e.g., Arend & Reeves, 1986 Brainard, Brunt, & Speigle, 1997 Burnham, Evans, & Newhall, 1957) or achromatic adjustment (e.g., Brainard, 1998 Helson & Michels, 1948). A variety of experimental methods have been developed to measure color constancy. ![]() However, laboratory measurements of the magnitude of color constancy vary with the experimental conditions and indicate that it is rarely perfect (for reviews, see Brainard & Radonjić, 2014 Foster, 2011 Hurlbert, 1998 Maloney, 1999 Smithson, 2005). They also suggest it will be worthwhile to explore whether and how the human visual system has internalized the statistics of natural illumination changes.Ĭolor constancy is the perceptual phenomenon by which object colors remain relatively stable despite spatial and temporal changes in the illumination spectrum, which alter the spectrum of light reflected from object surfaces to the eye. For the four chromatic references (blue, yellow, red, and green), the change in illumination toward the neutral reference is less well discriminated than changes in the other directions: a “neutral bias.” The results have implications for color constancy: In considering the stability of surface appearance under changes in illumination, both the starting chromaticity of the illumination and direction of change must be considered, as well as the chromatic characteristics of the surface reflectance ensemble. For the neutral reference, there is a trend for thresholds to be highest in the bluer illumination-change direction, replicating our previous reports of a “blue bias” for neutral reference illuminations. We find that relative discrimination thresholds for different chromatic directions of illumination change vary with the reference illumination. Nine participants completed the task for five different reference illumination conditions (neutral, blue, yellow, red, and green). A staircase procedure found thresholds for discrimination of an illumination change along each axis of chromatic change. The participant's task was to correctly select the target. One comparison matched the reference (the target) the other (the test) varied from the reference, nominally either bluer, yellower, redder, or greener. The scene was first illuminated by a reference illumination, followed by two comparisons. Participants viewed a Mondrian-papered scene illuminated by LED lamps. We measured discrimination thresholds for illumination changes along different chromatic directions starting from chromatically biased reference illuminations.
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