Orientation sensitivity of ganglion cells in primate retina

Peripheral Choroidal Response to Localized Defocus Blur: Influence of Native Peripheral Aberrations

Purpose

This study aims to examine the short-term peripheral choroidal thickness (PChT) response to signed defocus blur, both with and without native peripheral aberrations. This examination will provide insights into the role of peripheral aberration in detecting signs of defocus.

Methods

The peripheral retina (temporal 15°) of the right eye was exposed to a localized video stimulus in 11 young adults. An adaptive optics system induced 2D myopic or hyperopic defocus onto the stimulus, with or without correcting native peripheral ocular aberrations (adaptive optics [AO] or NoAO defocus conditions). Choroidal scans were captured using Heidelberg Spectralis OCT at baseline, exposure (10, 20, and 30 minutes), and recovery phases (4, 8, and 15 minutes). Neural network-based automated MATLAB segmentation program measured PChT changes from OCT scans, and statistical analysis evaluated the effects of different optical conditions over time.

Results

During the exposure phase, NoAO myopic and hyperopic defocus conditions exhibited distinct bidirectional PChT alterations, showing average thickening (10.0 ± 5.3 µm) and thinning (-9.1 ± 5.5 µm), respectively. In contrast, induced AO defocus conditions did not demonstrate a significant change from baseline. PChT recovery to baseline occurred for all conditions. The unexposed fovea did not show any significant ChT change, indicating a localized ChT response to retinal blur.

Conclusions

We discovered that the PChT response serves as a marker for detecting peripheral retinal myopic and hyperopic defocus blur, especially in the presence of peripheral aberrations. These findings highlight the significant role of peripheral oriented blur in cueing peripheral defocus sign detection.

Bounded contribution of human early visual cortex to the topographic anisotropy in spatial extent perception

To interact successfully with objects, it is crucial to accurately perceive their spatial extent, an enclosed region they occupy in space. Although the topographic representation of space in the early visual cortex (EVC) has been favored as a neural correlate of spatial extent perception, its exact nature and contribution to perception remain unclear. Here, we inspect the topographic representations of human individuals' EVC and perception in terms of how much their anisotropy is influenced by the orientation (co-axiality) and radial position (radiality) of stimuli. We report that while the anisotropy is influenced by both factors, its direction is primarily determined by radiality in EVC but by co-axiality in perception. Despite this mismatch, the individual differences in both radial and co-axial anisotropy are substantially shared between EVC and perception. Our findings suggest that spatial extent perception builds on EVC's spatial representation but requires an additional mechanism to transform its topographic bias.

Electronic Retinal Prostheses

Retinal prostheses are a promising means for restoring sight to patients blinded by photoreceptor atrophy. They introduce visual information by electrical stimulation of the surviving inner retinal neurons. Subretinal implants target the graded-response secondary neurons, primarily the bipolar cells, which then transfer the information to the ganglion cells via the retinal neural network. Therefore, many features of natural retinal signal processing can be preserved in this approach if the inner retinal network is retained. Epiretinal implants stimulate primarily the ganglion cells, and hence should encode the visual information in spiking patterns, which, ideally, should match the target cell types. Currently, subretinal arrays are being developed primarily for restoration of central vision in patients impaired by age-related macular degeneration (AMD), while epiretinal implants-for patients blinded by retinitis pigmentosa, where the inner retina is less preserved. This review describes the concepts and technologies, preclinical characterization of prosthetic vision and clinical outcomes, and provides a glimpse into future developments.

High-Fidelity Reproduction of Visual Signals by Electrical Stimulation in the Central Primate Retina

Electrical stimulation of retinal ganglion cells (RGCs) with electronic implants provides rudimentary artificial vision to people blinded by retinal degeneration. However, current devices stimulate indiscriminately and therefore cannot reproduce the intricate neural code of the retina. Recent work has demonstrated more precise activation of RGCs using focal electrical stimulation with multielectrode arrays in the peripheral macaque retina, but it is unclear how effective this can be in the central retina, which is required for high-resolution vision. This work probes the neural code and effectiveness of focal epiretinal stimulation in the central macaque retina, using large-scale electrical recording and stimulation ex vivo The functional organization, light response properties, and electrical properties of the major RGC types in the central retina were mostly similar to the peripheral retina, with some notable differences in density, kinetics, linearity, spiking statistics, and correlations. The major RGC types could be distinguished by their intrinsic electrical properties. Electrical stimulation targeting parasol cells revealed similar activation thresholds and reduced axon bundle activation in the central retina, but lower stimulation selectivity. Quantitative evaluation of the potential for image reconstruction from electrically evoked parasol cell signals revealed higher overall expected image quality in the central retina. An exploration of inadvertent midget cell activation suggested that it could contribute high spatial frequency noise to the visual signal carried by parasol cells. These results support the possibility of reproducing high-acuity visual signals in the central retina with an epiretinal implant.SIGNIFICANCE STATEMENT Artificial restoration of vision with retinal implants is a major treatment for blindness. However, present-day implants do not provide high-resolution visual perception, in part because they do not reproduce the natural neural code of the retina. Here, we demonstrate the level of visual signal reproduction that is possible with a future implant by examining how accurately responses to electrical stimulation of parasol retinal ganglion cells can convey visual signals. Although the precision of electrical stimulation in the central retina was diminished relative to the peripheral retina, the quality of expected visual signal reconstruction in parasol cells was greater. These findings suggest that visual signals could be restored with high fidelity in the central retina using a future retinal implant.

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.

Bi-directional Refractive Compensation for With-the-Rule and Against-the-Rule Astigmatism in Young Adults

Purpose

The purpose of this study was to investigate the short-term effect of imposing astigmatism on the refractive states of young adults.

Methods

Nineteen visually healthy low-astigmatic young adults (age = 20.94 ± 0.37 years; spherical-equivalent errors [M] = -1.47 ± 0.23 diopters [D]; cylindrical errors = -0.32 ± 0.05 D) were recruited. They were asked to wear a trial frame with treated and control lenses while watching a video for an hour. In three separate visits, the treated eye was exposed to one of three defocused conditions in random sequence: (1) with-the-rule (WTR) astigmatism = +3.00 DC × 180 degrees; (2) against-the-rule (ATR) astigmatism = +3.00 DC × 90 degrees; and (3) spherical defocus (SPH) = +3.00 DS. The control eye was fully corrected optically. Before and after watching the video, non-cycloplegic autorefraction was performed over the trial lenses. Refractive errors were decomposed into M, J0, and J45 astigmatism. Interocular differences in refractions (treated eye - control eye) were analyzed.

Results

After participants watched the video with monocular astigmatic defocus for an hour, the magnitude of the J0 astigmatism was significantly reduced by 0.25 ± 0.10 D in both WTR (from +1.53 ± 0.07 D to +1.28 ± 0.09 D) and 0.39 ± 0.15 D in ATR conditions (from -1.33 ± 0.06 D to -0.94 ± 0.18 D), suggesting an active compensation. In contrast, changes in J0 astigmatism were not significant in the SPH condition. No compensatory changes in J45 astigmatism or M were found under any conditions.

Conclusions

Watching a video for an hour with astigmatic defocus induced bidirectional, compensatory changes in astigmatic components, suggesting that refractive components of young adults are moldable to compensate for orientation-specific astigmatic blur over a short period.

Mechanism underpinning the sharpening of orientation and spatial frequency selectivities in the tree shrew (Tupaia belangeri) primary visual cortex

Most neurons in the primary visual cortex (V1) of mammals show sharp orientation selectivity and band-pass spatial frequency tuning. Here, we examine whether sharpening of the broad tuning that exists subcortically, namely in the retina and the lateral geniculate nucleus (LGN), underlie the sharper tuning seen for both the above features in tree shrew V1. Since the transition from poor feature selectivity to sharp tuning occurs entirely within V1 in tree shrews, we examined the orientation selectivity and spatial frequency tuning of neurons within individual electrode penetrations. We found that most layer 4 and layer 2/3 neurons in the same cortical column preferred the same stimulus orientation. However, a subset of layer 3c neurons close to the layer 4 border preferred near orthogonal orientations, suggesting that layer 2/3 neurons may inherit the orientation preferences of their layer 4 input neurons and also receive cross-orientation inhibition from layer 3c neurons. We also found that layer 4 neurons showed sharper orientation selectivity at higher spatial frequencies, suggesting that attenuation of low spatial frequency responses by spatially broad inhibition acting on layer 4 inputs to layer 2/3 neurons can enhance both orientation and spatial frequency selectivities. However, in a proportion of layer 2/3 neurons, the sharper tuning of layer 2/3 neurons appeared to arise also or even mainly from inhibition specific to high spatial frequencies acting on the layer 4 inputs to layer 2/3. Overall, our results are consistent with the suggestion that in tree shrews, sharp feature selectivity in layer 2/3 can be established by intracortical mechanisms that sharpen biases observed in layer 4, which are in turn inherited presumably from thalamic afferents.

Retinal Responses to Simulated Optical Blur Using a Novel Dead Leaves ERG Stimulus

Purpose

The purpose of this study was to evaluate retinal responses to different types and magnitudes of simulated optical blur presented at specific retinal eccentricities using naturalistic images.

Methods

Electroretinograms (ERGs) were recorded from 27 adults using 30-degree dead leaves naturalistic images, digitally blurred with one of three types of optical blur (defocus, astigmatism, and spherical aberrations), and one of three magnitudes (0.1, 0.3, or 0.5 µm) of blur. Digitally computed blur was applied to the entire image, or on an area outside the central 6 degrees or 12 degrees of retinal eccentricity.

Results

ERGs were significantly affected by blur type, magnitude, and retinal eccentricity. ERGs were differentially affected by defocus and spherical aberrations; however, astigmatism had no effect on the ERGs. When blur was applied only beyond the central 12 degrees eccentricity, the ERGs were unaffected. However, when blur was applied outside the central 6 degrees, the ERG responses were significantly reduced and were no different from the ERGs recorded with entirely blurred images.

Conclusions

Blur type, magnitude, and location all affect the retinal responses. Our data indicate that the retinal area between 6 and 12 degrees eccentricity has the largest effect on the retinal responses to blur. In addition, certain optical blur types appear to have a more detrimental effect on the ERGs than others. These results cannot be solely explained by changes to image contrast and spatial frequency content, suggesting that retinal neurons might be sensitive to spatial cues in order to differentiate between different blur types.

Astigmatic Defocus Leads to Short-Term Changes in Human Choroidal Thickness

Purpose

To examine the choroidal thickness (ChT) response to short-term with-the-rule (WTR) and against-the-rule (ATR) simple myopic astigmatic defocus, with the response to spherical myopic defocus and clear vision used as control conditions.

Methods

The left eye of 18 healthy adults aged 28 ± 6 years was exposed to clear vision, +3 D spherical myopic defocus, +3 D × 180 WTR, or +3 D × 90 ATR astigmatic defocus for 60 minutes, over four randomly ordered visits, while their right eye was optimally corrected. The macular ChT was measured with optical coherence tomography along the vertical and horizontal meridians before and after 20, 40, and 60 minutes of defocus.

Results

After 60 minutes of defocus, ChT increased by +8 ± 5 µm (P < 0.001) with spherical myopic defocus, but varied with simple myopic astigmatic defocus, depending on the axis of astigmatism (P < 0.001), increasing by +5 ± 6 µm (P = 0.037) with WTR and decreasing by −4 ± 5 µm (P = 0.011) with ATR astigmatic defocus. These changes were similar across the vertical and horizontal meridians (P = 0.22). The ChT changes were greater than the change during the clear vision control condition (−1 ± 4 µm) for WTR (+5 ± 5 µm, P = 0.002) but not ATR (−4 ± 6 µm, P = 0.09) astigmatic defocus.

Conclusions

These results provide insights into the human ChT response to short-term astigmatic defocus and highlight a potential difference in the myopiagenic signal associated with the orientation of astigmatic blur.

Diversity of Feature Selectivity in Macaque Visual Cortex Arising from a Limited Number of Broadly Tuned Input Channels

Spike (action potential) responses of most primary visual cortical cells in the macaque are sharply tuned for the orientation of a line or an edge, and neurons preferring similar orientations are clustered together in cortical columns. The preferred stimulus orientation of these columns span the full range of orientations, as observed in recordings of spikes and in classical optical imaging of intrinsic signals. However, when we imaged the putative thalamic input to striate cortical cells that can be seen in imaging of intrinsic signals when they are analyzed on a larger spatial scale, we found that the orientation domain map of the primary visual cortex did not show the same diversity of orientations. This map was dominated by just the one orientation that is most commonly preferred by neurons in the retina and the lateral geniculate nucleus. This supports cortical feature selectivity and columnar architecture being built upon feed-forward signals transmitted from the thalamus in a very limited number of broadly tuned input channels.

2-D Peripheral image quality metrics with different types of multifocal contact lenses

To evaluate the impact of multifocal contact lens wear on the image quality metrics across the visual field in the context of eye growth and myopia control. Two-dimensional cross-correlation coefficients were estimated by comparing a reference image against the computed retinal images for every location. Retinal images were simulated based on the measured optical aberrations of the naked eye and a set of multifocal contact lenses (centre-near and centre-distance designs), and images were spatially filtered to match the resolution limit at each eccentricity. Value maps showing the reduction in the quality of the image through each optical condition were obtained by subtracting the optical image quality from the theoretical physiological limits. Results indicate that multifocal contact lenses degrade the image quality independently from their optical design, though this result depends on the type of analysis conducted. Analysis of the image quality across the visual field should not be oversimplified to a single number but split into regional and groups because it provides more insightful information and can avoid misinterpretation of the results. The decay of the image quality caused by the multifocal contacts alone, cannot explain the translation of peripheral defocus towards protection on myopia progression, and a different explanation needs to be found.

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.

Linkage between retinal ganglion cell density and the nonuniform spatial integration across the visual field

The integration of visual information over space is critical to human pattern vision. For either luminance detection or object recognition, the position of the target in the visual field governs the size of a window within which visual information is integrated. Here we analyze the relationship between the topographic distribution of ganglion cell density and the nonuniform spatial integration across the visual field. We find that the variation in the retinal ganglion cell (RGC) density across the human retina is closely matched to the variation in the extent of spatial integration. Our study suggests that a fixed number of RGCs subserves spatial integration of visual input, independent of the visual-field location.

The ability to integrate visual information over space is a fundamental component of human pattern vision. Regardless of whether it is for detecting luminance contrast or for recognizing objects in a cluttered scene, the position of the target in the visual field governs the size of a window within which visual information is integrated. Here we analyze the relationship between the topographic distribution of ganglion cell density and the nonuniform spatial integration across the visual field. The extent of spatial integration for luminance detection (Ricco’s area) and object recognition (crowding zone) are measured at various target locations. The number of retinal ganglion cells (RGCs) underlying Ricco’s area or crowding zone is estimated by computing the product of Ricco’s area (or crowding zone) and RGC density for a given target location. We find a quantitative agreement between the behavioral data and the RGC density: The variation in the sampling density of RGCs across the human retina is closely matched to the variation in the extent of spatial integration required for either luminance detection or object recognition. Our empirical data combined with the simulation results of computational models suggest that a fixed number of RGCs subserves spatial integration of visual input, independent of the visual-field location.

Eccentricity dependence of orientation anisotropy of surround suppression of contrast-detection threshold

Both neurophysiological and psychophysical data provide evidence for orientation biases in nonfoveal vision-specifically, a tendency for a Cartesian horizontal and vertical bias close to fixation, changing to a radial bias with increasing retinal eccentricity. We explore whether the strength of surround suppression of contrast detection also depends on retinotopic location and relative surround configuration (horizontal, vertical, radial, tangential) in parafoveal vision. Three visual-field locations were tested (0°, 225°, and 270°, angle increasing anticlockwise from 0° horizontal axis) at viewing eccentricities of 6° and 15°. Contrast-detection threshold was estimated with and without a surrounding annulus. At 6° eccentricity, horizontally oriented parallel center-surround (C-S) configurations resulted in greater surround suppression compared to vertically oriented parallel center-surround configurations (p = 0.001). At 15° eccentricity, radially oriented parallel center-surround stimuli conferred greater suppression than tangentially oriented stimuli (p = 0.027). Parallel surrounds resulted in greater suppression than orthogonal surrounds at both eccentricities (p < 0.05). At 6° the horizontal center was more susceptible to suppression than a vertical center (p < 0.001) for both parallel and orthogonal surrounds, while at 15° a radial center was more susceptible to suppression (relative to a tangential center), but only if the surround was parallel (p = 0.005). Our data show that orientation anisotropy of surround suppression alters with eccentricity, reflecting a link between suppression strength and visual-field retinotopy.

Orientation-Selective Retinal Circuits in Vertebrates

Visual information is already processed in the retina before it is transmitted to higher visual centers in the brain. This includes the extraction of salient features from visual scenes, such as motion directionality or contrast, through neurons belonging to distinct neural circuits. Some retinal neurons are tuned to the orientation of elongated visual stimuli. Such ‘orientation-selective’ neurons are present in the retinae of most, if not all, vertebrate species analyzed to date, with species-specific differences in frequency and degree of tuning. In some cases, orientation-selective neurons have very stereotyped functional and morphological properties suggesting that they represent distinct cell types. In this review, we describe the retinal cell types underlying orientation selectivity found in various vertebrate species, and highlight their commonalities and differences. In addition, we discuss recent studies that revealed the cellular, synaptic and circuit mechanisms at the basis of retinal orientation selectivity. Finally, we outline the significance of these findings in shaping our current understanding of how this fundamental neural computation is implemented in the visual systems of vertebrates.

Functional characterization and spatial clustering of visual cortical neurons in the predatory grasshopper mouse Onychomys arenicola

Mammalian neocortical circuits are functionally organized such that the selectivity of individual neurons systematically shifts across the cortical surface, forming a continuous map. Maps of the sensory space exist in cortex, such as retinotopic maps in the visual system or tonotopic maps in the auditory system, but other functional response properties also may be similarly organized. For example, many carnivores and primates possess a map for orientation selectivity in primary visual cortex (V1), whereas mice, rabbits, and the gray squirrel lack orientation maps. In this report we show that a carnivorous rodent with predatory behaviors, the grasshopper mouse (Onychomys arenicola), lacks a canonical columnar organization of orientation preference in V1; however, neighboring neurons within 50 μm exhibit related tuning preference. Using a combination of two-photon microscopy and extracellular electrophysiology, we demonstrate that the functional organization of visual cortical neurons in the grasshopper mouse is largely the same as in the C57/BL6 laboratory mouse. We also find similarity in the selectivity for stimulus orientation, direction, and spatial frequency. Our results suggest that the properties of V1 neurons across rodent species are largely conserved.NEW & NOTEWORTHY Carnivores and primates possess a map for orientation selectivity in primary visual cortex (V1), whereas rodents and lagomorphs lack this organization. We examine, for the first time, V1 of a wild carnivorous rodent with predatory behaviors, the grasshopper mouse (Onychomys arenicola). We demonstrate the cellular organization of V1 in the grasshopper mouse is largely the same as the C57/BL6 laboratory mouse, suggesting that V1 neuron properties across rodent species are largely conserved.

Oculomotor inhibition covaries with conscious detection

Saccadic eye movements occur frequently even during attempted fixation, but they halt momentarily when a new stimulus appears. Here, we demonstrate that this rapid, involuntary "oculomotor freezing" reflex is yoked to fluctuations in explicit visual perception. Human observers reported the presence or absence of a brief visual stimulus while we recorded microsaccades, small spontaneous eye movements. We found that microsaccades were reflexively inhibited if and only if the observer reported seeing the stimulus, even when none was present. By applying a novel Bayesian classification technique to patterns of microsaccades on individual trials, we were able to decode the reported state of perception more accurately than the state of the stimulus (present vs. absent). Moreover, explicit perceptual sensitivity and the oculomotor reflex were both susceptible to orientation-specific adaptation. The adaptation effects suggest that the freezing reflex is mediated by signals processed in the visual cortex before reaching oculomotor control centers rather than relying on a direct subcortical route, as some previous research has suggested. We conclude that the reflexive inhibition of microsaccades immediately and inadvertently reveals when the observer becomes aware of a change in the environment. By providing an objective measure of conscious perceptual detection that does not require explicit reports, this finding opens doors to clinical applications and further investigations of perceptual awareness.

Electronic approaches to restoration of sight

Retinal prostheses are a promising means for restoring sight to patients blinded by the gradual atrophy of photoreceptors due to retinal degeneration. They are designed to reintroduce information into the visual system by electrically stimulating surviving neurons in the retina. This review outlines the concepts and technologies behind two major approaches to retinal prosthetics: epiretinal and subretinal. We describe how the visual system responds to electrical stimulation. We highlight major differences between direct encoding of the retinal output with epiretinal stimulation, and network-mediated response with subretinal stimulation. We summarize results of pre-clinical evaluation of prosthetic visual functions in- and ex vivo, as well as the outcomes of current clinical trials of various retinal implants. We also briefly review alternative, non-electronic, approaches to restoration of sight to the blind, and conclude by suggesting some perspectives for future advancement in the field.

Cardinal Orientation Selectivity Is Represented by Two Distinct Ganglion Cell Types in Mouse Retina

Orientation selectivity (OS) is a prominent and well studied feature of early visual processing in mammals, but recent work has highlighted the possibility that parallel OS circuits might exist in multiple brain locations. Although both classic and modern work has identified an OS mechanism in selective wiring from lateral geniculate nucleus (LGN) to primary visual cortex, OS responses have now been found upstream of cortex in mouse LGN and superior colliculus, suggesting a possible origin in the retina. Indeed, retinal OS responses have been reported for decades in rabbit and more recently in mouse. However, we still know very little about the properties and mechanisms of retinal OS in the mouse, including whether there is a distinct OS ganglion cell type, which orientations are represented, and what are the synaptic mechanisms of retinal OS. We have identified two novel types of OS ganglion cells in the mouse retina that are highly selective for horizontal and vertical cardinal orientations. Reconstructions of the dendritic trees of these OS ganglion cells and measurements of their synaptic conductances offer insights into the mechanism of the OS computation at the earliest stage of the visual system.

SIGNIFICANCE STATEMENT

Orientation selectivity (OS) is one of the most well studied computations in the brain and has become a prominent model system in various areas of sensory neuroscience. Although the cortical mechanism of OS suggested by Hubel and Wiesel (1962) has been investigated intensely, other OS cells exist upstream of cortex as early as the retina and the mechanisms of OS in subcortical regions are much less well understood. We identified two ON retinal ganglion cells (RGCs) in mouse that compute OS along the horizontal (nasal-temporal) and vertical (dorsoventral) axes of visual space. We show the relationship between dendritic morphology and OS for each RGC type and reveal new synaptic mechanisms of OS computation in the retina.

Synaptic Mechanisms Generating Orientation Selectivity in the ON Pathway of the Rabbit Retina

Neurons that signal the orientation of edges within the visual field have been widely studied in primary visual cortex. Much less is known about the mechanisms of orientation selectivity that arise earlier in the visual stream. Here we examine the synaptic and morphological properties of a subtype of orientation-selective ganglion cell in the rabbit retina. The receptive field has an excitatory ON center, flanked by excitatory OFF regions, a structure similar to simple cell receptive fields in primary visual cortex. Examination of the light-evoked postsynaptic currents in these ON-type orientation-selective ganglion cells (ON-OSGCs) reveals that synaptic input is mediated almost exclusively through the ON pathway. Orientation selectivity is generated by larger excitation for preferred relative to orthogonal stimuli, and conversely larger inhibition for orthogonal relative to preferred stimuli. Excitatory orientation selectivity arises in part from the morphology of the dendritic arbors. Blocking GABAA receptors reduces orientation selectivity of the inhibitory synaptic inputs and the spiking responses. Negative contrast stimuli in the flanking regions produce orientation-selective excitation in part by disinhibition of a tonic NMDA receptor-mediated input arising from ON bipolar cells. Comparison with earlier studies of OFF-type OSGCs indicates that diverse synaptic circuits have evolved in the retina to detect the orientation of edges in the visual input.

SIGNIFICANCE STATEMENT

A core goal for visual neuroscientists is to understand how neural circuits at each stage of the visual system extract and encode features from the visual scene. This study documents a novel type of orientation-selective ganglion cell in the retina and shows that the receptive field structure is remarkably similar to that of simple cells in primary visual cortex. However, the data indicate that, unlike in the cortex, orientation selectivity in the retina depends on the activity of inhibitory interneurons. The results further reveal the physiological basis for feature detection in the visual system, elucidate the synaptic mechanisms that generate orientation selectivity at an early stage of visual processing, and illustrate a novel role for NMDA receptors in retinal processing.

Neural Mechanisms Generating Orientation Selectivity in the Retina

The orientation of visual stimuli is a salient feature of visual scenes. In vertebrates, the first neural processing steps generating orientation selectivity take place in the retina. Here, we dissect an orientation-selective circuit in the larval zebrafish retina and describe its underlying synaptic, cellular, and molecular mechanisms. We genetically identify a class of amacrine cells (ACs) with elongated dendritic arbors that show orientation tuning. Both selective optogenetic ablation of ACs marked by the cell-adhesion molecule Teneurin-3 (Tenm3) and pharmacological interference with their function demonstrate that these cells are critical components for orientation selectivity in retinal ganglion cells (RGCs) by being a source of tuned GABAergic inhibition. Moreover, our morphological analyses reveal that Tenm3⁺ ACs and orientation-selective RGCs co-stratify their dendrites in the inner plexiform layer, and that Tenm3⁺ ACs require Tenm3 to acquire their correct dendritic stratification. Finally, we show that orientation tuning is present also among bipolar cell presynaptic terminals. Our results define a neural circuit underlying orientation selectivity in the vertebrate retina and characterize cellular and molecular requirements for its assembly.

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We identify Tenm3⁺ ACs with elongated dendritic arbors showing orientation tuning

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Tenm3⁺ AC GABAergic inhibition is crucial for orientation-selective RGC tuning

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Orientation tuning is present also among some bipolar cell presynaptic terminals

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We propose a model of how orientation selectivity is generated in ganglion cells

Contributions of Retinal Ganglion Cells to Subcortical Visual Processing and Behaviors

Every aspect of visual perception and behavior is built from the neural activity of retinal ganglion cells (RGCs), the output neurons of the eye. Here, we review progress toward understanding the many types of RGCs that communicate visual signals to the brain, along with the subcortical brain regions that use those signals to build and respond to representations of the outside world. We emphasize recent progress in the use of mouse genetics, viral circuit tracing, and behavioral psychophysics to define and map the various RGCs and their associated networks. We also address questions about the homology of RGC types in mice and other species including nonhuman primates and humans. Finally, we propose a framework for understanding RGC typology and for highlighting the relationship between RGC type-specific circuitry and the processing stations in the brain that support and give rise to the perception of sight.

Origins of feature selectivities and maps in the mammalian primary visual cortex

A common feature of the mammalian striate cortex is the arrangement of 'orientation domains' containing neurons preferring similar stimulus orientations. They are arranged as spokes of a pinwheel that converge at singularities known as 'pinwheel centers'. We propose that a cortical network of feedforward and intracortical lateral connections elaborates a full set of optimum orientations from geniculate inputs that show a bias to stimulus orientation and form a set of two or a small number of 'Cartesian' coordinates. Because each geniculate afferent carries signals only from one eye and its receptive field (RF) is either ON or OFF center, the network constructs also ocular dominance columns and a quasi-segregation of ON and OFF responses across the horizontal extent of the striate cortex.

Ambient illumination switches contrast preference of specific retinal processing streams

Contrast, a fundamental feature of visual scenes, is encoded in a distributed manner by ∼ 20 retinal ganglion cell (RGC) types, which stream visual information to the brain. RGC types respond preferentially to positive (ON(pref)) or negative (OFF(pref)) contrast and differ in their sensitivity to preferred contrast and responsiveness to nonpreferred stimuli. Vision operates over an enormous range of mean light levels. The influence of ambient illumination on contrast encoding across RGC types is not well understood. Here, we used large-scale multielectrode array recordings to characterize responses of mouse RGCs under lighting conditions spanning five orders in brightness magnitude. We identify three functional RGC types that switch contrast preference in a luminance-dependent manner (Sw1-, Sw2-, and Sw3-RGCs). As ambient illumination increases, Sw1- and Sw2-RGCs shift from ON(pref) to OFF(pref) and Sw3-RGCs from OFF(pref) to ON(pref). In all cases, transitions in contrast preference are reversible and track light levels. By mapping spatiotemporal receptive fields at different mean light levels, we find that changes in input from ON and OFF pathways in receptive field centers underlie shifts in contrast preference. Sw2-RGCs exhibit direction-selective responses to motion stimuli. Despite changing contrast preference, direction selectivity of Sw2-RGCs and other RGCs as well as orientation-selective responses of RGCs remain stable across light levels.

Subcortical orientation biases explain orientation selectivity of visual cortical cells

The primary visual cortex of carnivores and primates shows an orderly progression of domains of neurons that are selective to a particular orientation of visual stimuli such as bars and gratings. We recorded from single-thalamic afferent fibers that terminate in these domains to address the issue whether the orientation sensitivity of these fibers could form the basis of the remarkable orientation selectivity exhibited by most cortical cells. We first performed optical imaging of intrinsic signals to obtain a map of orientation domains on the dorsal aspect of the anaesthetized cat's area 17. After confirming using electrophysiological recordings the orientation preferences of single neurons within one or two domains in each animal, we pharmacologically silenced the cortex to leave only the afferent terminals active. The inactivation of cortical neurons was achieved by the superfusion of either kainic acid or muscimol. Responses of single geniculate afferents were then recorded by the use of high impedance electrodes. We found that the orientation preferences of the afferents matched closely with those of the cells in the orientation domains that they terminated in (Pearson's r = 0.633, n = 22, P = 0.002). This suggests a possible subcortical origin for cortical orientation selectivity.

Subtype-dependent postnatal development of direction- and orientation-selective retinal ganglion cells in mice

The direction-selective ganglion cells (DSGCs) and orientation-selective ganglion cells (OSGCs) encode the directional and the orientational information of a moving object, respectively. It is unclear how DSGCs and OSGCs mature in the mouse retina during postnatal development. Here we investigated the development of DSGCs and OSGCs after eye-opening. We show that 1) DSGCs and OSGCs are present at postnatal day 12 (P12), just before eye-opening; 2) the fractions of both DSGCs and OSGCs increase from P12 to P30; 3) the development of DSGCs and OSGCs is subtype dependent; and 4) direction and orientation selectivity are two separate features of retinal ganglion cells (RGCs) in the mouse retina. We classified RGCs into different functional subtypes based on their light response properties. Compared with P12, the direction and orientation selectivity of ON-OFF RGCs but not ON RGCs became stronger at P30. The tuning width of DSGCs for both ON and ON-OFF subtypes decreased with age. For OSGCs, we divided them into non-direction-selective (non-DS) OSGCs and direction-selective OSGCs (DS&OSGCs). For DS&OSGCs, we found that there was no correlation between the direction and orientation selectivity, and that the tuning width of both ON and ON-OFF subtypes remained unchanged with age. For non-DS OSGCs, the tuning width of ON but not ON-OFF subtype decreased with development. These findings provide a foundation to reveal the molecular and synaptic mechanisms underlying the development of the direction and orientation selectivity in the retina.

Spatiotemporal receptive field structures in retinogeniculate connections of cat

The spatial structure of the receptive field (RF) of cat lateral geniculate nucleus (LGN) neurons is significantly elliptical, which may provide a basis for the orientation tuning of LGN neurons, especially at high spatial frequency stimuli. However, the input mechanisms generating this elliptical RF structure are poorly defined. We therefore compared the spatiotemporal RF structures of pairs of retinal ganglion cells (RGCs) and LGN neurons that form monosynaptic connections based on the cross-correlation analysis of their firing activities. We found that the spatial RF structure of both RGCs and LGN neurons were comparably elliptical and oriented in a direction toward the area centralis. Additionally, the spatial RF structures of pairs with the same response sign were often overlapped and similarly oriented. We also found there was a small population of pairs with RF structures that had the opposite response sign and were spatially displaced and independently oriented. Finally, the temporal RF structure of an RGC was tightly correlated with that of its target LGN neuron, though the response duration of the LGN neuron was significantly longer. Our results suggest that the elliptical RF structure of an LGN neuron is mainly inherited from the primary projecting RGC and is affected by convergent inputs from multiple RGCs. We discuss how the convergent inputs may enhance the stimulus feature sensitivity of LGN neurons.

Retinal ganglion cell maps in the brain: implications for visual processing

Everything the brain knows about the content of the visual world is built from the spiking activity of retinal ganglion cells (RGCs). As the output neurons of the eye, RGCs include ∼20 different subtypes, each responding best to a specific feature in the visual scene. Here we discuss recent advances in identifying where different RGC subtypes route visual information in the brain, including which targets they connect to and how their organization within those targets influences visual processing. We also highlight examples where causal links have been established between specific RGC subtypes, their maps of central connections and defined aspects of light-mediated behavior and we suggest the use of techniques that stand to extend these sorts of analyses to circuits underlying visual perception.

Orientation-selective responses in the mouse lateral geniculate nucleus

The dorsal lateral geniculate nucleus (dLGN) receives visual information from the retina and transmits it to the cortex. In this study, we made extracellular recordings in the dLGN of both anesthetized and awake mice, and found that a surprisingly high proportion of cells were selective for stimulus orientation. The orientation selectivity of dLGN cells was unchanged after silencing the visual cortex pharmacologically, indicating that it is not due to cortical feedback. The orientation tuning of some dLGN cells correlated with their elongated receptive fields, while in others orientation selectivity was observed despite the fact that their receptive fields were circular, suggesting that their retinal input might already be orientation selective. Consistently, we revealed orientation/axis-selective ganglion cells in the mouse retina using multielectrode arrays in an in vitro preparation. Furthermore, the orientation tuning of dLGN cells was largely maintained at different stimulus contrasts, which could be sufficiently explained by a simple linear feedforward model. We also compared the degree of orientation selectivity in different visual structures under the same recording condition. Compared with the dLGN, orientation selectivity is greatly improved in the visual cortex, but is similar in the superior colliculus, another major retinal target. Together, our results demonstrate prominent orientation selectivity in the mouse dLGN, which may potentially contribute to visual processing in the cortex.

Retinal visual processing constrains human ocular following response

Ocular following responses (OFRs) are the initial tracking eye movements elicited at ultra-short latency by sudden motion of a textured pattern. We wished to evaluate quantitatively the impact that subcortical stages of visual processing might have on the OFRs. In three experiments we recorded the OFRs of human subjects to brief horizontal motion of 1D vertical sine-wave gratings restricted to an elongated horizontal aperture. Gratings were composed of a variable number of abutting horizontal strips where alternate strips were in counterphase. In one of the experiments we also utilized gratings occupying a variable number of horizontal strips separated vertically by mean-luminance gaps. We modeled retinal center/surround receptive fields as a difference of two 2-D Gaussian functions. When the characteristics of such local filters were selected in accord with the known properties of primate retinal ganglion cells, a single-layer model was capable to quantitatively account for the observed changes in the OFR amplitude for stimuli composed of counterphase strips of different heights (Experiment 1), for a wide range of stimulus contrasts (Experiment 2) and spatial frequencies (Experiment 3). A similar model using oriented filters that resemble cortical simple cells was also able to account for these data. Since similar filters can be constructed from the linear summation of retinal filters, and these filters alone can explain the data, we conclude that retinal processing determines the response to these stimuli. Thus, with appropriately chosen stimuli, OFRs can be used to study visual spatial integration processes as early as in the retina.

Effects of stimulus spatial frequency, size, and luminance contrast on orientation tuning of neurons in the dorsal lateral geniculate nucleus of cat

It is generally thought that orientation selectivity first appears in the primary visual cortex (V1), whereas neurons in the lateral geniculate nucleus (LGN), an input source for V1, are thought to be insensitive to stimulus orientation. Here we show that increasing both the spatial frequency and size of the grating stimuli beyond their respective optimal values strongly enhance the orientation tuning of LGN neurons. The resulting orientation tuning was clearly contrast-invariant. Furthermore, blocking intrathalamic inhibition by iontophoretically administering γ-aminobutyric acid (GABA)A receptor antagonists, such as bicuculline and GABAzine, slightly but significantly weakened the contrast invariance. Our results suggest that orientation tuning in the LGN is caused by an elliptical classical receptive field and orientation-tuned surround suppression, and that its contrast invariance is ensured by local GABAA inhibition. This contrast-invariant orientation tuning in LGN neurons may contribute to the contrast-invariant orientation tuning seen in V1 neurons.

Cortical-like receptive fields in the lateral geniculate nucleus of marmoset monkeys

Most neurons in primary visual cortex (V1) exhibit high selectivity for the orientation of visual stimuli. In contrast, neurons in the main thalamic input to V1, the lateral geniculate nucleus (LGN), are considered to be only weakly orientation selective. Here we characterize a sparse population of cells in marmoset LGN that show orientation and spatial frequency selectivity as great as that of cells in V1. The recording position in LGN and histological reconstruction of these cells shows that they are part of the koniocellular (K) pathways. Accordingly we have named them K-o ("koniocellular-orientation") cells. Most K-o cells prefer vertically oriented gratings; their contrast sensitivity and TF tuning are similar to those of parvocellular cells, and they receive negligible functional input from short wavelength-sensitive ("blue") cone photoreceptors. Four K-o cells tested displayed binocular responses. Our results provide further evidence that in primates as in nonprimate mammals the cortical input streams include a diversity of visual representations. The presence of K-o cells increases functional homologies between K pathways in primates and "sluggish/W" pathways in nonprimate visual systems.

The spatial structure of a nonlinear receptive field

Understanding a sensory system implies the ability to predict responses to a variety of inputs from a common model. In the retina, this includes predicting how the integration of signals across visual space shapes the outputs of retinal ganglion cells. Existing models of this process generalize poorly to predict responses to new stimuli. This failure arises in part from properties of the ganglion cell response that are not well captured by standard receptive-field mapping techniques: nonlinear spatial integration and fine-scale heterogeneities in spatial sampling. Here we characterize a ganglion cell's spatial receptive field using a mechanistic model based on measurements of the physiological properties and connectivity of only the primary excitatory circuitry of the retina. The resulting simplified circuit model successfully predicts ganglion-cell responses to a variety of spatial patterns and thus provides a direct correspondence between circuit connectivity and retinal output.

The complex interactions of retinal, optical and environmental factors in myopia aetiology

Myopia is the commonest ocular abnormality but as a research topic remains at the margins of mainstream ophthalmology. The concept that most myopes fall into the category of 'physiological myopia' undoubtedly contributes to this position. Yet detailed analysis of epidemiological data linking myopia with a range of ocular pathologies from glaucoma to retinal detachment demonstrates statistically significant disease association in the 0 to -6 D range of 'physiological myopia'. The calculated risks from myopia are comparable to those between hypertension, smoking and cardiovascular disease. In the case of myopic maculopathy and retinal detachment the risks are an order of magnitude greater. This finding highlights the potential benefits of interventions that can limit or prevent myopia progression. Our understanding of the regulatory processes that guide an eye to emmetropia and, conversely how the failure of such mechanisms can lead to refractive errors, is certainly incomplete but has grown enormously in the last few decades. Animal studies, observational clinical studies and more recently randomized clinical trials have demonstrated that the retinal image can influence the eye's growth. To date human intervention trials in myopia progression using optical means have had limited success but have been designed on the basis of simple hypotheses regarding the amount of defocus at the fovea. Recent animal studies, backed by observational clinical studies, have revealed that the mechanisms of optically guided eye growth are influenced by the retinal image across a wide area of the retina and not solely the fovea. Such results necessitate a fundamental shift in how refractive errors are defined. In the context of understanding eye growth a single sphero-cylindrical definition of foveal refraction is insufficient. Instead refractive error must be considered across the curved surface of the retina. This carries the consequence that local retinal image defocus can only be determined once the 3D structure of the viewed scene, off axis performance of the eye and eye shape has been accurately defined. This, in turn, introduces an under-appreciated level of complexity and interaction between the environment, ocular optics and eye shape that needs to be considered when planning and interpreting the results of clinical trials on myopia prevention.

Spatial receptive field properties of rat retinal ganglion cells

The rat is a popular animal model for vision research, yet there is little quantitative information about the physiological properties of the cells that provide its brain with visual input, the retinal ganglion cells. It is not clear whether rats even possess the full complement of ganglion cell types found in other mammals. Since such information is important for evaluating rodent models of visual disease and elucidating the function of homologous and heterologous cells in different animals, we recorded from rat ganglion cells in vivo and systematically measured their spatial receptive field (RF) properties using spot, annulus, and grating patterns. Most of the recorded cells bore likeness to cat X and Y cells, exhibiting brisk responses, center-surround RFs, and linear or nonlinear spatial summation. The others resembled various types of mammalian W cell, including local-edge-detector cells, suppressed-by-contrast cells, and an unusual type with an ON-OFF surround. They generally exhibited sluggish responses, larger RFs, and lower responsiveness. The peak responsivity of brisk-nonlinear (Y-type) cells was around twice that of brisk-linear (X-type) cells and several fold that of sluggish cells. The RF size of brisk-linear and brisk-nonlinear cells was indistinguishable, with average center and surround diameters of 5.6 ± 1.3 and 26.4 ± 11.3 deg, respectively. In contrast, the center diameter of recorded sluggish cells averaged 12.8 ± 7.9 deg. The homogeneous RF size of rat brisk cells is unlike that of cat X and Y cells, and its implication regarding the putative roles of these two ganglion cell types in visual signaling is discussed.

Fine spatial information represented in a population of retinal ganglion cells

Detailed measurement of ganglion cell receptive fields often reveals significant deviations from a smooth, Gaussian profile. We studied the effect of these irregularities on the representation of fine spatial information in the retina. We recorded from nearby clusters of ganglion cells, testing their ability to determine the location of small flashed spots, and we compared the results to the prediction of a Gaussian receptive field model derived from reverse correlation. Despite considerable receptive field overlap, almost all ganglion cell pairs signaled nearly independently. For groups of five cells with highly overlapping receptive fields, the measured light-evoked currents encoded ∼33% more information than predicted by the Gaussian receptive field model. Including measured local irregularities in the receptive field model increased performance to the level observed experimentally. These results suggest that instead of being an unavoidable defect, irregularities may be a positive design feature of population neural codes.

Myopia, posture and the visual environment

Evidence for a possible role for the peripheral retina in the control of refractive development is discussed, together with Howland's suggestion (Paper presented at the 13th International Myopia Conference, Tubingen, Germany, July 26-29, 2010) that signals to generate appropriate growth might be derived from ocular oblique astigmatism. The dependence of this, or similar peripheral mechanisms, on exposure to a uniform field of near-zero dioptric vergence is emphasized: this is required to ensure a consistent relationship between the astigmatic image fields and the retina. This condition is satisfied by typical outdoor environments. In contrast, indoor environments are likely to be unfavourable to peripherally-based emmetropization, since dioptric stimuli may vary widely across the visual field. This is particularly the case when short working distances or markedly asymmetric head postures with respect to the visual task are adopted.

Orientation-Specific Modulation of Rat Retinal Ganglion Cell Responses and Its Dependence on Relative Orientations of the Center and Surround Gratings

In the primary visual cortex (V1), it has been shown that the neuronal response elicited by a grating patch in the receptive field (RF) center can be suppressed or facilitated by an annular grating presented in the RF surround area; the effect depends on the relative orientations of the two gratings. The effect is thought to play a role in figure-ground segregation. Here we have found that response modulation similar to that reported in cortical area V1 can also be found in all major classes of retinal ganglion cells (RGCs), including “concentric” cells. Orientation-specific response modulation of this kind cannot result from interactions of independent RF mechanisms; therefore more complex mechanism, which takes into account the relative orientations of the gratings in the RF center and surround, or sensing the borders between texture regions, has to be present in RFs of RGCs, even of the concentric type. This challenges the consensus notion that their responses to visual stimuli are governed entirely by a RF composed of separate mechanisms: center, antagonistic surround, and modulatory extraclassical surround. Our findings raise the question of whether initial stages of complex analysis of visual input, normally attributed to the visual cortex, can be achieved within the retina.

Segregation of short-wavelength-sensitive (S) cone signals in the macaque dorsal lateral geniculate nucleus

An important problem in the study of the mammalian visual system is whether functionally different retinal ganglion cell types are anatomically segregated further up along the central visual pathway. It was previously demonstrated that, in a New World diurnal monkey (marmoset), the neurones carrying signals from the short-wavelength-sensitive (S) cones [blue-yellow (B/Y)-opponent cells] are predominantly located in the koniocellular layers of the dorsal lateral geniculate nucleus (LGN), whereas the red-green (R/G)-opponent cells carrying signals from the medium- and long-wavelength-sensitive cones are segregated in the parvocellular layers. Here, we used extracellular single-unit recordings followed by histological reconstruction to investigate the distribution of color-selective cells in the LGN of the macaque, an Old World diurnal monkey. Cells were classified using cone-isolating stimuli to identify their cone inputs. Our results indicate that the majority of cells carrying signals from S-cones are located either in the koniocellular layers or in the 'koniocellular bridges' that fully or partially span the parvocellular layers. By contrast, the R/G-opponent cells are located in the parvocellular layers. We conclude that anatomical segregation of B/Y- and R/G-opponent afferent signals for color vision is common to the LGNs of New World and Old World diurnal monkeys.

Receptive fields in primate retina are coordinated to sample visual space more uniformly

In the visual system, large ensembles of neurons collectively sample visual space with receptive fields (RFs). A puzzling problem is how neural ensembles provide a uniform, high-resolution visual representation in spite of irregularities in the RFs of individual cells. This problem was approached by simultaneously mapping the RFs of hundreds of primate retinal ganglion cells. As observed in previous studies, RFs exhibited irregular shapes that deviated from standard Gaussian models. Surprisingly, these irregularities were coordinated at a fine spatial scale: RFs interlocked with their neighbors, filling in gaps and avoiding large variations in overlap. RF shapes were coordinated with high spatial precision: the observed uniformity was degraded by angular perturbations as small as 15 degrees, and the observed populations sampled visual space with more than 50% of the theoretical ideal uniformity. These results show that the primate retina encodes light with an exquisitely coordinated array of RF shapes, illustrating a higher degree of functional precision in the neural circuitry than previously appreciated.

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.

Amacrine cell contributions to red-green color opponency in central primate retina: a model study

To investigate the contributions of amacrine cells to red-green opponency, a linear computational model of the central macaque retina was developed based on a published cone mosaic. In the model, amacrine cells of ON and OFF types received input from all neighboring midget bipolar cells of the same polarity, but OFF amacrine cells had a bias toward bipolar cells whose center responses were mediated by middle wavelength sensitive cones. This bias might arise due to activity dependent plasticity because there are midget bipolar cells driven by short wavelength sensitive cones in the OFF pathway. The model midget ganglion cells received inputs from neighboring amacrine cells of both types. As in physiological experiments, the model ganglion cells showed spatially opponent responses to achromatic stimuli, but they responded to cone isolating stimuli as though center and surround were each driven by a single cone type. Without amacrine cell input, long and middle wavelength sensitive cones contributed to both the centers and surrounds of model ganglion cell receptive fields. According to the model, the summed amacrine cell input was red-green opponent even though inputs to individual amacrine cells were unselective. A key prediction is that GABA and glycine depolarize two of the four types of central midget ganglion cells; this may reflect lower levels of the potassium chloride co-transporter in their dendrites.

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

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

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

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