Scrambling procedure. a, Images were scrambled using a model introduced by Portilla and Simoncelli (2000). Briefly, the procedure begins by computing a number of image statistics. A Gaussian white-noise image is then iteratively adjusted until it has the same number and type of local, oriented elements but presented at random positions within the image (see Materials and Methods). b, Example natural images and their scrambled counterparts. Each set contained 50 images.