Função de sensibilidade ao contraste de luminância e de cor para estímulos de mosaico

Abstract

Natural images are compound of different blends of color and luminance. In the visual system, there are processing channels of color and luminance that have distinct sensitivities for both information. The parvocellular pathway has high sensitivity to red-green color contrast and low sensitivity to luminance contrast, and the magnocellular pathway has low sensitivity to red-green color contrast and high sensitivity to luminance contrast. Mosaic stimuli which combines color and luminance information may help us to understand how the visual system processes the information of color and luminance. This study aims to investigate the luminance and color contrast sensitivity function using stimuli that combine both information. Fifteen normal trichromats and 1 congenital dyschromatopsic participant were evaluated. To estimate the color contrast sensitivity function, we used stimuli with a pseudoisochromatic setting in which the test stimulus consists of a mosaic with size and luminance spatial noise. The target was compound by a red-green chromatic grating that differed from the background only by the chromaticity. To estimate the luminance contrast sensitivity function, we used stimuli with size and color spatial noise. The target was compound of a luminance contrast grating that differed from the background by the luminance. We used 9 spatial frequencies between 0,1 and 5,4 cpg. It was applied a twointerval forced choice method. The threshold was estimated by using a staircase of 20 reversions with a rule of2 hits to 1 error. The threshold was estimated with the last 14 reversions. To compare both functions, the thresholds estimated in each test were relativized in function of the higher sensitivity of each participant within each test. The observed color contrast sensitivity function showed a low-pass tuning, with higher contrast sensitivities on the medium and low spatial frequencies, while the luminance contrast sensitivity function presented a band-pass tuning, with decrease of the contrast sensitivity on the spatial frequencies that were higher and lower than 2,7 cpg. The results suggest that the threshold perception of both stimuli may have as physiological substrate, the activation of the parvocellular pathway or P pathway.