Navegando por Assunto "Luminance"

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Acesso aberto (Open Access)
Função de sensibilidade ao contraste de luminância e de cor para estímulos de mosaico
(Universidade Federal do Pará, 2017) SANTOS, Patricia Seixas Alves; GOULART, Paulo Roney Kilpp; http://lattes.cnpq.br/7800966999068746; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718; https://orcid.org/0000-0002-4525-3971
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
Acesso aberto (Open Access)
Influência da luminância de fundo em estímulos pseudoisocromáticos sobre a discriminação de cores
(Universidade Federal do Pará, 2017) MOREIRA, Rodrigo Canto; GOULART, Paulo Roney Kilpp; http://lattes.cnpq.br/7800966999068746; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718; https://orcid.org/0000-0002-4525-3971
Color perception is one of the most complex attributes of the human sensory system and contributes to survival in the expression of eating, reproductive, sexual, surveillance, and other behaviors. Pseudo-isochromatic stimuli are widely used for color vision assessment of humans and other primates. Despite the pervasive use, there are no norms for its application, each developer of these tests using varied stimulus configurations. The present study aims to evaluate the effects of background luminance on color discrimination in humans using pseudo-isochromatic stimuli. Ten trichromatic subjects of both sexes, aged 26 to 54 years (32.3 ± 8.3 years of age), normal visual acuity and no history of previous diseases that potentially affected the visual apparatus and / or nervous system were tested. To evaluate the influence of background luminance on color discrimination, a pseudo-isochromatic stimulus with spatial noise of size and luminance (luminance noise between 5 and 35 cd / m2) was used, with a background of 0 cd/m² illumination, 7.5 cd/m², 15 cd/m², 22.5 cd/m² and 30 cd/m 2. The target was composed of a set of central circles that formed a letter C, which presented different chromaticity in relation to the mosaic field in eight different chromatic axes (0º, 45º, 90º, 135º, 180º, 225º, 270°, 315°) around a chromaticity point of the CIE 1976 diagram (u '= 0.219; v' = 0.48). The color discrimination thresholds on each axis were estimated using a 4-alternative forced choice method and a staircase of 21 reversals, with a 2-hit rule for 1 error, and the threshold was calculated as the average of the last 15 reversals. The variation of color discrimination thresholds as a function of background luminance was dependent on the angle being studied. At angles 0°, 45°, 180° and 225°, the size of vectors in threshold perception were higher at 0 cd/m² and decreased sharply by 7.5 cd/m², where they reached their lowest values and remained low in conditions with higher background luminances. At angle 90°, the vector size was minimal in background luminance condition of 7.5 cd/m² and increased to higher and lower background luminance values. On vectors of angle 135°, 270° and 315° the color discrimination did not change significantly as a function of change background luminance. The area of color discrimination ellipse varied as a function of luminance stimulus background with a minimum value of 7.5 cd/m². The results show that the color discrimination is influenced by modification of luminance background of pseudoisochromatic stimulus. Activation of a red-green opposing mechanism and two blueyellow opposing mechanisms may explain the different influences of background variation in stimulus on discrimination of chromaticity in different axes. The results are important in understanding the physiology of perception of pseudo-isochromatic stimuli and search for a standardization use of these stimuli in clinical practice to facilitate comparison of results between different studies.
Acesso aberto (Open Access)
Opencards: uma nova proposta de ferramenta para estimava de limiares psicofísicos
(Universidade Federal do Pará, 2017-01-27) CAMPOS, Yuri Sobral; GOULART, Paulo Roney Kilpp; http://lattes.cnpq.br/7800966999068746; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718; https://orcid.org/0000-0002-4525-3971
The knowledge the value of psychophysical thresholds is essential for any sensory study. In order to define a threshold value, before knowing the limits of sensitivity and the results are produced with the same, it is necessary a use of methodologies and tests to obtain them. Currently, it is comprehensive and use of computerized and non-computerized tests for a visual evaluation, however, the standards of advantages and disadvantages. In general, all tests are done with methodologies around you. The present study proposes the development of a printed test for a visual evaluation of discrimination. For this was sought a product of easy portability and application. The OpenCards consists of a set of boards printed in high spatial resolution and high color quality. On the plates with a mosaic of randomly colored circles within 16 color options with a target that differs from the background by a luminance difference. Targets have a shape of square, triangle, circle and letter X. Targets differed from the background by 90%, 80%, 70%, 60%, 50%, 40%, 35%, 35%, 30%, 25% , 20%, 15%, 10%, 5% of luminance contrast. There was still no sign without the target. To show the use of the OpenCards two methods of application have been elaborated. The first method was performed in an environment with controlled lighting and standard for studies in visual system. Plates were shown in subjects in random order and a task of the subject to identify a shape of the object. The subject's response was recorded and the procedure repeated 8 times. The second method was performed in an ambient light environment. The experimenter showed the plates with the same target ordered in descending contrast and how they shuffled along with a blank plate and asked the subject tested how to order in decreasing contrasts using a higher contrast plate as a reference. The test was performed with all two-stroke sets. The response of the subject tested was recorded in both methods applied and a taxon of subject was adjusted to a Gaussian model for an estimate of the psychophysical threshold. The threshold was the value of the luminance contrast in which the Gaussian model estimated the accuracy of 50% accuracy for the first applied method and 75% for the second applied method. Eight subjects were evaluated in both methods and three subjects. The estimated average threshold with the first method was 23.5 ± 3 and the mean estimated method with the second method was 30.1 ± 6.5. The mean difference between the results obtained by the two methods was 7.3% ± 3. The statistical difference between the thresholds estimated by two methods (p <0.05). Since the following stations are to be used for different types of work, and especially for them, they are applied in each method. OpenCards proved to be easy to apply by the experimenter, portability and understanding of the task by the subject tested.