Billock, Cunningham & Tsou (2008) 5 APPROVED FOR PUBLIC RELEASEMethods Participants The four observers were all myopes corrected to at least 20/20 binocular acuity. All are professional psychophysicists and highly experienced observers, with prior work in the psychophysics of "white" and "colored" (spatiotemporally non-uniform) visual noise. One subject (SF) was naïve to the purpose of the experiment. Another subject (VB) has a diagnosed, mild, congenital visual condition-optic nerve hypoplasia (a low density of neurons in the optic nerve). Although his vision is considered normal by standard clinical measures (including acuity), his contrast sensitivity is slightly depressed (about 1 sd) at all spatial frequencies relative to a large sample of age-matched-normals; this depression in sensitivity is worse for higher spatial frequencies. His vision is relevant here because it provides us a gauge of the effects of spatial under-sampling in an otherwise intact visual system. Apparatus All stimuli were generated and presented on a Silicon Graphics O 2 graphics workstation with a linearized 30 Hz display. Stimuli were viewed binocularly with natural pupils in a well-lit room (ambient luminance in the plane of the monitor was 3. 5 cd/m2). Subjects were comfortably fixed in place by a chin rest at two viewing distances, 40 cm and 100 cm. The far distance was a limiting case (e. g., each pixel subtends 0. 016 deg at 100 cm, matching the subjects' 1 arc min spatial resolution). The stimuli consisted of static, grayscale, random-phase fractals (e. g., Fig. 2) whose Fourier amplitude spectra were described by A(fs) = kf sȕ Eq. 1 Where k is a constant and f s is spatial frequency. (In visual psychophysics, amplitude rather than power spectra are used, because amplitude is proportional to perceptual contrast for each spatial frequency component. ) For each stimulus, the average luminance was constant at 8. 57 cd/m2and the Root Mean Square Contrast (a good measure of perceptual contrast for noise-like textures; Moulden et al., 1990; Peli, 1990, 1997) was 10. 98%. For consistency with another study, each fractal contained 64x64 pixels (18x18mm). Thus, at 40 cm, the stimuli subtended 2. 58O embedded in a 43. 9O horizontal by 36. 4O vertical dark surround. At 100 cm, each stimulus subtended 1. 03O embedded in a 21. 1O horizontal by 16. 4O vertical dark surround. Both a reference and a comparison image were generated for each trial. The images were created by filling a 64X64 array with random white noise (256 gray-levels). This white-noise image was Fourier-transformed and the amplitudes of all spatial frequencies were equalized to ensure that the noise was uniformly flat. The resulting amplitude spectra were filtered so that they followed a power law relationship (Eq. 1), and then inverse-Fourier transformed to produce the stimuli. Procedure Just noticeable discrimination thresholds (79% correct criterion) for fractal spatial exponents were measured using a two-alternative forced-choice adaptive staircase procedure with a 1 db step size (Mac Millan & Creelman, 1991). Ten β exponents were used (0. 4, 0. 6, 0. 8, 1. 0, 1. 2, 1. 4, 1. 6, 1. 8, 2. 0, and 2. 2) for the reference images. For the comparison image, the fractal exponent was equal to the exponent, β, of its reference image plus a small increment, Δβ. Observers were asked to identify the image with the lower spectral exponent, and were provided with immediate