Contrast processing by ON and OFF bipolar cells

A pupillary contrast response in mice and humans: Neural mechanisms and visual functions

In the pupillary light response (PLR), increases in ambient light constrict the pupil to dampen increases in retinal illuminance. Here, we report that the pupillary reflex arc implements a second input-output transformation; it senses temporal contrast to enhance spatial contrast in the retinal image and increase visual acuity. The pupillary contrast response (PCoR) is driven by rod photoreceptors via type 6 bipolar cells and M1 ganglion cells. Temporal contrast is transformed into sustained pupil constriction by the M1's conversion of excitatory input into spike output. Computational modeling explains how the PCoR shapes retinal images. Pupil constriction improves acuity in gaze stabilization and predation in mice. Humans exhibit a PCoR with similar tuning properties to mice, which interacts with eye movements to optimize the statistics of the visual input for retinal encoding. Thus, we uncover a conserved component of active vision, its cell-type-specific pathway, computational mechanisms, and optical and behavioral significance.

Distributed feature representations of natural stimuli across parallel retinal pathways

How sensory systems extract salient features from natural environments and organize them across neural pathways is unclear. Combining single-cell and population two-photon calcium imaging in mice, we discover that retinal ON bipolar cells (second-order neurons of the visual system) are divided into two blocks of four types. The two blocks distribute temporal and spatial information encoding, respectively. ON bipolar cell axons co-stratify within each block, but separate laminarly between them (upper block: diverse temporal, uniform spatial tuning; lower block: diverse spatial, uniform temporal tuning). ON bipolar cells extract temporal and spatial features similarly from artificial and naturalistic stimuli. In addition, they differ in sensitivity to coherent motion in naturalistic movies. Motion information is distributed across ON bipolar cells in the upper and the lower blocks, multiplexed with temporal and spatial contrast, independent features of natural scenes. Comparing the responses of different boutons within the same arbor, we find that axons of all ON bipolar cell types function as computational units. Thus, our results provide insights into the visual feature extraction from naturalistic stimuli and reveal how structural and functional organization cooperate to generate parallel ON pathways for temporal and spatial information in the mammalian retina.

Self-Rectifying All-Optical Modulated Optoelectronic Multistates Memristor Crossbar Array for Neuromorphic Computing

Researching optoelectronic memristors capable of integrating sensory and processing functions is essential for advancing the development of efficient neuromorphic vision. Here, we experimentally demonstrated an all-optical controlled and self-rectifying optoelectronic memristor (OEM) crossbar array with the function of multilevel storage under light stimuli. The NiO/TiO2 device exhibits an ultrahigh (>104) rectifying ratio (RR) thus overcoming the presence of sneak current. The reversible conductance modulation without electric signal involvement provides a novel way to realize ultrafast information processing. The proposed OEM array realized synaptic functions observed in the human brain, including long-term potentiation (LTP), long-term depression (LTD), paired-pulse facilitation (PPF), the transition from short-term memory (STM) to long-term memory (LTM), and learning experience behaviors successfully. The authors present a novel OEM crossbar that possesses complete light-modulation capabilities, potentially advancing the future development of efficient neuromorphic vision.

Dynamic Recruitment of the Feedforward and Recurrent Mechanism for Black-White Asymmetry in the Primary Visual Cortex

Black and white information is asymmetrically distributed in natural scenes, evokes asymmetric neuronal responses, and causes asymmetric perceptions. Recognizing the universality and essentiality of black-white asymmetry in visual information processing, the neural substrates for black-white asymmetry remain unclear. To disentangle the role of the feedforward and recurrent mechanisms in the generation of cortical black-white asymmetry, we recorded the V1 laminar responses and LGN responses of anesthetized cats of both sexes. In a cortical column, we found that black-white asymmetry starts at the input layer and becomes more pronounced in the output layer. We also found distinct dynamics of black-white asymmetry between the output layer and the input layer. Specifically, black responses dominate in all layers after stimulus onset. After stimulus offset, black and white responses are balanced in the input layer, but black responses still dominate in the output layer. Compared with that in the input layer, the rebound response in the output layer is significantly suppressed. The relative suppression strength evoked by white stimuli is notably stronger and depends on the location within the ON-OFF cortical map. A model with delayed and polarity-selective cortical suppression explains black-white asymmetry in the output layer, within which prominent recurrent connections are identified by Granger causality analysis. In addition to black-white asymmetry in response strength, the interlaminar differences in spatial receptive field varied dynamically. Our findings suggest that the feedforward and recurrent mechanisms are dynamically recruited for the generation of black-white asymmetry in V1.SIGNIFICANCE STATEMENT Black-white asymmetry is universal and essential in visual information processing, yet the neural substrates for cortical black-white asymmetry remain unknown. Leveraging V1 laminar recordings, we provided the first laminar pattern of black-white asymmetry in cat V1 and found distinct dynamics of black-white asymmetry between the output layer and the input layer. Comparing black-white asymmetry across three visual hierarchies, the LGN, V1 input layer, and V1 output layer, we demonstrated that the feedforward and recurrent mechanisms are dynamically recruited for the generation of cortical black-white asymmetry. Our findings not only enhance our understanding of laminar processing within a cortical column but also elucidate how feedforward connections and recurrent connections interact to shape neuronal response properties.

From retina to motoneurons: A substrate for visuomotor transformation in salamanders

The transformation of visual input into motor output is essential to approach a target or avoid a predator. In salamanders, visually guided orientation behaviors have been extensively studied during prey capture. However, the neural circuitry involved is not resolved. Using salamander brain preparations, calcium imaging and tracing experiments, we describe a neural substrate through which retinal input is transformed into spinal motor output. We found that retina stimulation evoked responses in reticulospinal neurons of the middle reticular nucleus, known to control steering movements in salamanders. Microinjection of glutamatergic antagonists in the optic tectum (superior colliculus in mammals) decreased the reticulospinal responses. Using tracing, we found that retina projected to the dorsal layers of the contralateral tectum, where the dendrites of neurons projecting to the middle reticular nucleus were located. In slices, stimulation of the tectal dorsal layers evoked glutamatergic responses in deep tectal neurons retrogradely labeled from the middle reticular nucleus. We then examined how tectum activation translated into spinal motor output. Tectum stimulation evoked motoneuronal responses, which were decreased by microinjections of glutamatergic antagonists in the contralateral middle reticular nucleus. Reticulospinal fibers anterogradely labeled from tracer injection in the middle reticular nucleus were preferentially distributed in proximity with the dendrites of ipsilateral motoneurons. Our work establishes a neural substrate linking visual and motor centers in salamanders. This retino‐tecto‐reticulo‐spinal circuitry is well positioned to control orienting behaviors. Our study bridges the gap between the behavioral studies and the neural mechanisms involved in the transformation of visual input into motor output in salamanders.

Brain-wide ongoing activity is responsible for significant cross-trial BOLD variability

A notorious issue of task-based functional magnetic resonance imaging (fMRI) is its large cross-trial variability. To quantitatively characterize this variability, the blood oxygenation level-dependent (BOLD) signal can be modeled as a linear summation of a stimulation-relevant and an ongoing (i.e. stimulation-irrelevant) component. However, systematic investigation on the spatiotemporal features of the ongoing BOLD component and how these features affect the BOLD response is still lacking. Here we measured fMRI responses to light onsets and light offsets in awake rats. The neuronal response was simultaneously recorded with calcium-based fiber photometry. We established that between-region BOLD signals were highly correlated brain-wide at zero time lag, including regions that did not respond to visual stimulation, suggesting that the ongoing activity co-fluctuates across the brain. Removing this ongoing activity reduced cross-trial variability of the BOLD response by ~30% and increased its coherence with the Ca2+ signal. Additionally, the negative ongoing BOLD activity sometimes dominated over the stimulation-driven response and contributed to the post-stimulation BOLD undershoot. These results suggest that brain-wide ongoing activity is responsible for significant cross-trial BOLD variability, and this component can be reliably quantified and removed to improve the reliability of fMRI response. Importantly, this method can be generalized to virtually all fMRI experiments without changing stimulation paradigms.

Pulvinar Modulates Contrast Responses in the Visual Cortex as a Function of Cortical Hierarchy

The pulvinar is the largest extrageniculate visual nucleus in mammals. Given its extensive reciprocal connectivity with the visual cortex, it allows the cortico-thalamocortical transfer of visual information. Nonetheless, knowledge of the nature of the pulvinar inputs to the cortex remains elusive. We investigated the impact of silencing the pulvinar on the contrast response function of neurons in 2 distinct hierarchical cortical areas in the cat (areas 17 and 21a). Pulvinar inactivation altered the response gain in both areas, but with larger changes observed in area 21a. A theoretical model was proposed, simulating the pulvinar contribution to cortical contrast responses by modifying the excitation-inhibition balanced state of neurons across the cortical hierarchy. Our experimental and theoretical data showed that the pulvinar exerts a greater modulatory influence on neuronal activity in area 21a than in the primary visual cortex, indicating that the pulvinar impact on cortical visual neurons varies along the cortical hierarchy.

What the salamander eye has been telling the vision scientist's brain

Salamanders have been habitual residents of research laboratories for more than a century, and their history in science is tightly interwoven with vision research. Nevertheless, many vision scientists - even those working with salamanders - may be unaware of how much our knowledge about vision, and particularly the retina, has been shaped by studying salamanders. In this review, we take a tour through the salamander history in vision science, highlighting the main contributions of salamanders to our understanding of the vertebrate retina. We further point out specificities of the salamander visual system and discuss the perspectives of this animal system for future vision research.

Empowering Retinal Gene Therapy with a Specific Promoter for Human Rod and Cone ON-Bipolar Cells

Optogenetic gene therapy holds promise to restore high-quality vision in blind patients and recently reached clinical trials. Although the ON-bipolar cells, the first retinal interneurons, make the most attractive targets for optogenetic vision restoration, they have remained inaccessible to human gene therapy due to the lack of a robust cell-specific promoter. We describe the design and functional evaluation of 770En_454P(hGRM6), a human GRM6 gene-derived, short promoter that drives strong and highly specific expression in both the rod- and cone-type ON-bipolar cells of the human retina. Expression also in cone-type ON-bipolar cells is of importance, since the cone-dominated macula mediates high-acuity vision and is the primary target of gene therapies. 770En_454P(hGRM6)-driven middle-wave opsin expression in ON-bipolar cells achieved lasting restoration of high visual acuity in the rd1 mouse model of late retinal degeneration. The new promoter enables precise manipulation of the inner retinal network and paves the way for clinical application of gene therapies for high-resolution optogenetic vision restoration, raising hopes of significantly improving the life quality of people suffering from blindness.

Circadian Vision in Zebrafish: From Molecule to Cell and from Neural Network to Behavior

Most visual system functions, such as opsin gene expression, retinal neural transmission, light perception, and visual sensitivity, display robust day-night rhythms. The rhythms persist in constant lighting conditions, suggesting the involvement of endogenous circadian clocks. While the circadian pacemakers that control the rhythms of animal behaviors are mostly found in the forebrain and midbrain, self-sustained circadian oscillators are also present in the neural retina, where they play important roles in the regulation of circadian vision. This review highlights some of the correlative studies of the circadian control of visual system functions in zebrafish. Because zebrafish maintain a high evolutionary proximity to mammals, the findings from zebrafish research may provide insights for a better understanding of the mechanisms of circadian vision in other vertebrate species including humans.

Retinal Defocus and Form-Deprivation Exposure Duration Affects RPE BMP Gene Expression

In the context of ocular development and eye growth regulation, retinal defocus and/or image contrast appear key variables although the nature of the signal(s) relayed from the retina to the sclera remains poorly understood. Nonetheless, under optimal visual conditions, eye length is brought into alignment with its optical power to achieve approximate emmetropia, through appropriate adjustment to eye growth. The retinal pigment epithelium (RPE), which lies between the retina and choroid/sclera, appears to play a crucial role in this process. In the investigations reported here, we used a chick model system to assess the threshold duration of exposure to lens-imposed defocus and form-deprivation necessary for conversion of evoked retinal signals into changes in BMP gene expression in the RPE. Our study provides evidence for the following: 1) close-loop, optical defocus-guided (negative and positive lenses) bidirectional BMP gene expression regulation, 2) open-loop, form-deprivation (diffusers)-induced down-regulation of BMP gene expression, and 3) early, transient up-regulation of BMP gene expression in response to both types of lens and diffuser applications. The critical exposure for accurately encoding retinal images as biological signals at the level of the RPE is in the order of minutes to hours, depending on the nature of the visual manipulations.

A single retinal circuit model for multiple computations

Vision is dependent on extracting intricate features of the visual information from the outside world, and complex visual computations begin to take place as soon as at the retinal level. In multiple studies on salamander retinas, the responses of a subtype of retinal ganglion cells, i.e., fast/biphasic-OFF ganglion cells, have been shown to be able to realize multiple functions, such as the segregation of a moving object from its background, motion anticipation, and rapid encoding of the spatial features of a new visual scene. For each of these visual functions, modeling approaches using extended linear-nonlinear cascade models suggest specific preceding retinal circuitries merging onto fast/biphasic-OFF ganglion cells. However, whether multiple visual functions can be accommodated together in a certain retinal circuitry and how specific mechanisms for each visual function interact with each other have not been investigated. Here, we propose a physiologically consistent, detailed computational model of the retinal circuit based on the spatiotemporal dynamics and connections of each class of retinal neurons to implement object motion sensitivity, motion anticipation, and rapid coding in the same circuit. Simulations suggest that multiple computations can be accommodated together, thereby implying that the fast/biphasic-OFF ganglion cell has potential to output a train of spikes carrying multiple pieces of information on distinct features of the visual stimuli.

How Do Efficient Coding Strategies Depend on Origins of Noise in Neural Circuits?

Neural circuits reliably encode and transmit signals despite the presence of noise at multiple stages of processing. The efficient coding hypothesis, a guiding principle in computational neuroscience, suggests that a neuron or population of neurons allocates its limited range of responses as efficiently as possible to best encode inputs while mitigating the effects of noise. Previous work on this question relies on specific assumptions about where noise enters a circuit, limiting the generality of the resulting conclusions. Here we systematically investigate how noise introduced at different stages of neural processing impacts optimal coding strategies. Using simulations and a flexible analytical approach, we show how these strategies depend on the strength of each noise source, revealing under what conditions the different noise sources have competing or complementary effects. We draw two primary conclusions: (1) differences in encoding strategies between sensory systems-or even adaptational changes in encoding properties within a given system-may be produced by changes in the structure or location of neural noise, and (2) characterization of both circuit nonlinearities as well as noise are necessary to evaluate whether a circuit is performing efficiently.

Information persistence evaluated with low-density dot patterns

After more than a century of study, we do not yet fully understand how shapes and patterns are encoded and identified. Greater progress might result from quantifying stimulus information, thus allowing manipulation of the degree to which a shape or pattern can elicit recognition. The present work used discrete dot patterns that are seen as letters of the alphabet. By adjusting the density of the dots in each pattern, one can determine the probability that it will be recognized. The experiments displayed low-density dot patterns to human respondents, assessing the interval across which non-redundant information provided by two compatible subsets would combine to elicit recognition. This provided a measure of the time required for decay of information persistence. Viewed in the context of prior work, the evidence indicates that the retina mediates initial visibility of the stimulus trace, but the longer-duration persistence required for memory retrieval is mediated by visual cortex.

Speeding rod recovery improves temporal resolution in the retina

The temporal resolution of the visual system progressively increases with light intensity. Under scotopic conditions, temporal resolution is relatively poor, and may be limited by both retinal and cortical processes. Rod photoresponses themselves are quite slow because of the slowly deactivating biochemical cascade needed for light transduction. Here, we have used a transgenic mouse line with faster than normal rod phototransduction deactivation (RGS9-overexpressors) to test whether rod signaling to second-order retinal neurons is rate-limited by phototransduction or by other mechanisms. We compared electrical responses of individual wild-type and RGS9-overexpressing (RGS9-ox) rods to steady illumination and found that RGS9-ox rods required 2-fold brighter light for comparable activation, owing to faster G-protein deactivation. When presented with flickering stimuli, RGS9-ox rods showed greater magnitude fluctuations around a given steady-state current amplitude. Likewise, in vivo electroretinography (ERG) and whole-cell recording from OFF-bipolar, rod bipolar, and horizontal cells of RGS9-ox mice displayed larger than normal magnitude flicker responses, demonstrating an improved ability to transmit frequency information across the rod synapse. Slow phototransduction recovery therefore limits synaptic transmission of increments and decrements of light intensity across the first retinal synapse in normal retinas, apparently sacrificing temporal responsiveness for greater overall sensitivity in ambient light.

Targeting channelrhodopsin-2 to ON-bipolar cells with vitreally administered AAV Restores ON and OFF visual responses in blind mice

Most inherited retinal dystrophies display progressive photoreceptor cell degeneration leading to severe visual impairment. Optogenetic reactivation of retinal neurons mediated by adeno-associated virus (AAV) gene therapy has the potential to restore vision regardless of patient-specific mutations. The challenge for clinical translatability is to restore a vision as close to natural vision as possible, while using a surgically safe delivery route for the fragile degenerated retina. To preserve the visual processing of the inner retina, we targeted ON bipolar cells, which are still present at late stages of disease. For safe gene delivery, we used a recently engineered AAV variant that can transduce the bipolar cells after injection into the eye's easily accessible vitreous humor. We show that AAV encoding channelrhodopsin under the ON bipolar cell-specific promoter mediates long-term gene delivery restricted to ON-bipolar cells after intravitreal administration. Channelrhodopsin expression in ON bipolar cells leads to restoration of ON and OFF responses at the retinal and cortical levels. Moreover, light-induced locomotory behavior is restored in treated blind mice. Our results support the clinical relevance of a minimally invasive AAV-mediated optogenetic therapy for visual restoration.

Neuronal and perceptual differences in the temporal processing of darks and lights

Visual information is mediated by two major thalamic pathways that signal light decrements (OFF) and increments (ON) in visual scenes, the OFF pathway being faster than the ON. Here, we demonstrate that this OFF temporal advantage is transferred to visual cortex and has a correlate in human perception. OFF-dominated cortical neurons in cats responded ∼3 ms faster to visual stimuli than ON-dominated cortical neurons, and dark-mediated suppression in ON-dominated neurons peaked ∼14 ms faster than light-mediated suppression in OFF-dominated neurons. Consistent with the neuronal differences, human observers were 6-14 ms faster at detecting darks than lights and better at discriminating dark than light flickers. Neuronal and perceptual differences both vanished if backgrounds were biased toward darks. Our results suggest that the cortical OFF pathway is faster than the ON pathway at increasing and suppressing visual responses, and these differences have parallels in the human visual perception of lights and darks.

Cone-isolating ON-OFF electroretinogram for studying chromatic pathways in the retina

The electroretinogram (ERG) provides information about outer retina function in both clinical and research applications. ERG components elicited by light increments and decrements can be separated using a long-flash paradigm in which periods of light ON and OFF are alternated. Here, the ON-OFF ERG is combined with a silent substitution technique to elicit responses from individual cone photoreceptor classes by modulating the intensities of three color lights between the two periods. The results focus on the short wavelength (S) cone pathways since they are vulnerable to disease and because there are many unanswered questions about S-cone ON and OFF circuitry.

Interacting linear and nonlinear characteristics produce population coding asymmetries between ON and OFF cells in the retina

The early visual system is a model for understanding the roles of cell populations in parallel processing. Cells in this system can be classified according to their responsiveness to different stimuli; a prominent example is the division between cells that respond to stimuli of opposite contrasts (ON vs OFF cells). These two cell classes display many asymmetries in their physiological characteristics (including temporal characteristics, spatial characteristics, and nonlinear characteristics) that, individually, are known to have important roles in population coding. Here we describe a novel distinction between the information that ON and OFF ganglion cell populations carry in mouse--that OFF cells are able to signal motion information about both light and dark objects, while ON cells have a selective deficit at signaling the motion of dark objects. We found that none of the previously reported asymmetries in physiological characteristics could account for this distinction. We therefore analyzed its basis via a recently developed linear-nonlinear-Poisson model that faithfully captures input/output relationships for a broad range of stimuli (Bomash et al., 2013). While the coding differences between ON and OFF cell populations could not be ascribed to the linear or nonlinear components of the model individually, they had a simple explanation in the way that these components interact. Sensory transformations in other systems can likewise be described by these models, and thus our findings suggest that similar interactions between component properties may help account for the roles of cell classes in population coding more generally.

Novel flicker-sensitive visual circuit neurons inhibited by stationary patterns

Many animals use visual motion cues for navigating within their surroundings. Both flies and vertebrates compute local motion by temporal correlation of neighboring photoreceptors, via so-called elementary motion detectors (EMDs). In the fly lobula plate and the vertebrate visual cortex the output from many EMDs is pooled in neurons sensitive to wide-field optic flow. Although the EMD has been the preferred model for more than 50 years, recent work has highlighted its limitations in describing some visual behaviors, such as responses to higher-order motion stimuli. Non-EMD motion processing may therefore serve an important function in vision. Here, we describe a novel neuron class in the fly lobula plate that clearly does not derive its input from classic EMDs. The centrifugal stationary inhibited flicker excited (cSIFE) neuron is strongly excited by flicker, up to very high temporal frequencies. The non-EMD driven flicker sensitivity leads to strong, nondirectional responses to high-speed, wide-field motion. Furthermore, cSIFE is strongly inhibited by stationary patterns, within a narrow wavelength band. cSIFE's outputs overlap with the inputs of well described optic flow-sensitive lobula plate tangential cells (LPTCs). Driving cSIFE affects the active dendrites of LPTCs, and cSIFE may therefore play a large role in motion vision.

Differential effects of alcohol on contrast processing mediated by the magnocellular and parvocellular pathways

This study investigated how acute alcohol intake affects contrast processing mediated by inferred magnocellular (MC) and parvocellular (PC) pathways. Achromatic contrast discrimination thresholds were measured in 16 young healthy participants using a steady-pedestal, pulsed-pedestal or pedestal-Δ-pedestal paradigm designed to favor the inferred MC or the PC pathway. Each participant completed two randomized sessions that included consumption of either 0.8 g/kg alcohol or a placebo beverage, with each session consisting of contrast discrimination measurements at baseline and at 60 min following beverage consumption. The results showed that, compared to placebo, alcohol significantly reduced MC contrast sensitivity and PC contrast gain but had no effect on PC contrast sensitivity for the majority of the participants; and did not alter MC contrast gain consistently across participants. The decrease in contrast gain in the PC pathway can be interpreted as a degradation of the postretinal signal-to-noise ratio, whereas the decrease of sensitivity in the MC pathway likely results from a change of cortical processing.

The slope of the psychometric function and non-stationarity of thresholds in spatiotemporal contrast vision

The slope of the two-interval, forced-choice psychometric function (e.g. the Weibull parameter, β) provides valuable information about the relationship between contrast sensitivity and signal strength. However, little is known about how or whether β varies with stimulus parameters such as spatiotemporal frequency and stimulus size and shape. A second unresolved issue concerns the best way to estimate the slope of the psychometric function. For example, if an observer is non-stationary (e.g. their threshold drifts between experimental sessions), β will be underestimated if curve fitting is performed after collapsing the data across experimental sessions. We measured psychometric functions for 2 experienced observers for 14 different spatiotemporal configurations of pulsed or flickering grating patches and bars on each of 8 days. We found β≈3 to be fairly constant across almost all conditions, consistent with a fixed nonlinear contrast transducer and/or a constant level of intrinsic stimulus uncertainty (e.g. a square law transducer and a low level of intrinsic uncertainty). Our analysis showed that estimating a single β from results averaged over several experimental sessions was slightly more accurate than averaging multiple estimates from several experimental sessions. However, the small levels of non-stationarity (SD≈0.8dB) meant that the difference between the estimates was, in practice, negligible.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Faster thalamocortical processing for dark than light visual targets

ON and OFF visual pathways originate in the retina at the synapse between photoreceptor and bipolar cells. OFF bipolar cells are shorter in length and use receptors with faster kinetics than ON bipolar cells and, therefore, process information faster. Here, we demonstrate that this temporal advantage is maintained through thalamocortical processing, with OFF visual responses reaching cortex ~3-6 ms before ON visual responses. Faster OFF visual responses could be demonstrated in recordings from large populations of cat thalamic neurons representing the center of vision (both X and Y) and from subpopulations making connection with the same cortical orientation column. While the OFF temporal advantage diminished as visual responses reached their peak, the integral of the impulse response was greater in OFF than ON neurons. Given the stimulus preferences from OFF and ON channels, our results indicate that darks are processed faster than lights in the thalamocortical pathway.

Contrast sensitivity, ocular blood flow and their potential role in assessing ischaemic retinal disease

PURPOSE

To examine the definition, evaluation methodology, association to ocular blood flow and potential clinical value of contrast sensitivity (CS) testing in clinical and research settings, focusing in patients with ischemic retinal disease.

METHODS

A review of the medical literature focusing on CS and ocular blood flow in ischemic retinal disease.

RESULTS

CS may be more sensitive than other methods at detecting subtle defects or improvements in primarily central retinal ganglion cell function early on in a disease process. CS testing attempts to provide spatial detection differences which are not directly assessed with standard visual acuity chart testing. Analyzing all studies that have assessed both CS change and ocular blood flow, it is apparent that both choroidal circulation and retinal circulation may have an important role in influencing CS.

CONCLUSION

The concept that CS is directly influenced by ocular blood flow is supported by reviewing the studies involving both. Although the studies in the literature have not established a direct cause and effect relationship per se, the literature review makes it logical to assume that changes in retinal and choroidal blood flow influence CS. This raises the possibility that a subjective visual characteristic, specifically CS, may be able to be evaluated more objectively by studying blood flow. It appears appropriate to study the relationship between blood flow and CS more extensively to develop improved ways of measuring various aspects of blood flow to the eye and to best quantify early changes in visual function.

Center/surround organization of retinal bipolar cells: High correlation of fundamental responses of center and surround to sinusoidal contrasts

Receptive field organization of cone-driven bipolar cells was investigated by intracellular recording in the intact light-adapted retina of the tiger salamander (Ambystoma tigrinum). Centered spots and concentric annuli of optimum dimensions were used to selectively stimulate the receptive field center and surround with sinusoidal modulations of contrast at 3 Hz. At low contrasts, responses of both the center and surround of both ON and OFF bipolar cells were linear, showing high gain and thus contrast enhancement relative to cones. The contrast/response curves for the fundamental response, measured by a Fast Fourier Transform, reached half maximum amplitude quickly at 13% contrast followed by saturation at high contrasts. The variation of the normalized amplitude of the center and surround responses was remarkably similar, showing linear regression over the entire response range with very high correlations, r2 = 0.97 for both ON and OFF cells. The contrast/response curves of both center and surround for both ON and OFF cells were well fit (r2 = 0.98) by an equation for single-site binding. In about half the cells studied, the nonlinear waveforms of center and surround could be brought into coincidence by scaling and shifting the surround response in time. This implies that a nonlinearity, common to both center and surround, occurs after polarity inversion at the cone feedback synapse. Evidence from paired whole-cell recordings between single cones and OFF bipolar cells suggests that substantial nonlinearity is not due to transmission at the cone synapse but instead arises from intrinsic bipolar cell and network mechanisms. When sinusoidal contrast modulations were applied to the center and surround simultaneously, clear additivity was observed for small responses in both ON and OFF cells, whereas the interaction was strikingly nonadditive for large responses. The contribution of the surround was then greatly reduced, suggesting attenuation at the cone feedback synapse.