Origins of direction selectivity in the primate retina.

From mouse to primate, there is a striking discontinuity in our current understanding of the neural coding of motion direction. In non-primate mammals, directionally selective cell types and circuits are a signature feature of the retina, situated at the earliest stage of the visual process. In primates, by contrast, direction selectivity is a hallmark of motion processing areas in visual cortex, but has not been found in the retina, despite significant effort. Here we combined functional recordings of light-evoked responses and connectomic reconstruction to identify diverse direction-selective cell types in the macaque monkey retina with distinctive physiological properties and synaptic motifs. This circuitry includes an ON-OFF ganglion cell type, a spiking, ON-OFF polyaxonal amacrine cell and the starburst amacrine cell, all of which show direction selectivity. Moreover, we discovered that macaque starburst cells possess a strong, non-GABAergic, antagonistic surround mediated by input from excitatory bipolar cells that is critical for the generation of radial motion sensitivity in these cells. Our findings open a door to investigation of a precortical circuitry that computes motion direction in the primate visual system.

Melanopsin-expressing ganglion cells on macaque and human retinas form two morphologically distinct populations.

The long-term goal of this research is to understand how retinal ganglion cells that express the photopigment melanopsin, also known as OPN4, contribute to vision in humans and other primates. Here we report the results of anatomical studies using our polyclonal antibody specifically against human melanopsin that confirm and extend previous descriptions of melanopsin cells in primates. In macaque and human retina, two distinct populations of melanopsin cells were identified based on dendritic stratification in either the inner or the outer portion of the inner plexiform layer (IPL). Variation in dendritic field size and cell density with eccentricity was confirmed, and dendritic spines, a new feature of melanopsin cells, were described. The spines were the sites of input from DB6 diffuse bipolar cell axon terminals to the inner stratifying type of melanopsin cells. The outer stratifying melanopsin type received inputs from DB6 bipolar cells via a sparse outer axonal arbor. Outer stratifying melanopsin cells also received inputs from axon terminals of dopaminergic amacrine cells. On the outer stratifying melanopsin cells, ribbon synapses from bipolar cells and conventional synapses from amacrine cells were identified in electron microscopic immunolabeling experiments. Both inner and outer stratifying melanopsin cell types were retrogradely labeled following tracer injection in the lateral geniculate nucleus (LGN). In addition, a method for targeting melanopsin cells for intracellular injection using their intrinsic fluorescence was developed. This technique was used to demonstrate that melanopsin cells were tracer coupled to amacrine cells and would be applicable to electrophysiological experiments in the future. J. Comp. Neurol. 524:2845-2872, 2016. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

Recurrent axon collaterals of intrinsically photosensitive retinal ganglion cells.

Retinal ganglion cells (RGCs), the output neurons of the retina, have axons that project via the optic nerve to diverse targets in the brain. Typically, RGC axons do not branch before exiting the retina and thus do not provide it with synaptic feedback. Although a small subset of RGCs with intraretinal axon collaterals has been previously observed in human, monkey, cat, and turtle, their function remains unknown. A small, more recently identified population of RGCs expresses the photopigment melanopsin. These intrinsically photosensitive retinal ganglion cells (ipRGCs) transmit an irradiance-coding signal to visual nuclei in the brain, contributing both to image-forming vision and to several nonimage-forming functions, including circadian photoentrainment and the pupillary light reflex. In this study, using melanopsin immunolabeling in monkey and a genetic method to sparsely label the melanopsin cells in mouse, we show that a subgroup of ipRGCs have axons that branch en route to the optic disc, forming intraretinal axon collaterals that terminate in the inner plexiform layer of the retina. The previously described collateral-bearing population identified by intracellular dye injection is anatomically indistinguishable from the collateral-bearing melanopsin cells identified here, suggesting they are a subset of the melanopsin-expressing RGC type and may therefore share its functional properties. Identification of an anatomically distinct subpopulation in mouse, monkey, and human suggests this pathway may be conserved in these and other species (turtle and cat) with intraretinal axon collaterals. We speculate that ipRGC axon collaterals constitute a likely synaptic pathway for feedback of an irradiance signal to modulate retinal light responses.

Characterization of a novel large-field cone bipolar cell type in the primate retina: evidence for selective cone connections.

Parallel processing of visual information begins at the first synapse in the retina between the photoreceptors and bipolar cells. Ten bipolar cell types have been previously described in the primate retina: one rod and nine cone bipolar types. In this paper, we describe an 11th type of bipolar cell identified in Golgi-stained macaque retinal whole mount and vertical section. Axonal stratification depth, in addition to dendritic and axonal morphology, distinguished the "giant" cell from all previously well-recognized bipolar cell types. The giant bipolar cell had a very large and sparsely branched dendritic tree and a relatively large axonal arbor that costratified with the DB4 bipolar cell near the center of the inner plexiform layer. The sparseness of the giant bipolar's dendritic arbor indicates that, like the blue cone bipolar, it does not contact all the cones in its dendritic field. Giant cells contacting the same cones as midget bipolar cells, which are known to contact single long-wavelength (L) or medium-wavelength (M) cones, demonstrate that the giant cell does not exclusively contact short-wavelength (S) cones and, therefore, is not a variant of the previously described blue cone bipolar. This conclusion is further supported by measurement of the cone contact spacing for the giant bipolar. The giant cell contacts an average of about half the cones in its dendritic field (mean ± S.D. = 52 ± 17.6%; n = 6), with a range of 27-82%. The dendrites from single or neighboring giant cells that converge onto the same cones suggest that the giant cell may selectively target a subset of cones with a highly variable local density, such as the L or M cones.

Parallel ON and OFF cone bipolar inputs establish spatially coextensive receptive field structure of blue-yellow ganglion cells in primate retina.

In the primate retina the small bistratified, "blue-yellow" color-opponent ganglion cell receives parallel ON-depolarizing and OFF-hyperpolarizing inputs from short (S)-wavelength sensitive and combined long (L)- and middle (M)-wavelength sensitive cone photoreceptors, respectively. However, the synaptic pathways that create S versus LM cone-opponent receptive field structure remain controversial. Here, we show in the macaque monkey retina in vitro that at photopic light levels, when an identified rod input is excluded, the small bistratified cell displays a spatially coextensive receptive field in which the S-ON-input is in spatial, temporal, and chromatic balance with the LM-OFF-input. ON pathway block with l-AP-4, the mGluR6 receptor agonist, abolished the S-ON response but spared the LM-OFF response. The isolated LM component showed a center-surround receptive field structure consistent with an input from OFF-center, ON-surround "diffuse" cone bipolar cells. Increasing retinal buffering capacity with HEPES attenuated the LM-ON surround component, consistent with a non-GABAergic outer retina feedback mechanism for the bipolar surround. The GABAa/c receptor antagonist picrotoxin and the glycine receptor antagonist strychnine did not affect chromatic balance or the basic coextensive receptive field structure, suggesting that the LM-OFF field is not generated by an inner retinal inhibitory pathway. We conclude that the opponent S-ON and LM-OFF responses originate from the excitatory receptive field centers of S-ON and LM-OFF cone bipolar cells, and that the LM-OFF- and ON-surrounds of these parallel bipolar inputs largely cancel, explaining the small, spatially coextensive but spectrally antagonistic receptive field structure of the blue-ON ganglion cell.

The smooth monostratified ganglion cell: evidence for spatial diversity in the Y-cell pathway to the lateral geniculate nucleus and superior colliculus in the macaque monkey.

In the primate visual system approximately 20 morphologically distinct pathways originate from retinal ganglion cells and project in parallel to the lateral geniculate nucleus (LGN) and/or the superior colliculus. Understanding of the properties of these pathways and the significance of such extreme early pathway diversity for later visual processing is limited. In a companion study we found that the magnocellular LGN-projecting parasol ganglion cells also projected to the superior colliculus and showed Y-cell receptive field structure supporting the hypothesis that the parasol cells are analogous to the well studied alpha-Y cell of the cat's retina. We here identify a novel ganglion cell class, the smooth monostratified cells, that share many properties with the parasol cells. Smooth cells were retrogradely stained from tracer injections made into either the LGN or superior colliculus and formed inner-ON and outer-OFF populations with narrowly monostratified dendritic trees that surprisingly appeared to perfectly costratify with the dendrites of parasol cells. Also like parasol cells, smooth cells summed input from L- and M-cones, lacked measurable S-cone input, showed high spike discharge rates, high contrast and temporal sensitivity, and a Y-cell type nonlinear spatial summation. Smooth cells were distinguished from parasol cells however by smaller cell body and axon diameters but approximately 2 times larger dendritic tree and receptive field diameters that formed a regular but lower density mosaic organization. We suggest that the smooth and parasol populations may sample a common presynaptic circuitry but give rise to distinct, parallel achromatic spatial channels in the primate retinogeniculate pathway.

Y-cell receptive field and collicular projection of parasol ganglion cells in macaque monkey retina.

The distinctive parasol ganglion cell of the primate retina transmits a transient, spectrally nonopponent signal to the magnocellular layers of the lateral geniculate nucleus. Parasol cells show well-recognized parallels with the alpha-Y cell of other mammals, yet two key alpha-Y cell properties, a collateral projection to the superior colliculus and nonlinear spatial summation, have not been clearly established for parasol cells. Here, we show by retrograde photodynamic staining that parasol cells project to the superior colliculus. Photostained dendritic trees formed characteristic spatial mosaics and afforded unequivocal identification of the parasol cells among diverse collicular-projecting cell types. Loose-patch recordings were used to demonstrate for all parasol cells a distinct Y-cell receptive field "signature" marked by a nonlinear mechanism that responded to contrast-reversing gratings at twice the stimulus temporal frequency [second Fourier harmonic (F2)] independent of stimulus spatial phase. The F2 component showed high contrast gain and temporal sensitivity and appeared to originate from a region coextensive with that of the linear receptive field center. The F2 spatial frequency response peaked well beyond the resolution limit of the linear receptive field center, showing a Gaussian center radius of approximately 15 microm. Blocking inner retinal inhibition elevated the F2 response, suggesting that amacrine circuitry does not generate this nonlinearity. Our data are consistent with a pooled-subunit model of the parasol Y-cell receptive field in which summation from an array of transient, partially rectifying cone bipolar cells accounts for both linear and nonlinear components of the receptive field.

Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN.

Human vision starts with the activation of rod photoreceptors in dim light and short (S)-, medium (M)-, and long (L)- wavelength-sensitive cone photoreceptors in daylight. Recently a parallel, non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered in nocturnal rodents. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, 'non-image-forming' functions of circadian photoentrainment and pupil constriction. Here we show an anatomically distinct population of 'giant', melanopsin-expressing ganglion cells in the primate retina that, in addition to being intrinsically photosensitive, are strongly activated by rods and cones, and display a rare, S-Off, (L + M)-On type of colour-opponent receptive field. The intrinsic, rod and (L + M) cone-derived light responses combine in these giant cells to signal irradiance over the full dynamic range of human vision. In accordance with cone-based colour opponency, the giant cells project to the lateral geniculate nucleus, the thalamic relay to primary visual cortex. Thus, in the diurnal trichromatic primate, 'non-image-forming' and conventional 'image-forming' retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.

Fireworks in the primate retina: in vitro photodynamics reveals diverse LGN-projecting ganglion cell types.

Diverse cell types and parallel pathways are characteristic of the vertebrate nervous system, yet it remains a challenge to define the basic components of most neural structures. We describe a process termed retrograde photodynamics that allowed us to rapidly make the link between morphology, physiology, and connectivity for ganglion cells in the macaque retina that project to the lateral geniculate nucleus (LGN). Rhodamine dextran injected into the LGN was transported retrogradely and sequestered within the cytoplasm of ganglion cell bodies. Exposure of the retina to light in vitro liberated the tracer and allowed it to diffuse throughout the dendrites, revealing the cell's complete morphology. Eight previously unknown LGN-projecting cell types were identified. Cells could also be targeted in vitro for intracellular recording and physiological analysis. The photodynamic process was also observed in pyramidal cells in a rat neocortical slice.