Visual discomfort and natural image statistics

Images with excessive energy at medium spatial frequencies (Fernandez and Wilkins, 2008 Perception 37 1098-1113), or that have high color contrast and little or no luminance contrast (Wilkins et al, 2008 Perception 37 Supplement, 144-145) appear uncomfortable or aversive and can induce headaches in hypersensitive observers. Such stimuli are uncharacteristic of natural images, and we examined whether visual discomfort more generally increases with deviations from the spatial and chromatic properties of natural scenes. Observers rated the level of discomfort or artistic merit in color images generated from noise or random overlapping rectangles (Mondrians). In one set, the slopes of the amplitude spectra for luminance or chromatic contrast were varied independently to create images ranging from strongly blurred to sharpened relative to a 'natural' 1/f spectrum. Perceived blur was dominated by the luminance slopes, with discomfort rated lowest for similar to 1/f spectra. In a second set of focused Mondrians, color was varied along different axes in the L-M versus S-LM chromatic plane. Discomfort ratings were lowest around bluish-yellowish axes that are again typical of natural outdoor scenes. A final set varied in the levels of both luminance and chromatic contrast. Discomfort increased with increasing color contrast or decreasing luminance contrast, and tended toward lower ratings for images with balanced levels of luminance and color contrast. Notably, these ratings of discomfort were not related to judgments of artistic merit. Thus, for both spatial and chromatic content, the least aversive images corresponded to characteristic properties of the natural visual environment, and may reflect a normalization of visual coding to the natural world.