Modulation frequency and orientation tuning of second-order texture mechanisms.

Modulation frequency and orientation tuning of second-order mechanisms underlying the detection of modulation in local spatial-frequency information are assessed by using an oblique-masking paradigm. Stimuli were Gabor-filtered noise patterns in which the local carrier spatial frequency was modulated about an average value of 4.7 cycles per degree (cpd) according to sinusoidal function. Thresholds were determined for spatial-frequency modulated test patterns (0.2 and 0.8 cpd) with fixed vertical carrier and modulation orientations presented alone and in the presence of spatiotemporally superimposed masks. Mask modulation frequency (0.1, 0.2, 0.4, 0.8, or 1.6 cpd), modulation orientation (0 degree, 45 degrees, or 90 degrees relative to vertical), and carrier orientation (18.5 degrees or 90 degrees relative to vertical) were manipulated independently while the mask modulation amplitude remained fixed at 0.25. Manipulating the modulation frequency of the mask revealed some modulation frequency specificity, particularly at lower test modulation frequencies. Spatial-frequency modulated masks produced threshold elevations regardless of the local carrier orientation. However, there was no evidence of threshold elevation when the mask modulation orientation was orthogonal to that of the test pattern. These results suggest a second-order texture mechanism that is tuned to both modulation frequency and modulation orientation but is not selective in terms of the orientation of first-order inputs.