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.

Apparent contrast and spatial frequency of local texture elements.

We measured the apparent contrast and spatial frequency of a parafoveal Gabor signal located at the center of an array of Gabor signals as a function of both element density and the direction of contrast and spatial frequency of the surrounding elements. The target Gabor appeared lower in contrast and higher in spatial frequency when the elements were in close proximity, regardless of the direction of contrast and spatial frequency of the surrounding elements. Overall, the evidence suggests that the appearance of a parafoveal target is strongly affected by its visual context. These findings provide additional support for the existence of spatial interactions among neurons implicated in textural processing.

Two-stage model of visual backward masking: sensory transmission and accrual of effective information as a function of target intensity and similarity.

A functional model is used to describe the effect of target intensity and target-set similarity on backward visual masking. The model consists of two distinct stages of visual information processing. The first stage is related to sensory transduction and transmission and is assumed to require a finite and measurable amount of time during which performance remains at chance. The second stage, associated with central processing, is characterized by a negatively accelerated growth function reflecting the accrual of effective information. Results show that the duration of the transmission stage is inversely related to target intensity. Surprisingly, the rate of information accrual is an interactive function of both target intensity and target-set similarity. The pattern of results is consistent with the interpretation that both intensity and similarity mediate their effect through a common mechanism--the accrual of effective information.