Michael S. Landy 5/20/03 (w/ Charles Chubb & John
Econopouly) <br><br>

Blackshot: An Unexpected Dimension of Human Sensitivity to Contrast
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PURPOSE. We studied the perceptual segregation of texture pairs
consisting of a grid of uniform, square texture elements, differing only
in the distribution of intensities across those elements. If two textures
differ in mean intensity (lightness) or in the variance of intensity
(contrast), then they are easily segregated. Chubb, Econopouly &
Landy (JOSA A, 11, 2350, 1994) demonstrated the existence of a 3rd
mechanism B (in addition to mechanisms L and C, coding lightness and
contrast respectively) that was solely responsible for the segregation of
textures equated for mean and contrast. Here, we determine the
sensitivity of B to textures differing in mean or variance, thus fully
specifying its (highly nonlinear) contrast response function.
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METHOD.
Two textures have gray-level histograms H1 and H2 chosen so as to equate
both mean and variance so that B alone discriminates them.  The magnitude
of the difference D=H1-H2 was varied to find a segregation threshold
t*D.  Then, D was perturbed (e.g., D=t*D+P) so that the two textures
differed in mean or variance, and segregation performance was assessed.
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RESULTS. Differences in texture mean and in texture variance traded off
linearly with changes in D.  This implies that even when the textures
differ slightly in mean or variance, it is still B alone that
discriminates between them.  The slopes of the lines relating changes in
mean and variance to changes in the amplitude of D reflect the
sensitivity of B to texture mean and variance. Combining these findings
with our previous results reveals that B is (i) highly sensitive to
texture elements of the lowest contrast (near -1), and (ii) has a
response that saturates by a contrast of 3/4 (dark gray).  That is, this
blackshot mechanism discriminates textures by comparing the number of
the blackest pixels in each.