Metacontrast masking within and between visual channels: effects of orientation and spatial frequency contrasts.

We measured the strength and optimal target-mask onset asynchrony (SOA(max)) of metacontrast masking using Gabor patches as targets and sinusoidal rings with Gaussian envelopes as masks. We varied spatial frequencies (f) between 0.5 and 8 cpd to manipulate the degree to which spatial frequency channels in the visual system are triggered. By varying spatial frequencies as well as spatial frequency contrast (Δf) between target and mask, we measured the properties of inter- as well as intra-channel inhibition. We found that an increase of the mask's spatial frequency decreased its effectiveness but did not change its SOA(max). When orientation contrast was introduced between targets and masks with the same spatial frequency, SOA(max) increased with orientation contrast. An effect of orientation contrast was not observed with low spatial frequency-on-high spatial frequency masking, indicating that orientation selectivity is a unique feature of within-channel masking. Spatial frequency contrast affects SOA(max) and effectiveness in an asymmetric fashion: low-on-high masking is strong and yields a longer SOA(max), compared to low-on-low and high-on-high masking; high-on-low masking is ineffective.