EFFECT OF BACKGROUND COMPONENTS ON SPATIAL-FREQUENCY MASKING

Previous studies of spatial-frequency masking and adaptation have show n that the contrast-detection threshold elevates maximally when the te st spatial frequency is the same as the masking (or adapting) frequenc y but changes only slightly when they are separated by two or more oct aves. At low spatial frequencies, however, the peak of the threshold-e levation function does not obey this rule: there is a well-established peak shift in the threshold-elevation functions toward higher spatial frequencies. We investigated whether this shift. might be due to the masking effects caused by the background field, which contributes ener gy at the very low end of the spectrum. We first measured the effect o f a 3-cycles/deg (c/deg) mask on detection of a range of test frequenc ies, compared with unmasked detection thresholds. We then measured the combined effect of a 2-c/deg and a 3-c/deg mask on detection, compare d with detection with just the 2-c/deg mask. The comparison in the sec ond case still tests the effect of the 3-c/deg mask, but the presence of the hidden 2-c/deg mask causes the peak masking effect to shift tow ard higher frequencies. This result provides a proof of concept for th e hypothesis that the peak shift, at low spatial frequencies is caused by the low-frequency energy in the background field, which is present in both masked and unmasked conditions. A five-parameter quantitative model of frequency masking is presented that describes the pure contr ast-detection function, the frequency-masking functions at mask freque ncies of 0.25, 0.5, 2, and 3 c/deg, and the peak-shift, phenomenon. (C ) 1998 Optical Society of America.