Billock, Cunningham & Tsou (2008) 8 APPROVED FOR PUBLIC RELEASEEffect of Viewing Distance For ideal 1/f images, there should be little effect of viewing distance, because increasing viewing distance would simply shift a lower spatial frequency component into a higher spatial frequency, but the relationship between the spatial frequencies would be preserved. However, all physically obtainable fractals are limited to a range of spatial scales set at the lower end by the size of the image and at the upper end by the size of the pixels. Shifting the viewing distance from 40 to 100 cm therefore shifts the spatial frequency range of the fractal image by a factor of 2. 5, but no information is lost because the stimuli were designed so that the individual pixels were resolvable at the far viewing distance by all observers. Accordingly, for three of the observers (DC, PH, and SF), viewing distance had little effect, although there is a slight trend suggesting lower thresholds in the nearer viewing distances (see Figure 4a-c). For VB, however, the Far thresholds (and their variability) were noticeably elevated compared to either his Near thresholds or to the other observers' Far thresholds (see Fig. 4). VB's anomalous results may be due to sampling problems induced by a mild congenital defect-a developmental paucity of retinal ganglion cells (optic nerve hypoplasia). Electrophysiological studies in VB and other hypoplastic subjects and post-mortem histology in other hypoplastics indicate that both retinal pre-processing and cortical post-processing seem to be normal (Billock et al., 1994) and point to reduction in retinal ganglion cell numbers as the sole cause of abnormal vision in hypoplasia. In the case of VB, perimetric thresholds are flattened relative to normals, suggesting the subject did not gain a full measure of the elevated density of foveal ganglion cells that develops in normals. Since pixel size and stimulus size are fixed, any sampling problems would more likely manifest as a threshold elevation at the far viewing conditions. Moreover, if the reduced sampling is not homogeneous, then this could increase variability (because, from trial to trial, filtered noise features would fall on neighboring retinal locations with different retinal sampling densities). Effect of Presentation Style Simultaneous viewing simulates the task of making a side-by-side comparison of fractal camouflaged targets, while sequential viewing simulates the task of comparing a target to one that is in memory; in theory and experiment the two paradigms can lead to somewhat different results (Garcia-Perez et al., 2005; Hansen & Hess, 2006). The discrimination function is similar for both conditions (Figs. 3 and 4) but there is a small advantage for simultaneously-viewed images, relative to sequentially-viewed ones, especially for small values of ȕ. This tendency can be clearly seen when the data from the 4 subjects are pooled (Fig. 3). This is contrary to Hansen & Hess (2006), who found an advantage for sequential viewing, and attributed differences between the two conditions to differences in the portions of retina they cover. However, our near-sequential and far-simultaneous stimuli covered very similar regions of central retina (2. 6o and 2. 3o respectively), and yet simultaneous viewing yielded lower thresholds for nine of ten exponents (and tied for the tenth). This suggests that for our experiment, the memory demands of sequential viewing were disadvantageous, a design consideration for combat target displays. Discussion Comparison to Related Studies: Static Fractals Some prior studies of fractal discrimination overlap our work. Our discrimination functions resemble those of Knill et al. (1990), particularly their low-contrast near condition (17. 5% RMS