Eliza Maria da Costa Brito Lacerda

Amplitude of the transient visual evoked potential (tVEP) as a function of achromatic and chromatic contrast : Contribution of different visual pathways

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5 degrees diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.

Colour Vision Impairment in Young Alcohol Consumers.

Alcohol consumption among young adults is widely accepted in modern society and may be the starting point for abusive use of alcohol at later stages of life. Chronic alcohol exposure can lead to visual function impairment. In the present study, we investigated the spatial luminance contrast sensitivity, colour arrangement ability, and colour discrimination thresholds on young adults that weekly consume alcoholic beverages without clinical concerns. Twenty-four young adults were evaluated by an ophthalmologist and performed three psychophysical tests to evaluate their vision functions. We estimated the spatial luminance contrast sensitivity function at 11 spatial frequencies ranging from 0.1 to 30 cycles/degree. No difference in contrast sensitivity was observed comparing alcohol consumers and control subjects. For the evaluation of colour vision, we used the Farnsworth-Munsell 100 hue test (FM 100 test) to test subject’s ability to perform a colour arrangement task and the Mollon-Reffin test (MR test) to measure subject’s colour discrimination thresholds. Alcohol consumers made more mistakes than controls in the FM100 test, and their mistakes were diffusely distributed in the FM colour space without any colour axis preference. Alcohol consumers also performed worse than controls in the MR test and had higher colour discrimination thresholds compared to controls around three different reference points of a perceptually homogeneous colour space, the CIE 1976 chromaticity diagram. There was no colour axis preference in the threshold elevation observed among alcoholic subjects. Young adult weekly alcohol consumers showed subclinical colour vision losses with preservation of spatial luminance contrast sensitivity. Adolescence and young adult age are periods of important neurological development and alcohol exposure during this period of life might be responsible for deficits in visual functions, especially colour vision that is very sensitive to neurotoxicants.

A visão através dos contrastes

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.

Color Discrimination Is Affected by Modulation of Luminance Noise in Pseudoisochromatic Stimuli.

Pseudoisochromatic stimuli have been widely used to evaluate color discrimination and to identify color vision deficits. Luminance noise is one of the stimulus parameters used to ensure that subjects response is due to their ability to discriminate target stimulus from the background based solely on the hue between the colors that compose such stimuli. We studied the influence of contrast modulation of the stimulus luminance noise on threshold and reaction time color discrimination. We evaluated color discrimination thresholds using the Cambridge Color Test (CCT) at six different stimulus mean luminances. Each mean luminance condition was tested using two protocols: constant absolute difference between maximum and minimum luminance of the luminance noise (constant delta protocol, CDP), and constant contrast modulation of the luminance noise (constant contrast protocol, CCP). MacAdam ellipses were fitted to the color discrimination thresholds in the CIE 1976 color space to quantify the color discrimination ellipses at threshold level. The same CDP and CCP protocols were applied in the experiment measuring RTs at three levels of stimulus mean luminance. The color threshold measurements show that for the CDP, ellipse areas decreased as a function of the mean luminance and they were significantly larger at the two lowest mean luminances, 10 cd/m2 and 13 cd/m2, compared to the highest one, 25 cd/m2. For the CCP, the ellipses areas also decreased as a function of the mean luminance, but there was no significant difference between ellipses areas estimated at six stimulus mean luminances. The exponent of the decrease of ellipse areas as a function of stimulus mean luminance was steeper in the CDP than CCP. Further, reaction time increased linearly with the reciprocal of the length of the chromatic vectors varying along the four chromatic half-axes. It decreased as a function of stimulus mean luminance in the CDP but not in the CCP. The findings indicated that visual performance using pseudoisochromatic stimuli was dependent on the Webers contrast of the luminance noise. Low Webers contrast in the luminance noise is suggested to have a reduced effect on chromatic information and, hence, facilitate desegregation of the hue-defined target from the background.

Avaliação visual de sujeitos expostos de forma ocupacional a solventes orgânicos através de métodos psicofísicos

Os solventes orgânicos sao substâncias neurotoxicas que podem causar efeitos danosos sobre as funcoes visuais. E comum a exposicao ocupacional a esses tipos de substâncias, pois elas apresentam grande aplicabilidade em inumeras atividades. Os efeitos podem ser quantificados por testes psicofisicos. Os testes mais usados nesse tipo de avaliacao sao de ordenamento de matizes, como o Teste D15 Dessaturado de Lanthony e o Teste dos 100 Matizes de Farnsworth-Munsell, e avaliacao de sensibilidade ao contraste espacial de luminância, como MCT 8000 Vistech, VCTS 6500 Vistech e FACT 101. Em geral esses testes descrevem perda de discriminacao de cores, afetando tanto o eixo azul-amarelo quanto o eixo verde-vermelho, e diminuicao da sensibilidade ao contraste de luminância principalmente para as frequencias espaciais mais baixas. Existe correlacao positiva entre os resultados da avaliacao psicofisica e varios marcadores biologicos e ambientais, mas essa correlacao depende do marcador e do tipo de solvente ao qual os individuos sao expostos. Fatores como alcoolismo cronico e tabagismo inveterado podem interferir no processo de correlacionar esses resultados.

Visual assessment by psychophysical methods of people subjected to occupational exposure to organic solvents

Os solventes orgânicos sao substâncias neurotoxicas que podem causar efeitos danosos sobre as funcoes visuais. E comum a exposicao ocupacional a esses tipos de substâncias, pois elas apresentam grande aplicabilidade em inumeras atividades. Os efeitos podem ser quantificados por testes psicofisicos. Os testes mais usados nesse tipo de avaliacao sao de ordenamento de matizes, como o Teste D15 Dessaturado de Lanthony e o Teste dos 100 Matizes de Farnsworth-Munsell, e avaliacao de sensibilidade ao contraste espacial de luminância, como MCT 8000 Vistech, VCTS 6500 Vistech e FACT 101. Em geral esses testes descrevem perda de discriminacao de cores, afetando tanto o eixo azul-amarelo quanto o eixo verde-vermelho, e diminuicao da sensibilidade ao contraste de luminância principalmente para as frequencias espaciais mais baixas. Existe correlacao positiva entre os resultados da avaliacao psicofisica e varios marcadores biologicos e ambientais, mas essa correlacao depende do marcador e do tipo de solvente ao qual os individuos sao expostos. Fatores como alcoolismo cronico e tabagismo inveterado podem interferir no processo de correlacionar esses resultados.

Visual psychophysics of congenital ocular toxoplasmosis case

A toxoplasmose ocular congênita é uma manifestação da infecção pelo parasita Toxoplasma gondii que ocorrer por meio placentário. Essa doença pode provocar importantes sequelas visuais. Este trabalho descreve um estudo de caso que utilizou avaliação psicofísica visual para descrever alterações funcionais decorrentes da toxoplasmose ocular congênita. A avaliação foi realizada em paciente de 30 anos, sexo masculino, residente em São Luís (MA), hígido com histórico da mãe ter apresentado infecção por toxoplasmose no período gestacional. Somente aos 26 anos foi feita uma avaliação médica detalhada que descreveu lesão na fóvea do olho direito. Na avaliação clínica clássica e na avaliação psicofísica visual, o paciente apresentou visão normal para o olho esquerdo. O olho direito apresentou baixa acuidade visual (valor decimal: 0,028), campo visual com escotomas localizados até 20o de ângulo visual e alteração da visão de cor. O estudo fornece informações médicas seguras e relevantes para o diagnóstico de toxoplasmose ocular congênita utilizando testes eficazes, de baixo custo e boa portabilidade, possibilitando uma alternativa de diagnóstico funcional para ser aplicada em locais de difícil acesso pelo interior do Brasil. Descritores: Toxoplasmose ocular; Transmissão vertical; Psicofísica visual AbstrAct

Cross-sectional study to assess the association of color vision with mercury hair concentration in children from Brazilian Amazonian riverine communities

HighlightsStudy of color vision and its relation to Hg exposure.Children from the Tapajós basin had higher mercury exposure than those from the Tocantins.Tapajos children with higher Hg levels performed slightly worse on color vision testing. Background: Mercury exposure in the Brazilian Amazon region has been an important concern since the 1980s, when gold mining activities contaminated many Amazonian river basins and the fish therein. Mercury exposure in humans can lead to changes in neural function. The visual system has been used as a functional indicator of methylmercury (organic) and mercury vapour (inorganic) toxicity. Children are particularly vulnerable to this metal exposure. Objective: To compare the color vision of children from riverine communities of mercury‐exposed (Tapajós basin) and non‐exposed Amazonian rivers (Tocantins basin). Methods: The study sample was 176 children, aged 7–14 years old. Children from two locations in the mercury‐exposed Tapajós river basin, Barreiras (n = 71) and São Luiz do Tapajos (n = 41), were compared to children from Limoeiro do Ajuru (n = 64), a non‐exposed area in the Tocantins river basin. No caregiver reported that any children had contact with mercury vapour during their lifetime, and probably most of the mercury in their bodies was obtained by fish consumption. Because of this, we decided to evaluate the mercury exposure by total mercury levels in hair samples, a good marker for organic mercury, and not in the urine, a marker for inorganic mercury. Color vision was assessed by the Lanthony Desaturated D‐15 test. We used the Vingrys and King‐Smith method (1988) to quantify the hue ordering test. The primary visual outcomes from this analysis were C‐index (magnitude of the hue ordering error) and angle of the hue ordering. Results: The Tapajós children had a higher mean hair mercury level (mean: 4.5 &mgr;g/g; range: 0.26–22.38 &mgr;g/g) than that of Tocantins children (mean: 0.49 &mgr;g/g; range: 0.03–1.91 &mgr;g/g) (p < 0.05). Mean difference was approximately 4.01 &mgr;g/g with a 95% confidence interval of 2.79–5.23. The results of the Lanthony D‐15d test showed no significant difference between the C‐index mean values of the Tapajós and Tocantins groups (p > 0.05). There was a weak linear correlation in the average C‐index obtained from both eyes and the total mercury concentration. Multiple logistic regression analysis indicated that the location of the community and the age had a greater influence on the visual outcomes than the sex of the children and within‐locale variation in mercury concentration. Conclusion: Our results suggest a difference in one aspect of vision, that is, color vision, between children living in two different river basins in the Brazilian Amazon. The association may be related to Hg exposure but also appeared related to the location of the community and age.

Influence of Spatial and Chromatic Noise on Luminance Discrimination

Pseudoisochromatic figures are designed to base discrimination of a chromatic target from a background solely on the chromatic differences. This is accomplished by the introduction of luminance and spatial noise thereby eliminating these two dimensions as cues. The inverse rationale could also be applied to luminance discrimination, if spatial and chromatic noise are used to mask those cues. In this current study estimate of luminance contrast thresholds were conducted using a novel stimulus, based on the use of chromatic and spatial noise to mask the use of these cues in a luminance discrimination task. This was accomplished by presenting stimuli composed of a mosaic of circles colored randomly. A Landolt-C target differed from the background only by the luminance. The luminance contrast thresholds were estimated for different chromatic noise saturation conditions and compared to luminance contrast thresholds estimated using the same target in a non-mosaic stimulus. Moreover, the influence of the chromatic content in the noise on the luminance contrast threshold was also investigated. Luminance contrast threshold was dependent on the chromaticity noise strength. It was 10-fold higher than thresholds estimated from non-mosaic stimulus, but they were independent of colour space location in which the noise was modulated. The present study introduces a new method to investigate luminance vision intended for both basic science and clinical applications.

Evidence of Asymptomatic Visual Losses after Surgical Repair of Cerebral Aneurysm

Deficits in visual acuity, visual field, and oculomotor function are commonly detected after repair of cerebral aneurysms. However, when these deficits are absent, it does not mean that other potential visual deficits also are absent. Here, we report three cases that after complete recover from surgical repair of cerebral aneurysms presented minimal visual acuities of about 20/20 and no visual disturbances. While two of them (Cases 1 and 2) showed visual fields with no relevant central defects, two of them showed relevant impairments in spatial contrast sensitivity (Cases 2 and 3). This evidence supports that after complete recover from surgical repair of hemorrhagic cerebral aneurysms spatial contrast sensitivity can be asymptomatically impaired when visual acuity (Cases 2 and 3) and visual fields (Case 2) are not correlated with symptoms of visual disturbances. Hypothetical explanations and consequences of such evidence are discussed.