Aliased Frequencies Enable the Discrimination of Compound Gratings in Peripheral Vision

Although gratings beyond the Nyquist limit of the peripheral retina are visible as aliased percepts when presented in isolation, the reported lack of aliasing for targets with complex spectra (edges, lines, letters) suggests that aliased frequency components are invisible in the presence of sub-Nyquist image components. We tested this hypothesis by systematically exploring a range of stimulus parameters in search of conditions which would enable subjects to detect the supra-Nyquist components of a compound grating. A three-alternative forced-choice masking paradigm was used, which required subjects to discriminate a 2.5 deg patch of compound grating (mask + test) from a simple grating (mask only). Using a 2 c/deg grating as the masking component, which is well below the 4 c/deg Nyquist limit to veridical perception at 20 deg in the horizontal nasal field, we varied the spatial frequency of the test grating over a range extending above and below the Nyquist frequency. We found that aliased, supra-Nyquist components are reliably detected in the presence of high contrast, sub-Nyquist gratings, provided they have sufficient contrast. Contrast threshold for detecting the aliased gratings was higher when presented as a component of the compound grating than when presented in isolation. This masking effect of the sub-Nyquist component was not specific to supra-Nyquist test targets, however, since a similar masking effect also occurred for sub-Nyquist targets. These results suggest that the invisibility of aliasing described previously for edges or square wave gratings is due primarily to the combination of the low amplitudes of supra-Nyquist harmonics in such stimuli and a high contrast threshold of the peripheral retina. The additional masking effects of the sub-Nyquist, fundamental component of a square wave on the supra-Nyquist harmonics make the detection of aliasing impossible even for very high contrast square wave gratings presented in the periphery.