Spatial phase effects in pattern masking

Abstract

Masking of bandpass-test stimuli by cosine gratings is known to be selective for spatial frequency and orientation, thus indicating a cortical component the phenomenon. Here we report that within 200 ms following the onset of a cosine mask, a phase-dependent masking effect emerges: Threshold elevation for a 33 ms D6 pattern (sixth derivative of a Gaussian) is greater on a light bar (0 phase) than on a dark bar (180 phase). A plausible hypothesis is that this phase effect results from a pointwise retinal luminance-adaptation process. To test this, we compared threshold elevations on cosine masks with those produced by uniform fields at the same luminance levels as the peak and trough of the cosine mask. The bright uniform mask produced a small threshold elevation, but the dark uniform mask actually facilitated detection of the test pattern. Comparable results were obtained with both positive and negative D6 test patterns. The effect of spatial phase can therefore be explained by a mode] consisting of a pointwise retinal luminance-adaptation process, followed by a phase-independent cortical contrast-adaptation process that is orientation- and spatial-frequency selective.