Suppression without inhibition: how retinal computation contributes to saccadic suppression.

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

Dependence of perceptual saccadic suppression on peri-saccadic image flow properties and luminance contrast polarity.

Across saccades, perceptual detectability of brief visual stimuli is strongly diminished. We recently observed that this perceptual suppression phenomenon is jumpstarted in the retina, suggesting that the phenomenon might be significantly more visual in nature than normally acknowledged. Here, we explicitly compared saccadic suppression strength when saccades were made across a uniform image of constant luminance versus when saccades were made across image patches of different luminance, width, and trans-saccadic luminance polarity. We measured perceptual contrast thresholds of human subjects for brief peri-saccadic flashes of positive (luminance increments) or negative (luminance decrements) polarity. Thresholds were >6-7 times higher when saccades translated a luminance stripe or edge across the retina than when saccades were made over a completely uniform image patch. Critically, both background luminance and flash luminance polarity strongly modulated peri-saccadic contrast thresholds. In addition, all of these very same visual dependencies also occurred in the absence of any saccades, but with qualitatively similar rapid translations of image patches across the retina. This similarity of visual dependencies with and without saccades supports the notion that perceptual saccadic suppression may be fundamentally a visual phenomenon, which strongly motivates neurophysiological and theoretical investigations on the role of saccadic eye movement commands in modulating its properties.

Visual properties of human retinal ganglion cells.

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

Perceptual saccadic suppression starts in the retina.

Visual sensitivity, probed through perceptual detectability of very brief visual stimuli, is strongly impaired around the time of rapid eye movements. This robust perceptual phenomenon, called saccadic suppression, is frequently attributed to active suppressive signals that are directly derived from eye movement commands. Here we show instead that visual-only mechanisms, activated by saccade-induced image shifts, can account for all perceptual properties of saccadic suppression that we have investigated. Such mechanisms start at, but are not necessarily exclusive to, the very first stage of visual processing in the brain, the retina. Critically, neural suppression originating in the retina outlasts perceptual suppression around the time of saccades, suggesting that extra-retinal movement-related signals, rather than causing suppression, may instead act to shorten it. Our results demonstrate a far-reaching contribution of visual processing mechanisms to perceptual saccadic suppression, starting in the retina, without the need to invoke explicit motor-based suppression commands.

The contrast sensitivity function of a small cryptobenthic marine fish.

Spatial resolution is a key property of eyes when it comes to understanding how animals' visual signals are perceived. This property can be robustly estimated by measuring the contrast sensitivity as a function of different spatial frequencies, defined as the number of achromatic vertical bright and dark stripe pairs within one degree of visual angle. This contrast sensitivity function (CSF) has been estimated for different animal groups, but data on fish are limited to two free-swimming, freshwater species (i.e., goldfish and bluegill sunfish). In this study, we describe the CSF of a small marine cryptobenthic fish (Tripterygion delaisi) using an optokinetic reflex approach. Tripterygion delaisi features a contrast sensitivity that is as excellent as other fish species, up to 125 (reciprocal of Michelson contrast) at the optimal spatial frequency of 0.375 c/°. The maximum spatial resolution is instead relatively coarse, around 2.125 c/°. By comparing our results with acuity values derived from anatomical estimates of ganglion cells' density, we conclude that the optokinetic reflex seems to be adapted to process low spatial frequency information from stimuli in the peripheral visual field and show that small marine fish can feature excellent contrast sensitivity at optimal spatial frequency.

Rods progressively escape saturation to drive visual responses in daylight conditions.

Rod and cone photoreceptors support vision across large light intensity ranges. Rods, active under dim illumination, are thought to saturate at higher (photopic) irradiances. The extent of rod saturation is not well defined; some studies report rod activity well into the photopic range. Using electrophysiological recordings from retina and dorsal lateral geniculate nucleus of cone-deficient and visually intact mice, we describe stimulus and physiological factors that influence photopic rod-driven responses. We find that rod contrast sensitivity is initially strongly reduced at high irradiances, but progressively recovers to allow responses to moderate contrast stimuli. Surprisingly, rods recover faster at higher light levels. A model of rod phototransduction suggests that phototransduction gain adjustments and bleaching adaptation underlie rod recovery. Consistently, exogenous chromophore reduces rod responses at bright background. Thus, bleaching adaptation renders mouse rods responsive to modest contrast at any irradiance. Paradoxically, raising irradiance across the photopic range increases the robustness of rod responses.

Alteration of the microsaccadic velocity-amplitude main sequence relationship after visual transients: implications for models of saccade control.

Microsaccades occur during gaze fixation to correct for miniscule foveal motor errors. The mechanisms governing such fine oculomotor control are still not fully understood. In this study, we explored microsaccade control by analyzing the impacts of transient visual stimuli on these movements' kinematics. We found that such kinematics can be altered in systematic ways depending on the timing and spatial geometry of visual transients relative to the movement goals. In two male rhesus macaques, we presented peripheral or foveal visual transients during an otherwise stable period of fixation. Such transients resulted in well-known reductions in microsaccade frequency, and our goal was to investigate whether microsaccade kinematics would additionally be altered. We found that both microsaccade timing and amplitude were modulated by the visual transients, and in predictable manners by these transients' timing and geometry. Interestingly, modulations in the peak velocity of the same movements were not proportional to the observed amplitude modulations, suggesting a violation of the well-known "main sequence" relationship between microsaccade amplitude and peak velocity. We hypothesize that visual stimulation during movement preparation affects not only the saccadic "Go" system driving eye movements but also a "Pause" system inhibiting them. If the Pause system happens to be already turned off despite the new visual input, movement kinematics can be altered by the readout of additional visually evoked spikes in the Go system coding for the flash location. Our results demonstrate precise control over individual microscopic saccades and provide testable hypotheses for mechanisms of saccade control in general.NEW & NOTEWORTHY Microsaccadic eye movements play an important role in several aspects of visual perception and cognition. However, the mechanisms for microsaccade control are still not fully understood. We found that microsaccade kinematics can be altered in a systematic manner by visual transients, revealing a previously unappreciated and exquisite level of control by the oculomotor system of even the smallest saccades. Our results suggest precise temporal interaction between visual, motor, and inhibitory signals in microsaccade control.

Hypothermia Promotes Survival of Ischemic Retinal Ganglion Cells.

PURPOSE: Ischemic stroke in retinal arteries leads to death of neural tissue and ultimately to blindness. The retina is known to die within 4 hours after onset of ischemia. It is debated whether hypothermia might increase the time window for medical treatment and thereby the chance of recovering sight. In order to characterize the time course of cell death during ischemia and potential beneficial effects of hypothermia in more detail, we investigated the survival of ganglion cells in ischemic pig and human retina as a function of time and temperature. METHODS: Eyes were obtained from minipigs and from human donors post mortem. Enucleated minipig eyes were stored for defined durations at three different temperatures (37 °C, 21 °C, and 4 °C). In order to assess the viability of the tissue, we measured ganglion cell activity (spiking) with multielectrode arrays. RESULTS: Minipig retinal ganglion cell function was severely compromised after 2 hours of ischemia at body temperature. After 4 hours, ganglion cells did not fire action potentials anymore. However, at 21 °C, ganglion cell activity was maintained under ischemic conditions for up to 12 hours, and for at least 50 hours at 4 °C. In postmortem human retina, we recorded ganglion cell activity in retinas received up to 27 hours after death. CONCLUSIONS: Our results demonstrate that hypothermia greatly increases survival of retinal ganglion cells exposed to ischemia. These results might be relevant for the future treatment of retinal ischemia.

Influence of Opa1 Mutation on Survival and Function of Retinal Ganglion Cells.

PURPOSE: Mutations in the OPA1 gene cause autosomal dominant optic atrophy (ADOA), a visual disorder associated with degeneration of retinal ganglion cells (RGCs). Here, we characterized the disease progression in a homologous mouse model B6;C3-Opa1 329-355del and asked whether the pronounced cell death affects certain RGC types more than others. METHODS: The influence of the Opa1 mutation was assessed by morphologic (retina and optic nerve histology) and functional (multielectrode array) methods. RESULTS: The RGC loss of approximately 50% within 18 months was significantly more pronounced in RGCs with small-caliber axons. Small-caliber axon RGCs comprise a variety of functional RGC types. Accordingly, electrophysiological analyses of RGCs did not show a dropout of distinct functional RGC subgroups. However, the response properties of RGCs were affected significantly by the mutation. Surprisingly, these functional changes were different under different luminance conditions (scotopic, mesopic, and photopic). Finally, melanopsin cells are known to be less susceptible to retinal insults. We found that these cells are also spared in the Opa1 mouse model, and demonstrated for the first time that this resistance persisted even when the melanopsin gene had been knocked-out. CONCLUSIONS: Small-caliber axons show a higher vulnerability to the Opa1 mutation in our mouse model for ADOA. Luminance-dependent functional changes suggest an influence of the Opa1 mutation on the retinal circuitry upstream of RGCs. Photoresponsive RGCs are protected against cell death due to the Opa1 mutation, but not by melanopsin expression itself.

Effects of the jimpy mutation on mouse retinal structure and function.

The Jimpy mutant mouse has a point mutation in the proteolipid protein gene (plp1). The resulting misfolding of the protein leads to oligodendrocyte death, myelin destruction, and failure to produce adequately myelinated axons in the central nervous system (CNS). It is not known how the absence of normal myelination during development influences neural function. We characterized the Jimpy mouse retina to find out whether lack of myelination in the optic nerve during development has an effect on normal functioning and morphology of the retina. Optokinetic reflex measurements showed that Jimpy mice had, in general, a functional visual system. Both PLP1 antibody staining and reverse transcriptase-polymerase chain reaction for plp1 mRNA showed that plp1 is not expressed in the wild-type retina. However, in the optic nerve, plp1 is normally expressed, and consequently, in Jimpy mutant mice, myelination of axons in the optic nerve was mostly absent. Nevertheless, neither axon count nor axon ultrastructure in the optic nerve was affected. Physiological recordings of ganglion cell activity using microelectrode arrays revealed a decrease of stimulus-evoked activity at mesopic light levels. Morphological analysis of the retina did not show any significant differences in the gross morphology, such as thickness of retinal layers or cell number in the inner and outer nuclear layer. The cell bodies in the inner nuclear layer, however, were larger in the peripheral retina of Jimpy mutant mice. Antibody labeling against cell type-specific markers showed that the number of rod bipolar and horizontal cells was increased in Jimpy mice. In conclusion, whereas the Jimpy mutation has dramatic effects on the myelination of retinal ganglion cell axons, it has moderate effects on retinal morphology and function.

Retinal output changes qualitatively with every change in ambient illuminance.

The collective activity pattern of retinal ganglion cells, the retinal code, underlies higher visual processing. How does the ambient illuminance of the visual scene influence this retinal output? We recorded from isolated mouse and pig retina and from mouse dorsal lateral geniculate nucleus in vivo at up to seven ambient light levels covering the scotopic to photopic regimes. Across each luminance transition, most ganglion cells exhibited qualitative response changes, whereas they maintained stable responses within each luminance. We commonly observed the appearance and disappearance of ON responses in OFF cells and vice versa. Such qualitative response changes occurred for a variety of stimuli, including full-field and localized contrast steps and naturalistic movies. Our results suggest that the retinal code is not fixed but varies with every change of ambient luminance. This finding raises questions about signal processing within the retina and has implications for visual processing in higher brain areas.

Characterization of a mouse model with complete RPE loss and its use for RPE cell transplantation.

PURPOSE: Age-related macular degeneration (AMD) is a major leading cause of visual impairment and blindness with no cure currently established. Cell replacement of RPE is discussed as a potential therapy for AMD. Previous studies were performed in animal models with severe limitations in recapitulating the disease progression. In detail, we describe the effect of systemic injection of sodium iodate in the mouse retina. We further evaluate the usefulness of this animal model to analyze cell-specific effects following transplantation of human embryonic stem cell (hESC)-derived RPE cells. METHODS: Morphologic, functional, and behavioral changes following sodium iodate injection were monitored by histology, gene expression analysis, electroretinography, and optokinetic head tracking. Human embryonic stem cell-derived RPE cells were transplanted 1 week after sodium iodate injection and experimental retinae were analyzed 3 weeks later. RESULTS: Injection of sodium iodate caused complete RPE cell loss, photoreceptor degeneration, and altered gene and protein expression in outer and inner nuclear layers. Retinal function was severely affected by day 3 and abolished from day 14. Following transplantation, donor hESC-derived RPE cells formed extensive monolayers that displayed wild-type RPE cell morphology, organization, and function, including phagocytosis of host photoreceptor outer segments. CONCLUSIONS: Systemic injection of sodium iodate has considerable effects on RPE, photoreceptors, and inner nuclear layer neurons, and provides a model to assay reconstitution and maturation of RPE cell transplants. The availability of an RPE-free Bruch's membrane in this model likely allows the unprecedented formation of extensive polarized cell monolayers from donor hESC-derived RPE cell suspensions.

Step-by-step instructions for retina recordings with perforated multi electrode arrays.

Multi-electrode arrays are a state-of-the-art tool in electrophysiology, also in retina research. The output cells of the retina, the retinal ganglion cells, form a monolayer in many species and are well accessible due to their proximity to the inner retinal surface. This structure has allowed the use of multi-electrode arrays for high-throughput, parallel recordings of retinal responses to presented visual stimuli, and has led to significant new insights into retinal organization and function. However, using conventional arrays where electrodes are embedded into a glass or ceramic plate can be associated with three main problems: (1) low signal-to-noise ratio due to poor contact between electrodes and tissue, especially in the case of strongly curved retinas from small animals, e.g. rodents; (2) insufficient oxygen and nutrient supply to cells located on the bottom of the recording chamber; and (3) displacement of the tissue during recordings. Perforated multi-electrode arrays (pMEAs) have been found to alleviate all three issues in brain slice recordings. Over the last years, we have been using such perforated arrays to study light evoked activity in the retinas of various species including mouse, pig, and human. In this article, we provide detailed step-by-step instructions for the use of perforated MEAs to record visual responses from the retina, including spike recordings from retinal ganglion cells and in vitro electroretinograms (ERG). In addition, we provide in-depth technical and methodological troubleshooting information, and show example recordings of good quality as well as examples for the various problems which might be encountered. While our description is based on the specific equipment we use in our own lab, it may also prove useful when establishing retinal MEA recordings with other equipment.

Salvaging ruins: reverting blind retinas into functional visual sensors.

Blindness is one of the most devastating conditions affecting the quality of life. Hereditary degenerative diseases, such as retinitis pigmentosa, are characterized by the progressive loss of photoreceptors, leading to complete blindness. No treatment is known, the current state-of-the-art of restoring vision are implanted electrode arrays. As a recently discovered alternative, optical neuromodulators, such as channelrhodopsin, allow new strategies for treating these diseases by imparting light-sensitivity onto the remaining retinal neurons after photoreceptor cell death. Retinal degeneration is a heterogeneous set of diseases with diverse secondary effects on the retinal circuitry. Successful treatment strategies have to take into account this diversity, as only the existing retinal hardware can serve as substrate for optogenetic intervention. The goal is to salvage the retinal ruins and to revert the leftover tissue into a functional visual sensor that operates as optimally as possible. Here, we discuss three different successful approaches that have been applied to degenerated mouse retina.

Strategies for expanding the operational range of channelrhodopsin in optogenetic vision.

Some hereditary diseases, such as retinitis pigmentosa, lead to blindness due to the death of photoreceptors, though the rest of the visual system might be only slightly affected. Optogenetics is a promising tool for restoring vision after retinal degeneration. In optogenetics, light-sensitive ion channels ("channelrhodopsins") are expressed in neurons so that the neurons can be activated by light. Currently existing variants of channelrhodopsin--engineered for use in neurophysiological research--do not necessarily support the goal of vision restoration optimally, due to two factors: First, the nature of the light stimulus is fundamentally different in "optogenetic vision" compared to "optogenetic neuroscience". Second, the retinal target neurons have specific properties that need to be accounted for, e.g. most retinal neurons are non-spiking. In this study, by using a computational model, we investigate properties of channelrhodopsin that might improve successful vision restoration. We pay particular attention to the operational brightness range and suggest strategies that would allow optogenetic vision over a wider intensity range than currently possible, spanning the brightest 5 orders of naturally occurring luminance. We also discuss the biophysical limitations of channelrhodopsin, and of the expressing cells, that prevent further expansion of this operational range, and we suggest design strategies for optogenetic tools which might help overcoming these limitations. Furthermore, the computational model used for this study is provided as an interactive tool for the research community.

Visual behavior: mice run from overhead danger.

Mice show an innate protective behavior to looming shadows approaching from above: they either run for cover or freeze in place. This newly discovered 'looming response' adds to the repertoire of stereotyped behaviors that can be utilized to study visual pathways.

Characterizing visual performance in mice: an objective and automated system based on the optokinetic reflex.

Testing optokinetic head or eye movements is an established method to determine visual performance of laboratory animals, including chickens, guinea pigs, mice, or fish. It is based on the optokinetic reflex which causes the animals to track a drifting stripe pattern with eye and head movements. We have developed an improved version of the optomotor test with better control over the stimulus parameters, as well as a high degree of automation. The stripe pattern is presented on computer monitors surrounding the animal. By tracking the head position of freely moving animals in real time, the visual angle under which the stripes of the pattern appeared was kept constant even for changing head positions. Furthermore, an algorithm was developed for automated evaluation of the tracking performance of the animal. Comparing the automatically determined behavioral score with manual assessment of the animals' tracking behavior confirmed the reliability of our methodology. As an example, we reproduced the known contrast sensitivity function of wild type mice. Furthermore, the progressive decline in visual performance of a mouse model of retinal degeneration, rd10, was demonstrated.

Relevance of exocytotic glutamate release from retinal glia.

Glial cells release molecules that influence brain development, function, and disease. Calcium-dependent exocytosis has been proposed as potential release mechanism in astroglia, but the physiological relevance of "gliotransmission" in vivo remains controversial. We focused on the impact of glial exocytosis on sensory transduction in the retina. To this end, we generated transgenic mice to block exocytosis by Cre recombinase-dependent expression of the clostridial botulinum neurotoxin serotype B light chain, which cleaves vesicle-associated membrane protein 1-3. Ubiquitous and neuronal toxin expression caused perinatal lethality and a reduction of synaptic transmission thus validating transgene function. Toxin expression in Müller cells inhibited vesicular glutamate release and impaired glial volume regulation but left retinal histology and visual processing unaffected. Our model to study gliotransmission in vivo reveals specific functions of exocytotic glutamate release in retinal glia.

Neuropeptides regulate swimming depth of Platynereis larvae.

Cilia-based locomotion is the major form of locomotion for microscopic planktonic organisms in the ocean. Given their negative buoyancy, these organisms must control ciliary activity to maintain an appropriate depth. The neuronal bases of depth regulation in ciliary swimmers are unknown. To gain insights into depth regulation we studied ciliary locomotor control in the planktonic larva of the marine annelid, Platynereis. We found several neuropeptides expressed in distinct sensory neurons that innervate locomotor cilia. Neuropeptides altered ciliary beat frequency and the rate of calcium-evoked ciliary arrests. These changes influenced larval orientation, vertical swimming, and sinking, resulting in upward or downward shifts in the steady-state vertical distribution of larvae. Our findings indicate that Platynereis larvae have depth-regulating peptidergic neurons that directly translate sensory inputs into locomotor output on effector cilia. We propose that the simple circuitry found in these ciliated larvae represents an ancestral state in nervous system evolution.

Approach sensitivity in the retina processed by a multifunctional neural circuit.

The detection of approaching objects, such as looming predators, is necessary for survival. Which neurons and circuits mediate this function? We combined genetic labeling of cell types, two-photon microscopy, electrophysiology and theoretical modeling to address this question. We identify an approach-sensitive ganglion cell type in the mouse retina, resolve elements of its afferent neural circuit, and describe how these confer approach sensitivity on the ganglion cell. The circuit's essential building block is a rapid inhibitory pathway: it selectively suppresses responses to non-approaching objects. This rapid inhibitory pathway, which includes AII amacrine cells connected to bipolar cells through electrical synapses, was previously described in the context of night-time vision. In the daytime conditions of our experiments, the same pathway conveys signals in the reverse direction. The dual use of a neural pathway in different physiological conditions illustrates the efficiency with which several functions can be accommodated in a single circuit.