A fair test of the effect of a shadow-incompatible luminance gradient on the simultaneous lightness contrast

Multiresolution wavelet framework models brightness induction effects

A new multiresolution wavelet model is presented here, which accounts for brightness assimilation and contrast effects in a unified framework, and includes known psychophysical and physiological attributes of the primate visual system (such as spatial frequency channels, oriented receptive fields, contrast sensitivity function, contrast non-linearities, and a unified set of parameters). Like other low-level models, such as the ODOG model [Blakeslee, B., & McCourt, M. E. (1999). A multiscale spatial filtering account of the white effect, simultaneous brightness contrast and grating induction. Vision Research, 39, 4361-4377], this formulation reproduces visual effects such as simultaneous contrast, the White effect, grating induction, the Todorović effect, Mach bands, the Chevreul effect and the Adelson-Logvinenko tile effects, but it also reproduces other previously unexplained effects such as the dungeon illusion, all using a single set of parameters.

Separate effects of background and illumination on lightness

Four experiments attempted to establish an effect of context on lightness. Lightness is one of the dimensions of color and it varies from black to white. Most of our stimuli were inspired by simultaneous lightness contrast illusion. First two experiments contrast the size of an effect produced by the change of background color vs. the change in illumination. The third experiment deals with different type of illusions, where the effect is obtained through the appearance of multiple illumination levels. The last experiment takes into account the ratio of the target and the background. The results reveal the size of effects produced separately by the background color and illumination level and suggest the prime importance of background. Also there are other factors such as reflectance range in the scene, incremental and decremental targets, and 2D vs. 3D representation.

The discoloration illusion

The discoloration illusion, a new visual phenomenon, is described. This phenomenon originates from the juxtaposition of eight chromatic parallel contours on a white background, creating a luminance gradient and enclosing a light red region. Under these conditions, the inner region appears white: the light red discolors and appears white with both surface color and luminous qualities. In two experiments, the discoloration illusion was (i) compared with the coloration effect of the watercolor illusion, obtained when the number of adjacent contours was reduced to at least two, and (ii) tested under several conditions useful for understanding the roles of the luminance gradient profile. The results suggest that discoloration is not a lightness illusion and does not depend on simultaneous contrast or on achromatic mechanisms, but more likely on chromatic mechanisms that, through the luminance chromatic gradient, provide cues about the interactions of light and surface and model the volume by depicting lights and shades. The discoloration illusion suggests a possible neural scenario where multiple juxtaposed contours may stimulate neurons, selective for different asymmetric luminance profiles and signaling not only the unilateral belongingness of the boundaries and the coloration effect but also the volumetric and the illumination effects.

Chromatic shadow compatibility and cone-excitation ratios

Logvinenko [Perception 31, 201 (2002)] asserts that Adelson's wall-of-blocks illusion [Science 262, 2042 (1993)], where identical gray-cube surface tops appear to differ in brightness, arises when the surfaces surrounding the cube tops are shadow compatible, creating a concomitant illusion of transparency. We replicated Logvinenko's main findings in the chromatic domain across three experiments in which observers match cube tops in hue, saturation, and brightness. A second set of stimuli adjusted cone-excitation ratios across the apparent transparency border [Proc. R. Soc. London 257, 115 (1994)], which enhanced lightness and brightness constancy but only when the stimuli varied in both chromaticity and intensity.