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Pentti I. Laurinen, Lynn A. Olzak, Toni P. Saarela; Summation processes in contrast-contrast. Journal of Vision 2005;5(8):454. doi: 10.1167/5.8.454.
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We have previously shown that the apparent contrast of a grating patch first increases and then decreases as a function of the size of the patch. This can be explained by assuming two antagonist mechanisms. A center part sums the contrast energy over some small area and a larger surround part inhibits the center. The summing center mechanism prevails in the rising portion of apparent contrast curve and the inhibiting surround mechanism dominates after the peak apparent contrast has been reached. To further test the idea of contrast-summing antagonistic mechanisms, we measured the apparent contrast of a center grating surrounded by a similar annular grating under two spatial arrangements. The size of the stimulus (center+surround) was either within the size of the measured summation area or the center of the stimulus was matched to the summation area, with the surround falling outside, on the inhibition-dominating area. The annulus either had higher or low contrast than the center. When the whole stimulus fell within the summation field, the apparent contrast of the center was roughly the average of center and surround contrasts. When the center was the size of the summation area and the surround fell outside the summation area, the apparent contrast of the center was reduced in all surround contrasts. These results support the idea of a center mechanisms integrating contrast information over limited range of space, opposed by a spatially wider mechanism inhibiting the center one.
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