Visual masking at different polar angles in the two-dimensional Fourier plane.

Human contrast thresholds were measured at 86 points in the two-dimensional Fourier plane with and without masks. The test stimuli were sinusoidal gratings in a 2.55-deg circular field. The superimposed masks were sinusoidal gratings having a polar spatial frequency of 8 cycles/deg, a contrast of 0.31, and one of five polar angles: 90, 105, 120, 135, and 180 deg. The test grating contrast for 75.5% correct detection in a three-alternative, forced-choice paradigm was determined by a maximum-likelihood adaptive psychophysical procedure. Three other observers were tested on subsets of these conditions. The threshold elevation surfaces produced by the masks lead to the following conclusions: spatial-frequency bandwidth is independent of mask orientation and has a value of approximately 2 octaves, orientation bandwidth is wider for oblique masks than for horizontal and vertical masks, the principle of spectral polar separability is violated, and two-dimensional Gabor functions in the frequency domain account for the masking effects. Individual differences were found among the observers.