Abstract
An assumption commonly made in the study of visual perception is that the lower the contrast threshold for a given stimulus, the more sensitive and selective will be the mechanism that processes it. On the basis of this consideration, we investigated contrast thresholds for two classes of stimuli: sine-wave gratings and radial frequency stimuli (i.e., j0 targets or stimuli modulated by spherical Bessel functions). Employing a suprathreshold summation method, we measured the selectivity of spatial and radial frequency filters using either sine-wave gratings or j0 target contrast profiles at either 1 or 4 cycles per degree of visual angle (cpd), as the test frequencies. Thus, in a forced-choice trial, observers chose between a background spatial (or radial) frequency alone and the given background stimulus plus the test frequency (1 or 4 cpd sine-wave grating or radial frequency). Contrary to our expectations, the results showed elevated thresholds (i.e., inhibition) for sine-wave gratings and decreased thresholds (i.e., summation) for radial frequencies when background and test frequencies were identical. This was true for both 1- and 4-cpd test frequencies. This finding suggests that sine-wave gratings and radial frequency stimuli are processed by different quasi-linear systems, one working at low luminance and contrast level (sine-wave gratings) and the other at high luminance and contrast levels (radial frequency stimuli). We think that this interpretation is consistent with distinct foveal only and foveal-parafoveal mechanisms involving striate and/or other higher visual areas (i.e., V2 and V4).
Highlights
In 1960, Kelly [1] proposed the use of images having a concentric configuration for studying visual mechanisms and properties where the contrast is modulated by a Bessel J0 profile
On the basis of this consideration, we investigated contrast thresholds for two classes of stimuli: sine-wave gratings and radial frequency stimuli (i.e., j0 targets or stimuli modulated by spherical Bessel functions)
The results showed elevated thresholds for sine-wave gratings and decreased thresholds for radial frequencies when background and test frequencies were identical. This was true for both 1- and 4-cpd test frequencies. This finding suggests that sine-wave gratings and radial frequency stimuli are processed by different quasi-linear systems, one working at low luminance and contrast level and the other at high luminance and contrast levels
Summary
In 1960, Kelly [1] proposed the use of images having a concentric configuration for studying visual mechanisms and properties where the contrast is modulated by a Bessel J0 profile. Among the few that did attempt to conduct experiments with these stimuli are Kelly and Magnuski [2] and Kelly [3] These investigators reported measurements of the modulation transfer function, or the contrast sensitivity function (CSF), for sine-wave grat-. And Magnuski [2] reported that the human visual system is at least two times less sensitive to J0 targets when compared to sine-wave gratings. This indicates that the amount of contrast necessary for the detection of sine-wave gratings is at least two times lower than the amount necessary to detect J0 targets in its respective most sensitive range. We will call these stimuli radial (spatial) frequencies, use spherical Bessel function profile modulation, j0, and name its response curve rCSF (radial (frequency) CSF) [4]
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