Context-dependent lightness affects perceived contrast

Abstract

a b s t r a c tPerceived contrast of a grating varies with its background (or mean) luminance: of the two gratings with the same photometric contrast the one on higher luminance background appears to have higher contrast. Does perceived contrast also vary with context-dependent background lightness even when the luminance remains constant? We investigated this question using a stimulus in which two equiluminant patches (''context squares", CSs) appear different in lightness. First we measured the lightness effect in a behavioral experiment. After ensuring that it was present for all participants, we conducted perceived contrast experiments, where participants judged the contrast of rectified incremental and decremental square-wave gratings superimposed on the CSs. For the incremental gratings participants' settings were significantly different for the two CSs. Specifically, perceived contrast was higher when the gratings were placed on the context square that was perceived lighter. In a follow-up experiment we measured perceived contrast of rectified gratings on isolated patches that differed in luminance. The pattern of results of the two experiments was consistent, demonstrating that possibly shared mechanisms underpin the effects of background luminance and context-dependent lightness on perceived contrast.

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

“…On the other hand, several studies have dealt with the effectiveness of lower-and higher-luminance components with respect to the background that spanned the entire achromatic gamut of the visual field from black to white. It has been reported that the on and off signals with respect to the adapting field (gray) in the achromatic channel were mediated by separate mechanisms on unipolar contrast adaptation (Beer & MacLeod, 2000;Sato, Motoyoshi, & Sato, 2016) and spatial interactions in perceived contrast (Pamir & Boyaci, 2016;Sato, Motoyoshi, & Sato, 2012). In the present study, the average stimulus observation time was approximately 2.5 s, and thus, the light adaptation to the luminance levels of each image should be weak even if it occurred.…”

“…For example, the wavy samples have some overall luminance gradient from shading. Considering that changes in perceived lightness from the luminance gradient affect the apparent contrast of a grating (Pamir & Boyaci, 2016), the luminance gradient from global surface shape could affect the quality of material perception. These possible effects of macro-shading patterns could be investigated using samples with macroscale shapes but without strong specular reflections.…”

The effect of spatial structure on binocular contrast perception

“…Contextual effects are ubiquitous in perception and it is thus ultimately essential to consider them for understanding binocular percepts during natural vision. For example, the same patch of gray can be perceived to be a different shade depending on whether it is on a dark or light background ( Adelson, 1993 ) and the same grating can appear to have different contrast depending on its surrounding context ( Pamir & Boyaci, 2016 ; Xing & Heeger, 2001 ). An effect known as surround suppression, in which a pattern seems to have a higher or lower contrast depending on the visual similarity of the surrounding area, can be observed both in early cortical neurons and psychophysically ( Xing & Heeger, 2001 ).…”