Receptive-field microstructure of blue-yellow ganglion cells in primate retina

In vivo chromatic and spatial tuning of foveolar retinal ganglion cells in Macaca fascicularis

The primate fovea is specialized for high acuity chromatic vision, with the highest density of cone photoreceptors and a disproportionately large representation in visual cortex. The unique visual properties conferred by the fovea are conveyed to the brain by retinal ganglion cells, the somas of which lie at the margin of the foveal pit. Microelectrode recordings of these centermost retinal ganglion cells have been challenging due to the fragility of the fovea in the excised retina. Here we overcome this challenge by combining high resolution fluorescence adaptive optics ophthalmoscopy with calcium imaging to optically record functional responses of foveal retinal ganglion cells in the living eye. We use this approach to study the chromatic responses and spatial transfer functions of retinal ganglion cells using spatially uniform fields modulated in different directions in color space and monochromatic drifting gratings. We recorded from over 350 cells across three Macaca fascicularis primates over a time period of weeks to months. We find that the majority of the L vs. M cone opponent cells serving the most central foveolar cones have spatial transfer functions that peak at high spatial frequencies (20-40 c/deg), reflecting strong surround inhibition that sacrifices sensitivity at low spatial frequencies but preserves the transmission of fine detail in the retinal image. In addition, we fit to the drifting grating data a detailed model of how ganglion cell responses draw on the cone mosaic to derive receptive field properties of L vs. M cone opponent cells at the very center of the foveola. The fits are consistent with the hypothesis that foveal midget ganglion cells are specialized to preserve information at the resolution of the cone mosaic. By characterizing the functional properties of retinal ganglion cells in vivo through adaptive optics, we characterize the response characteristics of these cells in situ.

Suppression without inhibition: how retinal computation contributes to saccadic suppression

Visual perception remains stable across saccadic eye movements, despite the concurrent strongly disruptive visual flow. This stability is partially associated with a reduction in visual sensitivity, known as saccadic suppression, which already starts in the retina with reduced ganglion cell sensitivity. However, the retinal circuit mechanisms giving rise to such suppression remain unknown. Here, we describe these mechanisms using electrophysiology in mouse, pig, and macaque retina, 2-photon calcium imaging, computational modeling, and human psychophysics. We find that sequential stimuli, like those that naturally occur during saccades, trigger three independent suppressive mechanisms in the retina. The main mechanism is triggered by contrast-reversing sequential stimuli and originates within the receptive field center of ganglion cells. It does not involve inhibition or other known suppressive mechanisms like saturation or adaptation. Instead, it relies on temporal filtering of the inherently slow response of cone photoreceptors coupled with downstream nonlinearities. Two further mechanisms of suppression are present predominantly in ON ganglion cells and originate in the receptive field surround, highlighting another disparity between ON and OFF ganglion cells. The mechanisms uncovered here likely play a role in shaping the retinal output following eye movements and other natural viewing conditions where sequential stimulation is ubiquitous.

Feature Detection by Retinal Ganglion Cells

Retinal circuits transform the pixel representation of photoreceptors into the feature representations of ganglion cells, whose axons transmit these representations to the brain. Functional, morphological, and transcriptomic surveys have identified more than 40 retinal ganglion cell (RGC) types in mice. RGCs extract features of varying complexity; some simply signal local differences in brightness (i.e., luminance contrast), whereas others detect specific motion trajectories. To understand the retina, we need to know how retinal circuits give rise to the diverse RGC feature representations. A catalog of the RGC feature set, in turn, is fundamental to understanding visual processing in the brain. Anterograde tracing indicates that RGCs innervate more than 50 areas in the mouse brain. Current maps connecting RGC types to brain areas are rudimentary, as is our understanding of how retinal signals are transformed downstream to guide behavior. In this article, I review the feature selectivities of mouse RGCs, how they arise, and how they are utilized downstream. Not only is knowledge of the behavioral purpose of RGC signals critical for understanding the retinal contributions to vision; it can also guide us to the most relevant areas of visual feature space. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

The Relationship Between Visual Sensitivity and Eccentricity, Cone Density and Outer Segment Length in the Human Foveola

Purpose

The cellular topography of the human foveola, the central 1° diameter of the fovea, is strikingly non-uniform, with a steep increase of cone photoreceptor density and outer segment (OS) length toward its center. Here, we assessed to what extent the specific cellular organization of the foveola of an individual is reflected in visual sensitivity and if sensitivity peaks at the preferred retinal locus of fixation (PRL).

Methods

Increment sensitivity to small-spot, cone-targeted visual stimuli (1 × 1 arcmin, 543-nm light) was recorded psychophysically in four human participants at 17 locations concentric within a 0.2° diameter on and around the PRL with adaptive optics scanning laser ophthalmoscopy-based microstimulation. Sensitivity test spots were aligned with cell-resolved maps of cone density and cone OS length.

Results

Peak sensitivity was at neither the PRL nor the topographical center of the cone mosaic. Within the central 0.1° diameter, a plateau-like sensitivity profile was observed. Cone density and maximal OS length differed significantly across participants, correlating with their peak sensitivity. Based on these results, biophysical simulation allowed to develop a model of visual sensitivity in the foveola, with distance from the PRL (eccentricity), cone density, and OS length as parameters.

Conclusions

Small-spot sensitivity thresholds in healthy retinas will help to establish the range of normal foveolar function in cell-targeted vision testing. Because of the high reproducibility in replicate testing, threshold variability not explained by our model is assumed to be caused by individual cone and bipolar cell weighting at the specific target locations.

Visual properties of human retinal ganglion cells

The retinal output is the sole source of visual information for the brain. Studies in non-primate mammals estimate that this information is carried by several dozens of retinal ganglion cell types, each informing the brain about different aspects of a visual scene. Even though morphological studies of primate retina suggest a similar diversity of ganglion cell types, research has focused on the function of only a few cell types. In human retina, recordings from individual cells are anecdotal or focus on a small subset of identified types. Here, we present the first systematic ex-vivo recording of light responses from 342 ganglion cells in human retinas obtained from donors. We find a great variety in the human retinal output in terms of preferences for positive or negative contrast, spatio-temporal frequency encoding, contrast sensitivity, and speed tuning. Some human ganglion cells showed similar response behavior as known cell types in other primate retinas, while we also recorded light responses that have not been described previously. This first extensive description of the human retinal output should facilitate interpretation of primate data and comparison to other mammalian species, and it lays the basis for the use of ex-vivo human retina for in-vitro analysis of novel treatment approaches.

Spike-triggered Clustering for Retinal Ganglion Cell Classification

Retinal ganglion cells (RGCs), the retina's output neurons, encode visual information through spiking. The RGC receptive field (RF) represents the basic unit of visual information processing in the retina. RFs are commonly estimated using the spike-triggered average (STA), which is the average of the stimulus patterns to which a given RGC is sensitive. Whereas STA, based on the concept of the average, is simple and intuitive, it leaves more complex structures in the RFs undetected. Alternatively, spike-triggered covariance (STC) analysis provides information on second-order RF statistics. However, STC is computationally cumbersome and difficult to interpret. Thus, the objective of this study was to propose and validate a new computational method, called spike-triggered clustering (STCL), specific for multimodal RFs. Specifically, RFs were fit with a Gaussian mixture model, which provides the means and covariances of multiple RF clusters. The proposed method recovered bipolar stimulus patterns in the RFs of ON-OFF cells, while the STA identified only ON and OFF RGCs, and the remaining RGCs were labeled as unknown types. In contrast, our new STCL analysis distinguished ON-OFF RGCs from the ON, OFF, and unknown RGC types classified by STA. Thus, the proposed method enables us to include ON-OFF RGCs prior to retinal information analysis.

Poor peripheral binding depends in part on stimulus color

We have compared two explanations for poor peripheral binding. Binding is the ability to assign the correct features (e.g., color, direction of motion, orientation) to objects. Wu, Kanai, and Shimojo (Nature, 429(6989), 262, 2004) showed that subjects performed poorly on binding dot color with direction of motion in the periphery. Suzuki, Wolfe, Horowitz, and Noguchi (Vision Research, 82, 58-65, 2013) similarly showed that subjects had trouble binding color with line orientation in the periphery. These authors concluded that performance in the periphery was poor because binding is poor in the periphery. However, both studies used red and green stimuli. We tested an alternative hypothesis, that poor peripheral binding is in part due to poor peripheral red/green color vision. Eccentricity-dependent changes in visual processing cause peripheral red/green vision to be worse than foveal vision. In contrast, blue/yellow vision remains centrifugally more stable. We tested 9 subjects in a replication and extension of Suzuki and colleagues' line orientation judgment, in red and green, and in blue and yellow. There were three central conditions: (1) red (or blue) all horizontal, green (or yellow) all vertical; (2) red (or blue) all vertical, green (or yellow) all horizontal; or (3) random pairing of color and orientation. In both the red/green and the blue/yellow color schemes, peripheral performance was influenced by central line orientation, replicating Suzuki and colleagues. However, the effect with blue/yellow lines was smaller, indicating that poor peripheral "binding," as hypothesized by both Wu and colleagues and Suzuki and colleagues, is due in part to their use of red and green stimuli.

Efficient Coding by Midget and Parasol Ganglion Cells in the Human Retina

In humans, midget and parasol ganglion cells account for most of the input from the eyes to the brain. Yet, how they encode visual information is unknown. Here, we perform large-scale multi-electrode array recordings from retinas of treatment-naive patients who underwent enucleation surgery for choroidal malignant melanomas. We identify robust differences in the function of midget and parasol ganglion cells, consistent asymmetries between their ON and OFF types (that signal light increments and decrements, respectively) and divergence in the function of human versus non-human primate retinas. Our computational analyses reveal that the receptive fields of human midget and parasol ganglion cells divide naturalistic movies into adjacent spatiotemporal frequency domains with equal stimulus power, while the asymmetric response functions of their ON and OFF types simultaneously maximize stimulus coverage and information transmission and minimize metabolic cost. Thus, midget and parasol ganglion cells in the human retina efficiently encode our visual environment.

New Features of Receptive Fields in Mouse Retina through Spike-triggered Covariance

Retinal ganglion cells (RGCs) encode various spatiotemporal features of visual information into spiking patterns. The receptive field (RF) of each RGC is usually calculated by spike-triggered average (STA), which is fast and easy to understand, but limited to simple and unimodal RFs. As an alternative, spike-triggered covariance (STC) has been proposed to characterize more complex patterns in RFs. This study compares STA and STC for the characterization of RFs and demonstrates that STC has an advantage over STA for identifying novel spatiotemporal features of RFs in mouse RGCs. We first classified mouse RGCs into ON, OFF, and ON/OFF cells according to their response to full-field light stimulus, and then investigated the spatiotemporal patterns of RFs with random checkerboard stimulation, using both STA and STC analysis. We propose five sub-types (T1–T5) in the STC of mouse RGCs together with their physiological implications. In particular, the relatively slow biphasic pattern (T1) could be related to excitatory inputs from bipolar cells. The transient biphasic pattern (T2) allows one to characterize complex patterns in RFs of ON/OFF cells. The other patterns (T3–T5), which are contrasting, alternating, and monophasic patterns, could be related to inhibitory inputs from amacrine cells. Thus, combining STA and STC and considering the proposed sub-types unveil novel characteristics of RFs in the mouse retina and offer a more holistic understanding of the neural coding mechanisms of mouse RGCs.

The Retinal Basis of Vertebrate Color Vision

The jawless fish that were ancestral to all living vertebrates had four spectral cone types that were probably served by chromatic-opponent retinal circuits. Subsequent evolution of photoreceptor spectral sensitivities is documented for many vertebrate lineages, giving insight into the ecological adaptation of color vision. Beyond the photoreceptors, retinal color processing is best understood in mammals, especially the blueON system, which opposes short- against long-wavelength receptor responses. For other vertebrates that often have three or four types of cone pigment, new findings from zebrafish are extending older work on teleost fish and reptiles to reveal rich color circuitry. Here, horizontal cells establish diverse and complex spectral responses even in photoreceptor outputs. Cone-selective connections to bipolar cells then set up color-opponent synaptic layers in the inner retina, which lead to a large variety of color-opponent channels for transmission to the brain via retinal ganglion cells.

Unusual Physiological Properties of Smooth Monostratified Ganglion Cell Types in Primate Retina

The functions of the diverse retinal ganglion cell types in primates and the parallel visual pathways they initiate remain poorly understood. Here, unusual physiological and computational properties of the ON and OFF smooth monostratified ganglion cells are explored. Large-scale multi-electrode recordings from 48 macaque retinas revealed that these cells exhibit irregular receptive field structure composed of spatially segregated hotspots, quite different from the classic center-surround model of retinal receptive fields. Surprisingly, visual stimulation of different hotspots in the same cell produced spikes with subtly different spatiotemporal voltage signatures, consistent with a dendritic contribution to hotspot structure. Targeted visual stimulation and computational inference demonstrated strong nonlinear subunit properties associated with each hotspot, supporting a model in which the hotspots apply nonlinearities at a larger spatial scale than bipolar cells. These findings reveal a previously unreported nonlinear mechanism in the output of the primate retina that contributes to signaling spatial information.

Probing Computation in the Primate Visual System at Single-Cone Resolution

Daylight vision begins when light activates cone photoreceptors in the retina, creating spatial patterns of neural activity. These cone signals are then combined and processed in downstream neural circuits, ultimately producing visual perception. Recent technical advances have made it possible to deliver visual stimuli to the retina that probe this processing by the visual system at its elementary resolution of individual cones. Physiological recordings from nonhuman primate retinas reveal the spatial organization of cone signals in retinal ganglion cells, including how signals from cones of different types are combined to support both spatial and color vision. Psychophysical experiments with human subjects characterize the visual sensations evoked by stimulating a single cone, including the perception of color. Future combined physiological and psychophysical experiments focusing on probing the elementary visual inputs are likely to clarify how neural processing generates our perception of the visual world.

Functional architecture of the foveola revealed in the living primate

The primate foveola, with its high cone density and magnified cortical representation, is exquisitely specialized for high-resolution spatial vision. However, uncovering the wiring of retinal circuitry responsible for this performance has been challenging due to the difficulty in recording receptive fields of foveal retinal ganglion cells (RGCs) in vivo. In this study, we use adaptive optics scanning laser ophthalmoscopy (AOSLO) to image the calcium responses of RGCs in the living primate, with a stable, high precision visual stimulus that allowed us to localize the receptive fields of hundreds of foveal ganglion cells. This approach revealed a precisely radial organization of foveal RGCs, despite the many distortions possible during the extended developmental migration of foveal cells. By back projecting the line connecting RGC somas to their receptive fields, we have been able to define the ‘physiological center’ of the foveola, locating the vertical meridian separating left and right hemifields in vivo.

Human S-cone electroretinograms obtained by silent substitution stimulation

We used triple silent substitution stimuli to characterize human S-cone electroretinograms (ERGs) in normal trichromats. Short-wavelength-cone (S-cone) ERGs were found to have different morphological features and temporal frequency response characteristics compared to ERGs derived from L-cones, M-cones, and rod photoreceptors in normal participants. Furthermore, in two cases of retinal pathology, blue cone monochromatism (BCM) and enhanced S-cone syndrome (ESCS), S-cone ERGs elicited by our stimuli were preserved and enhanced, respectively. The results from both normal and pathological retinae demonstrate that triple silent substitution stimuli can be used to generate ERGs that provide an assay of human S-cone function.

Electrical receptive fields of retinal ganglion cells: Influence of presynaptic neurons

Implantable retinal stimulators activate surviving neurons to restore a sense of vision in people who have lost their photoreceptors through degenerative diseases. Complex spatial and temporal interactions occur in the retina during multi-electrode stimulation. Due to these complexities, most existing implants activate only a few electrodes at a time, limiting the repertoire of available stimulation patterns. Measuring the spatiotemporal interactions between electrodes and retinal cells, and incorporating them into a model may lead to improved stimulation algorithms that exploit the interactions. Here, we present a computational model that accurately predicts both the spatial and temporal nonlinear interactions of multi-electrode stimulation of rat retinal ganglion cells (RGCs). The model was verified using in vitro recordings of ON, OFF, and ON-OFF RGCs in response to subretinal multi-electrode stimulation with biphasic pulses at three stimulation frequencies (10, 20, 30 Hz). The model gives an estimate of each cell’s spatiotemporal electrical receptive fields (ERFs); i.e., the pattern of stimulation leading to excitation or suppression in the neuron. All cells had excitatory ERFs and many also had suppressive sub-regions of their ERFs. We show that the nonlinearities in observed responses arise largely from activation of presynaptic interneurons. When synaptic transmission was blocked, the number of sub-regions of the ERF was reduced, usually to a single excitatory ERF. This suggests that direct cell activation can be modeled accurately by a one-dimensional model with linear interactions between electrodes, whereas indirect stimulation due to summated presynaptic responses is nonlinear.

Implantable neural stimulation devices are being widely used and clinically tested for the restoration of lost function (e.g. cochlear implants) and the treatment of neurological disorders. Smart devices that can combine sensing and stimulation will dramatically improve future patient outcomes. To this end, mathematical models that can accurately predict neural responses to electrical stimulation will be critical for the development of smart stimulation devices. Here, we demonstrate a model that predicts neural responses to simultaneous stimulation across multiple electrodes in the retina. We show that the activation of presynaptic neurons leads to nonlinearities in the responses of postsynaptic retinal ganglion cells. The model is accurate and is applicable to a wide range of neural stimulation devices.

Measurements of neuronal color tuning: Procedures, pitfalls, and alternatives

Measuring the color tuning of visual neurons is important for understanding the neural basis of vision, but it is challenging because of the inherently three-dimensional nature of color. Color tuning cannot be represented by a one-dimensional curve, and measuring three-dimensional tuning curves is difficult. One approach to addressing this challenge is to analyze neuronal color tuning data through the lens of mathematical models that make assumptions about the shapes of tuning curves. In this paper, we discuss the linear-nonlinear cascade model as a platform for measuring neuronal color tuning. We compare fitting this model by three techniques: two using response-weighted averaging and one using numerical optimization of likelihood. We highlight the advantages and disadvantages of each technique and emphasize the effects of the stimulus distribution on color tuning measurements.

Spatiochromatic Interactions between Individual Cone Photoreceptors in the Human Retina

A remarkable feature of human vision is that the retina and brain have evolved circuitry to extract useful spatial and spectral information from signals originating in a photoreceptor mosaic with trichromatic constituents that vary widely in their relative numbers and local spatial configurations. A critical early transformation applied to cone signals is horizontal-cell-mediated lateral inhibition, which imparts a spatially antagonistic surround to individual cone receptive fields, a signature inherited by downstream neurons and implicated in color signaling. In the peripheral retina, the functional connectivity of cone inputs to the circuitry that mediates lateral inhibition is not cone-type specific, but whether these wiring schemes are maintained closer to the fovea remains unsettled, in part because central retinal anatomy is not easily amenable to direct physiological assessment. Here, we demonstrate how the precise topography of the long (L)-, middle (M)-, and short (S)-wavelength-sensitive cones in the human parafovea (1.5° eccentricity) shapes perceptual sensitivity. We used adaptive optics microstimulation to measure psychophysical detection thresholds from individual cones with spectral types that had been classified independently by absorptance imaging. Measured against chromatic adapting backgrounds, the sensitivities of L and M cones were, on average, receptor-type specific, but individual cone thresholds varied systematically with the number of preferentially activated cones in the immediate neighborhood. The spatial and spectral patterns of these interactions suggest that interneurons mediating lateral inhibition in the central retina, likely horizontal cells, establish functional connections with L and M cones indiscriminately, implying that the cone-selective circuitry supporting red-green color vision emerges after the first retinal synapse.SIGNIFICANCE STATEMENT We present evidence for spatially antagonistic interactions between individual, spectrally typed cones in the central retina of human observers using adaptive optics. Using chromatic adapting fields to modulate the relative steady-state activity of long (L)- and middle (M)-wavelength-sensitive cones, we found that single-cone detection thresholds varied predictably with the spectral demographics of the surrounding cones. The spatial scale and spectral pattern of these photoreceptor interactions were consistent with lateral inhibition mediated by retinal horizontal cells that receive nonselective input from L and M cones. These results demonstrate a clear link between the neural architecture of the visual system inputs-cone photoreceptors-and visual perception and have implications for the neural locus of the cone-specific circuitry supporting color vision.

Temporal properties of colour opponent receptive fields in the cat lateral geniculate nucleus

The primordial form of mammalian colour vision relies on opponent interactions between inputs from just two cone types, 'blue' (S-) and 'green' (ML-) cones. We recently described the spatial receptive field structure of colour opponent blue-ON cells from the lateral geniculate nucleus of cats. Functional inputs from the opponent cone types were spatially coextensive and equally weighted, supporting their high chromatic and low achromatic sensitivity. Here, we studied relative cone weights, temporal frequency tuning and visual latency of cat blue-ON cells and non-opponent achromatic cells to temporally modulated cone-isolating and achromatic stimuli. We confirmed that blue-ON cells receive equally weighted antagonistic inputs from S- and ML-cones whereas achromatic cells receive exclusive ML-cone input. The temporal frequency tuning curves of S- and ML-cone inputs to blue-ON cells were tightly correlated between 1 and 48 Hz. Optimal temporal frequencies of blue-ON cells were around 3 Hz, whereas the frequency optimum of achromatic cells was close to 10 Hz. Most blue-ON cells showed negligible response to achromatic flicker across all frequencies tested. Latency to visual stimulation was significantly greater in blue-ON than in achromatic cells. The S- and ML-cone responses of blue-ON cells had on average, similar latencies to each other. Altogether, cat blue-ON cells showed remarkable balance of opponent cone inputs. Our results also confirm similarities to primate blue-ON cells suggesting that colour vision in mammals evolved on the basis of a sluggish pathway that is optimized for chromatic sensitivity at a wide range of spatial and temporal frequencies.

Influence of stimulus size on revealing non-cardinal color mechanisms

Multiple studies have shown that performance of subjects on a number of visual tasks is worse for non-cardinal than cardinal colors, especially in the red-green/luminance (RG/LUM) and tritan/luminance (TRIT/LUM) color planes. Inspired by neurophysiological evidence that suppressive surround input to receptive fields is particularly sensitive to luminance, we hypothesized that non-cardinal mechanisms in the RG/LUM and TRIT/LUM planes would be more sensitive to stimulus size than are isoluminant non-cardinal mechanisms. In Experiment 1 we tested 9-10 color-normal subjects in each of the three color planes (RG/TRIT, RG/LUM, and TRIT/LUM) on visual search at four bull's-eye dot sizes (0.5°/1°, 1°/2°, 2°/4°, and 3°/6° center/annulus dot diameter). This study yielded a significant main effect of dot size in each of the three color planes. In Experiment 2 we tested the same hypothesis using noise masking, at three stimulus sizes (3°, 6° and 9° diameter Gabors), again in all three color planes (5 subjects per color plane). This experiment yielded, in the RG/TRIT plane, a significant main effect of stimulus size; in the RG/LUM plane, significant evidence for non-cardinal mechanisms only for the 9° stimulus; but in the TRIT/LUM plane no evidence for non-cardinal mechanisms at any stimulus size. These results suggest that non-cardinal mechanisms, particularly in the RG/LUM color plane, are more sensitive to stimulus size than are non-cardinals in the RG/TRIT plane, supporting our hypothesis.

Anatomical identification of extracellularly recorded cells in large-scale multielectrode recordings

This study combines for the first time two major approaches to understanding the function and structure of neural circuits: large-scale multielectrode recordings, and confocal imaging of labeled neurons. To achieve this end, we develop a novel approach to the central problem of anatomically identifying recorded cells, based on the electrical image: the spatiotemporal pattern of voltage deflections induced by spikes on a large-scale, high-density multielectrode array. Recordings were performed from identified ganglion cell types in the macaque retina. Anatomical images of cells in the same preparation were obtained using virally transfected fluorescent labeling or by immunolabeling after fixation. The electrical image was then used to locate recorded cell somas, axon initial segments, and axon trajectories, and these signatures were used to identify recorded cells. Comparison of anatomical and physiological measurements permitted visualization and physiological characterization of numerically dominant ganglion cell types with high efficiency in a single preparation.

Tools for probing local circuits: high-density silicon probes combined with optogenetics

To understand how function arises from the interactions between neurons, it is necessary to use methods that allow the monitoring of brain activity at the single-neuron, single-spike level and the targeted manipulation of the diverse neuron types selectively in a closed-loop manner. Large-scale recordings of neuronal spiking combined with optogenetic perturbation of identified individual neurons has emerged as a suitable method for such tasks in behaving animals. To fully exploit the potential power of these methods, multiple steps of technical innovation are needed. We highlight the current state of the art in electrophysiological recording methods, combined with optogenetics, and discuss directions for progress. In addition, we point to areas where rapid development is in progress and discuss topics where near-term improvements are possible and needed.

NeuroGrid: recording action potentials from the surface of the brain

Recording from neural networks at the resolution of action potentials is critical for understanding how information is processed in the brain. Here, we address this challenge by developing an organic material-based, ultraconformable, biocompatible and scalable neural interface array (the 'NeuroGrid') that can record both local field potentials(LFPs) and action potentials from superficial cortical neurons without penetrating the brain surface. Spikes with features of interneurons and pyramidal cells were simultaneously acquired by multiple neighboring electrodes of the NeuroGrid, allowing for the isolation of putative single neurons in rats. Spiking activity demonstrated consistent phase modulation by ongoing brain oscillations and was stable in recordings exceeding 1 week's duration. We also recorded LFP-modulated spiking activity intraoperatively in patients undergoing epilepsy surgery. The NeuroGrid constitutes an effective method for large-scale, stable recording of neuronal spikes in concert with local population synaptic activity, enhancing comprehension of neural processes across spatiotemporal scales and potentially facilitating diagnosis and therapy for brain disorders.

Short-wavelength cone-opponent retinal ganglion cells in mammals

In all of the mammalian species studied to date, the short-wavelength-sensitive (S) cones and the S-cone bipolar cells that receive their input are very similar, but the retinal ganglion cells that receive synapses from the S-cone bipolar cells appear to be quite different. Here, we review the literature on mammalian retinal ganglion cells that respond selectively to stimulation of S-cones and respond with opposite polarity to longer wavelength stimuli. There are at least three basic mechanisms to generate these color-opponent responses, including: (1) opponency is generated in the outer plexiform layer by horizontal cells and is conveyed to the ganglion cells via S-cone bipolar cells, (2) inputs from bipolar cells with different cone inputs and opposite response polarity converge directly on the ganglion cells, and (3) inputs from S-cone bipolar cells are inverted by S-cone amacrine cells. These are not mutually exclusive; some mammalian ganglion cells that respond selectively to S-cone stimulation seem to utilize at least two of them. Based on these findings, we suggest that the small bistratified ganglion cells described in primates are not the ancestral type, as proposed previously. Instead, the known types of ganglion cells in this pathway evolved from monostratified ancestral types and became bistratified in some mammalian lineages.

Temporal response properties of koniocellular (blue-on and blue-off) cells in marmoset lateral geniculate nucleus

Visual perception requires integrating signals arriving at different times from parallel visual streams. For example, signals carried on the phasic-magnocellular (MC) pathway reach the cerebral cortex pathways some tens of milliseconds before signals traveling on the tonic-parvocellular (PC) pathway. Visual latencies of cells in the koniocellular (KC) pathway have not been specifically studied in simian primates. Here we compared MC and PC cells to “blue-on” (BON) and “blue-off” (BOF) KC cells; these cells carry visual signals originating in short-wavelength-sensitive (S) cones. We made extracellular recordings in the lateral geniculate nucleus (LGN) of anesthetized marmosets. We found that BON visual latencies are 10–20 ms longer than those of PC or MC cells. A small number of recorded BOF cells ( n = 7) had latencies 10–20 ms longer than those of BON cells. Within all cell groups, latencies of foveal receptive fields (<10° eccentricity) were longer (by 3–8 ms) than latencies of peripheral receptive fields (>10°). Latencies of yellow-off inputs to BON cells lagged the blue-on inputs by up to 30 ms, but no differences in visual latency were seen on comparing marmosets expressing dichromatic (“red-green color-blind”) or trichromatic color vision phenotype. We conclude that S-cone signals leaving the LGN on KC pathways are delayed with respect to signals traveling on PC and MC pathways. Cortical circuits serving color vision must therefore integrate across delays in (red-green) chromatic signals carried by PC cells and (blue-yellow) signals carried by KC cells.

A polyaxonal amacrine cell population in the primate retina

Amacrine cells are the most diverse and least understood cell class in the retina. Polyaxonal amacrine cells (PACs) are a unique subset identified by multiple long axonal processes. To explore their functional properties, populations of PACs were identified by their distinctive radially propagating spikes in large-scale high-density multielectrode recordings of isolated macaque retina. One group of PACs exhibited stereotyped functional properties and receptive field mosaic organization similar to that of parasol ganglion cells. These PACs had receptive fields coincident with their dendritic fields, but much larger axonal fields, and slow radial spike propagation. They also exhibited ON-OFF light responses, transient response kinetics, sparse and coordinated firing during image transitions, receptive fields with antagonistic surrounds and fine spatial structure, nonlinear spatial summation, and strong homotypic neighbor electrical coupling. These findings reveal the functional organization and collective visual signaling by a distinctive, high-density amacrine cell population.

Retinal representation of the elementary visual signal

The propagation of visual signals from individual cone photoreceptors through parallel neural circuits was examined in the primate retina. Targeted stimulation of individual cones was combined with simultaneous recording from multiple retinal ganglion cells of identified types. The visual signal initiated by an individual cone produced strong responses with different kinetics in three of the four numerically dominant ganglion cell types. The magnitude and kinetics of light responses in each ganglion cell varied nonlinearly with stimulus strength but in a manner that was independent of the cone of origin after accounting for the overall input strength of each cone. Based on this property of independence, the receptive field profile of an individual ganglion cell could be well estimated from responses to stimulation of each cone individually. Together, these findings provide a quantitative account of how elementary visual inputs form the ganglion cell receptive field.

Distribution and specificity of S-cone (“blue cone”) signals in subcortical visual pathways

We review here the distribution of S-cone signals and properties of S-cone recipient receptive fields in subcortical pathways. Nearly everything we know about S-cone signals in the subcortical visual system comes from the study of visual systems in cats and primates (monkeys); in this review, we concentrate on results from macaque and marmoset monkeys. We discuss segregation of S-cone recipient (blue-on and blue-off) receptive fields in the dorsal lateral geniculate nucleus and describe their receptive field properties. We treat in some detail the question of detecting weak S-cone signals as an introduction for newcomers to the field. Finally, we briefly consider the question on how S-cone signals are distributed among nongeniculate targets.

Mixing of Chromatic and Luminance Retinal Signals in Primate Area V1

Vision emerges from activation of chromatic and achromatic retinal channels whose interaction in visual cortex is still poorly understood. To investigate this interaction, we recorded neuronal activity from retinal ganglion cells and V1 cortical cells in macaques and measured their visual responses to grating stimuli that had either luminance contrast (luminance grating), chromatic contrast (chromatic grating), or a combination of the two (compound grating). As with parvocellular or koniocellular retinal ganglion cells, some V1 cells responded mostly to the chromatic contrast of the compound grating. As with magnocellular retinal ganglion cells, other V1 cells responded mostly to the luminance contrast and generated a frequency-doubled response to equiluminant chromatic gratings. Unlike magnocellular and parvocellular retinal ganglion cells, V1 cells formed a unimodal distribution for luminance/color preference with a 2- to 4-fold bias toward luminance. V1 cells associated with positive local field potentials in deep layers showed the strongest combined responses to color and luminance and, as a population, V1 cells encoded a diverse combination of luminance/color edges that matched edge distributions of natural scenes. Taken together, these results suggest that the primary visual cortex combines magnocellular and parvocellular retinal inputs to increase cortical receptive field diversity and to optimize visual processing of our natural environment.

Receptive field properties of color opponent neurons in the cat lateral geniculate nucleus

Most nonprimate mammals possess dichromatic ("red-green color blind") color vision based on short-wavelength-sensitive (S) and medium/long-wavelength-sensitive (ML) cone photoreceptor classes. However, the neural pathways carrying signals underlying the primitive "blue-yellow" axis of color vision in nonprimate mammals are largely unexplored. Here, we have characterized a population of color opponent (blue-ON) cells in recordings from the dorsal lateral geniculate nucleus of anesthetized cats. We found five points of similarity to previous descriptions of primate blue-ON cells. First, cat blue-ON cells receive ON-type excitation from S-cones, and OFF-type excitation from ML-cones. We found no blue-OFF cells. Second, the S- and ML-cone-driven receptive field regions of cat blue-ON cells are closely matched in size, consistent with specialization for detecting color contrast. Third, the receptive field center diameter of cat blue-ON cells is approximately three times larger than the center diameter of non-color opponent receptive fields at any eccentricity. Fourth, S- and ML-cones contribute weak surround inhibition to cat blue-ON cells. These data show that blue-ON receptive fields in cats are functionally very similar to blue-ON type receptive fields previously described in macaque and marmoset monkeys. Finally, cat blue-ON cells are found in the same layers as W-cells, which are thought to be homologous to the primate koniocellular system. Based on these data, we suggest that cat blue-ON cells are part of a "blue-yellow" color opponent system that is the evolutionary homolog of the blue-ON division of the koniocellular pathway in primates.

Attaining higher resolution visual prosthetics: a review of the factors and limitations

Visual prosthetics is an expanding subfield of functional electrical stimulation which has gained increased interest recently in light of new advances in treatments and technology. These treatments and technology represent a major improvement over prior art, but are still subject to a host of limitations which are dependent on the manner in which one approaches the topic of visual prosthetics. These limitations pose new research challenges whose solutions are directly applicable to the well-being of blind individuals everywhere. In this review, we will outline and critically compare major current approaches to visual prosthetics, and in particular retinal prosthetics. Then, we will engage in an in-depth discussion of the limitations imposed by current technology, physics, and the underlying biology of the retina to highlight several of the challenges currently facing researchers.

A color-coding amacrine cell may provide a blue-off signal in a mammalian retina

Retinal amacrine cells are thought to lack chromatic or color–selective light responses and play only a minor role in color processing. We now show that a type of mammalian (Ictidomys tridecemlineatus) amacrine cell selectively carries a blue–On signal, which is received from a blue or short–wavelength sensitive (S–) cone On bipolar cell. This glycinergic inhibitory “S–cone amacrine cell” is ideally placed for driving “blue–Off” responses in downstream ganglion cells.

Mesopic rod and S-cone interactions revealed by modulation thresholds

We analyzed mesopic rod and S-cone interactions in terms of their contributions to the blue-yellow opponent pathway. Stimuli were generated using a four-primary colorimeter. Mixed rod and S-cone modulation thresholds (constant L-, M-cone excitation) were measured as a function of their phase difference. Modulation amplitude was equated using threshold units and contrast ratios. This study identified three interaction types: (1) a linear and antagonistic rod:S-cone interaction, (2) probability summation, and (3) a previously unidentified mutual nonlinear reinforcement. Linear rod:S-cone interactions occur within the blue-yellow opponent pathway. Probability summation involves signaling by different postreceptoral pathways. The origin of the nonlinear reinforcement is possibly at the photoreceptors.

A tritan Waldo would be easier to detect in the periphery than a red/green one: evidence from visual search

In a color naming task from 0° to 55° eccentricity, we found that red/green performance (n=10 subjects) declines around 40° eccentricity, 5° earlier than does tritan performance (main effect of color, p=0.009; eccentricity, p<0.001; interaction, p=0.005). In a feature visual search task (e.g., red target dot among green distractor dots; twelve 2.5° diameter dots; 0, 20, and 45° eccentricity; 12 subjects), performance was significantly more impaired for red/green than for tritan stimuli, especially in the periphery (main effect of color, p=0.007; eccentricity, p<0.001; interaction, p=0.003). This effect occurred even following a rod bleach. Our results are consistent with influences from both the retina (especially random rather than selective peripheral cone input to midget ganglion cells for red/green perception, and selective cone input to small bistratified cells for tritan perception) and the cortex (differential cortical magnification across the two chromatic axes).

Functional connectivity in the retina at the resolution of photoreceptors

To understand a neural circuit requires knowing its connectivity. This paper reports measurements of functional connectivity between the input and ouput layers of the retina at single cell resolution and its implications for color vision. Multi-electrode technology was employed to record simultaneously from complete populations of the retinal ganglion cell types (midget, parasol, small bistratified) that transmit high-resolution visual signals to the brain. Fine-grained visual stimulation was used to identify the location, type and strength of the functional input of each cone photoreceptor to each ganglion cell. The populations of ON and OFF midget and parasol cells each sampled the complete population of long and middle wavelength sensitive cones. However, only OFF midget cells frequently received strong input from short wavelength sensitive cones. ON and OFF midget cells exhibited a small non-random tendency to selectively sample from either long or middle wavelength sensitive cones, to a degree not explained by clumping in the cone mosaic. These measurements reveal computations in a neural circuit at the elementary resolution of individual neurons.

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.

A framework for simulating and estimating the state and functional topology of complex dynamic geometric networks

We introduce a framework for simulating signal propagation in geometric networks (networks that can be mapped to geometric graphs in some space) and developing algorithms that estimate (i.e., map) the state and functional topology of complex dynamic geometric networks. Within the framework, we define the key features typically present in such networks and of particular relevance to biological cellular neural networks: dynamics, signaling, observation, and control. The framework is particularly well suited for estimating functional connectivity in cellular neural networks from experimentally observable data and has been implemented using graphics processing unit high-performance computing. Computationally, the framework can simulate cellular network signaling close to or faster than real time. We further propose a standard test set of networks to measure performance and compare different mapping algorithms.

High-sensitivity rod photoreceptor input to the blue-yellow color opponent pathway in macaque retina

Small bistratified cells (SBCs) in the primate retina carry a major blue-yellow opponent signal to the brain. Here we show that SBCs also carry signals from rod photoreceptors, with the same sign as S cone input. SBCs exhibited robust responses under low scotopic conditions (<0.01 P*/rod/s). Physiological and anatomical experiments indicated that this rod input arose from the AII amacrine cell mediated rod pathway. Rod and cone signals were both present in SBCs at mesopic light levels. We discuss three implications of these findings. First, more retinal circuits than previously thought may multiplex rod and cone signals, efficiently exploiting the limited number of optic nerve fibers. Second, signals from AII amacrine cells may diverge to most or all of the <20 RGC types in the peripheral primate retina. Third, rod input to SBCs may be the substrate for behavioral biases toward perception of blue at mesopic light levels.

Neuroligin 2 controls the maturation of GABAergic synapses and information processing in the retina

In the present study, we investigated the role of Neuroligin 2 (NL2) in synaptic transmission and network function using the mouse retina as a model circuit. We show that NL2 is preferentially located at GABAergic rather than glycinergic or glutamatergic postsynapses. The absence of NL2 from the retina resulted in a severe reduction of GABA(A) receptor clustering, and in subtle alterations of the retinal circuitry. Light processing was impaired accordingly, and retinal ganglion cells, the output neurons of the retina, showed increased basal activity and altered coding of visual information. Together, our data indicate that NL2 is essential for the functional integrity of GABAergic signaling and as a consequence, for information processing in the retina.

Color vision, cones, and color-coding in the cortex

Color processing begins with the absorption of light by cone photoreceptors, and progresses through a series of hierarchical stages: Retinal signals carrying color information are transmitted through the lateral geniculate nucleus of the thalamus (LGN) up to the primary visual cortex (V1). From V1, the signals are processed by the second visual area (V2); then by cells located in subcompartments ("globs") within the posterior inferior temporal (PIT) cortex, a brain region that encompasses area V4 and brain regions immediately anterior to V4. Color signals are then processed by regions deep within the inferior temporal (IT) cortex including area TE. As a heuristic, one can consider each of these stages to be involved in constructing a distinct aspect of the color percept. The three cone types are the basis for trichromacy; retinal ganglion cells that respond in an opponent fashion to activation of different cone classes are the basis for color opponency (these "cone-opponent" cells increase their firing rate above baseline to activation of one cone class and decrease their firing rate below baseline to activation of a different cone class); double-opponent neurons in the V1 generate local color contrast and are the building blocks for color constancy; glob cells elaborate the perception of hue; and IT integrates color perception in the context of behavior. Finally, though nothing is known, these signals presumably interface with motor programs and emotional centers of the brain to mediate the widely acknowledged emotional salience of color.

Transmission of blue (S) cone signals through the primate lateral geniculate nucleus

This study concerns the transmission of short-wavelength-sensitive (S) cone signals through the primate dorsal lateral geniculate nucleus. The principal cell classes, magnocellular (MC) and parvocellular (PC), are traditionally segregated into on- and off-subtypes on the basis of the sign of their response to luminance variation. Cells dominated by input from S-cones ('blue-on and blue-off') are less frequently encountered and their properties are less well understood. Here we characterize the spatial and chromatic properties of a large sample of blue-on and blue-off neurons and contrast them with those of PC and MC neurons. The results confirm that blue-on and blue-off cells have larger receptive fields than PC and MC neurons at equivalent eccentricities. Relative to blue-on cells, blue-off cells are less sensitive to S-cone contrast, have larger receptive fields, and show more low-pass spatial frequency tuning. Thus, blue-on and blue-off neurons lack the functional symmetry characteristic of on- and off-subtypes in the MC and PC pathways. The majority of MC and PC cells received no detectible input from S-cones. Where present, input from S-cones tended to provide weak inhibition to PC cells. All cell types showed evidence of a suppressive extra-classical receptive field driven largely or exclusively by ML-cones. These data indicate that S-cone signals are isolated to supply the classical receptive field mechanisms of blue-on and blue-off cells in the LGN, and that the low spatial precision of S-cone vision has origins in both classical and extraclassical receptive field properties of subcortical pathways.

Functional asymmetries in visual pathways carrying S-cone signals in macaque

In the lateral geniculate nucleus of macaque, we recorded from neurons with substantial input from S-cones and found that, on several important dimensions, the properties of neurons that receive inhibitory input from S-cones ("S-") are quite unlike those of neurons that receive excitatory input from S-cones ("S+"). First, the organization of chromatic inputs differs substantially: in S+ cells, S-cone signals were usually opposed by those of L- and M-cones; in S- cells, signals from L-cones were usually opposed to those of S- and M-cones. Second, to pure S-cone modulation, S+ cells are twice as sensitive as S- cells, but S- cells were much more susceptible to contrast adaptation. Third, in S- cells but not S+ cells, the spatial frequency resolution for achromatic modulation was often greater, the tuning curve and more bandpass, than that for S-cone modulation. Along the dimensions on which we measured, the properties of the S+ cells were relatively tightly clustered, suggesting a homogenous class. Although the chromatic properties of S- cells are heterogeneous, the distribution of their tuning along other stimulus dimensions does not suggest multiple subtypes.