Responses to a smaller version of the image from page 578 by cells sensitive to all 16 possible 2×2 blocks, as well as their repetitive 3×3 extensions. Patches which appear to have different textures in the original image are seen to contain characteristically different densities of these various blocks. I strongly suspect that it is density differences such as these that allow our visual system to distinguish textures.

From Stephen Wolfram: A New Kind of Science [citation]