Responses of cat horizontal cells to sinusoidal gratings

Inhibitory mechanisms that generate centre and surround properties in ON and OFF brisk-sustained ganglion cells in the rabbit retina

Lateral inhibition produces the centre-surround organization of retinal receptive fields, in which inhibition driven by the mean luminance enhances the sensitivity of ganglion cells to spatial and temporal contrast. Surround inhibition is generated in both synaptic layers; however, the synaptic mechanisms within the inner plexiform layer are not well characterized within specific classes of retinal ganglion cell. Here, we compared the synaptic circuits generating concentric centre-surround receptive fields in ON and OFF brisk-sustained ganglion cells (BSGCs) in the rabbit retina. We first characterized the synaptic inputs to the centre of ON BSGCs, for comparison with previous results from OFF BSGCs. Similar to wide-field ganglion cells, the spatial extent of the excitatory centre and inhibitory surround was larger for the ON than the OFF BSGCs. The results indicate that the surrounds of ON and OFF BSGCs are generated in both the outer and the inner plexiform layers. The inner plexiform layer surround inhibition comprised GABAergic suppression of excitatory inputs from bipolar cells. However, ON and OFF BSGCs displayed notable differences. Surround suppression of excitatory inputs was weaker in ON than OFF BSGCs, and was mediated largely by GABA(C) receptors in ON BSGCs, and by both GABA(A) and GABA(C) receptors in OFF BSGCs. Large ON pathway-mediated glycinergic inputs to ON and OFF BSGCs also showed surround suppression, while much smaller GABAergic inputs showed weak, if any, spatial tuning. Unlike OFF BSGCs, which receive strong glycinergic crossover inhibition from the ON pathway, the ON BSGCs do not receive crossover inhibition from the OFF pathway. We compare and discuss possible roles for glycinergic inhibition in the two cell types.

Light- and dopamine-regulated receptive field plasticity in primate horizontal cells

Center-surround antagonistic receptive fields (CSARFs) are building blocks for spatial vision and contrast perception. Retinal horizontal cells (HCs) are the first lateral elements along the visual pathway, and are thought to contribute to receptive field surrounds of higher order neurons. Primate HC receptive fields have not been found to change with light, and dopaminergic modulation has not been investigated. Recording intracellularly from HCs in dark-adapted macaque retina, we found that H1-HCs had large receptive fields (λ = 1,158 ± 137 μm) that were reduced by background light (-45%), gap junction closure (-53%), and D1 dopamine receptor activation (-48%). Tracer coupling was modulated in a correlative manner, suggesting that coupling resistance plays a dominant role in receptive field formation under low light conditions. The D1 antagonist SCH23390 increased the size of receptive fields (+13%), suggesting tonic dopamine release in the dark. Because light elevates dopamine release in primate retina, our results support a dopaminergic role in post-receptoral light adaptation by decreasing HC receptive field diameters, which influences the center-surround receptive field organization of higher-order neurons and thereby spatial contrast sensitivity.

A two pathway model for tonic suppressed-by-contrast cells in the cat retina

A two pathway spatiotemporal model is proposed to describe the function of tonic suppressed-by-contrast cells of the cat retina. The model is able to describe the experimentally determined responses of such neurons to drifting sinusoidal gratings. It is also able to predict their responses to alternating sinusoidal gratings and flashing or moving spots of light, and these predictions resemble experimental observations, at least qualitatively. The model is physiologically plausible, it can be used to summarize the dynamic responses of the tonic suppressed-by-contrast cells of the cat and potentially to account for the responses of the suppressed-by-contrast cells of other species.

Functional circuitry for peripheral suppression in Mammalian Y-type retinal ganglion cells

A retinal ganglion cell receptive field is made up of an excitatory center and an inhibitory surround. The surround has two components: one driven by horizontal cells at the first synaptic layer and one driven by amacrine cells at the second synaptic layer. Here we characterized how amacrine cells inhibit the center response of on- and off-center Y-type ganglion cells in the in vitro guinea pig retina. A high spatial frequency grating (4-5 cyc/mm), beyond the spatial resolution of horizontal cells, drifted in the ganglion cell receptive field periphery to stimulate amacrine cells. The peripheral grating suppressed the ganglion cell spiking response to a central spot. Suppression of spiking was strongest and observed most consistently in off cells. In intracellular recordings, the grating suppressed the subthreshold membrane potential in two ways: a reduced slope (gain) of the stimulus-response curve by approximately 20-30% and, in off cells, a tonic approximately 1-mV hyperpolarization. In voltage clamp, the grating increased an inhibitory conductance in all cells and simultaneously decreased an excitatory conductance in off cells. To determine whether center response inhibition was presynaptic or postsynaptic (shunting), we measured center response gain under voltage-clamp and current-clamp conditions. Under both conditions, the peripheral grating reduced center response gain similarly. This result suggests that reduced gain in the ganglion cell subthreshold center response reflects inhibition of presynaptic bipolar terminals. Thus amacrine cells suppressed ganglion cell center response gain primarily by inhibiting bipolar cell glutamate release.

Cellular basis for contrast gain control over the receptive field center of mammalian retinal ganglion cells

Retinal ganglion cells fire spikes to an appropriate contrast presented over their receptive field center. These center responses undergo dynamic changes in sensitivity depending on the ongoing level of contrast, a process known as "contrast gain control." Extracellular recordings suggested that gain control is driven by a single wide-field mechanism, extending across the center and beyond, that depends on inhibitory interneurons: amacrine cells. However, recordings in salamander suggested that the excitatory bipolar cells, which drive the center, may themselves show gain control independently of amacrine cell mechanisms. Here, we tested in mammalian ganglion cells whether amacrine cells are critical for gain control over the receptive field center. We made extracellular and whole-cell recordings of guinea pig Y-type cells in vitro and quantified the gain change between contrasts using a linear-nonlinear analysis. For spikes, tripling contrast reduced gain by approximately 40%. With spikes blocked, ganglion cells showed similar levels of gain control in membrane currents and voltages and under conditions of low and high calcium buffering: tripling contrast reduced gain by approximately 20-25%. Gain control persisted under voltage-clamp conditions that minimize inhibitory conductances and pharmacological conditions that block inhibitory neurotransmitter receptors. Gain control depended on adequate stimulation, not of ganglion cells but of presynaptic bipolar cells. Furthermore, horizontal cell measurements showed a lack of gain control in photoreceptor synaptic release. Thus, the mechanism for gain control over the ganglion cell receptive field center, as measured in the subthreshold response, originates in the presynaptic bipolar cells and does not require amacrine cell signaling.

Chromatic induction in humans: how are the cone signals combined to provide opponent processing?

We investigated the effect of 16 isoluminant chromatic surrounds on the perceived colour of an enclosed grey test-field at photopic (43 cd/m2) conditions. Stimuli were shown on a grey background identical to the test-field. Use of these stimuli implies that activations of receptoral (cS, cM, and cL) and postreceptoral (cM-cL, cS-(cM+cL)) mechanisms by surround colours are known quantitatively. This allows to predict shifts in colour of the test-field in terms of receptoral (adaptation) as well as postreceptoral (contrast) mechanisms assuming a standard two-stage model. Predictions are tested using matching and hue compensation procedures. Both procedures yield comparable results that are consistent with the assumption that postreceptoral mechanisms explain the observed shifts in perceived colour.

The influence of different retinal subcircuits on the nonlinearity of ganglion cell behavior

Y-type retinal ganglion cells show a pronounced, nonlinear, frequency-doubling behavior in response to modulated sinewave gratings. This is not observed in X-type cells. The source of this spatial nonlinear summation is still under debate. We have designed a realistic biophysical model of the cat retina to test the influence of different retinal cell classes and subcircuits on the linearity of ganglion cell responses. The intraretinal connectivity consists of the fundamental feedforward pathway via bipolar cells, lateral horizontal cell connectivity, and two amacrine circuits. The wiring diagram of X- and Y-cells is identical apart from two aspects: (1) Y-cells have a wider receptive field and (2) they receive input from a nested amacrine circuit consisting of narrow- and wide-field amacrine cells. The model was tested with contrast-reversed gratings. First and second harmonic response components were determined to estimate the degree of nonlinearity. By means of circuit dissection, we found that a high degree of the Y-cell nonlinear behavior arises from the spatial integration of temporal photoreceptor nonlinearities. Furthermore, we found a weaker and less uniform influence of the nested amacrine circuit. Different sources of nonlinearities interact in a multiplicative manner, and the influence of the amacrine circuit is approximately 25% weaker than that of the photoreceptor. The model predicts that significant nonlinearities occur already at the level of horizontal cell responses. Pharmacological inactivation of the amacrine circuit is expected to exert a milder effect in reducing ganglion cell nonlinearity.

A neural model of foveal light adaptation and afterimage formation

Psychophysical research has documented the existence of three processes in light adaptation: a fast subtractive process, a divisive process that is fast at light onset and slower at light offset, and a very slow subtractive process (Hayhoe et al., 1987). In the neural model developed here, the fast subtractive process is identified with horizontal cell feedback onto cones and the divisive process with amacrine cell feedback onto bipolar cells. The very slow subtractive process is identified with the modulatory feedback circuit from amacrines via interplexiform cells to horizontal cells. A nonlinear dynamical model is developed incorporating these aspects of retinal circuitry along with both ON- and OFF-center M and P pathways. This model is shown to account for many aspects of foveal light adaptation, including negative afterimage formation, and to explain a number of the physiological differences between M and P ganglion cells, including their differing contrast-response functions.

Gain control and hyperpolarization level in cat horizontal cells as a function of light and dark adaptation

First a model is presented that accurately summarizes the dynamic properties of cat horizontal (H-) cells under photopic conditions as measured in our previous work. The model predicts that asymmetries in response to dark as compared to light flashes are flash-duration dependent. This somewhat surprising prediction is tested and confirmed in intracellular recordings from the optically intact in vivo eye of the cat (Experiment 1). The model implies that the gain of H-cells should be related rather directly to the sustained (baseline) membrane potential. We performed three additional experiments to test this idea. Experiment 2 concerns response vs intensity (R-I-) curves for various flash-diameters and background-sizes with background luminance varying over a 4 log unit range. Results support the assumption of a rather strict coupling between flash sensitivity (gain) and the sustained level of hyperpolarization. In Experiment 3 we investigate this relation for both dark and light flashes given on each of four background light levels. The results suggest that there are fixed minimum and maximum hyperpolarization levels, and that the baseline hyperpolarization for a given illumination thus also sets the available range for dark and light flash-responses. The question then arises whether, or how this changes during dark adaptation, when the rod contribution to H-cell responses gradually increases. The fourth experiment therefore studies the relationship between gain and hyperpolarization level during prolonged dark-adaptation. The results show that the rod contribution increases the polarization range of H-cells, but that the gain and polarization level nevertheless remain directly coupled. H-cell models relying on a close coupling between polarization level and gain thus remain attractive options.

Parallel cone bipolar to on-beta ganglion cell pathways in the cat retina: spatial responses, spatial aliasing, and spatial variance

An important issue in understanding the retina is finding candidate functional roles for different cell pathways and the details of their anatomy and physiology. We consider various spatial properties of the three main cone ==> cone bipolar cell ==> on-beta ganglion cell pathways in the cat retina and possible roles for the particulars of their anatomy. The cone bipolar cells in these pathways have distinct morphologies and modest differences in their convergence, divergence, densities, and synaptic weighting; and it is unclear whether the pathways differ in their spatial properties or in some other manner. Since differences in spatial processing of cells are best studied on a systemwide level, we developed the multirate filter-based method of retinal modeling, a technique for relating the anatomy of multiple cell layers to its systemic effects. We demonstrate that (1) despite the anatomic distinctions among the three main cone bipolar cell pathways, their spatial responses are essentially identical; (2) despite the spatial averaging in the pathways, there is essentially no filtering of the nonaliasing signal components after the cone layer; (3) instead, this averaging combined with prefiltering by the eye's optics and cone gap junctions prevents spatial aliasing; and (4) the averaging and prefiltering combined allow cell responses to be similar despite significant cell-to-cell anatomic differences.

Modelling the spatio-temporal modulation response of ganglion cells with difference-of-Gaussians receptive fields: relation to photoreceptor response kinetics

Difference-of-Gaussians (DOG) models for the receptive fields of retinal ganglion cells accurately predict linear responses to both periodic stimuli (typically moving sinusoidal gratings) and aperiodic stimuli (typically circular fields presented as square-wave pulses). While the relation of spatial organization to retinal anatomy has received considerable attention, temporal characteristics have been only loosely connected to retinal physiology. Here we integrate realistic photoreceptor response waveforms into the DOG model to clarify how far a single set of physiological parameters predict temporal aspects of linear responses to both periodic and aperiodic stimuli. Traditional filter-cascade models provide a useful first-order approximation of the single-photon response in photoreceptors. The absolute time scale of these, plus a time for retinal transmission, here construed as a fixed delay, are obtained from flash/step data. Using these values, we find that the DOG model predicts the main features of both the amplitude and phase response of linear cat ganglion cells to sinusoidal flicker. Where the simplest model formulation fails, it serves to reveal additional mechanisms. Unforeseen facts are the attenuation of low temporal frequencies even in pure center-type responses, and the phase advance of the response relative to the stimulus at low frequencies. Neither can be explained by any experimentally documented cone response waveform, but both would be explained by signal differentiation, e.g. in the retinal transmission pathway, as demonstrated at least in turtle retina.

Simulations of X and Y retinal ganglion cell behavior with a nonlinear push-pull model of spatiotemporal retinal processing

This article describes a nonlinear model of neural processing in the vertebrate retina, comprising model photoreceptors, model push-pull bipolar cells, and model ganglion cells. Previous analyses and simulations have shown that with a choice of parameters that mimics beta cells, the model exhibits X-like linear spatial summation (null response to contrast-reversed gratings) in spite of photoreceptor nonlinearities; on the other hand, a choice of parameters that mimics alpha cells leads to Y-like frequency doubling. This article extends the previous work by showing that the model can replicate qualitatively many of the original findings on X and Y cells with a fixed choice of parameters. The results generally support the hypothesis that X and Y cells can be seen as functional variants of a single neural circuit. The model also suggests that both depolarizing and hyperpolarizing bipolar cells converge onto both ON and OFF ganglion cell types. The push-pull connectivity enables ganglion cells to remain sensitive to deviations about the mean output level of nonlinear photoreceptors. These and other properties of the push-pull model are discussed in the general context of retinal processing of spatiotemporal luminance patterns.

The lateral spread of light adaptation in cat horizontal cell responses

To investigate the sites of light adaptation processes in the mammalian distal retina, we studied the lateral spread of adaptation signals in cone-driven cat horizontal (H-) cell responses. The size of the adaptation pool is compared to the receptive field for H-cell responses. H-cell activity was recorded intracellularly in the optically intact, in vivo eye. It is demonstrated that light adaptation as measured in H-cells is not a strictly local process. Background light falling outside a central test region effectively modulates the responses to a small test light, flashed on the receptive field center. The integration area for adaptation signals was quantitatively compared to the H-cell receptive field size by measuring the desensitizing effect of background light on the responses to a small centered test spot, as a function of background spot size. The area-adaptation function is comparable to the area-response function but has a slightly smaller length constant. Light adaptation in H-cell responses, therefore, reveals spread of adaptation over a large distance and is probably mediated through lateral interactions in the H-cell network rather than in the cones.