The watercolor effect: Quantitative evidence for luminance-dependent mechanisms of long-range color assimilation

Abstract

When a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour, the brighter color will spread into the entire enclosed area. This is known as the watercolor effect (WCE). Here we quantified the effect of color spreading using both color-matching and hue-cancellation tasks. Over a wide range of stimulus chromaticities, there was a reliable shift in color appearance that closely followed the direction of the inducing contour. When the contours were equated in luminance, the WCE was still present, but weak. The magnitude of the color spreading increased with increases in luminance contrast between the two contours. Additionally, as the luminance contrast between the contours increased, the chromaticity of the induced color more closely resembled that of the inside contour. The results support the hypothesis that the WCE is mediated by luminance-dependent mechanisms of long-range color assimilation.

Spatial Dependence of Color Assimilation by the Watercolor Effect

“…For example, while relatively large chromaticity shifts of 38.11% and 18.04% were needed for the two narrowest columns (5.4 and 9.3 min of arc, respectively), shifts of only 4.61% and 1.75% were required for wider columns. Note that the magnitude of the effect for a stimulus width of 1.43 deg is similar to that found by Devinck et al (2005) under comparable conditions. We conclude that the WCE is most salient for stimuli with smaller enclosed areas.…”

“…They were asked to adjust the chromaticity of the enclosed stimulus area until it appeared achromatic (hue cancellation). The results of an earlier study (Devinck et al 2005) showed no difference between color-matching and hue-cancellation tasks for the pattern used in the present paper. In addition, observers generally found the hue-cancellation task to be easier than matching and were able to perform this task relatively quickly.…”

“…They were composed of two different colors: an orange (u′, v′ = 0.231, 0.512; Y = 55 cd m −2 ) inner contour, and purple (u′, v′ = 0.211, 0.375; Y = 20 cd m −2 ) outer contour, presented on a neutral white background (u′, v′ = 0.189, 0.467; Y = 80 cd m −2 ). The WCE is dependent on the luminance contrast between the two borders (Devinck et al 2005;Pinna 2005). Thus, the luminance of the orange contour was higher than that of the purple contour with a luminance ratio of 2.75 corresponding to the higher contrast used in a previous paper; this setting allowed us to obtain a stronger WCE (Devinck et al 2005).…”

“…The WCE is dependent on the luminance contrast between the two borders (Devinck et al 2005;Pinna 2005). Thus, the luminance of the orange contour was higher than that of the purple contour with a luminance ratio of 2.75 corresponding to the higher contrast used in a previous paper; this setting allowed us to obtain a stronger WCE (Devinck et al 2005). Under these conditions, uniform orange color spreading was readily seen within the enclosed surface area for all pattern widths tested.…”

Dissociation of color and figure–ground effects in the watercolor illusion

“…Based on pilot experiments we assumed that the illusory color could be matched by a color located in color space on the straight line connecting the background white with the color of the orange line of the watercolor pattern. A recent study published after the present results were submitted confirmed this assumption (Devinck et al, 2005). A set of 20 comparison colors distributed along that line was used.…”