The effect of contrast intensity and polarity in the achromatic watercolor effect

Abstract

The watercolor effect (WCE) is a filling-in phenomenon in a region demarcated by two thin abutting lines. The perceived chromaticity of the region is similar to that of the interior line. We develop a series of achromatic WCE stimuli to induce lightness changes analogous to the induced chromaticity in the chromatic version of the WCE. We use a variation of the paired-comparison paradigm to quantify the induced lightness of the filled-in regions to regions with real luminance variations. The luminance of the inner line is fixed, while the luminance of the outer line varies across stimuli. Data from seven subjects (five naive) confirm that an achromatic WCE exists. Moreover, outer lines with both high and low luminances can generate a WCE with an inner line of a moderate luminance. All subjects show a single peak of the effect strength for both polarity conditions, which is never at the extreme luminance levels. Most subjects show an inverted U curve for effect strength as a function of the contrast of the outer lines against the background. Results suggest that the contrast difference between the outer line and the inner line affects the existence and the strength of the achromatic WCE in a nonlinear way.

A common signal detection model accounts for both perception and discrimination of the watercolor effect

“…Free viewing has been used to study the WCE in numerous previous studies (Devinck et al, 2005;Devinck & Spillmann, 2009;Pinna, 1987;Pinna et al, 2001). von der Heydt and Pierson (2006) used a duration of 1 s, not too much shorter than our estimated average duration, and Cao et al (2011) used 150 ms to study an achromatic variant of the WCE.…”

“…It is not surprising that in a discrimination experiment, observers might exploit alternative cues, if present. Cao et al (2011) used the degree of inconsistency in judging pairs of stimuli evoking different strengths of an achromatic version of the WCE to demonstrate that the WCE can be quantified. For this method to work, stimuli must be spaced closely enough so that the observer's judgments will be probabilistic, e.g., a given pair (a, b) will, on some trials, be judged a 9 b and on others b 9 a.…”

“…Previous attempts have used, for example, matching (Devinck et al, 2005;von der Heydt & Pierson, 2006), hue cancellation Devinck & Spillmann, 2009), and paired comparisons (Cao, Yazdanbakhsh, & Mingolla, 2011). Matching and cancellation measure the phenomenon by specifying stimuli of equivalent appearance or strength to the fill-in color, but the results show large within and between individual variability (Devinck et al, 2005;von der Heydt & Pierson, 2006).…”

Assimilative and non-assimilative color spreading in the watercolor configuration

“…Nevertheless, the color spreading does not involve large brightness changes (Devinck et al, 2005). Recent studies revealed that the achromatic watercolor effect does exist and is measurable, but the magnitude is small (Takashima, 2008; Cao et al, 2011). Regarding the effects of luminance conditions, previous studies showed that the spreading was very weak when the luminance of the outer contour (OC) was in between the luminances of the inner contour (IC) and of the background (Devinck et al, 2005; Cao et al, 2011).…”

“…Recent studies revealed that the achromatic watercolor effect does exist and is measurable, but the magnitude is small (Takashima, 2008; Cao et al, 2011). Regarding the effects of luminance conditions, previous studies showed that the spreading was very weak when the luminance of the outer contour (OC) was in between the luminances of the inner contour (IC) and of the background (Devinck et al, 2005; Cao et al, 2011). The magnitude of color spreading increased with the luminance ratio of the IC to the OC (Devinck et al, 2005; Devinck and Knoblauch, 2012).…”