Induction from a below-threshold chromatic pattern

Abstract

Patterned backgrounds can induce large shifts in color appearance, even with patterns of only 10% S-cone contrast (S. K. Shevell & P. Monnier, 2005). The present study tested whether a background pattern could induce color shifts even at a below-threshold contrast. In the first experiment, S-cone contrast threshold for discriminating a pattern from a homogenous background was measured by a 2AFC procedure. Next, a test ring was inserted within the patterned background. With the test ring present, six of eight observers reliably distinguished trials with a patterned background from trials with a homogeneous field, even though the S-cone contrast in the pattern was too low to be discriminated from a homogeneous background. This suggested that a below-threshold S-cone pattern shifted the color appearance of the test ring; that is, the appearance of the test was used to discriminate whether the background was patterned or homogeneous. This was corroborated by asymmetric color matches, which revealed a color shift caused by subthreshold S-cone contrast within the patterned background.

Figure 1

(a–b) Pattern-only condition. Panel a shows a non-signal interval and b a signal interval. (c–d) Pattern-with-test-ring condition. Panel c shows a non-signal interval and d a signal interval. The S-cone patterns were composed of 13 purple/lime rings (12 in the pattern-with-test-ring condition) at spatial frequency 3 cycles per degree.

Figure 2

Ratios of threshold for pattern-only and pattern-with-test-ring conditions. Threshold S-cone contrast ratio (vertical axis) is plotted as a function of the test-ring chromaticity (horizontal axis). Ratios greater than 1.0 indicate superior discrimination in the pattern-with-test-ring condition compared to the pattern-only condition. Each line shows measurements for a different observer. Symbols show the means and standard errors for the eight observers. The dashed lines denote the two observers with a non-significant effect of introducing a test ring.

Figure 3

Ratios of the pattern-only to pattern-with-test-ring S-cone contrast thresholds, averaged across the three test-ring chromaticities. The asterisks denote observers with a statistically significant change in threshold due to adding a test ring. The median S-cone contrast ratio for the eight observers was 1.43 (arrow on vertical axis).

Figure 4

Color appearance measurements expressed as shifts form the isomeric match along the S/(L + M) chromatic direction. Each line represents measurements from a single observer; symbols show means and standard errors for the five observers. Measurements above (below) zero were for patterns with the test ring flanked by purple (lime) inducing circles.

Figure 5

Ratios of the pattern-only to pattern-with-test-ring S-cone contrast threshold with equiluminant stimuli. The asterisks denote observers with a statistically significant change in threshold due to adding an equiluminant test ring. The median S-cone contrast ratio for the five observers was 1.83 (arrow on vertical axis).