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. 2009 Jun 15;587(Pt 12):2753–2767. doi: 10.1113/jphysiol.2009.170704

Figure 2. Natural images contain more negative than positive contrasts at all scales. Correspondingly the optimal mosaic contains more OFF cells.

Figure 2

A, centres of difference-of-Gaussians filters superimposed on an image. B, contrast distributions at three scales: the distributions are skewed, and negative contrasts are more abundant. C, proportion of negative, positive and subthreshold (<3%) contrasts. At all scales negative contrasts are ∼50% more numerous. The proportion of subthreshold contrasts declines with scale because distributions in B flatten with increasing centre radius. D, in the optimal mosaic OFF filters are more numerous and smaller than the ON filters. Here 16 filters cover 25 image pixels, and information in bits is maximized when 12 of the filters are of the OFF type (adapted from C. Ratliff, Y.-H. Kao, P. Sterling & V. Balasubramanian, unpublished observations).