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Cheryl Olman, Huseyin Boyaci, Fang Fang, Katja Doerschner; V1 responses to different types of luminance histogram contrast. Journal of Vision 2008;8(6):345. doi: 10.1167/8.6.345.
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It has been suggested that there exist neural mechanisms which are sensitive to luminance histogram skew, and that these mechanisms play a role in estimating surface properties (Motoyoshi, Nishida, Sharan & Adelson, 2007), or texture discrimination (Chubb, Landy, Econopouly, 2004). However, to the best of our knowledge, no systematic measurements of neural activity in early visual cortex in response to stimuli with skewed luminance histogram have been made to corroborate the existence of the postulated mechanisms.
Using fMRI, we measured BOLD signal in early visual cortex in response to noise images with either positive, negative or zero skewness of the luminance histogram.
Stimuli were circular (radius = 6.35 deg visual angle) random noise images whose histogram mean (M=128) and standard deviation (SD=43) were equated. Histogram contrast was manipulated by adjusting the third moment (skewness) of the luminance histogram to take on one of three values: positive (+1.4), negative (−1.4), or zero (0). Images were presented at a rate of 10Hz, in blocks of 24s separated by 24s presentation of a static image with uniform mean luminance (M=128, SD=0). The order of presentation during each scan (TR=2s) was: blank, positive skew, blank, negative skew, blank, zero skew. This sequence was repeated twice during each scan; there were a total of six scans per session. Observers fixated at the center of each stimulus, and performed a demanding fixation task during each scan.
The BOLD signal was analyzed within an independently determined ROI corresponding to an annulus located within the stimulus area (inner radius= 2 deg, outer radius 4 deg). On average V1 responses to noise images with positively and negatively skewed luminance histogram were significantly larger than those to images with zero skew. Furthermore, the V1 BOLD signal in response to images with negative contrast was larger than that to those with positive luminance histogram contrast.
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