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Abstract
Using computer graphics and a two-alternative forced-choice method we measured threshold
contrast as a function of viewing distance, spatial frequency, and eccentricity for
gratings with and without added, white two-dimensional spatial noise. Our experiments
showed that in spatial noise contrast sensitivity was independent of viewing distance
as long as contrast sensitivity was lower with noise than without. With increasing
spatial frequency (f) the grating area (A) was reduced in order to keep the relative
grating size (Af2) constant. At all spatial frequencies the test gratings thus had
the same amount of detail and contour. Noise spectral density was reduced in direct
proportion to grating area in order to keep the physical signal-to-noise ratio constant.
An increase in spatial frequency was thus accompanied with reductions in grating area
and noise spectral density similar to those produced by a corresponding increase in
viewing distance. In agreement, contrast detection in spatial noise was found to be
independent of spatial frequency as long as contrast sensitivity was lower with noise
than without. The effect of increasing eccentricity on visual performance can be compensated
for by reducing the viewing distance (M-scaling). Hence, without M-scaling the effect
of increasing eccentricity is similar to that of increasing viewing distance. In agreement,
we found that contrast sensitivity in spatial noise was independent of eccentricity
as long as contrast sensitivity was lower with noise than without.