Abstract Abstract: Abstract?? With eleven different types of stimuli that exercise a wide gamut of spatial and temporal visual processes, negative perturbations from mean luminance are found to be typically 25% more effective visually than positive perturbations of the same magnitude (range 8a??67%). In Experiment 12, the magnitude of the blacka??white asymmetry is shown to be a saturating function of stimulus contrast. Experiment 13 shows blacka??white asymmetry primarily involves a nonlinearity in the visual representation of decrements. Blacka??white asymmetry in early visual processing produces even-harmonic distortion frequencies in all ordinary stimuli and in illusions such as the perceived asymmetry of optically perfect sine wave gratings. In stimuli intended to stimulate exclusively second-order processing in which motion or shape are defined not by luminance differences but by differences in texture contrast, the blacka??white asymmetry typically generates artifactual luminance (first-order) motion and shape components. Because blacka??white asymmetry pervades psychophysical and neurophysiological procedures that utilize spatial or temporal variations of luminance, it frequently needs to be considered in the design and evaluation of experiments that involve visual stimuli. Simple procedures to compensate for blacka??white asymmetry are proposed.
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