Navegando por Assunto "Sensibilidade ao contraste"

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Acesso aberto (Open Access)
Avaliação psicofísica da visão espacial cromática e acromática de pacientes pós-operatórios de microcirurgia vascular para aneurismas intracranianos do sistema carotídeo
(Universidade Federal do Pará, 2017-07-01) BASTOS, Albedy Moreira; RODRIGUES, Anderson Raiol; http://lattes.cnpq.br/4030747999301402; SILVEIRA, Luiz Carlos de Lima; http://lattes.cnpq.br/9383834641490219
The intracranial saccular aneurysms constitute the main cause of spontaneous meningeal or subarachnoid hemorrhage. The complications derived from the sprouting of such aneurysms hemorrhage, classified as sensorial (acuity and visual field), motor (motility of extra-ocular muscle), and perceptual, are considered as well studied by neurosurgery due to relative similarity between cases. However, little is known about possible postoperative perceptual and cognitive alterations. In the present study, the visual sensitivity of patients in postoperative condition of intracranial aneurysm of the carotid system to achromatic and chromatic spatial stimuli was evaluated. The contrast sensitivity to luminance spatial gratings was measured in eleven spatial frequencies (0,2, 0,5, 0,8, 1, 2, 4, 8, 10, 15, 20, and 30 cycles/degree). The color discrimination ability was measured by a version of the Farnsworth- Munsell test with 85 hues, divided in four series of stimuli presented sequentially, being one serie with 22 stimuli and three series with 24 stimuli. For the measure of color discrimination thresholds, 5 MacAdam’s ellipses were determined and, for each one of them, discrimination thresholds had been measured in 20 directions of the CIE 1976 color space, through the test of Mollon-Reffin. The studied variables were compared with the statistical norms of agematched control subjects. The results show that, individually, there are not significant losses of spatial contrast sensitivity. However, individual losses in color discrimination ability and individual alterations in color discrimination thresholds in all color confusion axis (protanopic, deuteranopic and tritanopic) were detected. Thus, it can be concluded that color vision have sensorial parameters which measurement can be useful in the evaluation of patients postoperative of intracranial aneurysm of the carotid system.
Acesso aberto (Open Access)
Avaliação visual de sujeitos expostos de forma ocupacional a solventes orgânicos através de métodos psicofísicos
(2011) LACERDA, Eliza Maria da Costa Brito; VENTURA, Dora Selma Fix; SILVEIRA, Luiz Carlos de Lima
Organic solvents are neurotoxic substances that can cause damaging effects in the visual system. Occupational exposure to these substances is common because they are used in a wide variety of activities. These effects can be quantified by specific psychophysical tests. The most commonly used tests for color vision assessment are arrangement tests, such as the Lanthony Panel D-15 desaturated test and the Farnsworth-Munsell 100-hue test, and for contrast sensitivity assessment, printed panel tests such as the MCT 8000 VISTECH, VCTS 6500VISTECH, and FACT 101 tests. Generally, these tests show color discrimination losses in the blue-yellow and red-green axis, and a decrease in contrast sensitivity, mainly at low spatial frequencies. There is a positive correlation between psychophysical results and biological environmental markers, but this correlation depends on the marker and on the kind of solvent to which the individuals are exposed. Factors such as alcohol and tobacco can interfere with the correlation of the results.
Acesso aberto (Open Access)
Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics
(2013-02) BARBONI, Mirella Telles Salgueiro; GOMES, Bruno Duarte; SOUZA, Givago da Silva; RODRIGUES, Anderson Raiol; VENTURA, Dora Selma Fix; SILVEIRA, Luiz Carlos de Lima
The purpose of the present study was to measure contrast sensitivity to equiluminant gratings using steady-state visual evoked cortical potential (ssVECP) and psychophysics. Six healthy volunteers were evaluated with ssVECPs and psychophysics. The visual stimuli were red-green or blue-yellow horizontal sinusoidal gratings, 5° × 5°, 34.3 cd/m2 mean luminance, presented at 6 Hz. Eight spatial frequencies from 0.2 to 8 cpd were used, each presented at 8 contrast levels. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. Psychophysical contrast thresholds were measured using stimuli at 6 Hz and static presentation. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. ssVECP and both psychophysical contrast sensitivity functions (CSFs) were low-pass functions for red-green gratings. For electrophysiology, the highest contrast sensitivity values were found at 0.4 cpd (1.95 ± 0.15). ssVECP CSF was similar to dynamic psychophysical CSF, while static CSF had higher values ranging from 0.4 to 6 cpd (P < 0.05, ANOVA). Blue-yellow chromatic functions showed no specific tuning shape; however, at high spatial frequencies the evoked potentials showed higher contrast sensitivity than the psychophysical methods (P < 0.05, ANOVA). Evoked potentials can be used reliably to evaluate chromatic red-green CSFs in agreement with psychophysical thresholds, mainly if the same temporal properties are applied to the stimulus. For blue-yellow CSF, correlation between electrophysiology and psychophysics was poor at high spatial frequency, possibly due to a greater effect of chromatic aberration on this kind of stimulus.
Acesso aberto (Open Access)
Comparative neurophysiology of spatial luminance contrast sensitivity
(2011-06) SOUZA, Givago da Silva; GOMES, Bruno Duarte; SILVEIRA, Luiz Carlos de Lima
The luminance contrast sensitivity function has been investigated using behavioral and electrophysiological methods in many vertebrate species. Some features are conserved across species as a shape of the function, but other features, such as the contrast sensitivity peak value, spatial frequency contrast sensitivity peak, and visual acuity have changed. Here, we review contrast sensitivity across different classes of vertebrates, with an emphasis on the frequency contrast sensitivity peak and visual acuity. We also correlate the data obtained from the literature to test the power of the association between visual acuity and the spatial frequency of the contrast sensitivity function peak.
Acesso aberto (Open Access)
Contrast sensitivity of pattern transient VEP components: contribution from M and P pathways
(2013-12) SOUZA, Givago da Silva; GOMES, Bruno Duarte; LACERDA, Eliza Maria da Costa Brito; SAITO, Cézar Akiyoshi; SILVA FILHO, Manoel da; SILVEIRA, Luiz Carlos de Lima
The purpose of this study was to compare contrast sensitivity estimated from transient visual evoked potentials (VEPs) elicited by achromatic pattern-reversal and pattern-onset/offset modes. The stimuli were 2-cpd, achromatic horizontal gratings presented either as a 1 Hz pattern reversal or a 300 ms onset/700 ms offset stimulus. Contrast thresholds were estimated by linear regression to amplitudes of VEP components vs. the logarithm of the stimulus contrasts, and these regressions were extrapolated to the zero amplitude level. Contrast sensitivity was defined as the inverse of contrast threshold. For pattern reversal, the relation between the P100 amplitude and log of the stimulus contrast was best described by two separate linear regressions. For the N135 component, a single straight line was sufficient. In the case of pattern onset/offset for both the C1 and C2 components, single straight lines described their amplitude vs. log contrast relations in the medium-to-low contrast range. Some saturation was observed for C2 components. The contrast sensitivity estimated from the low-contrast limb of the P100, from the N135, and from the C2 were all similar but higher than those obtained from the high-contrast limb of the P100 and C1 data, which were also similar to each other. With 2 cpd stimuli, a mechanism possibly driven by the M pathway appeared to contribute to the P100 component at medium-to-low contrasts and to the N135 and C2 components at all contrast levels, whereas another mechanism, possibly driven by the P and M pathways, appeared to contribute to the P100 component at high contrast and C1 component at all contrast levels.
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)
Impairment of color spatial vision in chronic alcoholism measured by psychophysical methods
(2009-12) CASTRO, Antonio José de Oliveira; RODRIGUES, Anderson Raiol; CÔRTES, Maria Izabel Tentes; SILVEIRA, Luiz Carlos de Lima
We used psychophysical tests to evaluate spatial vision in 15 subjects with a clinical history of chronic alcoholism by measuring luminance contrast sensitivity and color discrimination. The subjects were initially subjected to clinical inquiry and ophthalmological exam. Subjects then performed psychophysical tests to measure spatial contrast thresholds using sine wave gratings of different spatial frequencies and contrasts and chromatic discrimination thresholds using the Mollon-Reffin test. For the analysis, subjects were divided into three groups according to age and compared with age-matched controls. Ten subjects had some degree of color vision loss, which was quite severe in seven cases. All subjects had normal luminance contrast sensitivity. The results suggest that color vision changes related to chronic alcoholism can occur in the absence of impairment of spatial luminance contrast sensitivity and thus is an important aspect to be considered in the clinical evaluation of this condition.
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)
Propriedades espaciais das respostas isoladas de cones L e M ao eletrorretinograma: implicações sobre a atividade das vias visuais paralelas
(Universidade Federal do Pará, 2015-06-16) JACOB, Mellina Monteiro; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718; GOMES, Bruno Duarte; http://lattes.cnpq.br/4932238030330851
We studied the spatial arrangement of L- and M-cone driven electroretinograms (ERGs) reflecting the activity of magno- and parvocellular pathways. L- and M-cone isolating sine wave stimuli were created with a four primary LED stimulator using triple silent substitution paradigms. Temporal frequencies were 8 and 12 Hz, to reflect cone opponent activity, and 30, 36 and 48 Hz to reflect luminance activity. The responses were measured for full-field stimuli and for different circular and annular stimuli. The ERG data confirm the presence of two different mechanisms at intermediate and high temporal frequencies. The responses measured at high temporal frequencies strongly depended upon spatial stimulus configuration. In the full-field conditions, the L-cone driven responses were substantially larger than the full-field M-cone driven responses and also than the L-cone driven responses with smaller stimuli. The M-cone driven responses at full-field and with 70° diameter stimuli displayed similar amplitudes. The L- and M-cone driven responses measured at 8 and 12 Hz were of similar amplitude and approximately in counter-phase. The amplitudes were constant for most stimulus configurations. The results indicate that, when the ERG reflects luminance activity, it is positively correlated with stimulus size. Beyond 35° retinal eccentricity, the retina mainly contains L-cones. Small stimuli are sufficient to obtain maximal ERGs at low temporal frequencies where the ERGs are also sensitive to cone-opponent processing.
Acesso aberto (Open Access)
A visão através dos contrastes
(2013) SOUZA, Givago da Silva; LACERDA, Eliza Maria da Costa Brito; SILVEIRA, Vladímir de Aquino; ARAÚJO, Carolina dos Santos; SILVEIRA, Luiz Carlos de Lima
The first step in the information processing of visual stimuli corresponds to foton counting by photorreceptor cells. In the post-receptoral steps, information on the stimulus absolute intensity is converted in comparisons between information coming from adjacent retinal areas or successive moments. This metrics implemented by the visual system to quantify the stimulus is called contrast - spatial or simultaneous contrast and temporal or successive contrast. Contrast is essential to the generation of conscious visual perception in the domain of space and time and in three orthogonal color dimensions - black and white, blue and yellow, and green and red. A Bell-shaped curve delimits the thresholds of contrast detection as a function of spatial or temporal frequency. It is called contrast sensitivity function and is affected by several optical and neural factors. Different classes of neurons contribute to different regions of the contrast sensitivity function and their activities represent the work of visual processing pathways that begin in the retina and end in the visual cortex. Basic and clinical investigations have given support to the importance of the study of luminance (black and white) spatial contrast sensitivity as a tool to evaluate the visual function in normal and subjects affected by neuro-ophthalmologic dysfunctions.