Classical definitions of color appearance and color-difference metrics (defined through discrimination thresholds) have relied on the use of uniform color stimuli. Many studies in the past have measured discrimination thresholds for uniformly colored light fields and color patches (MacAdam,
1942; Melgosa, Hita, Poza, Alman, & Berns,
1997; Poirson & Wandell,
1990; Poirson, Wandell, Varner, & Brainard,
1990). These measurements have been used to develop color-appearance spaces such as CIE 1976 UCS (International Commission on Illumination [CIE],
2004), CIELAB (CIE,
2004), and CIECAM02 (Moroney et al.,
2002), some of which are also associated with color-difference metrics such as Δ
ELAB and Δ
ECAM02. The aim of these color spaces is to propose a description of color based on appearance, where equal distances traversed in the color space correspond to roughly equal perceived differences in appearance. Although these theories offer critical insights into the early mechanisms of human color vision, such as color opponency, they do not provide a convincing framework to study natural polychromatic stimuli. The response of the visual system to these stimuli is more complex, and relatively less understood. For instance, Webster and Mollon (
1997) showed that the human visual system adapts to color distributions in natural scenes. In particular, when natural (or naturallike) stimuli are presented to observers, their color perception has been shown to be affected by factors such as the object's textural properties (Vurro, Ling, & Hurlbert,
2013) and the observer's memory of the object (Olkkonen, Hansen, & Gegenfurtner,
2008). Consequently, attempts to define and estimate discrimination surfaces for polychromatic stimuli have been relatively fewer and more recent. Montag and Berns (
2000) compared luminance thresholds for textures and uniform patches and found the luminance thresholds for textures to be higher by a factor of 2. Hansen, Giesel, and Gegenfurtner (
2008) and Giesel, Hansen, and Gegenfurtner (
2009) estimated chromatic thresholds in an isoluminant plane for uniform patches, natural objects, and polychromatic textures with color distributions similar to natural stimuli.