Chromatic discrimination in fixed saturation levels from tufted capuchin monkeys with different color vision genotypes.

Recent research has proposed new approaches to investigate color vision in Old World Monkeys by measuring suprathreshold chromatic discrimination. In this study, we aimed to extend this approach to New World Monkeys with different color vision genotypes by examining their performance in chromatic discrimination tasks along different fixed chromatic saturation axes. Four tufted capuchin monkeys were included in the study, and their color vision genotypes were one classical protanope, one classical deuteranope, one non-classical protanope, and a normal trichromat. During the experiments, the monkeys were required to perform a chromatic discrimination task using pseudoisochromatic stimuli with varying target saturations of 0.06, 0.04, 0.03, and 0.02 u'v' units. The number of errors made by the monkeys along different chromatic axes was recorded, and their performance was quantified using the binomial probability of their hits during the tests. Our results showed that dichromatic monkeys made more errors near the color confusion lines associated with their specific color vision genotypes, while the trichromatic monkey did not demonstrate any systematic errors. At high chromatic saturation, the trichromatic monkey had significant hits in the chromatic axes around the 180° chromatic axis, whereas the dichromatic monkeys had errors in colors around the color confusion lines. At lower saturation, the performance of the dichromatic monkeys became more challenging to differentiate among the three types, but it was still distinct from that of the trichromatic monkey. In conclusion, our findings suggest that high saturation conditions can be used to identify the color vision dichromatic phenotype of capuchin monkeys, while low chromatic saturation conditions enable the distinction between trichromats and dichromats. These results extend the understanding of color vision in New World Monkeys and highlight the usefulness of suprathreshold chromatic discrimination measures in exploring color vision in non-human primates.

Uniform trichromacy in Alouatta caraya and Alouatta seniculus: behavioural and genetic colour vision evaluation.

Primate colour vision depends on a matrix of photoreceptors, a neuronal post receptoral structure and a combination of genes that culminate in different sensitivity through the visual spectrum. Along with a common cone opsin gene for short wavelengths (sws1), Neotropical primates (Platyrrhini) have only one cone opsin gene for medium-long wavelengths (mws/lws) per X chromosome while Paleotropical primates (Catarrhini), including humans, have two active genes. Therefore, while female platyrrhines may be trichromats, males are always dichromats. The genus Alouatta is inferred to be an exception to this rule, as electrophysiological, behavioural and molecular analyses indicated a potential for male trichromacy in this genus. However, it is very important to ascertain by a combination of genetic and behavioural analyses whether this potential translates in terms of colour discrimination capability. We evaluated two howler monkeys (Alouatta spp.), one male A. caraya and one female A. seniculus, using a combination of genetic analysis of the opsin gene sequences and a behavioral colour discrimination test not previously used in this genus. Both individuals completed the behavioural test with performances typical of trichromatic colour vision and the genetic analysis of the sws1, mws, and lws opsin genes revealed three different opsin sequences in both subjects. These results are consistent with uniform trichromacy in both male and female, with presumed spectral sensitivity peaks similar to Catarrhini, at ~ 430 nm, 532 nm, and 563 nm for S-, M- and L-cones, respectively.

Specificity of the chromatic noise influence on the luminance contrast discrimination to the color vision phenotype.

Many studies have examined how color and luminance information are processed in the visual system. It has been observed that chromatic noise masked luminance discrimination in trichromats and that luminance thresholds increased as a function of noise saturation. Here, we aimed to compare chromatic noise inhibition on the luminance thresholds of trichromats and subjects with severe deutan or protan losses. Twenty-two age-matched subjects were evaluated, 12 trichromats and 10 with congenital color vision impairment: 5 protanopes/protanomalous, and 5 deuteranopes/deuteranomalous. We used a mosaic of circles containing chromatic noise consisting of 8 chromaticities around protan, deutan, and tritan confusion lines. A subset of the circles differed in the remaining circles by the luminance arising from a C-shaped central target. All the participants were tested in 4 chromatic noise saturation conditions (0.04, 0.02, 0.01, 0.005 u'v' units) and 1 condition without chromatic noise. We observed that trichromats had an increasing luminance threshold as a function of chromatic noise saturation under all chromatic noise conditions. The subjects with color vision deficiencies displayed no changes in the luminance threshold across the different chromatic noise saturations when the noise was composed of chromaticities close to their color confusion lines (protan and deutan chromatic noise). However, for tritan chromatic noise, they were found to have similar results to the trichromats. The use of chromatic noise masking on luminance threshold estimates could help to simultaneously examine the processing of luminance and color information. A comparison between luminance contrast discrimination obtained from no chromatic and high-saturated chromatic noise conditions could be initially undertaken in this double-duty test.

Distribution of rods and cones in the red-eared turtle retina (Trachemys scripta elegans).

We have identified the photoreceptors of Trachemys scripta elegans, an intensely studied species that is a model for color vision work. To recognize and count the different photoreceptor types, we labeled them with a combination of morphological and immunohistochemistry markers. The counts for the determination of the density of each photoreceptor type were made in wholemount retinas. The percentages found for each cone type were 29, 23, 21, 12, and 6%, respectively, for L (both types), double, M, S, and ultraviolet cones. The cones were found to be organized horizontally in a visual streak, a linear region with a higher density of photoreceptors that ends temporally in the periphery and more centrally in the nasal side. This region of high density of photoreceptors was not symmetrical along its extension; there was a region with conspicuous central density peaks in the temporal area, suggestive of an area centralis. We also observed a dorsoventral asymmetry in photoreceptor density, with greater density in the ventral region. This asymmetry was observed in cones and rods, but it was more pronounced in the rods. Our results corroborate and extend the findings of previous work in the literature describing the retinal photoreceptors of T. s. elegans and their spatial organization. The higher cone density within the visual streak reflects increased spatial resolution and its existence suggests the possibility of binocular vision. It is remarkable that within this region the entire potential for color vision is also present.

LWS visual pigment in owls: Spectral tuning inferred by genetics.

Owls constitute a diverse group of raptors, active at different times of the day with distinct light conditions that might be associated with multiple visual adaptations. We investigated whether shifts in the spectral sensitivity of the L cone visual pigment, as inferred by analysis of gene structure, could be one such adaptive mechanism. Using Sanger sequencing approach, we characterized the long wavelength-sensitive (LWS) opsin gene expressed in the retina of five owl species, specifically chosen to represent distinct patterns of activity. Nocturnality was epitomized by the American barn owl (Tyto furcata), the striped owl (Asio clamator), and the tropical screech owl (Megascops choliba); diurnality, by the ferruginous pygmy owl (Glaucudium brasilianum); and cathemerality, by the burrowing owl (Athene cunicularia). We also analyzed the presence of the L cone in the retinas of four species of owl (T. furcata, A. cunicularia, G. brasilianum and M. choliba) using immnunohistochemistry. Five critical sites for the spectral tuning of the LWS opsin (164, 181, 261, 269, and 292) were analyzed and compared to the sequence of other birds. The sequence of A. cunicularia showed a substitution on residue 269, with the presence of an alanine instead threonine, which generates an estimated maximum absorption (λmax) around 537 nm. No other variation was found in the spectral tuning sites of the LWS opsin, among the other species, and the λmax was estimated at around 555 nm. The presence of L cones in the retinas of the four species of owls was revealed using immunohistochemistry and we observed a reduced number of L cones in T. furcata compared to A. cunicularia, G. brasilianum and M. choliba.

Characterization of the melanopsin gene (Opn4x) of diurnal and nocturnal snakes.

BACKGROUND: A number of non-visual responses to light in vertebrates, such as circadian rhythm control and pupillary light reflex, are mediated by melanopsins, G-protein coupled membrane receptors, conjugated to a retinal chromophore. In non-mammalian vertebrates, melanopsin expression is variable within the retina and extra-ocular tissues. Two paralog melanopsin genes were classified in vertebrates, Opn4x and Opn4m. Snakes are highly diversified vertebrates with a wide range of daily activity patterns, which raises questions about differences in structure, function and expression pattern of their melanopsin genes. In this study, we analyzed the melanopsin genes expressed in the retinas of 18 snake species from three families (Viperidae, Elapidae, and Colubridae), and also investigated extra-retinal tissue expression. RESULTS: Phylogenetic analysis revealed that the amplified gene belongs to the Opn4x group, and no expression of the Opn4m was found. The same paralog is expressed in the iris, but no extra-ocular expression was detected. Molecular evolutionary analysis indicated that melanopsins are evolving primarily under strong purifying selection, although lower evolutionary constraint was detected in snake lineages (ω = 0.2), compared to non-snake Opn4x and Opn4m (ω = 0.1). Statistical analysis of selective constraint suggests that snake phylogenetic relationships have driven stronger effects on melanopsin evolution, than the species activity pattern. In situ hybridization revealed the presence of melanopsin within cells in the outer and inner nuclear layers, in the ganglion cell layer, and intense labeling in the optic nerve. CONCLUSIONS: The loss of the Opn4m gene and extra-ocular photosensitive tissues in snakes may be associated with a prolonged nocturnal/mesopic bottleneck in the early history of snake evolution. The presence of melanopsin-containing cells in all retinal nuclear layers indicates a globally photosensitive retina, and the expression in classic photoreceptor cells suggest a regionalized co-expression of melanopsin and visual opsins.

Photoreceptors morphology and genetics of the visual pigments of Bothrops jararaca and Crotalus durissus terrificus (Serpentes, Viperidae).

Snakes inhabit a great variety of habitats, whose spectral quality of light may vary a lot and influence specific adaptations of their visual system. In this study, we investigated the genetics of the visual opsins and the morphology of retinal photoreceptors, of two nocturnal snakes from the Viperidae family, Bothrops jararaca and Crotalus durissus terrificus, which inhabit preferentially the Atlantic Rain Forest and the Brazilian Savannah, respectively. Total RNA was extracted from homogenized retinas and converted to cDNA. The opsin genes expressed in snake retinas, LWS, RH1, and SWS1, were amplified by polymerase chain reactions (PCRs) and sequenced. The absorption peak (λmax) of the opsins were estimated based on amino acids located at specific spectral tuning sites. Photoreceptor cell populations were analyzed using immunohistochemistry with anti-opsin antibodies. Results showed the same morphological cell populations and same opsins absorption peaks, in both viperid species: double and single cones with LWS photopigment and λmax at ∼555 nm; single cones with SWS1 photopigment and λmax at ∼360 nm; and rods with the rhodopsin RH1 photopigment and λmax at ∼500 nm. The results indicate adaptations to nocturnal habit in both species despite the differences in habitat, and the possibility of a dichromatic color vision at photopic conditions.

Effect of the Decrease in Luminance Noise Range on Color Discrimination of Dichromats and Trichromats.

Color vision assessment can be done using pseudoisochromatic stimuli, which has a luminance noise to eliminate brightness differences between the target and background of the stimulus. It is not clear the influence of the luminance noise on color discrimination. We investigated the effect of change in the luminance noise limits on color discrimination. Eighteen trichromats and ten congenital dichromats (eight protans, two deutans) had their color vision evaluated by the Cambridge Colour Test, and were genetically tested for diagnostic confirmation. The stimuli were composed of a mosaic of circles in a 5° circular field. A subset of the circles differed in chromaticity from the remaining field, forming a letter C. Color discrimination was estimated in stimulus conditions differing in luminance noise range: (i) 6-20 cd/m2; (ii) 8-18 cd/m2; (iii) 10-16 cd/m2; and (iv) 12-14 cd/m2. Six equidistant luminance values were used within the luminance noise limits with the mean stimulus luminance maintained constant under all conditions. A four-alternative, forced-choice method was applied to feed a staircase procedure to estimate color discrimination thresholds along eight chromatic axes. An ellipse model was adjusted to the eight color discrimination thresholds. The parameters of performance were threshold vector lengths and the ellipse area. Results were compared using the Kruskal-Wallis test with a significance level of 5%. The linear function model was applied to analyze the dependence of the discrimination parameters on the noise luminance limits. The first derivative of linear function was used as an indicator of the rate of change in color discrimination as a function of luminance noise changes. The rate of change of the ellipse area as a function of the luminance range in dichromats was higher than in trichromats (p < 0.05). Significant difference was also found for individual thresholds in half of the axes we tested. Luminance noise had a greater effect on color discrimination ability of dichromats than the trichromats, especially when the chromaticities were close to their protan and deutan color confusion lines.

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

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 µg/g; range: 0.26-22.38 µg/g) than that of Tocantins children (mean: 0.49 µg/g; range: 0.03-1.91 µg/g) (p < 0.05). Mean difference was approximately 4.01 µg/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.

The genetics of New World monkey visual pigments

To have color vision, having at least two cone photopigment types with different spectral sensitivities present in distinct photoreceptors is necessary together with the neural circuitry necessary to extract color information. Visual pigments are highly conserved molecules, but differences can be found among vertebrate groups. Primates have a variety of cone photopigments (i.e., opsins) that are expressed by polymorphic genes. This article examines the diversity of cone photopigments in New World monkeys and their behavioral relevance...

Color discrimination in the tufted capuchin monkey, Sapajus spp.

The present study evaluated the efficacy of an adapted version of the Mollon-Reffin test for the behavioral investigation of color vision in capuchin monkeys. Ten tufted capuchin monkeys (Sapajus spp., formerly referred to as Cebus apella) had their DNA analyzed and were characterized as the following: one trichromat female, seven deuteranope dichromats (six males and one female), and two protanope males, one of which was identified as an "ML protanope." For their behavioral characterization, all of the subjects were tested at three regions of the Commission International de l'Eclairage (CIE) 1976 u'v' diagram, with each test consisting of 20 chromatic variation vectors that were radially distributed around the chromaticity point set as the test background. The phenotypes inferred from the behavioral data were in complete agreement with those predicted from the genetic analysis, with the threshold distribution clearly differentiating between trichromats and dichromats and the estimated confusion lines characteristically converging for deuteranopes and the "classic" protanope. The discrimination pattern of the ML protanope was intermediate between protan and deutan, with confusion lines horizontally oriented and parallel to each other. The observed phenotypic differentiation confirmed the efficacy of the Mollon-Reffin test paradigm as a useful tool for evaluating color discrimination in nonhuman primates. Especially noteworthy was the demonstration of behavioral segregation between the "classic" and "ML" protanopes, suggesting identifiable behavioral consequences of even slight variations in the spectral sensitivity of M/L photopigments in dichromats.

The genetics of New World monkey visual pigments

To have color vision, having at least two cone photopigment types with different spectral sensitivities present in distinct photoreceptors is necessary together with the neural circuitry necessary to extract color information. Visual pigments are highly conserved molecules, but differences can be found among vertebrate groups. Primates have a variety of cone photopigments (i.e., opsins) that are expressed by polymorphic genes. This article examines the diversity of cone photopigments in New World monkeys and their behavioral relevance.(AU)

Long-term occupational exposure to organic solvents affects color vision, contrast sensitivity and visual fields.

The purpose of this study was to evaluate the visual outcome of chronic occupational exposure to a mixture of organic solvents by measuring color discrimination, achromatic contrast sensitivity and visual fields in a group of gas station workers. We tested 25 workers (20 males) and 25 controls with no history of chronic exposure to solvents (10 males). All participants had normal ophthalmologic exams. Subjects had worked in gas stations on an average of 9.6 ± 6.2 years. Color vision was evaluated with the Lanthony D15d and Cambridge Colour Test (CCT). Visual field assessment consisted of white-on-white 24-2 automatic perimetry (Humphrey II-750i). Contrast sensitivity was measured for sinusoidal gratings of 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 20.0 cycles per degree (cpd). Results from both groups were compared using the Mann-Whitney U test. The number of errors in the D15d was higher for workers relative to controls (p<0.01). Their CCT color discrimination thresholds were elevated compared to the control group along the protan, deutan and tritan confusion axes (p<0.01), and their ellipse area and ellipticity were higher (p<0.01). Genetic analysis of subjects with very elevated color discrimination thresholds excluded congenital causes for the visual losses. Automated perimetry thresholds showed elevation in the 9°, 15° and 21° of eccentricity (p<0.01) and in MD and PSD indexes (p<0.01). Contrast sensitivity losses were found for all spatial frequencies measured (p<0.01) except for 0.5 cpd. Significant correlation was found between previous working years and deutan axis thresholds (rho = 0.59; p<0.05), indexes of the Lanthony D15d (rho=0.52; p<0.05), perimetry results in the fovea (rho= -0.51; p<0.05) and at 3, 9 and 15 degrees of eccentricity (rho= -0.46; p<0.05). Extensive and diffuse visual changes were found, suggesting that specific occupational limits should be created.

Estudos dos pigmentos visuais de macaco-prego (cebus sp) e da relação entre a psicofísica e a genética da visão de cores em humanos / Study of the visual pigments from capuchin monkey (Cebus sp) and the psychophysics and the genetics colour vision relationship in humans

A visão de cores é possível devido aos diferentes tipos de fotorreceptores e ao processamento pós-receptoral da informação gerada após ativação destas células pela luz. Análises genéticas, funcionais e morfológicas podem ser utilizadas no estudo da visão de cores. Neste trabalho, foram estudadas a genética dos pigmentos visuais de macacos do gênero Cebus e a genética e a psicofísica da visão de cores de humanos com e sem discromatopsias hereditárias. O sequenciamento dos genes que expressam as opsinas presentes nos cones L e M foi realizado em dois grupos de macacos-prego mantidos na UFRJ e na UFPA. Treze animais do Instituto de Biofísica Carlos Chagas Filho da UFRJ e vinte animais da Escola de Primatas da UFPA tiveram o sangue coletado, o material genético extraído e os éxons 3 e 5 dos genes que codificam as opsinas L/M sequenciados após amplificação por PCR. Os aminoácidos localizados nas posições 180, 277 e 285 das opsinas foram identificados e com este resultado foi determinada a curva de absorção espectral das opsinas. O resultado foi a caracterização de seis machos dicromatas, quatro fêmeas tricromatas e três fêmeas dicromatas entre os animais da UFRJ. Entre os animais da UFPA, dezesseis machos eram dicromatas, uma fêmea era tricromata e três fêmeas eram dicromatas. Entre os alelos encontrados nesses grupos, a combinação SFT, cujo pico de absorção espectral da opsina é de 546-553, foi descrita pela primeira vez no gênero Cebus. A variedade de alelos encontrada entre os dois grupos é devida às diferentes condições ambientais nos locais de origem dos animais. (...)

No estudo com humanos, a visão de cores de 19 sujeitos controles (5 homens e 14 mulheres) e 16 sujeitos daltônicos (14 homens e 1 mulher), moradores no estado de São Paulo (SP) foi avaliada através de quatro testes psicofísicos e análises genéticas dos pigmentos visuais. Estes resultados foram comparados com os dados descritos por Gonçalves (2006) em sujeitos daltônicos do estado do Pará (PA). Na correlação entre o fenótipo e o genótipo dos sujeitos estudados neste trabalho, foi observada uma correspondência de 100% entre a classificação da discromatopsia obtida com as análises genéticas e os testes psicofísicos

Color vision is possible due to different types of photoreceptors and to the post receptoral processing of the information generated after cell activation by the light. Genetic, functional and morphological analysis can be used as tools in the color vision studies. In the present work the genetic of the visual pigments from Cebus sp and the genetic and the psychophysics of normal and defective human color vision were evaluated. The sequencing of the L and M genes of 33 capuchin monkeys was performed. Blood samples were collected from 13 monkeys from Instituto de Biofísica Carlos Chagas Filho (UFRJ) and 20 monkeys from Escola de Primatas (UFPA), the DNA was extracted and exons 3 and 5 from L/M genes were sequenced after PCR amplification. For each animal the 180, 277 and 285 aminoacids were identified and the spectral absorbance curve was estimated based on their sequence. In the UFRJ group, six males and three females were identified as dichromats and four females were identified as thrichromats. In the UFPA group sixteen males and three females were dichromats and one female was thrichromat. The SFT aminoacid combination was described for the first time in the Cebus genera. The allelic diversity described in these animals is probably due to the distinct environmental conditions from the place of birth of the animals. In the human study 19 control subjects (5 men and 14 women), (...)

... and sixteen daltonic subjects (14 men and 1 woman), all from São Paulo city, had the colour vision evaluated with 4 colour vision tests and with the visual pigment genetics. The results were compared to the data described by Gonçalves (2006) in daltonic subjects from Pará state. The correlation between phenotype and genotype of the subjects from SP had a 100% of correspondence in the genetic and psychophysical classifications of dischromatopsia

Voronoi analysis uncovers relationship between mosaics of normally placed and displaced amacrine cells in the thraira retina.

Although neuronal dynamics is to a high extent a function of synapse strength, the spatial distribution of neurons is also known to play an important role, which is evidenced by the topographical organization of the main stations of the visual system: retina, lateral geniculate nucleus, and cortex. The coexisting systems of normally placed and displaced amacrine cells in the vertebrate retina provide interesting examples of retinotopic spatial organization. However, it is not clear whether these two systems are spatially interrelated or not. The current work applies two mathematical-computational methods-a new method involving Voronoi diagrams for local density quantification and a more traditional approach, the Ripley K function-in order to characterize the mosaics of normally placed and displaced amacrine cells in the retina of Hoplias malabaricus and search for possible spatial relationships between these two types of mosaics. The results obtained by the Voronoi local density analysis suggest that the two systems of amacrine cells are spatially interrelated through nearly constant local density ratios.

Análise morfológica de células amácrinas PV-IR e bipolares PKC-IR em retina de peixe tropical (Hoplias malabaricus) intoxicado com metilmercúrio / Morphologic analyzes of PV-IR amacrine cells and PKC-IR bipolar cells in retina of tropical fish (Hoplias malabaricus) intoxicated whit metylmercury

Os mecanismos fisiológicos e morfológicos que causam essas alterações visuais em decorrência da intoxicação mercurial ainda não foram totalmente esclarecidos. O presente trabalho analisou o efeito do metilmercúrio na retina do peixe tropical, traíra (Hoplias malabaricus) provocado por intoxicação aguda e sub-crônica. A intoxicação aguda foi realizada através de uma injeção intraperitoneal nas doses de 0,01/0,05/0,1/1/2 e 6ug/g, com tempo de depuração de 15 dias. A intoxicação sub-crônica foi obtida através da ingestão de peixes (lambaris) intoxicados com doses fixas de 0,0075/0,075 e O,75ug/g durante 70 dias com intervalos de 5 dias. Após o final dos procedimentos de intoxicação as retinas foram analisadas através da técnica de imunocitoquímica para proteína ligante de cálcio parvalbumina e para enzima proteína cinase C, a fim de identificar e analisar quantitativamente células amácrinas e células bipolares, respectivamente. Os resultados obtidos mostram uma redução no número de células amácrinas PV -IR e bipolares PKC-IR. A redução do número de células observada nas retinas de ambos grupos intoxicados pode ser causada por um processo de morte neuronal apoptótica ou necrótica. Essas perdas justificam morfologicamente os déficits observados na avaliação da discriminação de cores e de contrastes em pacientes intoxicados por metilmercúrio (AU)