Abstract
The cause of the low-frequency roll-off in the contrast sensitivity function of human vision was investigated. Evidence was found that low-frequency structure in the visual world is perceived not by harmonic analysis but by detection of contrast gradients. This hypothesis is supported by the thresholds of trapezoidal gratings, i.e. gratings which have regions of uniform luminance alternating with linear luminance ramps. Doubling the ramp slope halves the threshold at ramp widths of 0.5° and greater. Trapezoids with ramp widths less than 0.5° all have the same threshold which is equal to a square grating threshold. The visual world therefore is analyzed by two different mechanisms. Above the peak of the contrast sensitivity function harmonic analysis takes place and below it, contrast gradient analysis.
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