Navegando por Assunto "Cor - Percepção"

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Acesso aberto (Open Access)
O efeito do ruído espacial de cor sobre a discriminação limiar de luminância: investigação básica e aplicada em populações expostas ao mercúrio
(Universidade Federal do Pará, 2018-05-22) FARIAS, Letícia Miquilini de Arruda; CÔRTES, Maria Izabel Tentes; http://lattes.cnpq.br/3913689546568227; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
Natural images are a complex set of color and luminance contrast that when combined in visual scene helps to create the discrimination of objects from the surrounding visual environment. A series of neural streams transmits the color and luminance information from the retina to the higher cortical centers. Several proposals have been made to study how the visual system processes the stimuli that combining color and luminance contrasts. This thesis has as main proposal to introduce a new stimulus to be used luminance discrimination task under chromatic noise masking. Thus, five experiments were carried out with focus this new paradigm to explore basic and applied questions about its use. Study 1 investigated the effect of color noise saturation on the threshold discrimination of luminance contrast. Study 2 investigated how the mosaic arrangement contributed to the contrast values of luminance contrast thresholds under the chromatic noise masking. Study 3 investigated the influence of the color content of the noise on the threshold discrimination of the luminance contrast. Study 4 investigated the influence of the polarity of the luminance contrast under the chromatic noise masking on the estimated threshold contrasts. Study 5 compared the values of threshold luminance contrasts under chromatic noise masking of two riverine populations of different Amazonian regions of the Pará State and exposed to different levels of mercury by feeding. The main finding of this thesis was that the luminance contrast thresholds varied as a function of the vector length of the chromatic noise values. The higher chromatic noise length, the higher luminance contrast threshold. The contrast threshold estimated by the non-mosaic stimulus exhibited significantly lower values than those estimated with mosaic stimuli (p <0.01). No statistical difference was observed between the contrasts threshold estimated around the five reference chromaticities at different saturation conditions (p> 0.05). The luminance contrasts thresholds estimated in the luminance decrement protocol were xiii significantly lower at all saturation levels than those estimated using the luminance increase protocol (p <0.05). There is no statistical difference between the thresholds of estimated luminance discrimination among riverine communities that were differently exposed to mercury (p> 0.05). The luminance contrasts threshold estimated by the new stimulus, described in this thesis, were influenced by chromatic and spatial noise, and by the polarity of the stimulus of luminance contrast. However, the different chromatic noise compositions did not exhibit any influence on the luminance discrimination. The presence of one or more color-sensitive visual pathways and luminance may be the physiological substrate of the mechanism underlying the luminance contrast perception of this new stimulus.
Acesso aberto (Open Access)
Mascaramento por ruído de luminância sobre a discriminação de cor e luminância
(Universidade Federal do Pará, 2019-03-13) LOUREIRO, Terezinha Medeiros Gonçalves de; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
Several psychophysical experiments were developed using pseudoisochromatics stimuli to evaluate color vision. It was observed that the color perception depends on the characteristics of the luminance noise present in the stimulus. In this thesis were developed two sets of experiments that study how the effects of spatial noise of luminance influence in the visual perception. In the first experiment we investigated the effect of the change in luminance noise amplitude in color discrimination. Eighteen trichromates and ten congenital discromatopics had their color vision evaluated by stimuli adapted from the Cambridge Colour Test and were genetically tested to diagnose mutations associated with congenital color vision deficiency. The stimuli were composed of mosaics of circles in a circular field of 5 ° of visual angle. A subset of the circles differed from the remaining field by chromaticity. The color discrimination was estimated in 4 stimulus conditions that differed in the amplitude of the luminance noise: (i) between 6-20 cd/m²; (ii) between 8 and 18 cd/m²; (iii) between 10 and 16 cd/m²; and (iv) between 12 and 14 cd/m². Six equidistant luminance values were used between the luminance noise limits and the mean luminance of the stimulus maintained in all four conditions. The color discrimination thresholds were estimated through a staircase procedure in 8 different chromatic axes. An ellipse function was fitted to the chromaticity data. The indicators of color discrimination were the area of the ellipse and the values of the eight thresholds of color discrimination. The rate of change of these indicators as a function of the luminance noise amplitude values was calculated as the value of the derivative of the linear function that best fit the function. In the second experiment, a subset of the circles differed from the remaining field by the luminance noise difference, forming the perception of a letter C. In this experiment we tried to evaluate the luminance discrimination in different luminance noise conditions (levels of 2, 4, 6, 10 and 14). Thirty healthy subjects were tested. A ladder procedure was used to control the average luminance of the luminance noise of the target. The luminance discrimination ranges between the mean luminance of the target and the background were the visual functional indicators. The results of the first experiment showed that the rate of variation of the ellipse area as a function of the luminance amplitude in dichromates was higher than in trichromates (p <0.05). It was observed that the low amplitude of the luminance noise (condition of 2 cd/m²) improves the color discrimination of the trichrome and dichromate subjects. Regarding chromatic axes, it was observed that there was a significant difference between the rates of variation of the threshold vector size as a function of the amplitude of the luminance noise of trichromates and dichromates in the axes 0º, 45º, 90º and 135º. The results of the second experiment showed that at lower luminance levels, noise significantly impaired the luminance discrimination (p <0.05) compared to the conditions of higher levels of luminance noise. It was also observed that the higher the luminance contrast present within the noise the worse the luminance discrimination. It is concluded that the modification of luminance noise can lead to significant modifications of the luminance discrimination for color discrimination.