Contrast Adaptation and the Spatial Structure of Natural Images

Abstract

Contrast (pattern-selective) adaptation influences perception by adjusting sensitivity to the prevailing pattern of stimulation. We asked how the state of adaptation might depend on the patterns of spatial contrast typical of the natural visual environment. In one set of experiments, we examined whether adaptation to the characteristic amplitude spectra of natural images (which tend to decrease with frequency as 1/f) induces characteristic changes in contrast sensitivity. Contrast thresholds and suprathreshold contrast matches were measured after adaptation to random samples from an ensemble of images of natural outdoor scenes, or synthetic images formed by filtering the amplitude spectra of noise over a range of slopes. Adaptation differentially reduced sensitivity at low to medium spatial frequencies, but losses were not strongly dependent on the slope of the adapting spectra. In a second set of experiments, we examined the figural aftereffects induced by adaptation to naturalistic stimuli, by adapting and testing with images of human faces, for which small configural changes are highly discriminable. Observers adapted to frontal-view images of faces that were distorted by local expansions or contractions about the centre, and then adjusted distortions in test images to try to select the original face. Adaptation strongly biased perception in a direction opposite to the adapting distortion, with strongest aftereffects when test and adapting stimuli were derived from the same face image. Our results suggest that adaptation to the stimuli encountered in the course of normal viewing may exert ubiquitous and selective influences that are important in characterising the normal operating state of the visual system.