Natural scene statistics at the centre of gaze

Early stages of visual processing may exploit the characteristic structure of natural visual stimuli. This structure may differ from the intrinsic str ucture of natural scenes, because sampling of the environment is an active process. For example, humans move their eyes several times a second when lo oking at a scene. The portions of a scene that fall on the fovea are sample d at high spatial resolution, and receive a disproportionate fraction of co rtical processing. We recorded the eye positions of human subjects while th ey viewed images of natural scenes. We report that active selection affecte d the statistics of the stimuli encountered by the fovea, and also by the p arafovea up to eccentricities of 4 degrees. We found two related effects. F irst, subjects looked at image regions that had high spatial contrast. Seco nd, in these regions, the intensities of nearby image points (pixels) were less correlated with each other than in images selected at random. These ef fects could serve to increase the information available to the visual syste m for further processing. We show that both of these effects can be simply obtained by constructing an artificial ensemble comprised of the highest-co ntrast regions of images.