Induction from a below-threshold chromatic pattern.

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.