Local luminance nonlinearity and receptor aliasing in the detection of high-frequency gratings

Contrast sensitivity for orientation discrimination is limited to spatial frequencies below 50-60 cycles per degree by neural spatial integration, and we find that contrast sensitivity, measured using an orientation-discrimination criterion, declines sharply with increasing spatial frequencies in that range. Yet interference fringe patterns pulsed at constant mean luminance can be detected at spatial frequencies far above that resolution limit [D.R. Williams, Vision Res. 25, 195 (1985)]. This is due at least in part to aliasing by the receptor mosaic, but another possible cue is provided by nonlinear distortion, which can create spatially uniform temporal transients when a pulsed fringe pattern is presented. We investigated the contribution of these spatially unstructured distortion products to grating detection by superimposing the pulsed fringe patterns on a randomly flickering uniform field. This manipulation has almost no effect on contrast sensitivity well below the resolution limit. Just above the resolution limit, however, the random luminance mask greatly elevates the fringe pattern detection threshold, suggesting that spatially unstructured cues provide the basis for detection in this range. At still higher fringe pattern frequencies, which approximately match the cone mosaic, the random luminance flicker again becomes ineffective for some observers, creating a clear secondary peak in contrast sensitivity in the flicker mask condition, which is presumably due to spatially structured cues provided by aliasing with the cone mosaic. The aliasing peak is still more clearly demarcated if the subject sets contrast thresholds for the perception of pattern as such. The contrast sensitivity function then has a notch or gap between the normal sensitivity range and the aliasing range. Apparently, in the unmasked case, spatially uniform cues (changes in overall color and brightness) bridge this gap.