Givago da Silva Souza

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.

Normal and dichromatic color discrimination measured with transient visual evoked potential.

It would be informative to have an electrophysiological method to study, in an objective way, the effects of mercury exposure and other neurotoxics on human color vision performance. The purpose of the present work was to study human color discrimination by measuring chromatic difference thresholds with visual evoked potential (VEP). Six young normal trichromats (24 +/- 1 years old) and one deutan (26 years old) were tested. The stimuli consisted of sinusoidal isoluminant chromatic gratings made from chromaticity pairs located along four different color directions centered on two reference points. Heterochromatic flicker photometry (HFP) protocol was used to obtain the isoluminance condition for every subject and for all chromaticity pairs. Spatial frequency was 2 cycles/deg. Presentation mode comprised onset (300 ms)/offset (700 ms) periods. As previously described, we found a negative deflection in the VEP which was related to the chromatic difference: as chromatic difference increased, amplitude increased and latency decreased. VEP response amplitude was plotted against distance in the CIE 1976 color space between the grating chromaticities and fitted with a regression line. We found color thresholds by extrapolating the fitting to null amplitude values. The thresholds were plotted in the CIE 1976 color space as MacAdam ellipses. In normal trichromats the ellipses had small size, low ellipticity, and were vertically oriented. In the deutan subject, the ellipses had large size, high ellipticity, and were oriented towards the deutan copunctal locus. The VEP thresholds were similar to those obtained using grating stimuli and psychophysical procedures, however smaller than those obtained using pseudoisochromatic stimuli (Mollon-Reffin method). We concluded that transient VEP amplitude as a function of contrast can be reliably used in objective studies of chromatic discrimination performance in normal and altered human subjects.

Influence of retinopathy on the achromatic and chromatic vision of patients with type 2 diabetes

BackgroundLuminance contrast sensitivity and colour vision are considered to have great predictive value in the evaluation of type 2 diabetic retinopathy. However, these two visual characteristics have seldom been investigated in the same group of patients. In the present study we measured contrast sensitivity and colour vision in a group of patients with type 2 diabetes and correlated the results with estimates of common metabolic markers for the disease. A subgroup of the patients had no clinical signs of retinopathy.MethodsThe vision of 27 patients (n = 50 eyes) with type 2 diabetes, with retinopathy (n = 20 eyes), or without retinopathy (n = 30 eyes) were evaluated using two psychophysical tests, the Farnsworth–Munsell 100 hue test (FM 100), and measurements of the luminance contrast sensitivity at 11 spatial frequencies. The results were compared with measurements obtained from an age-matched control group (n = 32), and were correlated with the level of glycated haemoglobin, glycaemic level, and time of disease onset. Signs of retinopathy were identified during the ophthalmological examinations.ResultsContrast sensitivity and colour vision impairments were present at different levels in diabetes patients. Eyes with retinopathy showed more severe vision loss than eyes without retinopathy. The FM 100 test was more sensitive for separation of patients from controls. Colour vision loss had no colour axes preference. The contrast sensitivity test appeared to have some advantage in differentiating patients with retinopathy from patients without retinopathy.ConclusionsBoth methods can be useful to follow the visual function of diabetic patients and should be used together to discriminate patients from controls, as well as to identify early signs of retinal damage.

Cone contrast influence on components of the pattern onset/offset VECP

Transient visual evoked cortical potentials (VECP) were recorded from the scalp of healthy normal trichromats (n = 12). VECPs were elicited by onset/offset presentation of patterned stimuli of two kinds: isochromatic luminance‐modulated, and equiluminant red‐green modulated, sine wave gratings. The amplitude and latency of the major onset components of the onset/offset VECP were measured and plotted as a function of the logarithm of pooled cone contrast. The early onset components, achromatic C1 and chromatic N1, increase linearly with log contrast, but N1 has a higher contrast gain than C1. The late onset components, achromatic C2 and chromatic N2, have similar contrast gain, and similar response as a function of contrast level: both increase in the low‐to‐medium range of contrasts and saturate at high contrast levels. In the range of pooled cone contrast tested, C1 and N1 show similar latencies, whilst C2 shows shorter latencies than N2. We suggest that C1 and N1 are generated by the same visual mechanism with high red‐green contrast gain and low luminance contrast gain, whilst C2 and N2 are generated by different visual mechanisms.

The Spatial Properties of L- and M-cone Inputs to Electroretinograms that Reflect Different Types of Post-receptoral Processing

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.

Retinal disorders in northern Brazilian patients treated with chloroquine assessed by multifocal ERG.

The effects of chloroquine intake on the retinal function in a Brazilian population of patients were assessed by multifocal electroretinography. Twenty-four randomly chosen eyes of patients treated with chloroquine for rheumatoid arthritis and systemic lupus erythematosus were examined using multifocal electroretinography (mfERG). Control measurements were acquired from 21 randomly chosen eyes of age-matched healthy subjects. None of the study participants had an inherited retinal disease or a Snellen visual acuity reduced to less than 20/40. In patients and control subjects, cumulative chloroquine dose, total daily dose, duration of treatment, retinal examination, visual field defects, visual acuity, and the mfERG were assessed. The average amplitudes and implicit times of the N1, P1, and P2 components of the mfERGs were measured in the central hexagon (R1) and in five rings (R2–R6). The values measured in patients and normal subjects were compared. The P1 amplitudes in R2 were significantly decreased in the patients. In addition, the amplitudes of N1 and N2 in R1 were significantly smaller in the patients. The implicit times of none of the components were significantly different between patients and controls. The response amplitude was not significantly correlated with cumulative dose and duration of intake. There was no correlation with retinal appearance, visual field, and visual acuity. In agreement with earlier data, the central mfERG amplitudes were decreased in chloroquine patients indicating functional alterations in the retina. These changes are also present in a Brazilian population suggesting that the effects of chloroquine are general and that genetic background and life circumstances probably have, if at all, only little effect.

Manifestações emocionais e motoras de ribeirinhos expostos ao mercúrio na Amazônia

Introduction: The investigation of clinical and neurological impactations associated with exposure to mercury levels in exposed populations is necessary in the Amazon. Objective: To analyze emotional and motor symptoms of riverside dwellers exposed by diet in the municipalities of Itaituba and Acará, in Pará, Brazil. Methods: Hair samples were collected to assess the total mercury (HgT). Demographic data as well as emotional (depression, anxiety and insomnia) and motor (paresthesia, muscle weakness, loss of balance when walking, tremors, limb pain and dysarthria) symptomatology data were obtained. Results: Mean levels of HgT in Itaituba were significantly higher (p < 0.0001) than in Acará. Emotional symptoms were identified in 26 (26.5%) participants from Itaituba and in 24 (52.2%) from Acará. Specific motor complaints in Itaituba occurred in 63 (64.3%) volunteers; the most frequently mentioned afflictions were limb pain (36.7%), paresthesia (32.6%) and muscle weakness (27.5%). In Acará, 33 (71.7%) participants had motor symptoms, the majority of which complained of paresthesia (54.3%), limb pain (52.2%) and tremors (34.8%). Average HgT levels in Itaituba in those with emotional and motor symptoms were above the tolerable levels (6 µg/g) determined by the World Health Organization. Conclusion: Results showed that mercury levels in emotional and motor symptoms in Itaituba are higher than in riverside dwellers in Acará. Further studies, including the application of specific qualitative and/or quantitative standard tests, as well as the investigation of other clinical signs are necessary.

Somatosensory Psychophysical Losses in Inhabitants of Riverside Communities of the Tapajós River Basin, Amazon, Brazil: Exposure to Methylmercury Is Possibly Involved

The purpose of this work was to evaluate the somatosensory system of methylmercury-exposed inhabitants living in the communities of the Tapajós river basin by using psychophysical tests and to compare with measurements performed in inhabitants of the Tocantins river basin. We studied 108 subjects from Barreiras and São Luiz do Tapajós, two communities of the Tapajós river basin, State of Pará, Amazon, Brazil, aged 13–53 years old. Mercury analysis was performed in head hair samples weighting 0.1–0.2 g by using atomic absorption spectrometry. Three somatosensory psychophysical tests were performed: tactile sensation threshold, vibration sensation duration, and two-point discrimination. Semmes-Weinstein 20 monofilaments with different diameters were used to test the tactile sensation in the lower lip, right and left breasts, right and left index fingers, and right and left hallux. The threshold was the thinner monofilament perceived by the subject. Vibration sensation was investigated using a 128 Hz diapason applied to the sternum, right and left radial sides of the wrist, and right and left outer malleoli. Two trials were performed at each place. A stopwatch recorded the vibration sensation duration. The two-point discrimination test was performed using a two-point discriminator. Head hair mercury concentration was significantly higher in mercury-exposed inhabitants of Tapajós than in non-exposed inhabitants of Tocantins (p < 0.01). When all subjects were divided in two groups independently of age—mercury-exposed and non-exposed—the following results were found: tactile sensation thresholds in mercury-exposed subjects were higher than in non-exposed subjects at all body parts, except at the left chest; vibration sensation durations were shorter in mercury-exposed than in non-exposed subjects, at all locations except in the upper sternum; two-point discrimination thresholds were higher in mercury-exposed than in non-exposed subjects at all body parts. There was a weak linear correlation between tactile sensation threshold and mercury concentration in the head hair samples. No correlation was found for the other two measurements. Mercury-exposed subjects had impaired somatosensory function compared with non-exposed control subjects. Long-term mercury exposure of riverside communities in the Tapajós river basin is a possible but not a definitely proven cause for psychophysical somatosensory losses observed in their population. Additionally, the relatively simple psychophysical measures used in this work should be followed by more rigorous measures of the same population.

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.

Visual Dysfunction of Type I and VI Mucopolysaccharidosis Patients Evaluated with Visual Evoked Cortical Potential

Purpose: To evaluate the visual system of patients suffering from type I or VI mucopolysaccharidosis (MPS) by recording the visual evoked cortical potential (VECP). Methods: Two patients with MPS VI and 2 patients with MPS I were tested before and after enzyme replacement therapy (ERT). A control group of 20 subjects was tested for statistical comparison. VECP was elicited by monocular stimulation with 1-Hz phase-reversal checkerboard patterns at 0.5 and 2 cycles per degree and with 16° of visual field. In all patients, both eyes were tested. VECP amplitude and latency were measured and compared with tolerance limits obtained from controls. Results: MPS I and VI patients have a severe visual impairment that can be quantified by measuring VECPs. Even after several weeks of ERT, the visual impairment remained unaltered, indicating that the treatment had no significant influence on the visual conditions of MPS patients. Visual responses to high spatial frequencies were more deeply impaired than responses to low spatial frequencies. This can be explained by the kind of damage in the visual system that preferentially targets the eye optics. Conclusion: VECPs can be used to monitor the degree of visual impairment of MPS patients and to check ERT efficacy.