Context-dependent lightness affects perceived contrast

The effect of spatial structure on binocular contrast perception

To obtain a single percept of the world, the visual system must combine inputs from the two eyes. Understanding the principles that govern this binocular combination process has important real-world clinical and technological applications. However, most research examining binocular combination has relied on relatively simple visual stimuli and it is unclear how well the findings apply to real-world scenarios. For example, it is well-known that, when the two eyes view sine wave gratings with differing contrast (dichoptic stimuli), the binocular percept often matches the higher contrast grating. Does this winner-take-all property of binocular contrast combination apply to more naturalistic imagery, which include broadband structure and spatially varying contrast? To better understand binocular combination during naturalistic viewing, we conducted psychophysical experiments characterizing binocular contrast perception for a range of visual stimuli. In two experiments, we measured the binocular contrast perception of dichoptic sine wave gratings and naturalistic stimuli, and asked how the contrast of the surrounding context affected percepts. Binocular contrast percepts were close to winner-take-all across many of the stimuli when the surrounding context was the average contrast of the two eyes. However, we found that changing the surrounding context modulated the binocular percept of some patterns and not others. We show evidence that this contextual effect may be due to the spatial orientation structure of the stimuli. These findings provide a step toward understanding binocular combination in the natural world and highlight the importance of considering the effect of the spatial interactions in complex stimuli.

Relative contributions of low- and high-luminance components to material perception

Besides specular highlights, image pixels that represent clues to recognizing the object material, such as shading between threads of fabrics, often yield relatively lower luminance in the image. Here, we psychophysically examined how lower and higher luminance components contribute to material perception. We created two types of luminance-modulated images-low- and high-luminance-preserved (LLP and HLP) images-and instructed observers to choose which modified image resulted in a material impression closer to the original. LLP images were created by compressing the luminance contrast of the higher half of the histogram in each original photograph and vice versa. The stimuli were photographs of various samples of stone, wood, leather, and fabric. Although the LLP and HLP images were equally chosen, the choice ratios of the HLP images largely differed across the samples and categories and moderately correlated with the luminance statistics of higher-spatial-frequency sub-bands. These results suggest that either the lower- or higher-luminance components play an important role in material perception, depending on the material category. However, the correlation with sub-band image statistics for stone/wood samples was much weaker than for leather/fabric samples, suggesting that more intricate image characteristics may be involved in evaluating the material impressions of the stone/wood samples.