Agostini and Galmonte (1997 Investigative Ophthalmology & Visual Science 38 S895; 2002 Psychonomic Bulletin & Review 9 264 - 269) showed that a linear luminance gradient can largely modify the lightness of a target region. In the present work, a version of the gradient configuration is offered where the luminance range has been drastically reduced. In this condition, the gradient is almost unnoticeable. Comparing the lightness of a gray target placed at the centre of this gradient with that of an identical target surrounded by a surface having a homogenous luminance value equal to the highest luminance of the gradient, they appear quite different even though their backgrounds appear 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 by 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 colours taking into account the global spatial distribution of luminance gradients.
The phantom contrast effect
GALMONTE, Alessandra
2005-01-01
Abstract
Agostini and Galmonte (1997 Investigative Ophthalmology & Visual Science 38 S895; 2002 Psychonomic Bulletin & Review 9 264 - 269) showed that a linear luminance gradient can largely modify the lightness of a target region. In the present work, a version of the gradient configuration is offered where the luminance range has been drastically reduced. In this condition, the gradient is almost unnoticeable. Comparing the lightness of a gray target placed at the centre of this gradient with that of an identical target surrounded by a surface having a homogenous luminance value equal to the highest luminance of the gradient, they appear quite different even though their backgrounds appear 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 by 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 colours taking into account the global spatial distribution of luminance gradients.
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