A luminance gradient paradox

Agostini & Galmonte (ARVO, 1997) showed that a linear luminance gradient can largely modify the lightness of a target region. In the present work a new version of the gradient configuration in offered where the luminance range has been drastically reduced. In these conditions the gradient is almost unnoticeable. Comparing the lightness of a gray target placed at the center of this gradient with an identical target surrounded by a surface having a homogeneous luminance value equal to that of the highest luminance of the gradient, they appear quite different even though their backgrounds appear quite the same. This perceptual paradox is remarkable because it suggests that smooth changes in luminance, even when difficult to detect, can affect lightness perception. This paradox could be due to the local contrast between the target and its closest surrounding luminance. But, by narrowing the spatial distribution of an identical reduced range luminance gradient the direction of the effect is reversed. It is not possible to account for this result considering only the local contrast between the last luminance of the gradient and that of the target. This suggests that the visual system computes surface colors taking into account the global spatial distribution of luminance gradients