Navegando por Assunto "Sensibilidade de contraste (Visão)"

Agora exibindo 1 - 11 de 11

Acesso aberto (Open Access)
Color vision loss in patients treated with chloroquine
(2003) VENTURA, Dora Selma Fix; SILVEIRA, Luiz Carlos de Lima; NISHI, Mauro; COSTA, Marcelo Fernandes da; GUALTIERI, Mirella; ALEXANDRE, Ruth Mayanna Araújo dos Santos; PINTO, Carolina Trindade; MOURA, Ana Laura de Araújo; RODRIGUES, Anderson Raiol; SAKURADA, Claudio; SAUMA, Maria de Fátima L. C.; SOUZA, John Manuel de
Patients that make use of chloroquine or hydroxychloroquine, drugs which are frequently administered for treatment of rheumatoid arthritis, lupus erithromatosus or malaria, may suffer alterations in color vision and in contrast sensitivity. The present work evaluates the visual function of these patients in a joint study of the University of São Paulo (USP), in São Paulo, and of the Federal University of Pará (UFPA), in Belém. Thirty two chloroquine user patients without alterations in the eye fundus exam were evaluated in São Paulo (n=10; aged 38 to 71 years; mean=55,8 years) and in Belém (n=22; aged 20 to 67; mean=40 years). The prescribed accumulated chloroquine dose was 45 to 430 g (mean=213 g; sd = 152 g) for the São Paulo group, and 36 to 540 g (mean=174 g; sd=183 g) for the Belém group. Tests were performed monocularly with corrected eye refractive state. Color discrimination was evaluated using the Cambridge Colour Test (CCT): the color discrimination threshold was measured first in the protan, deutan and tritan axes and, in succession, three MacAdam's ellipses were determined. The patient's color vision was also evaluated with color arrangement tests: the Farnsworth-Munsell 100 Hue (FM100), the Farnsworth-Munsell D15, and the Lanthony Desaturated (D15d) tests. We also measured the contrast sensitivity for black-and-white sine wave grating of twenty two patients. The results were compared with controls without ophthalmologic or neuro-ophthalmologic pathologies. Twenty four patients presented acquired dyschromatopsia. There were cases of selective loss (11 patients) and of diffuse loss (13 patients). Although losses were present in the FM100 there was no correlation between the FM100 error score and the ellipse area measured by the CCT. Moreover, three patients that scored normal in the FM100, failed to reach normal threshold in the CCT. The Lanthony test was less sensitive than the other two tests, since it failed to indicate loss in about half the patients, and the D15 was the least sensitive test, having failed to indicate loss in 9 out of 10 patients. Contrast sensitivity was within normal values for patients submitted to this test. The extent of losses in color discrimination was positively correlated with the accumulated dose. The CCT is recommended for follow up since it provides quantitative data that can be directly interpreted in CIE (Commission Internationalle d'Éclairage) color space.
Acesso aberto (Open Access)
Comparison of the reliability of multifocal visual evoked cortical potentials generated by pattern reversal and pattern pulse stimulation
(2012-10) SOUZA, Givago da Silva; SCHAKELFORD, H.B.; MOURA, Ana Laura de Araújo; GOMES, Bruno Duarte; VENTURA, Dora Selma Fix; FITZGERALD, M.E.C.; SILVEIRA, Luiz Carlos de Lima
This study compared the effectiveness of the multifocal visual evoked cortical potentials (mfVEP) elicited by pattern pulse stimulation with that of pattern reversal in producing reliable responses (signal-to-noise ratio >1.359). Participants were 14 healthy subjects. Visual stimulation was obtained using a 60-sector dartboard display consisting of 6 concentric rings presented in either pulse or reversal mode. Each sector, consisting of 16 checks at 99% Michelson contrast and 80 cd/m2 mean luminance, was controlled by a binary m-sequence in the time domain. The signal-to-noise ratio was generally larger in the pattern reversal than in the pattern pulse mode. The number of reliable responses was similar in the central sectors for the two stimulation modes. At the periphery, pattern reversal showed a larger number of reliable responses. Pattern pulse stimuli performed similarly to pattern reversal stimuli to generate reliable waveforms in R1 and R2. The advantage of using both protocols to study mfVEP responses is their complementarity: in some patients, reliable waveforms in specific sectors may be obtained with only one of the two methods. The joint analysis of pattern reversal and pattern pulse stimuli increased the rate of reliability for central sectors by 7.14% in R1, 5.35% in R2, 4.76% in R3, 3.57% in R4, 2.97% in R5, and 1.78% in R6. From R1 to R4 the reliability to generate mfVEPs was above 70% when using both protocols. Thus, for a very high reliability and thorough examination of visual performance, it is recommended to use both stimulation protocols.
Acesso aberto (Open Access)
Efeitos da adaptação ao flicker de luminância sobre o potencial cortical provocado visual
(Universidade Federal do Pará, 2015-08-20) LOUREIRO, Terezinha Medeiros Gonçalves de; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
Visual Evoked Potential (VEP) has been a useful method to evaluate spatial vision in humans. Sustained observation of a visual stimulus produces several changes in neural responses at different processing levels in visual system. Previous studies has elucidated how primary visual cortex processing spatial information. Many others studies has also suggested about the contribution of parallel pathways M and P activation on the visual cortical responses evoked by a stimuli that excite only one of these pathways. Cortical excitation through a kind of stimulus that promotes one or both preferential adaptation could be a valuable approach to study activity from M and P pathways interactions on the visual responses. The purpose of this study is to evaluate the effects of luminance flicker adaptation on cortical responses elicited under favorable conditions of joint or differential M and P pathways activation, leading to an increase or decrease cortical responses. Eight subjects (20.25 ± 1.5) with normal vision acuity or corrected to 20/20 were tested. VEPs were recorded under three conditions of visual stimulation with no adaptation: sinusoidal gratings at 0.4, 2 and 10cpd presented at 1 Hz pattern-reversal stimulus (test stimuli). Other conditions was elicited by two-dimensional Gaussian mask adaptation stimulus with luminance variation in time domain (flicker) presented at 5 Hz, 10 Hz and 30 Hz temporal modulation. The experiment consisted on VEPs records above occipital scalp elicited by 8 seconds of adaptation stimulus followed by 2 seconds test stimuli. Cortical responses were evaluated in the time and temporal frequencies domain. In the time domain were measured latency and the P1 component amplitude (peak-line), while in the temporal frequency domain were evaluated amplitudes of alpha, beta and gamma frequency bands present in the in the records. VEPs elicited by the test stimuli were compared between flicker adaptation and no adaptation conditions. Main findings consisted on flicker adaptation that occurred differently at spatial frequencies domain. Results showed P1 component in all stimulation conditions and flicker adaptation at lower spatial frequency (0.4 cpd) in all time conditions. It has also showed a reduction at alpha band energy and an increase in the gamma band at same condition. This study concluded that flicker adaptation led to VEP amplitude decreased due to loss of alpha oscillations energy and gamma band energy increased at 0.4 cpd, and it represented a modification on the balance between M and P visual pathways.
Acesso aberto (Open Access)
Estimativa da sensibilidade ao contraste espacial de luminância e discriminação de cores por meio do potencial provocado visual transiente
(2006) GOMES, Bruno Duarte; SOUZA, Givago da Silva; RODRIGUES, Anderson Raiol; SAITO, Cézar Akiyoshi; SILVA FILHO, Manoel da; SILVEIRA, Luiz Carlos de Lima
The Visual Evoked Potential (VEP) is a cortical response obtainable on the scalp. It usually reflects the activity from V1 neurons. It is classified in transient or steady-state, according with the temporal frequency of stimulation. Other stimuli properties evoke a selective activity from different neuronal groups found in V1. This way, VEP have been used to study luminance and chromatic human vision. Several studies used VEP to estimate luminance contrast sensitivity in the spatial frequency domain. More recently, some studies used VEP to measure color discrimination thresholds. The transient VEP shows a good agreement with psychophysical measurements of spatial luminance contrast sensitivity and color discrimination, being a noninvasive method to study vision from subjects with difficulty to perform psychophysical tests.
Acesso aberto (Open Access)
Estudos de mecanismos cromáticos e acromáticos para o potencial cortical provocado visual (VECP) e multifocal (mfVEP)
(Universidade Federal do Pará, 2017-10-06) ARAÚJO, Carolina dos Santos; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
Visual evoked potentials (VECPs) and multifocal visual evoked potentials (mfVEPs) are being widely used to investigate cortical information processing in response to stimuli at different sites of the visual field and present the potential for complement information to conventional VEPs on chromatic and achromatic mechanisms of human vision. The objective of this work was to investigate the contribution of chromatic and achromatic mechanisms to VECP and mfVEP from two experiments: in the first, 9 healthy trichromates subjects with normal or corrected to 20/20 visual acuity were submitted to a visual stimulation of 8º of size, formed by achromatic sinusoidal gratings in 7 spatial frequencies (from 0.4 to 10 cpd) and in six contrast levels (from 3.12% to 99%); in the second, a multifocal dartboard with 60 sectors covering 40° of visual angle was shown for 14 healthy subjects (12 trichromates, 1 protan and 1 deutan discromatopsic type) in 7 different ratios of red-green luminance (R/R+G) and in one achromatic condition (99%). The two stimuli were presented as a reverse pattern, temporally controlled by m-sequences. The first slice (K2.1) and the second slice (K2.2) of the second order kernel were extracted. In experiment 1, were analyzed the main components of the waveforms recorded and in experiment 2 the signal-to-noise ratio (SNR) of the waveforms was analyzed to classify them as reliable (SNR> 1.35) or unreliable (SNR <1.35) and the number of reliable waveforms in 6 different rings with the same visual eccentricity (R1 being the central ring and R6 the most peripheral ring) was quantified. The results of experiment 1 indicated that responses in K2.1 were dominated by M pathway, and responses in K2.2 reflected the contribution of P pathway. The results of experiment 2 were similar for K2.1 and K2.2. In the R1-R4 rings, all red-green luminance ratios showed a similar number of reliable waveforms. In the R5-R6 rings, there are more reliable waveforms in the red-green luminance ratios with high luminance contrast, while the equiluminant condition has the fewest reliable responses. Protan and deutan subjects showed inverted results: stimulus conditions with green brighter than red generated more reliable waveforms in the protan subject (0.2-0.4), while the opposite combination generated more reliable waveforms in the deutan subject. The two second-order kernel slices are useful for studying the chromatic and achromatic mfVEP mechanisms. The results in R1-R4 indicated a similar contribution of chromatic and achromatic mechanisms for mfVEP, while R5-R6 show the most pronounced contribution of the achromatic mechanism to mfVEP. The method used allowed to identificate specific characteristics of protan and deutan discromatopsic type from obtained data. discromatopsic type from obtained data.
Acesso aberto (Open Access)
Ganho de contraste do potencial cortical provocado visual multifocal: efeitos da excentricidade e do modo de estimulação
(Universidade Federal do Pará, 2016-11-29) SILVA, Veronica Gabriela Ribeiro da; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
This study evaluated effects of eccentricity and mode presentation on the multifocal visual evoked potential (mfVEPS) recordings extracted by second-order kernels and its possible contributions from parallel visual pathways. Nine subjects (22.5 ± 3.7 years-old) were studied. All the subjects had 20/20 or corrected visual acuity and no previous history of neuro-ophtahlmic diseases or degenerative diseases. The subjects were tested with non dilated pupil in a monocular way. All the experimental procedures agreed to the tenets of Helsinki and were approved by Committee for Ethic in Research of Nucleus of Tropical Medicine (023/2011 protocol, Federal University of Pará, Belém, PA, Brazil). A CRT monitor displayed a 22º radius, 60 sectors dartboard, each sector with 16 checks (8 white and 8 black), pattern mean luminance of 40 cd/m2. The pattern selection to be shown in each sector was temporally modulated according to a binary pseudorandom m-sequence. Two stimulation protocols were used and we called them as pattern reversal and pattern pulse. Stimulus was presented at five Michelson contrast levels (100%, 50%, 25%, 12.5%, and 6.25%) in two trials with increasing and decreasing contrast order. The subject was instructed to keep the eye in a red cross (1º) placed at the center of the screen. Veris 6.01 was used to configure the stimuli. mfVEPs were recorded with gold cup electrodes: the reference electrode was placed at the inion; the recording electrodes were placed at, 4 cm above the inion (channel 1), 1 cm above and 4 cm to the right of the inion (channel 2), 1 cm above and 4 cm to the left of the inion (channel 3). Ground surface electrode was placed at the forehead. Skin impedance was kept below 5 KOhm. Recordings were amplified 100.000x, band-pass filtered between 3 and 100 Hz. The Veris 6.1 performed an offline low-pass filtering at 35 Hz. Veris 6.1 was used to extract first (K2.1) and second (K2.2) slices from second-order kernels data from original channels. Using MATLAB routines three additional channels were computed from the subtraction of the three original channels. For each subject, a signal-to-noise ratio (SNR) evaluation was performed over the averaged data of two trials in each one of the 6 channels. We measured the RMS amplitude of signal and noise interval of each recording. Finally, we analyzed the waveforms with best SNR for each sector. Mean RMS amplitude for each of six eccentric rings (R1 and R6 are the inner and outer rings, respectively) and for all rings together as a function of stimulus contrast was modeled using Michaelis-Menten functions. Semi-saturation constant (C50) of the contrast-response function was used as indicator of response contrast gain. For pattern reversal protocol contrast-response functions from K2.1/K2.2 had the following C50 values: R1: 35,5% ± 9,3; R2: 26,5% ± 6,5; R3: 22,4% ± 8,8; R4: 18,4% ± 4,4; R5: 20,6% ± 9,3; R6: 26,7% ± 12 / R1: 38,4% ± 4,2; R2: 27,4% ± 7,4; R3: 20,2% ± 4,9; R4: 22,4% ± 4,2; R5: 18,7% ± 3,2; R6: 23,1% ± 8,9. For pattern pulse protocol contrast-response functions from K2.1/K2.2 had the following C50 values: R1: 0; R2: 44,7% ± 10,5; R3: 38,3% ± 12,1; R4: 45,8% ± 12,1; R5: 49,4% ± 16,1; R6: 47,8% ± 14,7 / R1: 0; R2: 50,2% ± 10,3; R3: 48,2% ± 11,1; R4: 28,5% ± 4,2; R5: 54,3% ± 16,2; R6: 0. Two contrast sensitivity mechanisms contribute to mfVEPs elicited by stimuli located in the central visual field, one mechanism with higher contrast gain (pattern reversal mfVEP) and other mechanism with low contrast gain (pattern pulse). For stimulus at the periphery visual field, mechanism with high contrast gain contributed to the generation of mfVEPs elicited by all stimulation modes.
Acesso aberto (Open Access)
Influência de parâmetros espaciais sobre potenciais corticais provocados visuais gerados por estimulação pseudoaleatória
(Universidade Federal do Pará, 2013-03-08) ARAÚJO, Carolina dos Santos; GOMES, Bruno Duarte; http://lattes.cnpq.br/4932238030330851; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
The contributions of contrast detection mechanisms to the visual cortical evoked potential (VECP) have been investigated studying the contrast-response and spatial frequency-response functions. Previously, the use of m-sequences for stimulus control has been almost restricted to multifocal electrophysiology stimulation and, in some aspects, it substantially differs from conventional VECP. Single stimulation with spatial contrast temporally controlled by msequences was not extensively tested or compared to multifocal techniques. Our purpose was to evaluate the influence of spatial frequency and contrast of sinusoidal gratings on the VECP elicited by pseudo-random stimulation. Nine normal subjects were stimulated by achromatic sinusoidal gratings driven by a pseudo-random binary m-sequence at seven spatial frequencies (0.4-10 cpd) and three stimulus sizes (4º, 8º, and 16º of visual angle). At 8º of visual angle, it was also used six contrasts levels (3.12-99%). First order kernel had not provided a consistent measurable signal across spatial frequencies and contrasts that were tested – signal was very small or absent – while the second order kernel first and second slices exhibited reliable responses for the stimulus range. The main differences between results obtained with the first and second slices of the second order kernel were the shape of the amplitude versus contrast and amplitude versus spatial frequency functions. The results indicated that the second order kernel first slice was dominated by M pathway, but for some stimulus condition some P pathway contribution could be found, while the second order kernel second slice reflected the P pathway contribution. The present work extended previous findings of the visual pathways contribution to VECP elicited by pseudo-random stimulation for a wider range of spatial frequencies.
Acesso aberto (Open Access)
Lateral interactions in visual perception of temporal signals: cortical and subcortical components
(2011-06) TEIXEIRA, Cláudio Eduardo Corrêa; SILVEIRA, Luiz Carlos de Lima; KREMERS, Jan
The aim of this work was to isolate and investigate subcortical and cortical lateral interactions involved in flicker perception. We quantified the perceived flicker strength (PFS) in the center of a test stimulus which was simultaneously modulated with a surround stimulus (50% Michelson contrast in both stimuli). Subjects were requested to adjust the modulation depth of a separate matching stimulus that was physically identical to the center of the test stimulus but without the surround. Using LCD goggles, synchronized to the frame rate of a CRT screen, the center and surround could be presented monoptically or dichoptically. In the monoptic condition, center-surround interactions can have both subcortical and cortical origins. In the dichoptic condition, center-surround interactions cannot occur in the retina and the LGN, therefore isolating a cortical mechanism. Results revealed both a strong monoptic (subcortical plus cortical) lateral interaction and a weaker dichoptic (cortical) lateral interaction. Subtraction of the dichoptic from the monoptic data revealed a subcortical mechanism of the lateral interaction. While the modulation of the cortical PFS component showed a low-pass temporal-frequency tuning, the modulation of the subcortical PFS component was maximal at 6 Hz. These findings are consistent with two separate temporal channels influencing the monoptic PFS, each with distinct lateral interactions strength and frequency tuning characteristics. We conclude that both subcortical and cortical lateral interactions modulate flicker perception.
Acesso aberto (Open Access)
Mercury toxicity in the Amazon: contrast sensitivity and color discrimination of subjects exposed to mercury
(2007-03) RODRIGUES, Anderson Raiol; SOUZA, Cleidson Ronald Botelho de; BRAGA, Alexandre Melo; RODRIGUES, Paulo Sergio Silva; SILVEIRA, Antonio Tobias; DAMIN, Enira Teresinha Braghirolli; CÔRTES, Maria Izabel Tentes; CASTRO, Antonio José de Oliveira; MELLO, Guilherme Arantes; VIEIRA, José Luiz Fernandes; PINHEIRO, Maria da Conceição Nascimento; VENTURA, Dora Selma Fix; SILVEIRA, Luiz Carlos de Lima
We measured visual performance in achromatic and chromatic spatial tasks of mercury-exposed subjects and compared the results with norms obtained from healthy individuals of similar age. Data were obtained for a group of 28 mercury-exposed subjects, comprising 20 Amazonian gold miners, 2 inhabitants of Amazonian riverside communities, and 6 laboratory technicians, who asked for medical care. Statistical norms were generated by testing healthy control subjects divided into three age groups. The performance of a substantial proportion of the mercury-exposed subjects was below the norms in all of these tasks. Eleven of 20 subjects (55%) performed below the norms in the achromatic contrast sensitivity task. The mercury-exposed subjects also had lower red-green contrast sensitivity deficits at all tested spatial frequencies (9/11 subjects; 81%). Three gold miners and 1 riverine (4/19 subjects, 21%) performed worse than normal subjects making more mistakes in the color arrangement test. Five of 10 subjects tested (50%), comprising 2 gold miners, 2 technicians, and 1 riverine, performed worse than normal in the color discrimination test, having areas of one or more MacAdam ellipse larger than normal subjects and high color discrimination thresholds at least in one color locus. These data indicate that psychophysical assessment can be used to quantify the degree of visual impairment of mercury-exposed subjects. They also suggest that some spatial tests such as the measurement of red-green chromatic contrast are sufficiently sensitive to detect visual dysfunction caused by mercury toxicity.
Acesso aberto (Open Access)
Perda de sensibilidade ao contraste espacial de luminância em sujeitos com história clínica de hipovitaminose B1
(Universidade Federal do Pará, 2012) LOBATO, Jaisane Santos Melo; SOUZA, Givago da Silva; http://lattes.cnpq.br/5705421011644718
Vitamin B1 acts directly in the energetic metabolism and its deficiency causes several deficits to the nervous system, including those that occurs in vision that is an important and fundamental sense in the quality of life of the human beings. Studies have been shown that the contrast sensitivity estimative is a good biomarker for evaluation of the visual system and the own nervous system. The present study purposed to compare the contrast sensitivity of subjects with history of hypovitaminosis B1 with healthy subjects living in the cities of Imperatriz and João Lisboa in the Maranhão state from 2006 to 2009. This is a transversal, analytic, and case control study. It was investigated 18 patients with history of hypovitaminosis B1 (13 males, 5 females) with mean age 33.77 ± 9.33 years-old, whose had 35 eyes. The control group was composed by 40 subjects (80 eyes) from both sexes, and mean age 33.25 ± 9.3 years-old. Both groups had similar way of life. For psychophysical evaluation, it was used cathodic ray tubes, 21”, 6 x 5 degrees of visual angle. It was performed a nutritional evaluation of all investigated subjects, further visual acuity and contrast sensitivity evaluation. There were statistical differences between the contrast sensitivity of the control group and the group of the subjects with history of hypovitaminosis B1 at 4, 10, 15 e 20 cpd (two-way ANOVA, α = 0,05, Tukey post-hoc test). Most of the subjects with changes in healthy nutritional condition had changes in the visual evaluation. Clinical complains also had important relationship with visual losses.
Acesso aberto (Open Access)
Single-pass measurement of the optical quality of the opossum eye
(2011-06) CRUZ, Eduardo Oswaldo; DINIZ, Cristovam Wanderley Picanço; SILVEIRA, Luiz Carlos de Lima
This paper reports the results that are part of a series of experiments designed to evaluate aspects of the spatial resolution of the visual system of the opossum, Didelphis marsupialis aurita. This nocturnal marsupial presents a well-developed eye, displaying features that reflect specialization for operation at low levels of luminosity. The species was shown to be slightly myopic, a feature that may prove to be valuable because of the increased depth of field. Opossum visual acuity has been previously evaluated by means of determining the Contrast Sensitivity Function (CSF). The results indicate rather poor visual acuity compared with other nocturnal animals. In this paper, we describe the results obtained for the optical quality of the opossum's eye using a single-pass method. The results suggest that the opossum's optical system is capable of forming images that can be resolved when separated by an angular distance on the order of 6 minutes of arc.