Contrast sensitivity for oriented patterns in 1/f noise: contrast response and the horizontal effect.

When observers detect an oriented, broadband contrast increment on a background of 1/f spatial noise, thresholds will be lowest for obliquely orientated stimuli and highest for horizontally oriented stimuli-an anisotropy termed the "horizontal effect." Here, we assessed what spatial frequencies within the broadband increment were relied on by observers in performing the original task and which spatial frequencies contribute to the anisotropic performance. We found that against a background of 1/f noise, contrast thresholds are lowest for content around 8 cycles per degree, and that at this spatial frequency a horizontal effect is seen which closely resembles the anisotropy observed in broadband masking. The magnitude of the horizontal effect decreased at lower and higher spatial frequencies. To allow for a fit to a standard "gain control" model of psychophysical contrast discrimination, threshold-versus-contrast (TvC) functions were measured for the 8-cpd noise broadband content against either an identical pattern (i.e., pedestal) or a broadband 1/f noise pattern, whose contrast was varied. Results and model application indicate that the threshold pattern for oriented noise around 8 cpd, and for oriented broadband content, is best explained as the result of an anisotropic contrast gain control process.