Billock, Cunningham & Tsou (2008) 9 APPROVED FOR PUBLIC RELEASEcontrast at 1 meter with 64x64 pixel images), which is similar to our Far Sequential condition. While the average exponent of natural scenes is around 1. 1 (Table 1), the greatest sensitivity to changes in a fractal image's exponent are consistently found to be around 1. 6 across a wide range of conditions. We see no evidence for a second minimum at low ȕ (circa 0. 6) reported by Tadmor and Tolhurst, even when we used a simultaneous viewing condition similar to Tadmor and Tolhurst (1994). Nor is the discrepancy due to the angular size of the image, as all three studies had stimuli that were similar in size. Hansen & Hess (2006) note that the spatial presentation task uses two different parafoveal patches of retina, while the temporal task uses the same patch of central fovea; they find that fovea and parafovea yield somewhat different patterns of discriminability as a function of ȕ, but none of their data show a second minimum at low ȕ (rather, they find a maximum at ȕ=0. 8, with better thresholds on either side, similar to our findings). Another possible source of this difference in discriminability functions may be the specific nature of Tadmor and Tolhurst's task. In both the present study and Knill et al. (1990), standard two-Alternative Forced-Choice psychophysical procedures were used. In contrast, Tadmor and Tolhurst (1994) used an odd-one-out task (i. e., three images were presented simultaneously, two of which had identical exponents-the task was to choose the image that was different from the other two). In other words, Tadmor and Tolhurst's task was one of simple discrimination, while our task (and Knill's) requires discrimination and some form of identification (once the two images could be told apart, the subjects had to decide which had a lower exponent). These tasks coincide in difficulty only if all information required for identification is present at the discrimination threshold, which will most often take place when a single channel mediates performance of the task. Indeed, Tolhurst and Tadmor (1997) have shown that simple discrimination data is often consistent with a single channel mediating discrimination. However, since a comparison of channel outputs is required to estimate the spectral exponent of an image, discrimination plus identification would likely require a comparison of channel outputs, perhaps raising the JND for β near 0. 4 sufficiently to eliminate the second minimum that Tadmor and Tolhurst (1994) found. Figure 5. Camouflage can be dynamic in several ways, including simple movement. A Jordanian F-16 painted in Hyper Stealth Biotechnology Corp. 's fractal-like camouflage.