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

Synaptic Origins of the Complex Receptive Field Structure in Primate Smooth Monostratified Retinal Ganglion Cells

Considerable progress has been made in studying the receptive fields of the most common primate retinal ganglion cell (RGC) types, such as parasol RGCs. Much less is known about the rarer primate RGC types and the circuitry that gives rise to noncanonical receptive field structures. The goal of this study was to analyze synaptic inputs to smooth monostratified RGCs to determine the origins of their complex spatial receptive fields, which contain isolated regions of high sensitivity called "hotspots." Interestingly, smooth monostratified RGCs co-stratify with the well-studied parasol RGCs and are thus constrained to receiving input from bipolar and amacrine cells with processes sharing the same layer, raising the question of how their functional differences originate. Through 3D reconstructions of circuitry and synapses onto ON smooth monostratified and ON parasol RGCs from central macaque retina, we identified four distinct sampling strategies employed by smooth and parasol RGCs to extract diverse response properties from co-stratifying bipolar and amacrine cells. The two RGC types differed in the proportion of amacrine cell input, relative contributions of co-stratifying bipolar cell types, amount of synaptic input per bipolar cell, and spatial distribution of bipolar cell synapses. Our results indicate that the smooth RGC's complex receptive field structure arises through spatial asymmetries in excitatory bipolar cell input which formed several discrete clusters comparable with physiologically measured hotspots. Taken together, our results demonstrate how the striking differences between ON parasol and ON smooth monostratified RGCs arise from distinct strategies for sampling a common set of synaptic inputs.

Morphology, Molecular Characterization, and Connections of Ganglion Cells in Primate Retina

The eye sends information about the visual world to the brain on over 20 parallel signal pathways, each specialized to signal features such as spectral reflection (color), edges, and motion of objects in the environment. Each pathway is formed by the axons of a separate type of retinal output neuron (retinal ganglion cell). In this review, we summarize what is known about the excitatory retinal inputs, brain targets, and gene expression patterns of ganglion cells in humans and nonhuman primates. We describe how most ganglion cell types receive their input from only one or two of the 11 types of cone bipolar cell and project selectively to only one or two target regions in the brain. We also highlight how genetic methods are providing tools to characterize ganglion cells and establish cross-species homologies.

Cellular-Scale Imaging of Transparent Retinal Structures and Processes Using Adaptive Optics Optical Coherence Tomography

High-resolution retinal imaging is revolutionizing how scientists and clinicians study the retina on the cellular scale. Its exquisite sensitivity enables time-lapse optical biopsies that capture minute changes in the structure and physiological processes of cells in the living eye. This information is increasingly used to detect disease onset and monitor disease progression during early stages, raising the possibility of personalized eye care. Powerful high-resolution imaging tools have been in development for more than two decades; one that has garnered considerable interest in recent years is optical coherence tomography enhanced with adaptive optics. State-of-the-art adaptive optics optical coherence tomography (AO-OCT) makes it possible to visualize even highly transparent cells and measure some of their internal processes at all depths within the retina, permitting reconstruction of a 3D view of the living microscopic retina. In this review, we report current AO-OCT performance and its success in visualizing and quantifying these once-invisible cells in human eyes.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

Diverse Cell Types, Circuits, and Mechanisms for Color Vision in the Vertebrate Retina

Synaptic interactions to extract information about wavelength, and thus color, begin in the vertebrate retina with three classes of light-sensitive cells: rod photoreceptors at low light levels, multiple types of cone photoreceptors that vary in spectral sensitivity, and intrinsically photosensitive ganglion cells that contain the photopigment melanopsin. When isolated from its neighbors, a photoreceptor confounds photon flux with wavelength and so by itself provides no information about color. The retina has evolved elaborate color opponent circuitry for extracting wavelength information by comparing the activities of different photoreceptor types broadly tuned to different parts of the visible spectrum. We review studies concerning the circuit mechanisms mediating opponent interactions in a range of species, from tetrachromatic fish with diverse color opponent cell types to common dichromatic mammals where cone opponency is restricted to a subset of specialized circuits. Distinct among mammals, primates have reinvented trichromatic color vision using novel strategies to incorporate evolution of an additional photopigment gene into the foveal structure and circuitry that supports high-resolution vision. Color vision is absent at scotopic light levels when only rods are active, but rods interact with cone signals to influence color perception at mesopic light levels. Recent evidence suggests melanopsin-mediated signals, which have been identified as a substrate for setting circadian rhythms, may also influence color perception. We consider circuits that may mediate these interactions. While cone opponency is a relatively simple neural computation, it has been implemented in vertebrates by diverse neural mechanisms that are not yet fully understood.

Cone synapses in mammalian retinal rod bipolar cells

Some mammalian rod bipolar cells (RBCs) can receive excitatory chemical synaptic inputs from both rods and cones (DBCR2 ), but anatomical evidence for mammalian cone-RBC contacts has been sparse. We examined anatomical cone-RBC contacts using neurobiotin (NB) to visualize individual mouse cones and standard immuno-markers to identify RBCs, cone pedicles and synapses in mouse and baboon retinas. Peanut agglutinin (PNA) stained the basal membrane of all cone pedicles, and mouse cones were positive for red/green (R/G)-opsin, whereas baboon cones were positive for calbindin D-28k. All synapses in the outer plexiform layer were labeled for synaptic vesicle protein 2 (SV2) and PSD (postsynaptic density)-95, and those that coincided with PNA resided closest to bipolar cell somas. Cone-RBC synaptic contacts were identified by: (a) RBC dendrites deeply invaginating into the center of cone pedicles (invaginating synapses), (b) RBC dendritic spines intruding into the surface of cone pedicles (superficial synapses), and (c) PKCα immunoreactivity coinciding with synaptic marker SV2, PSD-95, mGluR6, G protein beta 5 or PNA at cone pedicles. One RBC could form 0-1 invaginating and 1-3 superficial contacts with cones. 20.7% and 38.9% of mouse RBCs contacted cones in the peripheral and central retina (p < .05, n = 14 samples), respectively, while 34.4% (peripheral) and 48.5% (central) of cones contacted RBCs (p > .05). In baboon retinas (n = 4 samples), cone-RBC contacts involved 12.2% of RBCs (n = 416 cells) and 22.5% of cones (n = 225 cells). This suggests that rod and cone signals in the ON pathway are integrated in some RBCs before reaching AII amacrine cells.

Connectivity map of bipolar cells and photoreceptors in the mouse retina

In the mouse retina, three different types of photoreceptors provide input to 14 bipolar cell (BC) types. Classically, most BC types are thought to contact all cones within their dendritic field; ON-BCs would contact cones exclusively via so-called invaginating synapses, while OFF-BCs would form basal synapses. By mining publically available electron microscopy data, we discovered interesting violations of these rules of outer retinal connectivity: ON-BC type X contacted only ~20% of the cones in its dendritic field and made mostly atypical non-invaginating contacts. Types 5T, 5O and 8 also contacted fewer cones than expected. In addition, we found that rod BCs received input from cones, providing anatomical evidence that rod and cone pathways are interconnected in both directions. This suggests that the organization of the outer plexiform layer is more complex than classically thought.

ON Bipolar Cells in Macaque Retina: Type-Specific Synaptic Connectivity with Special Reference to OFF Counterparts

To date, 12 macaque bipolar cell types have been described. This list includes all morphology types first outlined by Polyak (1941) using the Golgi method in the primate retina and subsequently identified by other researchers using electron microscopy (EM) combined with the Golgi method, serial section transmission EM (SSTEM), and immunohistochemical imaging. We used SSTEM for the rod-dense perifoveal area of macaque retina, reconfirmed ON (cone) bipolar cells to be classified as invaginating midget bipolar (IMB), diffuse bipolar (DB)4, DB5, DB6, giant bipolar (GB), and blue bipolar (BB) types, and clarified their type-specific connectivity. DB4 cells made reciprocal synapses with a kind of ON-OFF lateral amacrine cell, similar to OFF DB2 cells. GB cells contacted rods and cones, similar to OFF DB3b cells. Retinal circuits formed by GB and DB3b cells are thought to substantiate the psychophysical finding of fast rod signals in mesopic vision. DB6 cell output synapses were directed to ON midget ganglion (MG) cells at 70% of ribbon contacts, similar to OFF DB1 cells that directed 60% of ribbon contacts to OFF MG cells. IMB cells contacted medium- or long-wavelength sensitive (M/L-) cones but not short-wavelength sensitive (S-) cones, while BB cells contacted S-cones but not M/L-cones. However, IMB and BB dendrites had similar morphological architectures, and a BB cell contacting a single S-cone resembled an IMB cell. Thus, both IMB and BB may be the ON bipolar counterparts of the OFF flat midget bipolar (FMB) type, likewise DB4 of DB2, DB5 of DB3a, DB6 of DB1, and GB of DB3b OFF bipolar type. The ON DB plus GB, and OFF DB cells predominantly contacted M/L-cones and their outputs were directed mainly to parasol ganglion (PG) cells but also moderately to MG cells. BB cells directed S-cone-driven outputs almost exclusively to small bistratified ganglion (SBG) cells. Some FMB cells predominantly contacted S-cones and their outputs were directed to OFF MG cells. Thus, two-step synaptic connections largely narrowed down the S-cone component to SBG and some OFF MG cells. The other OFF MG cells, ON MG cells, and ON and OFF PG cells constructed M/L-cone dominant pathways.

Connectivity between the OFF bipolar type DB3a and six types of ganglion cell in the marmoset retina

Parallel visual pathways originate at the first synapse in the retina, where cones make connections with cone bipolar cells that in turn contact ganglion cells. There are more ganglion cell types than bipolar types, suggesting that there must be divergence from bipolar to ganglion cells. Here we analyze the contacts between an OFF bipolar type (DB3a) and six ganglion cell types in the retina of the marmoset monkey (Callithrix jacchus). Ganglion cells were transfected via particle-mediated gene transfer of an expression plasmid for the postsynaptic density 95-green fluorescent protein (PSD95-GFP), and DB3a cells were labeled via immunohistochemistry. Ganglion cell types that fully or partially costratified with DB3a cells included OFF parasol, OFF midget, broad thorny, recursive bistratified, small bistratified, and large bistratified cells. On average, the number of DB3a contacts to parasol cells (18 contacts per axon terminal) is higher than that to other ganglion cell types (between four and seven contacts). We estimate that the DB3a output to OFF parasol cells accounts for at least 30% of the total DB3a output. Furthermore, we found that OFF parasol cells receive approximately 20% of their total bipolar input from DB3a cells, suggesting that other diffuse bipolar types also provide input to OFF parasol cells. We conclude that DB3a cells preferentially contact OFF parasol cells but also provide input to other ganglion cell types.

Cone bipolar cells in the retina of the microbat Carollia perspicillata

We studied the retinal cone bipolar cells of Carollia perspicillata, a microchiropteran bat of the phyllostomid family. Microchiroptera are strongly nocturnal, with small eyes and rod-dominated retinae. However, they also possess a significant cone population (2-4%) comprising two spectral types, which are hence the basis for daylight and color vision. We used antibodies against the calcium-binding protein recoverin and the carbohydrate epitope 15 (CD15) as reliable markers for certain cone bipolar cells. Dye injections of recoverin- or CD15-prelabeled cone bipolar cells in vertical slices revealed the morphology of the axon terminal system of individual bipolar cells. Seven distinct cone bipolar cell types were identified. They differed in the morphology and stratification level of their axon terminal system in the inner plexiform layer and in immunoreactivity for recoverin and/or CD15. Additional immunocytochemical markers were used to assess the functional ON/OFF subdivision of the inner plexiform layer. In line with the extended thickness of the ON sublayer of the inner plexiform layer in the microbat retina, more ON than OFF cone bipolar cell types were found, namely, four versus three. Most likely, in the bats' predominantly dark environment, ON signals have greater importance for contrast perception. We conclude that the microbat retina conforms to the general mammalian blueprint, in which light signals of intensities above rod sensitivity are detected by cones and transmitted to various types of ON and OFF cone bipolar cells.

S-cone Visual Stimuli Activate Superior Colliculus Neurons in Old World Monkeys: Implications for Understanding Blindsight

The superior colliculus (SC) is thought to be unresponsive to stimuli that activate only short wavelength-sensitive cones (S-cones) in the retina. The apparent lack of S-cone input to the SC was recognized by Sumner et al. [Sumner, P., Adamjee, T., & Mollon, J. D. Signals invisible to the collicular and magnocellular pathways can capture visual attention. Current Biology, 12, 1312–1316, 2002] as an opportunity to test SC function. The idea is that visual behavior dependent on the SC should be impaired when S-cone stimuli are used because they are invisible to the SC. The SC plays a critical role in blindsight. If the SC is insensitive to S-cone stimuli blindsight behavior should be impaired when S-cone stimuli are used. Many clinical and behavioral studies have been based on the assumption that S-cone-specific stimuli do not activate neurons in the SC. Our goal was to test whether single neurons in macaque SC respond to stimuli that activate only S-cones. Stimuli were calibrated psychophysically in each animal and at each individual spatial location used in experimental testing [Hall, N. J., & Colby, C. L. Psychophysical definition of S-cone stimuli in the macaque. Journal of Vision, 13, 2013]. We recorded from 178 visually responsive neurons in two awake, behaving rhesus monkeys. Contrary to the prevailing view, we found that nearly all visual SC neurons can be activated by S-cone-specific visual stimuli. Most of these neurons were sensitive to the degree of S-cone contrast. Of 178 visual SC neurons, 155 (87%) had stronger responses to a high than to a low S-cone contrast. Many of these neurons' responses (56/178 or 31%) significantly distinguished between the high and low S-cone contrast stimuli. The latency and amplitude of responses depended on S-cone contrast. These findings indicate that stimuli that activate only S-cones cannot be used to diagnose collicular mediation.

Neurobiological hypothesis of color appearance and hue perception

De Valois and De Valois [Vis. Res.33, 1053 (1993)] showed that to explain hue appearance, S-cone signals have to be combined with M versus L opponent signals in two different ways to produce red-green and yellow-blue axes, respectively. Recently, it has been shown that color appearance is normal for individuals with genetic mutations that block S-cone input to blue-ON ganglion cells. This is inconsistent with the De Valois hypothesis in which S-opponent konio-geniculate signals are combined with L-M signals at a third processing stage in cortex. Instead, here we show that color appearance, including individual differences never explained before, are predicted by a model in which S-cone signals are combined with L versus M signals in the outer retina.

NaV1.1 channels in axon initial segments of bipolar cells augment input to magnocellular visual pathways in the primate retina

In the primate visual system, the ganglion cells of the magnocellular pathway underlie motion and flicker detection and are relatively transient, while the more sustained ganglion cells of the parvocellular pathway have comparatively lower temporal resolution, but encode higher spatial frequencies. Although it is presumed that functional differences in bipolar cells contribute to the tuning of the two pathways, the properties of the relevant bipolar cells have not yet been examined in detail. Here, by making patch-clamp recordings in acute slices of macaque retina, we show that the bipolar cells within the magnocellular pathway, but not the parvocellular pathway, exhibit voltage-gated sodium (NaV), T-type calcium (CaV), and hyperpolarization-activated, cyclic nucleotide-gated (HCN) currents, and can generate action potentials. Using immunohistochemistry in macaque and human retinae, we show that NaV1.1 is concentrated in an axon initial segment (AIS)-like region of magnocellular pathway bipolar cells, a specialization not seen in transient bipolar cells of other vertebrates. In contrast, CaV3.1 channels were localized to the somatodendritic compartment and proximal axon, but were excluded from the AIS, while HCN1 channels were concentrated in the axon terminal boutons. Simulations using a compartmental model reproduced physiological results and indicate that magnocellular pathway bipolar cells initiate spikes in the AIS. Finally, we demonstrate that NaV channels in bipolar cells augment excitatory input to parasol ganglion cells of the magnocellular pathway. Overall, the results demonstrate that selective expression of voltage-gated channels contributes to the establishment of parallel processing in the major visual pathways of the primate retina.

Processing of S-cone signals in the inner plexiform layer of the mammalian retina

Color information is encoded by two parallel pathways in the mammalian retina. One pathway compares signals from long- and middle-wavelength sensitive cones and generates red-green opponency. The other compares signals from short- and middle-/long-wavelength sensitive cones and generates blue-green (yellow) opponency. Whereas both pathways operate in trichromatic primates (including humans), the fundamental, phylogenetically ancient color mechanism shared among most mammals is blue-green opponency. In this review, we summarize the current understanding of how signals from short-wavelength sensitive cones are processed in the primate and nonprimate mammalian retina, with a focus on the inner plexiform layer where bipolar, amacrine, and ganglion cell processes interact to facilitate the generation of blue-green opponency.

Connectomic reconstruction of the inner plexiform layer in the mouse retina

Comprehensive high-resolution structural maps are central to functional exploration and understanding in biology. For the nervous system, in which high resolution and large spatial extent are both needed, such maps are scarce as they challenge data acquisition and analysis capabilities. Here we present for the mouse inner plexiform layer--the main computational neuropil region in the mammalian retina--the dense reconstruction of 950 neurons and their mutual contacts. This was achieved by applying a combination of crowd-sourced manual annotation and machine-learning-based volume segmentation to serial block-face electron microscopy data. We characterize a new type of retinal bipolar interneuron and show that we can subdivide a known type based on connectivity. Circuit motifs that emerge from our data indicate a functional mechanism for a known cellular response in a ganglion cell that detects localized motion, and predict that another ganglion cell is motion sensitive.

Distinct synaptic mechanisms create parallel S-ON and S-OFF color opponent pathways in the primate retina

Anatomical and physiological approaches are beginning to reveal the synaptic origins of parallel ON- and OFF-pathway retinal circuits for the transmission of short (S-) wavelength sensitive cone signals in the primate retina. Anatomical data suggest that synaptic output from S-cones is largely segregated; central elements of synaptic triads arise almost exclusively from the "blue-cone" bipolar cell, a presumed ON bipolar, whereas triad-associated contacts derive primarily from the "flat" midget bipolar cell, a hyperpolarizing, OFF bipolar. Similarly, horizontal cell connectivity is also segregated, with only the H2 cell-type receiving numerous contacts from S-cones. Negative feedback from long (L-) and middle (M-) wavelength sensitive cones via the H2 horizontal cells elicits an antagonistic surround in S-cones demonstrating that S versus L + M or "blue-yellow" opponency is first established in the S-cone. However, the S-cone output utilizes distinct synaptic mechanisms to create color opponency at the ganglion cell level. The blue-cone bipolar cell is presynaptic to the small bistratified, "blue-ON" ganglion cell. S versus L + M cone opponency arises postsynaptically by converging S-ON and LM-OFF excitatory bipolar inputs to the ganglion cell's bistratified dendritic tree. The common L + M cone surrounds of the parallel S-ON and LM-OFF cone bipolar inputs appear to cancel resulting in "blue-yellow" antagonism without center-surround spatial opponency. By contrast, in midget ganglion cells, opponency arises by the differential weighting of cone inputs to the receptive field center versus surround. In the macula, the "private-line" connection from a midget ganglion cell to a single cone predicts that S versus L + M opponency is transmitted from the S-cone to the S-OFF midget bipolar and ganglion cell. Beyond the macula, OFF-midget ganglion cell dendritic trees enlarge and collect additional input from multiple L and M cones. Thus S-OFF opponency via the midget pathway would be expected to become more complex in the near retinal periphery as L and/or M and S cone inputs sum to the receptive field center. An important goal for further investigation will be to explore the hypothesis that distinct bistratified S-ON versus midget S-OFF retinal circuits are the substrates for human psychophysical detection mechanisms attributed to S-ON versus S-OFF perceptual channels.

Bipolar cell-photoreceptor connectivity in the zebrafish (Danio rerio) retina

Bipolar cells convey luminance, spatial, and color information from photoreceptors to amacrine and ganglion cells. We studied the photoreceptor connectivity of 321 bipolar cells in the adult zebrafish retina. 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was inserted into whole-mounted transgenic zebrafish retinas to label bipolar cells. The photoreceptors that connect to these DiI-labeled cells were identified by transgenic fluorescence or their positions relative to the fluorescent cones, as cones are arranged in a highly ordered mosaic: rows of alternating blue- (B) and ultraviolet-sensitive (UV) single cones alternate with rows of red-(R) and green-sensitive (G) double cones. Rod terminals intersperse among cone terminals. As many as 18 connectivity subtypes were observed, 9 of which-G, GBUV, RG, RGB, RGBUV, RGRod, RGBRod, RGBUVRod, and RRod bipolar cells-accounted for 96% of the population. Based on their axon terminal stratification, these bipolar cells could be further subdivided into ON, OFF, and ON-OFF cells. The dendritic spread size, soma depth and size, and photoreceptor connections of the 308 bipolar cells within the nine common connectivity subtypes were determined, and their dendritic tree morphologies and axonal stratification patterns compared. We found that bipolar cells with the same axonal stratification patterns could have heterogeneous photoreceptor connectivity whereas bipolar cells with the same dendritic tree morphology usually had the same photoreceptor connectivity, although their axons might stratify on different levels.

Organizational motifs for ground squirrel cone bipolar cells

In daylight vision, parallel processing starts at the cone synapse. Cone signals flow to On and Off bipolar cells, which are further divided into types according to morphology, immunocytochemistry, and function. The axons of the bipolar cell types stratify at different levels in the inner plexiform layer (IPL) and can interact with costratifying amacrine and ganglion cells. These interactions endow the ganglion cell types with unique functional properties. The wiring that underlies the interactions among bipolar, amacrine, and ganglion cells is poorly understood. It may be easier to elucidate this wiring if organizational rules can be established. We identify 13 types of cone bipolar cells in the ground squirrel, 11 of which contact contiguous cones, with the possible exception of short-wavelength-sensitive cones. Cells were identified by antibody labeling, tracer filling, and Golgi-like filling following transduction with an adeno-associated virus encoding for green fluorescent protein. The 11 bipolar cell types displayed two organizational patterns. In the first pattern, eight to 10 of the 11 types came in pairs with partially overlapping axonal stratification. Pairs shared morphological, immunocytochemical, and functional properties. The existence of similar pairs is a new motif that might have implications for how signals first diverge from a cone to bipolar cells and then reconverge onto a costratifying ganglion cell. The second pattern is a mirror symmetric organization about the middle of the IPL involving at least seven bipolar cell types. This anatomical symmetry may be associated with a functional symmetry in On and Off ganglion cell responses.

Cardinal axes are not independent in color discrimination

We measured chromatic discrimination under conditions where the target fields could be distinguished only by the ratio of excitation of the long- (L) and middle-wavelength (M) cones. The excitation level of the short-wavelength (S) cones was varied in the experiments, although for any given measurement the S-cone excitation was common to the two target fields and could not be directly used for discrimination. Adaptation was maintained by a steady neutral background metameric to Illuminant D65. Thresholds varied substantially and systematically with the S-cone level of the target probes, but in a complex way: when the ratio of L:M cone excitation was low, an increase in S-cone excitation reduced the thresholds, but when the L:M ratio was higher, an increase in S-cone excitation raised the thresholds. To account for the pattern of results, we postulate a neural channel that draws synergistic inputs from L and S cones and an opposed input from M cones. The proposed channel has a compressive response function and is most sensitive at the point set by the steady background.

Spatial distributions of cone inputs to cells of the parvocellular pathway investigated with cone-isolating gratings

Receptive fields of midget ganglion cells and parvocellular lateral geniculate nucleus (LGN) neurons show color-opponent responses because they receive antagonistic input from the middle- and long-wavelength sensitive cones. It has been controversial as to whether this opponency can derive from random connectivity; if receptive field centers of cells near the fovea are cone-specific due to midget morphology, this would confer some degree of color opponency even with random cone input to the surround. A simple test of this mixed surround hypothesis is to compare spatial frequency tuning curves for luminance gratings and gratings isolating cone input to the receptive field center. If tuning curves for luminance gratings were bandpass, then with the mixed surround hypothesis tuning curves for gratings isolating the receptive field center cone class should also be bandpass, but to a lesser extent than for luminance. Tuning curves for luminance, chromatic, and cone-isolating gratings were measured in macaque retinal ganglion cells and LGN cells. We defined and measured a bandpass index to compare luminance and center cone-isolating tuning curves. Midget retinal ganglion cells and parvocellular LGN cells had bandpass indices between 0.1 and 1 with luminance gratings, but the index was usually near 1 (meaning low-pass tuning) when the receptive field center cone class alone was modulated. This is strong evidence for a considerable degree of cone-specific input to the surround. A fraction of midget and parvocellular cells showed evidence of incomplete specificity. Fitting the data with receptive field models revealed considerable intercell variability, with indications in some cells of a more complex receptive structure than a simple difference of Gaussians model.