Abstract

Using computer graphics and a two-alternative forced-choice method we measured contrast detection thresholds in spatial noise as a function of spatial frequency (F). Grating area (A) decreased in inverse proportion to F² Hence, regardless of spatial frequency, viewing distance, or eccentricity, the test gratings had a constant number of square cycles (c²), calculated as AF². Noise was produced by adding to each pixel a random number drawn independently from an even distribution with zero mean. Thus, noise was white up to a cutoff frequency determined by the pixel density of our display. Noise spectral density decreased in direct proportion to grating area (A) and in inverse proportion to spatial frequency squared (F²). Hence, a change in viewing distance had the same effect on grating area and noise spectral density as a corresponding change in spatial frequency. Increasing eccentricity results in increasing cortical spatial frequency and decreasing cortical grating area. Their magnitude is determined by the cortical magnification factor and their effects can be compensated for by reducing the viewing distance (M-scaling). Thus, the effect of increasing eccentricity is similar to that of increasing viewing distance.

© 1989 Optical Society of America