Abstract
The human visual system provides a relatively stable representation of object lightness, an object intrinsic property that correlates with its surface reflectance, despite variation in object extrinsic properties of the visual scene. We study the effect of variation in two such independent extrinsic properties on human lightness perception, the intensity of light sources and the spectra of background objects. Human subjects viewed rendered images of a 1-degree achromatic sphere within a 2-degree scene with naturalistic background objects and light sources. On each trial, subjects viewed two images in sequence for 0.25s each and reported the interval containing the lighter sphere. The spectra of background objects and light sources in the scene were varied across trials and intervals. For light sources, the shape of the spectra was fixed to be CIE standard illuminant D65, but the overall intensity was sampled from an interval. The range of intensity variation was controlled by changing the size of the interval. The spectra of background objects were sampled from a distribution of naturalistic surface reflectances. The amount of variation in spectras was controlled by changing the size of the covariance matrix of the distribution. Lightness discrimination thresholds were measured for individual and simultaneous variation of these two properties. For individual variation in light source and background spectras, discrimination thresholds were constant for small amount of variation and then increased as the amount of variation increased. For simultaneous variation in both spectras, the discrimination threshold was higher by 0.043 ± 0.006 as compared to no variation condition. This increase was comparable to the sum of the corresponding threshold increase (light intensity 0.028 ± 0.007; background spectra 0.010 ± 0.002) for same amount of individual variations. These results suggest that discrimination thresholds depend not on the type of variation, but the total amount.