Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics

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.

Comparison of the reliability of multifocal visual evoked cortical potentials generated by pattern reversal and pattern pulse stimulation

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.

Mercury toxicity in the Amazon: contrast sensitivity and color discrimination of subjects exposed to mercury

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 percent) 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 percent). Three gold miners and 1 riverine (4/19 subjects, 21 percent) performed worse than normal subjects making more mistakes in the color arrangement test. Five of 10 subjects tested (50 percent), 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.

Comparative study of human exposure to mercury in riverside communities in the Amazon region

Four populations in the Amazon area were selected for a comparative study of mercury-exposed and non-exposed populations: São Luiz do Tapajós, Barreiras, Panacauera, and Pindobal Grande. The highest mercury levels in human hair samples were found in São Luiz do Tapajós and Barreiras, greatly exceeding the limits established by the World Health Organization. Panacauera showed an intermediate level below 9 µg/g. This was the first comparative and simultaneous evaluation of mercury exposure in the Amazon area. Also, thanks to this type of monitoring, we were able to eliminate the uncertainties about the reference dose. On the basis of these data, we can conclude that the mercury levels detected in exposed populations of the Tapajós River basin may be dangerous not only because they are above the World Health Organization limits, but also because the simultaneous mercury detection in non-exposed populations with similar characteristics provided a valid control and revealed lower mercury levels. Our results support the importance of continuous monitoring in both exposed and non-exposed populations.

Density, proportion, and dendritic coverage of retinal ganglion cells of the common marmoset (Callithrix jacchus jacchus)

We performed a quantitative analysis of M and P cell mosaics of the common-marmoset retina. Ganglion cells were labeled retrogradely from optic nerve deposits of Biocytin. The labeling was visualized using horseradish peroxidase (HRP) histochemistry and 3-3'diaminobenzidine as chromogen. M and P cells were morphologically similar to those found in Old- and New-World primates. Measurements were performed on well-stained cells from 4 retinas of different animals. We analyzed separate mosaics for inner and outer M and P cells at increasing distances from the fovea (2.5-9 mm of eccentricity) to estimate cell density, proportion, and dendritic coverage. M cell density decreased towards the retinal periphery in all quadrants. M cell density was higher in the nasal quadrant than in other retinal regions at similar eccentricities, reaching about 740 cells/mm² at 2.5 mm of temporal eccentricity, and representing 8-14 percent of all ganglion cells. P cell density increased from peripheral to more central regions, reaching about 5540 cells/mm² at 2.5 mm of temporal eccentricity. P cells represented a smaller proportion of all ganglion cells in the nasal quadrant than in other quadrants, and their numbers increased towards central retinal regions. The M cell coverage factor ranged from 5 to 12 and the P cell coverage factor ranged from 1 to 3 in the nasal quadrant and from 5 to 12 in the other quadrants. These results show that central and peripheral retinal regions differ in terms of cell class proportions and dendritic coverage, and their properties do not result from simply scaling down cell density. Therefore, differences in functional properties between central and peripheral vision should take these distinct regional retinal characteristics into account.

Parallel pathways in the retina of old and new world primates

Old-world simians are all trichromats, but in most new-world primates there is a polymorphism; males are dichromats but most females are trichromats. In the old world simian, luminance and red-green chromatic channels defined by psychophysical experiments have as a basis parasol ganglion cells of the magnocellular (MC) pathway and midget ganglion cells of the parvocellular (PC) pathway respectively. Small bistratified ganglion cells provide a basis for a blue-yellow chromatic channel, which should probably be considered a separate entity. In both dichromatic and trichromatic new-world animals, the MC pathway and the small bistratified, blue-yellow system seem anatomically and physiologically similar to those in their old-world relatives. The midget ganglion cells of the parvocellular pathway in trichromats are anatomically and physiologically similar to the old-world pattern. In dichromatic animals, they are anatomically similar and physiologically resemble those of trichromatic animals, except for the lack of chromatic opponency. We conclude that these three systems may form a basic pattern for the visual pathway of primates. However, the results from dichromats indicate that the evolution of trichromacy may be found to be more complex than presently supposed.

Effect of congenital hypothyroidism on cell density in the ganglion cell layer of the rat retina

The effect of congenital hypothyroidism on the visual system of Wistar rats was studied by determining neuron density in the retinal ganglion cell layer. Retinae of adult rats from mothers treated with propylthiouracil, 50 mg/day, starting on the l5th day of pregnancy (PTU group), and of adult rats from untreated mothers (control group) were examined. Retinae were prepared, and the neurons in the nasotemporal region located above the optic disc were counted. Hypothyroid rats showed a significant reduction in the retinal area (about 6.8 per cent), when compared to controls. The cell density in the retinal ganglion cell layer was significantly decreased in 6 PTUtreated compared to 5 control retinae in total (2,793 ñ 330 vs 3,704 ñ 662 neurons/mm2), nasal (3,031 ñ 580 vs 3,853 ñ 699 neurons/mm2) and temporal (2,555 ñ 156 vs 3,555 ñ 827 neurons/mm2) regions. These alterations in a region considered to be one of the most specialized in the visual process suggest a structural deficiency induced by congenital hypothyroidism, with a possible decrease in the vísual acuity of the rat.

The M-ganglion cell density gradient in New World monkeys

The M-ganglion cell distribution was studied in the retinae of New World monkeys, the diurnal Cebus and the nocturnal Aotus. Retinal whole mounts were stained by the neurofibrillar method of Gros-Schultze. The M-ganglion cell distribution in the Cebus and Aotus is similar to that reported for Old World primates, such as the Macaca. The M-ganglion cell density peak occurs in the foveal slope and declines towards the periphery. There is a nasotemporal asymmetry: in the Cebus retina the M-ganglion cell density is 1.2-4.3 times higher in the nasal when compared to the temporal region at the equivalent eccentricities; in the Aotus, the nasotemporal asymmetry is smaller, 1.6-2.2 higher in the nasal region. We compared the M-ganglion cell density with previously published values for the density of the total population of ganglion cells and observed a larger proportion of M-ganglion cells in the Aotus, when compared with Cebus or Macaca

Biocytin as a retrograde tracer in the mammalian visual system

We have successfully used biocytin as a retrograde tracer in the mammalian visual system. Retinal ganglion cells, pyramidal and stellate cortical neurons were labelled. Both pressure injections and gel implants were used successfully for retrograde labelling. Biocytin was detected using avidin conjugates and horseradish peroxidase histochemistry. Retrograde filling with biocytin proved to be more reliable and to allow better morphological resolution than other commonly used neurotracers such as horseradish peroxidase. The fine details of cell morphology observable by this method are comparable in many cases to the results obtained with intracellular tracer injections. The morphological resolution obtained with this method allows the study of brain microcircuits using extracellular deposits of biocytin

Topography of a-type horizontal cells in the retina of the capybara

A-type horizontal cells inh retinal flat mounts obtained from capybaras and stained by the method of Gros-Schultze were examined for horizontal cell density distribution and dendritic field size. The total number of A-type horizontal cells was 69,316,76,667 and 79,524 in three retinae. The A-type horizontal cell distribution presented a visual streakk that parallels that observed in the ganglion cell distribution. A-type horizontal cell density decreased from the retinal center toward the periphery, whereas the dendritic field size increased toward the periphery. However, the coverage factor did not remain constaant along the retina

The neurons of the retinal ganglion cell layer of the Guinea pig: quantitative analysis of their distribution and size

The topographical distribution of ganglion cells and displaced amacrine cells in the guinea pig retina is described. Neurons were counted in the ganglion cell layer of retinal whole mounts stained by the method of Nissl or retrogradely labeled with horseradish peroxidase. Neuronal soma size was estimated from samples taken from different retinal regions. We estimated that a total of 295,000 neurons comprise the guinea pig ganglion cell layer and they consist of 159,000 ganglion cells and 136,000 displaced amacrine cells. The visual streak is poorly differentiated. Ganglion cell density reaches a peak of 2,272 cells/mm* in a temporal expansion of the visual streak, 4-5 mm toward the optic disk. The visual streak temporal expansion may represent the analogue of the area centralis for this species. The ventral hemi-retina has a higher ganglion cell density than the dorsal hemi-retina. The displaced amacrine cells are more uniformly distributed than the ganglion cells. The present paper provides relevant data concerning the number and distribution of the neurons of the retinal ganglion cell which were not available or were very contradictory in the literature

The visual cortex of the agouti (Dasyprocta agouti): architectonic subdivisions

1. We have studied the cytoarchitecture and myeloarchitecture of the agouti's cortical surface that can be activated by visual stimulation. Five architectonic subdivisions that correspond to distinctive visuotopic representations were characterized. 2. The largest portion of the visual cortex is occupied by area 17 which is situated lateral to the cingulate cortex, medial to area 18, posterior to the parietal cortex, and anterior to the agranular retrosplenial cortex. Additionally, four architectonic subdivisions in the extrastriate visual cortex were distingished, i. e., from medial to lateral: area 18, area 19, anterior lateral area, and temporal posterior area. 3. Along the border of the extrastriate cortex a ring of nonvisual cortical fiels was encountered encompassing parietal (somatic sensorial) cortex, temporal anterior and temporal intermediate (auditory) areas, a band of pre-rhinal cortex, and agranular retrosplenial cortex