Previous studies of spatial-frequency masking and adaptation have shown that the contrast-detection threshold elevates maximally when the test spatial frequency is the same as the masking (or adapting) frequency but changes only slightly when they are separated by two or more octaves. At low spatial frequencies, however, the peak of the threshold-elevation 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 energy at the very low end of the spectrum. We first measured the effect of a 3-cycles/deg (c/deg) mask on detection of a range of test frequencies, compared with unmasked detection thresholds. We then measured the combined effect of a 2-c/deg and a 3-c/deg mask on detection, compared with detection with just the 2-c/deg mask. The comparison in the second 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 toward higher frequencies. This result provides a proof of concept for the 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 contrast-detection function, the frequency-masking functions at mask frequencies of 0.25, 0.5, 2, and 3 c/deg, and the peak-shift phenomenon.
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