Morphology, dendritic field size, somal size, density, and coverage of M and P retinal ganglion cells of dichromatic Cebus monkeys

Fractals in the Neurosciences: A Translational Geographical Approach

The chapter presents three new fractal indices (fractal fragmentation index, fractal tentacularity index, and fractal anisotropy index) and normalized Kolmogorov complexity with proven applicability in geographic research, developed by the authors, and the possibility of their future use in neuroscience. The research demonstrates the relevance of fractal analysis in different fields and the basic concepts and principles of fractal geometry being sufficient for the development of models relevant to the studied reality. Also, the research highlighted the need to continue interdisciplinary research based on known fractal indicators, as well as the development of new analysis methods with the translational potential between fields.

Contribution of parasol-magnocellular pathway ganglion cells to foveal retina in macaque monkey

Parasol-magnocellular pathway ganglion cells form an important output stream of the primate retina and make a major contribution to visual motion detection. They are known to comprise ON and OFF type response polarities but the relative numbers of ON and OFF parasol cells, and the overall contribution of parasol cells to high-acuity foveal vision are not well understood. Here we use antibodies against carbonic anhydrase 8 (CA8) and intracellular injections of the liphilic dye DiI to show that CA8 selectively labels OFF parasol cells in macaque retina. By combined labeling with CA8 antibodies and a previously-described marker for parasol cells (GABA_A receptor antibodies), we show that ON and OFF parasol cells each comprise ∼ 6% of all ganglion cells in central retina (each peak density ∼ 3000 cells/mm² at 5 deg.), and each population comprises ∼ 10% of all ganglion cells in peripheral temporal retina. Thus, the spatial density of parasol cells in central retina is greater than reported by previous anatomical studies, and the central-peripheral gradient in parasol cell density is shallower than previously reported. The data nevertheless predict decline in spatial acuity with visual field eccentricity for both midget-parvocellular pathway and parasol-magnocellular pathway mediated visual functions. The spatial resolving power of the OFF parasol array (peak ∼ 7 cpd) falls short of macaque behavioral grating acuity by at least a factor of three throughout the retina.

Modification and evaluation of the peripheral contrast sensitivity function models

We propose a series of modifications to the Barten contrast sensitivity function model for peripheral vision based on anatomical and psychophysical studies. These modifications result in a luminance pattern detection model that could quantitatively describe the extent of veridical pattern resolution and the aliasing zone. We evaluated our model against psychophysical measurements in peripheral vision. Our numerical assessment shows that the modified Barten leads to lower estimate errors than its original version.

Improving the spatial resolution of artificial vision using midget retinal ganglion cell populations modelled at the human fovea

OBJECTIVE

Retinal prostheses seek to create artificial vision by stimulating surviving retinal neurons of patients with profound vision impairment. Notwithstanding tremendous research efforts, the performance of all implants tested to date has remained rudimentary, incapable of overcoming the threshold for legal blindness. To maximize the perceptual efficacy of retinal prostheses, a device must be capable of controlling retinal neurons with greater spatiotemporal precision. Most studies of retinal stimulation were derived from either non-primate species or the peripheral primate retina. We investigated if artificial stimulation could leverage the high spatial resolution afforded by the neural substrates at the primate fovea and surrounding regions to achieve improved percept qualities.

APPROACH

We began by developing a new computational model capable of generating anatomically accurate retinal ganglion cell (RGC) populations within the human central retina. Next, multiple RGC populations across the central retina were stimulated in-silico to compare clinical and recently proposed neurostimulation configurations based on their ability to improve perceptual efficacy and reduce activation thresholds.

MAIN RESULTS

Our model uniquely upholds eccentricity-dependent characteristics such as RGC density and dendritic field diameter, whilst incorporating anatomically accurate features such as axon projection and three-dimensional RGC layering, features often forgone in favor of reduced computational complexity. Following epiretinal stimulation, the RGCs in our model produced response patterns in shapes akin to the complex percepts reported in clinical trials. Our results also demonstrated that even within the neuron-dense central retina, epiretinal stimulation using a multi-return hexapolar electrode arrangement could reliably achieve spatially focused RGC activation and could achieve single-cell excitation in 74% of all tested locations.

SIGNIFICANCE

This study establishes an anatomically accurate three-dimensional model of the human central retina and demonstrates the potential for an epiretinal hexapolar configuration to achieve consistent, spatially confined retinal responses, even within the neuron-dense foveal region. Our results promote the prospect and optimization of higher spatial resolution in future epiretinal implants.

Retinal Ganglion Cells-Diversity of Cell Types and Clinical Relevance

Retinal ganglion cells (RGCs) are the bridging neurons that connect the retinal input to the visual processing centres within the central nervous system. There is a remarkable diversity of RGCs and the various subtypes have unique morphological features, distinct functions, and characteristic pathways linking the inner retina to the relevant brain areas. A number of psychophysical and electrophysiological tests have been refined to investigate this large and varied population of RGCs. Technological advances, such as high-resolution optical coherence tomography imaging, have provided additional tools to define the pattern of RGC involvement and the chronological sequence of events in both inherited and acquired optic neuropathies. The mechanistic insights gained from these studies, in particular the selective vulnerability and relative resilience of particular RGC subtypes, are of fundamental importance as they are directly relevant to the development of targeted therapies for these invariably progressive blinding diseases. This review provides a comprehensive description of the various types of RGCs, the developments in proposed methods of classification, and the current gaps in our knowledge of how these RGCs are differentially affected depending on the underlying aetiology. The synthesis of the current body of knowledge on the diversity of RGCs and the pathways that are potentially amenable to therapeutic modulation will hopefully lead to much needed effective treatments for patients with optic neuropathies.

The Relationship Between the Photopic Negative Response and Retinal Ganglion Cell Topography

Purpose

To assess the topographic relationship between the photopic negative response (PhNR) and retinal ganglion cell distribution in healthy individuals.

Method

Data was recorded from 16 healthy participants. The amplitude of PhNRs obtained in response to focal long duration (250 ms) and brief flash (5 ms), red (660 nm) on blue (469 nm) stimuli of increasing size (5° - full field) were measured. The number of retinal ganglion cell receptive fields (RGCf) in each stimulus area was established from the literature and regression analysis used to determine the relationships between: PhNR amplitude and number of RGCfs stimulated, PhNR density and the RGCf density and response per RGCf as a function of eccentricity.

Results

The overall amplitude of the PhNR increased with stimulus size and the response density declined from ∼0.1 μV/deg in the macular region to ∼0.003 μV/deg approximately 45° from the fovea. Contrary to expectations, the relationship between the PhNR and number of RGCf was nonlinear, the response from more eccentric neurons being about three times greater than those in the macular region.

Conclusions

Although the amplitude of the PhNR broadly maps on to the topographic distribution of RGCf the increase in PhNR amplitude with increasing eccentricity is only partly explained by RGCf numbers. Increases in the PhNR amplitude may be due to topographic variations in the contributions from other non-RGC neurons, as well as eccentricity-related morphologic and physiologic differences in RGCs.

Survey of retinal ganglion cell morphology in marmoset

In primate retina, the midget, parasol, and small bistratified cell populations form the large majority of ganglion cells. In addition, there is a variety of low-density wide-field ganglion cell types that are less well characterized. Here we studied retinal ganglion cells in the common marmoset, Callithrix jacchus, using particle-mediated gene transfer. Ganglion cells were transfected with an expression plasmid for the postsynaptic density 95-green fluorescent protein. The retinas were processed with established immunohistochemical markers for bipolar and/or amacrine cells to determine ganglion cell dendritic stratification. In total over 500 ganglion cells were classified based on their dendritic field size, morphology, and stratification in the inner plexiform layer. Over 17 types were distinguished, including midget, parasol, broad thorny, small bistratified, large bistratified, recursive bistratified, recursive monostratified, narrow thorny, smooth monostratified, large sparse, giant sparse (melanopsin) ganglion cells, and a group that may contain several as yet uncharacterized types. Assuming each characterized type forms a hexagonal mosaic, the midget and parasol cells account for over 80% of all ganglion cells in the central retina but only ∼50% of cells in the peripheral (>2 mm) retina. We conclude that the fovea is dominated by midget and parasol cells, but outside the fovea the ganglion cell diversity in marmoset is likely as great as that reported for nonprimate retinas. Taken together, the ganglion cell types in marmoset retina resemble those described previously in macaque retina with respect to morphology, stratification, and change in proportion across the retina.

Ganglion cell and displaced amacrine cell density distribution in the retina of the howler monkey (Alouatta caraya)

Unlike all other New World (platyrrine) monkeys, both male and female howler monkeys (Alouatta sp.) are obligatory trichromats. In all other platyrrines, only females can be trichromats, while males are always dichromats, as determined by multiple behavioral, electrophysiological, and genetic studies. In addition to obligatory trichromacy, Alouatta has an unusual fovea, with substantially higher peak cone density in the foveal pit than every other diurnal anthropoid monkey (both platyrrhines and catarrhines) and great ape yet examined, including humans. In addition to documenting the general organization of the retinal ganglion cell layer in Alouatta, the distribution of cones is compared to retinal ganglion cells, to explore possible relationships between their atypical trichromacy and foveal specialization. The number and distribution of retinal ganglion cells and displaced amacrine cells were determined in six flat-mounted retinas from five Alouatta caraya. Ganglion cell density peaked at 0.5 mm between the fovea and optic nerve head, reaching 40,700-45,200 cells/mm2. Displaced amacrine cell density distribution peaked between 0.5-1.75 mm from the fovea, reaching mean values between 2,050-3,100 cells/mm2. The mean number of ganglion cells was 1,133,000±79,000 cells and the mean number of displaced amacrine cells was 537,000±61,800 cells, in retinas of mean area 641±62 mm2. Ganglion cell and displaced amacrine cell density distribution in the Alouatta retina was consistent with that observed among several species of diurnal Anthropoidea, both platyrrhines and catarrhines. The principal alteration in the Alouatta retina appears not to be in the number of any retinal cell class, but rather a marked gradient in cone density within the fovea, which could potentially support high chromatic acuity in a restricted central region.

Scaling the primate lateral geniculate nucleus: niche and neurodevelopment in the regulation of magnocellular and parvocellular cell number and nucleus volume

New stereological assessments of lateral geniculate nucleus (LGN) neuron numbers and volumes in five New World primates (Cebus apella, Saguinus midas niger, Alouatta caraya, Aotus azarae, and Callicebus moloch) and compiled LGN volumes for an additional 26 mammals were analyzed for a better understanding of visual system evolution. Both the magnocellular (M)- and the parvocellular (P)-cell populations scale allometrically with brain volume in primates, P cells with a significantly higher slope such that, for every increase in M neuron number, P neuron numbers more than double (ln scale; y = 0.89x + 2.42R(2) = 0.664). In diurnal primates, the ratio of P to M cells was slightly but significantly higher than in nocturnal primates. For all mammals, including primates, LGN volume was unrelated to nocturnal or diurnal niche but showed marked differences in slope and intercept depending on taxonomic group. The allometric scaling of M and P cells can be related to the order of neurogenesis, with late-generated P cells increasing with positive allometry compared with the earlier-generated M cells. This developmental regularity links relative foveal representation to relative isocortex enlargement, which is also generated late. The small increase in the P/M cell ratio in diurnal primates may result from increased developmental neuron loss in the M-cell population as it competes for limited termination zones in primary visual cortex.

Impaired saccadic eye movement in primary open-angle glaucoma

PURPOSE

Our study aimed at investigating the extent to which saccadic eye movements are disrupted in patients with primary open-angle glaucoma (POAG). This approach followed upon the discovery of differences in the eye-movement behavior of POAG patients during the exploration of complex visual scenes.

METHODS

The eye movements of 8 POAG patients and 4 healthy age-matched controls were recorded. Four of the patients had documented visual field scotoma, and 4 had no identifiable scotoma on visual field testing. The eye movements were monitored as the observers watched static and kinetic targets. The gain, latency, and velocity-peak latency of the saccades recorded were then analyzed.

RESULTS

In POAG patients, with abnormal visual fields, watching a static target, the saccades were delayed and their accuracy was reduced, compared with those of normal observers. In POAG patients, with normal and abnormal visual fields, watching a kinetic target, a task involving precise motion analysis, the latency and accuracy of the saccades were impaired, compared with those of normal observers.

CONCLUSIONS

Our findings suggest that POAG alters saccade programming and execution particularly in the case of moving targets.

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.

Amacrine and bipolar inputs to midget and parasol ganglion cells in marmoset retina

Retinal ganglion cells receive excitatory synapses from bipolar cells and inhibitory synapses from amacrine cells. Previous studies in primate suggest that the strength of inhibitory amacrine input is greater to cells in peripheral retina than to foveal (central) cells. A comprehensive study of a large number of ganglion cells at different eccentricities, however, is still lacking. Here, we compared the amacrine and bipolar input to midget and parasol ganglion cells in central and peripheral retina of marmosets (Callithrix jacchus). Ganglion cells were labeled by retrograde filling from the lateral geniculate nucleus or by intracellular injection. Presumed amacrine input was identified with antibodies against gephyrin; presumed bipolar input was identified with antibodies against the GluR4 subunit of the AMPA receptor. In vertical sections, about 40% of gephyrin immunoreactive (IR) puncta were colocalized with GABAA receptor subunits, whereas immunoreactivity for gephyrin and GluR4 was found at distinct sets of puncta. The density of gephyrin IR puncta associated with ganglion cell dendrites was comparable for midget and parasol cells at all eccentricities studied (up to 2 mm or about 16 degrees of visual angle for midget cells and up to 10 mm or >80 degrees of visual angle for parasol cells). In central retina, the densities of gephyrin IR and GluR4 IR puncta associated with the dendrites of midget and parasol cells are comparable, but the average density of GluR4 IR puncta decreased slightly in peripheral parasol cells. These anatomical results indicate that the ratio of amacrine to bipolar input does not account for the distinct functional properties of parasol and midget cells or for functional differences between cells of the same type in central and peripheral retina.

GABA inactivation of visual area MT modifies the responsiveness and direction selectivity of V2 neurons in Cebus monkeys

We investigated the contribution of the projections from area MT to the receptive field properties of cells in visual area V2 in anesthetized and paralyzed Cebus apella monkeys. We recorded extracellular single-unit activity using tungsten microelectrodes in three monkeys before and after pressure injection of a 0.25-mol/l GABA solution. The visual stimulus consisted of a single bar moving in one of eight directions. In total, 72 V2 neurons were studied in 18 sessions of GABA injection into area MT. A group of 22 neurons was investigated over a shorter period of time ranging from 15 to 60 min, during which the activity did not return to baseline levels. The remaining 50 neurons were studied over a period of at least 2 h, and no statistical difference was observed in the neuronal response before and long after GABA inactivation. The effects on these 50 neurons consisted of an early (1-20 min) significant general decrease in excitability with changes in either orientation or direction selectivity. The differential decrease in excitability resulted in an intermediate improvement (20-40 min) of the signal-to-noise ratio for the stimulus-driven activity. The inactivation depended on the quantity of GABA injected into area MT and persisted for a period of 2 h. The GABA inactivation in area MT produced inhibition of most cells (72%) and a significant change of direction tuning in the majority (56%) of V2 neurons. Both increases and also decreases in the direction tuning of V2 neurons were observed. These feedback projections are capable of modulating not only the levels of spontaneous and driven activity of V2 neurons but also the V2 receptive field properties, such as direction selectivity.

GABA inactivation of area V4 changes receptive-field properties of V2 neurons in Cebus monkeys

To investigate the contribution of feedback circuits from area V4 to the receptive-field properties of V2 neurons, we used tungsten microelectrodes to record extracellular single units in these visual areas, before and after pressure injections of a solution of 0.25 mol/L of GABA in two anesthetized and paralyzed Cebus apella monkeys. The visual stimulus consisted of a single bar moving in one of eight directions. Using a device made of four stainless steel pipettes and one central tungsten electrode, we inactivated, with different amounts of GABA, topographically corresponding areas of V4, while studying V2 neurons. We studied a total of 36 V2 neurons during six sessions of GABA injections into area V4. GABA inactivation of visual area V4 produced a general decrease in the excitability of the neurons, which included a decrease in spontaneous and driven activities, followed by changes in direction selectivity. The changes in selectivity were toward an increase in directional selectivity and decrease in orientation selectivity. Thus, feedback connections arising from V4, an area of the ventral steams of visual information processing, are capable of not only modulating the spontaneous and driven activity of V2 neurons, but also of modifying V2 receptive field properties, such as its direction and/or orientation selectivity.

Cone photopigment variations in Cebus apella monkeys evidenced by electroretinogram measurements and genetic analysis

We investigated the color vision pattern in Cebus apella monkeys by means of electroretinogram measurements (ERG) and genetic analysis. Based on ERG we could discriminate among three types of dichromatic males. Among females, this classification is more complex and requires additional genetic analysis. We found five among 10 possible different phenotypes, two trichromats and three dichromats. We also found that Cebus present a new allele with spectral peak near 552nm, with the amino acid combination SFT at positions 180, 277 and 285 of the opsin gene, in addition to the previously described SYT, AFT and AFA alleles.

Retinal connectivity and primate vision

The general principles of retinal organization are now well known. It may seem surprising that retinal organization in the primate, which has a complex visual behavioral repertoire, appears relatively simple. In this review, we primarily consider retinal structure and function in primate species. Photoreceptor distribution and connectivity are considered as are connectivity in the outer and inner retina. One key issue is the specificity of retinal connections; we suggest that the retina shows connectional specificity but this is seldom complete, and we consider here the functional consequences of imprecise wiring. Finally, we consider how retinal systems can be linked to psychophysical descriptions of different channels, chromatic and luminance, which are proposed to exist in the primate visual system.

Rod bipolar cells in the retina of the capuchin monkey (Cebus apella): Characterization and distribution

Rod bipolar cells in Cebus apella monkey retina were identified by an antibody against the alpha isoform of protein kinase C (PKCα), which has been shown to selectively identify rod bipolars in two other primates and various mammals. Vertical sections were used to confirm the identity of these cells by their characteristic morphology of dendrites and axons. Their topographic distribution was assessed in horizontal sections; counts taken along the dorsal, ventral, nasal, and temporal quadrants. The density of rod bipolar cells increased from 500 to 2900 cells/mm2 at 1 mm from the fovea to reach a peak of 10,000–12,000 cells/mm2 at 4 mm, approximately 5 deg of eccentricity, and then gradually decreased toward retinal periphery to values of 5000 cells/mm2 or less. Rod to rod bipolar density ratio remained between 10 and 20 across most of the retinal extension. The number of rod bipolar cells per retina was 6,360,000 ± 387,433 (mean ± s.d., n = 6). The anti-PKCα antibody has shown to be a good marker of rod bipolar cells of Cebus, and the cell distribution is similar to that described for other primates. In spite of the difference in the central retina, the density variation of rod bipolar cells in the Cebus and Macaca as well as the convergence from rod to rod bipolar cells are generally similar, suggesting that both retinae stabilize similar sensitivity (as measured by rod density) and convergence.

Importance of achromatic contrast in short-range fruit foraging of primates

Trichromatic primates have a ‘red-green’ chromatic channel in addition to luminance and ‘blue-yellow’ channels. It has been argued that the red-green channel evolved in primates as an adaptation for detecting reddish or yellowish objects, such as ripe fruits, against a background of foliage. However, foraging advantages to trichromatic primates remain unverified by behavioral observation of primates in their natural habitats. New World monkeys (platyrrhines) are an excellent model for this evaluation because of the highly polymorphic nature of their color vision due to allelic variation of the L-M opsin gene on the X chromosome. In this study we carried out field observations of a group of wild, frugivorous black-handed spider monkeys (Ateles geoffroyi frontatus, Gray 1842, Platyrrhini), consisting of both dichromats (n = 12) and trichromats (n = 9) in Santa Rosa National Park, Costa Rica. We determined the color vision types of individuals in this group by genotyping their L-M opsin and measured foraging efficiency of each individual for fruits located at a grasping distance. Contrary to the predicted advantage for trichromats, there was no significant difference between dichromats and trichromats in foraging efficiency and we found that the luminance contrast was the main determinant of the variation of foraging efficiency among red-green, blue-yellow and luminance contrasts. Our results suggest that luminance contrast can serve as an important cue in short-range foraging attempts despite other sensory cues that could be available. Additionally, the advantage of red-green color vision in primates may not be as salient as previously thought and needs to be evaluated in further field observations.

Specificity of M and L cone inputs to receptive fields in the parvocellular pathway: random wiring with functional bias

Many of the parvocellular pathway (PC) cells in primates show red-green spectral selectivity (cone opponency), but PC ganglion cells in the retina show no anatomical signs of cone selectivity. Here we asked whether responses of PC cells are compatible with "random wiring" of cone inputs. We measured long-wavelength-sensitive (L) and medium-wavelength-sensitive (M) cone inputs to PC receptive fields in the dorsal lateral geniculate of marmosets, using discrete stimuli (apertures and annuli) to achieve functional segregation of center and surround. Receptive fields between the fovea and 30 degrees eccentricity were measured. We show that, in opponent PC cells, the center is dominated by one (L or M) cone type, with normally <20% contribution from the other cone type (high "cone purity"), whereas non-opponent cells have mixed L and M cone inputs to the receptive field center. Furthermore, opponent response strength depends on the overall segregation of L and M cone inputs to center and surround rather than exclusive input from one cone type to either region. These data are consistent with random wiring. The majority of PC cells in both foveal (<8 degrees) and peripheral retina nevertheless show opponent responses. This arises because cone purity in the receptive field surround is at least as high as in the center, and the surround in nearly all opponent PC cells is dominated by the opposite cone type to that which dominates the center. These functional biases increase the proportion of opponent PC cells, but their anatomical basis is unclear.

Random wiring in the midget pathway of primate retina

The present study addresses the questions of how topographically organized neuronal populations are connected, and whether there is anatomical evidence for color-selective wiring in retinal pathways for red-green color vision. The connectivity of OFF midget bipolar and OFF midget ganglion cells was studied in the peripheral retina of dichromatic ("red-green color blind") and trichromatic ("color normal") marmosets (Callithrix jacchus). Midget bipolar cells were identified immunohistochemically. Midget ganglion cells were retrogradely labeled from the lateral geniculate nucleus and photofilled. Comparable results were obtained from all retinas studied. Between 3 and 16 bipolar terminals converge onto each ganglion cell. Nearly all bipolar terminals investigated show regions of colocalization (areas of presumed synaptic contacts) with ganglion cell dendrites. This contact area makes up approximately 14% of the axon surface area for a typical midget bipolar cell. The output from individual midget bipolar axons is often shared between midget ganglion cells so that, on average, <70% of the axon terminal area of a midget bipolar cell shows overlap with the dendritic field of a given midget ganglion cell. We conclude that there is no morphological evidence of red-green color selectivity in the connections between midget bipolar and midget ganglion cell mosaics. Furthermore, the results suggest that convergence is based on local interactions between axons and dendrites rather than cell-by-cell recognition between members of each mosaic.

Horizontal cell morphology in nocturnal and diurnal primates: A comparison between owl-monkey (Aotus) and capuchin monkey (Cebus)

Horizontal cell morphology was studied in the retina of the nocturnal owl-monkey,Aotus, and compared with that of its diurnal, close relative, the capuchin monkey,Cebus. Cells were initially labeled with DiI and the staining was later photoconverted in a stable precipitated using DAB as chromogen. The sizes of cell bodies, dendritic fields, and axon terminals, number of dendritic clusters, intercluster spacing, and intercone spacing were measured at increasing eccentricities. Two distinct morphological classes of horizontal cells were identified, which resembled those of H1 and H3 cells described in diurnal monkeys. A few examples of a third class, possibly corresponding to the H2 cells of diurnal monkeys, were labeled. Both H1 and H3 cells increased in size and had increasing numbers of dendritic clusters with eccentricity. H3 cells were larger and had a larger number of dendritic clusters than H1 cells. Owl-monkey H1 cells had larger dendritic fields than capuchin monkey H1 cells at all quadrants in the central and midperipheral retinal regions, but the difference disappeared in the far periphery. Owl-monkey and capuchin monkey H1 cells had about the same number of dendritic clusters across eccentricity. As owl-monkey H1 cells were larger than capuchin monkey H1 cells, the equal number of clusters in these two primates was due to the fact that they were more spaced in the owl-monkey cells. H1 intercluster distance closely matched intercone spacing for both the owl-monkey and capuchin monkey retinas. On the other hand, H3 intercluster distance was larger than intercone spacing in the retina of both primates. Owl-monkey H1 axon terminals had 2–3 times more knobs than capuchin monkey H1 axon terminals in spite of having about the same size and, consequently, knob density was 2–3 times higher for owl-monkey than capuchin monkey H1 axon terminals across all eccentricities. The differences observed between owl-monkey and capuchin monkey horizontal cells, regarding the morphology of their dendritic trees and axon terminals, may be related to the differences found in the cone-to-rod ratio in the retina of these two primates. They seem to represent retinal specializations to the nocturnal and diurnal life styles of the owl-monkey and capuchin monkey, respectively.

Mosaic properties of midget and parasol ganglion cells in the marmoset retina

We measured mosaic properties of midget and parasol ganglion cells in the retina of a New World monkey, the common marmosetCallithrix jacchus. We addressed the functional specialization of these populations for color and spatial vision, by comparing the mosaic of ganglion cells in dichromatic (“red–green color blind”) and trichromatic marmosets. Ganglion cells were labelled by photolytic amplification of retrograde marker (“photofilling”) following injections into the lateral geniculate nucleus, or by intracellular injection in anin vitroretinal preparation. The dendritic-field size, shape, and overlap of neighboring cells were measured. We show that in marmosets, both midget and parasol cells exhibit a radial bias, so that the long axis of the dendritic field points towards the fovea. The radial bias is similar for parasol cells and midget cells, despite the fact that midget cell dendritic fields are more elongated than are those of parasol cells. The dendritic fields of midget ganglion cells from the same (ON or OFF) response-type array show very little overlap, consistent with the low coverage of the midget mosaic in humans. No large differences in radial bias, or overlap, were seen on comparing retinae from dichromatic and trichromatic animals. These data suggest that radial bias in ganglion cell populations is a consistent feature of the primate retina. Furthermore, they suggest that the mosaic properties of the midget cell population are associated with high spatial resolution rather than being specifically associated with trichromatic color vision.

Topography of pig retinal ganglion cells

In the present work we analyzed the distribution of retinal ganglion cells (RGCs) in the pig retina. RGCs were retrogradely labeled in vivo by injecting Fluoro-Gold into the optic nerve. RGC density and the distribution of RGCs in terms of soma size were analyzed. Different regions of the porcine retina were identified following analysis of the distribution of RGCs in terms of cell density and soma size: in the central retina, we found a high-density horizontal RGC band lying dorsal to the optic disc. Moreover, in this region, a high proportion of RCGs with small soma size was observed. From the central to the more peripheral retina, we observed a decrease in RGC density, together with a greater presence of RGCs with larger somas. The results of this study should prove to be useful as a foundation for future studies with the porcine retina as a model in ophthalmic research. The study also highlights the necessity to label the RGC population specifically with retrograde tracers in order to quantify precisely alterations in the cell population associated with experimental treatments.

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/mm(2) at 2.5 mm of temporal eccentricity, and representing 8-14% of all ganglion cells. P cell density increased from peripheral to more central regions, reaching about 5540 cells/mm(2) 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.

Colour through the thalamus

Visual perception in humans and other primates depends on the retino-thalamo-cortical pathway. This pathway begins with retinal ganglion cells, which have axonal terminations in the lateral geniculate nucleus (LGN) of the thalamus. Each ganglion cell axon provides input to one or more LGN relay neurones and, in turn, nearly all the LGN relay neurones project to the primary visual cortex. Thus, this pathway forms the dominant functional input to cortical mechanisms for colour vision, as well as for other aspects of conscious visual perception. In this review, recent progress in understanding the transmission of signals for colour vision through the LGN is summarised, with emphasis on studies which provide links between function and structure.

Morphology and physiology of primate M- and P-cells

Catarrhines and platyrrhines, the so-called Old- and New-World anthropoids, have different cone photopigments. Postreceptoral mechanisms must have co-evolved with the receptors to provide trichromatic color vision, and so it is important to compare postreceptoral processes in these two primate groups, both from anatomical and physiological perspectives. The morphology of ganglion cells has been studied in the retina of catarrhines such as the diurnal and trichromatic Macaca, as well as platyrrhines such as the diurnal, di- or trichromatic Cebus, and the nocturnal, monochromatic Aotus. Diurnal platyrrhines, both di- and trichromats, have ganglion cell classes very similar to those found in catarrhines: M (parasol), P (midget), small-field bistratified, and several classes of wide-field ganglion cells. In the fovea of all diurnal anthropoids, P-cell dendritic trees contact single midget bipolars, which contact single cones. The Aotus retina has far fewer cones than diurnal species, but M- and P-cells are similar to those in diurnal primates although of larger size. As in diurnal anthropoids, in the Aotus, the majority of midget bipolar cells, found in the central 2 mm of eccentricity, receive input from a single cone and the sizes of their axon terminals match the sizes of P-cell dendritic fields in the same region. The visual responses of retinal ganglion cells of these species have been studied using single-unit electrophysiological recordings. Recordings from retinal ganglion cells in Cebus and Aotus showed that they have very similar properties as those in the macaque, except that P-cells of mono- and dichromatic animals lack cone opponency. Whatever the original role of the M- and P-cells was, they are likely to have evolved prior to the divergence of catarrhines and platyrrhines. M- and P-cell systems thus appear to be strongly conserved in the various primate species. The reasons for this may lie in the roles of these systems for both achromatic and chromatic vision.

Colors of primate pelage and skin: objective assessment of conspicuousness

We present a quantitative means of assessing the conspicuousness of animal coats or other objects in terms of the color vision of each possible observer. We measured reflectance spectra from the fur and skin of many primate species in order to provide an objective survey of the possibilities of pelage coloration found in extant primates. We show that the orange coloration displayed by many platyrrhine and some strepsirhine primates, while being conspicuous to humans, would be cryptic amongst foliage to all males and many females of their own species. In relation to this finding, we briefly review what is known of the color vision of birds that prey on primates, and assess how conspicuous the orange pelage would be to these predators.

Motion perception in glaucoma patients: a review

Most of the histopathological and psychophysical studies in glaucoma reveal a preferential damage to the magnocellular (M) pathway although a few of them support a damage to the parvocellular (P) pathway as well. In glaucoma, the visual fields are usually evaluated by conventional perimetry. However, it has been demonstrated that 20-40% of ganglion cells are lost before field defects are detected using conventional perimetry. Therefore, new psychophysical tests have recently been designed in order to specifically isolate and evaluate the visual mechanisms that are impaired at the early stages of glaucoma. In this context, several authors have addressed the issue of motion perception under the hypothesis of a predominant damage of the M pathway in glaucoma, and that motion perception is mediated mainly by M pathway. The results of these studies depict a large variation in the percentage of patients showing anomalous motion perception. Overall, motion thresholds are elevated in both glaucoma and ocular hypertensive patients as compared to control subjects, irrespective of the stimulus size and eccentricity. The test which discriminates best between patients and normal subjects is motion perimetry. The visual field defects in glaucoma patients identified by conventional perimetry and motion perimetry are similar, but the sizes of the defects are usually larger with motion perimetry. However, motion tests in central vision have no correlation with visual field defect on conventional perimetry. In glaucoma, loss of performance on motion perception tests does not necessarily support the existence of a specific deficit in the M pathway, because some behavioral studies suggest that the P pathway can also mediate motion perception. It is also difficult to conclude that motion perception is specifically affected in glaucoma because most of these studies do not yield a comparison with other visual functions. Despite these difficulties, localized motion perception tests at eccentricities of more than 15 degrees can be considered as a promising diagnostic tool.

Fruits, foliage and the evolution of primate colour vision

Primates are apparently unique amongst the mammals in possessing trichromatic colour vision. However, not all primates are trichromatic. Amongst the haplorhine (higher) primates, the catarrhines possess uniformly trichromatic colour vision, whereas most of the platyrrhine species exhibit polymorphic colour vision, with a variety of dichromatic and trichromatic phenotypes within the population. It has been suggested that trichromacy in primates and the reflectance functions of certain tropical fruits are aspects of a coevolved seed-dispersal system: primate colour vision has been shaped by the need to find coloured fruits amongst foliage, and the fruits themselves have evolved to be salient to primates and so secure dissemination of their seeds. We review the evidence for and against this hypothesis and we report an empirical test: we show that the spectral positioning of the cone pigments found in trichromatic South American primates is well matched to the task of detecting fruits against a background of leaves. We further report that particular trichromatic platyrrhine phenotypes may be better suited than others to foraging for particular fruits under particular conditions of illumination; and we discuss possible explanations for the maintenance of polymorphic colour vision amongst the platyrrhines.

M and P retinal ganglion cells of the owl monkey: morphology, size and photoreceptor convergence

We have estimated photoreceptor convergence to M and P retinal ganglion cells of two closely related nocturnal (owl monkey, Aotus) and diurnal (capuchin monkey, Cebus) anthropoids. Rod convergence is higher in the owl monkey retina while cone convergence to both M and P cells are very similar in the retinas of the owl monkey and the capuchin monkey. These results indicate that during evolution, the owl monkey retina has undergone changes compatible with a more nocturnal lifestyle, but kept a cone to ganglion cell relation similar to that found in diurnal primates.

Intraretinal axon diameters of a New World primate, the marmoset (Callithrix jacchus)

PURPOSE

Previously, measurements of retinal ganglion cell axon diameter have been used to make inferences about the physiology and clinical pathology of the visual pathway. However, few of these studies were able to unequivocally relate axon diameter to retinal ganglion cell type and other associated measurements. In this and our previous study we have examined intraretinal axon diameters to determine if differences in axon diameter may help to explain conduction velocity measurements found previously.

METHODS

Individual retinal ganglion cells of a New World primate, the common marmoset (Collithrix jacchus) were injected iontophoretically with 2% Lucifer yellow and 4% neurobiotin. Labelled cells were visualized by horseradish peroxidase immunohistochemistry and diaminobenzidine and then retinae were mounted vitreal side up on a glass slide. Cell measurements were made with the aid of a camera lucida attachment and computer-aided morphometry Axons were photographed under x 100 oil immersion and measured at a final magnification of x 4600.

RESULTS

A sample of 62 parasol cells, 22 midget cells, 16 hedge cells and 11 small bistratified cells were analysed. Dendritic field diameter of the different cell classes showed only moderate (non-significant) increases with eccentricity. Only the parasol cells demonstrated a significant increase in mean axon diameter with eccentricity. When the parasol class was examined more closely, it was found that only parasol cells of the superior, inferior and temporal retina (SIT group) showed significant positive correlations between different cell parameters (mean axon diameter, soma diameter, dendritic field diameter, eccentricity). Soma and dendritic field diameters of the SIT group were significantly larger than those of the nasal parasol cells. However, mean axon diameters of the SIT cells were not significantly different from nasal parasol cells. Axon diameters of nasal parasol cells were very variable and overlapped those of the midget and hedge cell classes to a large extent.

CONCLUSIONS

The present data show that for marmoset parasol cells there may not be a clearly defined distinction between nasal and superior, inferior and temporal parasol cells on the basis of axon size. Of particular interest in the present analysis is the clear separation of superior, inferior and temporal parasol cells and nasal parasol cells when comparing soma and dendritic field diameters which is not reflected in the distribution of axon diameters. We suggest that changes in diameter along the length of an axon, differences between retinal quadrants and the variability between cells may be related to minimization of spatiotemporal dispersion necessary for accurate perception of motion within the visual world.

Visual responses of ganglion cells of a New-World primate, the capuchin monkey, Cebus apella

1. The genetic basis of colour vision in New-World primates differs from that in humans and other Old-World primates. Most New-World primate species show a polymorphism; all males are dichromats and most females trichromats. 2. In the retina of Old-World primates such as the macaque, the physiological correlates of trichromacy are well established. Comparison of the retinae in New- and Old-World species may help constrain hypotheses as to the evolution of colour vision and the pathways associated with it. 3. Ganglion cell behaviour was recorded from trichromatic and dichromatic members of a New-World species (the capuchin monkey, Cebus apella) and compared with macaque data. Despite some differences in quantitative detail (such as a temporal response extended to higher frequencies), results from trichromatic animals strongly resembled those from the macaque. 4. In particular, cells of the parvocellular (PC) pathway showed characteristic frequency-dependent changes in responsivity to luminance and chromatic modulation, cells of the magnocellular (MC) pathway showed frequency-doubled responses to chromatic modulation, and the surround of MC cells received a chromatic input revealed on changing the phase of heterochromatically modulated lights. 5. Ganglion cells of dichromats were colour-blind versions of those of trichromats. 6. This strong physiological homology is consistent with a common origin of trichromacy in New- and Old-World monkeys; in the New-World primate the presence of two pigments in the middle-to-long wavelength range permits full expression of the retinal mechanisms of trichromatic vision.

Photoreceptor topography of the retina in the New World monkey Cebus apella

The number and topographical distribution of photoreceptors was studied in whole-mounted retinas of Cebus apella. It was estimated a total of 48 million rods and 3.8 million cones. The average peak foveal cone density and the Nyquist Limit at the foveola were estimated as 169, 127 cells/mm(2) and 46.77+/-7.98 cyc/deg, respectively. A cone-enriched rim was found near the ora serrata, more noticeable in the nasal retina. Rod distribution was asymmetrical along horizontal and vertical meridians with a higher density in the dorsal retina. The rod/cone ratio was variable and asymmetrical along both meridians.

Parallel pathways for spectral coding in primate retina

The primate retina is an exciting focus in neuroscience, where recent data from molecular genetics, adaptive optics, anatomy, and physiology, together with measures of human visual performance, are converging to provide new insights into the retinal origins of color vision. Trichromatic color vision begins when the image is sampled by short- (S), middle- (M) and long- (L) wavelength-sensitive cone photoreceptors. Diverse retinal cell types combine the cone signals to create separate luminance, red-green, and blue-yellow pathways. Each pathway is associated with distinctive retinal architectures. Thus a blue-yellow pathway originates in a bistratified ganglion cell type and associated interneurons that combine excitation from S cones and inhibition from L and M cones. By contrast, a red-green pathway, in which signals from L and M cones are opposed, is associated with the specialized anatomy of the primate fovea, in which the "midget" ganglion cells receive dominant excitatory input from a single L or M cone.

Environmental enrichment of brown capuchins (Cebus apella): behavioral and plasma and fecal cortisol measures of effectiveness

No consensus exists about the quantity and variety of environmental enrichment needed to achieve an acceptable level of psychological well-being among singly housed primates. Behavioral and plasma and fecal cortisol measures were used to evaluate the effectiveness of four levels of toy and foraging enrichment provided to eight wild-caught, singly housed adult male brown capuchins (Cebus apella). The 16-week-long study comprised six conditions and began with a 4-week-long preexperimental and ended with a 4-week-long postexperimental period during which the subjects were maintained at baseline enrichment levels. During the intervening 8 weeks, the subjects were randomly assigned to a sequence of four 2-week-long experimental conditions: control (baseline conditions), toy (the addition of two plastic toys to each cage), box (access to a foraging box with food treats hidden within crushed alfalfa), and box & toy (the addition of two plastic toys and access to a foraging box). Behavioral responses to changes in enrichment were rapid and extensive. Within-subject repeated-measure ANOVAs with planned post hoc contrasts identified highly significant reductions in abnormal and undesirable behaviors (and increases in normal behaviors) as the level of enrichment increased from control to toy to box to box & toy. No significant behavioral differences were found between the control and pre- and postexperimental conditions. Plasma and fecal cortisol measures revealed a different response to changing enrichment levels. Repeated-measure ANOVA models found significant changes in both these measures across the six conditions. The planned post hoc analyses, however, while finding dramatic increases in cortisol titers in both the pre- and postexperimental conditions relative to the control condition, did not distinguish cortisol responses among the four enrichment levels. Linear regressions among weekly group means in behavioral and cortisol measures (n=16) found that plasma cortisol was significantly predicted by the proportions of both normal and abnormal behaviors; as the proportion of normal behaviors increased, the plasma cortisol measures decreased. Plasma cortisol weekly group means were also significantly and positively predicted by fecal cortisol weekly group means, but no behavioral measure significantly predicted fecal cortisol weekly group means. In sum, these findings argue strongly that access to a variety of toy and foraging enrichment positively affects behavioral and physiological responses to stress and enhances psychological well-being in singly housed brown capuchins.

Recent techniques for tracing pathways in the central nervous system of developing and adult mammals

Over the last 20 years, the choice of neural tracers has increased manyfold, and includes newly introduced anterograde tracers that allow quantitation of single-axon morphologies, and retrograde tracers that can be combined with intracellular fills for the study of dendritic arbors of neurons which have a specific projection pattern. The combination of several different tracers now permits the comparison of multiple connections in the same animal, both quantitatively and qualitatively. Moreover, the finding of new virus strains, which infect neural cells without killing them, provides a tool for studying multisynaptic connections that participate in a circuit. In this paper, the labeling characteristics, mechanism of transport and advantages/disadvantages of use are discussed for the following recently introduced neural tracers: carbocyanine dyes, fluorescent latex microspheres, fluorescent dextrans, biocytin, dextran amines, Phaseolus vulgaris leucoagglutinin, cholera toxin and viruses. We also suggest the choice of specific tracers, depending on the experimental animal, age and type of connection to be studied, and discuss quantitative methodologies.

Spatial and temporal response properties of the major retino-geniculate pathways of Old and New World monkeys

Old World monkeys, apes and humans all enjoy trichromatic colour vision, and the absorption spectra of the photopigments are very similar in all species and all individuals. Colour vision in New World monkeys however, is very heterogeneous. In many species, the majority of individuals is dichromatic. Recently, anatomical and electrophysiological studies revealed that the retinal organisation in Old World monkeys and New World monkeys is very similar, although the cells belonging to the parvocellular pathway do not show any colour opponency and their spectral sensitivity is similar to that of the magnocellular cells. Apparently, the magnocellular and parvocellular pathways in the retina and the LGN have not developed as an adaptation to luminance and chromatic processing. It is more likely that the two pathways originally evolved to cover different ranges in the spatio-temporal domain. In the present paper, several spatial and temporal properties of parvo- and magnocellular cells (which are identical for dichromatic and trichromatic animals) are compared.

Comparative retinal physiology in anthropoids

During the last decade it has become clear that colour vision in platyrrhines (New World monkeys) differs from the uniform trichromatic pattern normally found in catarrhines (Old World monkeys, apes and human). Colour vision in most platyrrhine species is polymorphic, with many dichromatic individuals. The comparison of response properties in retinal ganglion cells and lateral geniculate cells between catarrhines and playrrhines elucidates how the evolution of trichromatic colour vision influenced the post-receptoral processing. We find that spatial and temporal processing is very similar in the platyrrhine and catarrhine retina, strongly suggesting that the retinal structure and function, found in living anthropoids, was already present in their common ancestor.

Morphology of P and M retinal ganglion cells of the bush baby

P/midget ganglion cells mediate red-green color opponency in anthropoids. It has been proposed that these cells evolved as a specialization to subserve color vision in primates. If that is correct, they must have evolved about the same time as the long-wavelength ('red') and medium-wavelength ('green') pigment genes diverged, thirty million years ago. Strepsirhines are another group of primates that diverged from the ancestor of the anthropoids at least 55 million years ago. If P/midget ganglion cells evolved to subserve color vision, they should be absent in strepsirhines. We tested this hypothesis in a nocturnal strepsirhine, the greater bush baby Otolemur. The retinal ganglion cells were labeled with the lipophilic tracer Dil and the results show that bush babies have P/midget and M/parasol cells similar to those found in the peripheral retinas of anthropoids. A number of studies have shown that the P and M pathways of bush babies share many similarities with those of anthropoids, and our results show that the same is true for their retinal ganglion cells. These results support the hypothesis that the P system evolved prior to the emergence of red-green color opponency.

Post-receptoral mechanisms of colour vision in New World primates

Diurnal platyrrhines, both di- and trichromats, have magnocellular (M-) and parvocellular (P-) retinal ganglion cells which are morphologically very similar to those found in catarrhines. Catarrhine central P ganglion cells contact single midget bipolar cells, which contact single cones. Physiological recordings of retinal ganglion cells of dichromatic Cebus monkeys showed very similar cell properties to the catarrhine macaque, except that P ganglion cells lacked colour-opponency. We describe the presence of single-headed midget bipolar cells in the Cebus retina. These midget bipolar cells have axon terminal sizes in the same range as the dendritic tree sizes of P ganglion cells as far as 2 mm of retinal eccentricity. This result supports the view that, as in catarrhines, central P ganglion cells of platyrrhines receive input from single midget bipolar cells which in turn, receive input from single cones. This finding is consistent with the idea that a P pathway with one-to-one connectivity was present in the anthropoid ancestor before the divergence between catarrhines and platyrrhines.