Human visual system is sensitive to both the first-order and the second-order variations in an image.
The latter one is especially important for the digital image processing as it allows human observers
to perceive the envelope of the pixel intensities as smooth surface instead of the discrete pixels. Here
we used pattern masking paradigm to measure the detection threshold of contrast modulated (CM)
stimuli, which comprise the modulation of the contrast of horizontal gratings by a vertical Gabor
function, under different modulation depth of the CM stimuli. The threshold function showed a
typical dipper shape: the threshold decreased with modulation depth (facilitation) at low pedestal
depth modulations and then increased (suppression) at high pedestal modulation. The data was well
explained by a modified divisive inhibition model that operated both on depth modulation and
carrier contrast in the input images. Hence the divisive inhibition, determined by both the first- and
the second-order information in the stimuli, is necessary to explain the discrimination between two
second-order stimuli.
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