Using eye movements to assess brain function in mice

Modeling Retinitis Pigmentosa 59: Dhdds T206A and Dhdds K42E knock-in mutant mice are phenotypically similar

D e h ydro d olichyl d iphosphate s ynthase (DHDDS) is an essential enzyme required for several forms of protein glycosylation in all eukaryotic cells. Surprisingly, three mutant alleles, ( Dhdds K42E/K42E (K42E/K42E), Dhdds T206A/K42E (T206A/K42E), and found in only one patient, Dhdds R98W/K42E (R98W/K42E) have been reported that cause non-syndromic retinitis pigmentosa (RP59), an inherited retinal degeneration (IRD). Because T206A was only observed heterozygously with the K42E allele in RP59 patients, we used CRISPR/CAS9 technology to generate T206A/T206A, and subsequently T206A/K42E alleles in mice to assess the contribution of the T206A allele to the disease phenotype, to model the human disease, and to compare resulting phenotypes to our homozygous K42E mouse model. By postnatal (PN) 12-mo, T206A/K42E mice exhibit significant reduction of inner nuclear layer thickness as was observed in K42E/K42E mice. No change in outer nuclear layer thickness is observed in all mutant phenotypes up to PN 12 mo. Electroretinography (ERG) showed a significantly reduced b-wave without a-wave decrement and by PN 3-mo, ERG c- and d-wave responses were significantly attenuated in all phenotypes. Consistent with a reduction in inner nuclear layer thickness seen by OCT and cell loss observed by histology, bipolar and amacrine cell densities were reduced in all Dhdds mutant phenotypes compared to PN 8-12 mo age-matched controls. These results indicate that the DHDDS T206A allele causes retinal disease independent of the K42E allele, and that there likely is a common disease mechanism involving RP59-associated DHDDS mutations. We conclude that the physiological basis of retinal dysfunction in RP59 involves defective signaling in the inner retina resulting in bipolar/amacrine cell degeneration.

Instrumented swim test for quantifying motor impairment in rodents

Swim tests are highly effective for identifying vestibular deficits in rodents by offering significant vestibular motor challenges with reduced proprioceptive input, unlike rotarod and balance beam tests. Traditional swim tests rely on subjective assessments, limiting objective quantification and reproducibility. We present a novel instrumented swim test using a miniature motion sensor with a 3D accelerometer and 3D gyroscope affixed to the rodent's head. This setup robustly quantifies six-dimensional motion-three translational and three rotational axes-during swimming with high temporal resolution. We demonstrate the test's capabilities by comparing head movements of Gpr156-/- mutant mice, which have impaired otolith organ development, to their heterozygous littermates. Our results show axis-specific differences in head movement probability distribution functions and dynamics that identify mice with the Gpr156 mutation. Axis-specific power spectrum analyses reveal selective movement alterations within distinct frequency ranges. Additionally, our spherical visualization and 3D analysis quantifies swimming performance based on head vector distance from upright. We use this analysis to generate a single classifier metric-a weighted average of an animal's head deviation from upright during swimming. This metric effectively distinguishes animals with vestibular dysfunction from those with normal vestibular function. Overall, this instrumented swim test provides quantitative metrics for assessing performance and identifying subtle, axis- and frequency-specific deficits not captured by existing systems. This novel quantitative approach can enhance understanding of rodent sensorimotor function including enabling more selective and reproducible studies of vestibular-motor deficits.

Tau P301S Transgenic Mice Develop Gait and Eye Movement Impairments That Mimic Progressive Supranuclear Palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative disorder with an estimated prevalence of 5-7 people in 100,000. Clinically characterized by impairments in gait, balance, and eye movements, as well as aggregated Tau pathology, PSP leads to death in approximately 5-8 years. No disease-modifying treatments are currently available. The contribution of Tau pathology to the symptoms of patients with PSP is poorly understood, in part due to lack of a rodent model that recapitulates characteristic aspects of PSP. Here, we assessed the hTau.P301S mouse for key clinical features of PSP, finding progressive impairments in balance and gait coordination. Additionally, we found impairments in fast vertical eye movements, one of the most distinctive features of PSP. Across animals, we found that Tau pathology in motor control regions correlated with motor deficits. These findings highlight the utility of the hP301S mouse in modeling key aspects of PSP.

Vestibular hair cells are more prone to damage by excessive acceleration insult in the mouse with KCNQ4 dysfunction

KCNQ4 is a voltage-gated K+ channel was reported to distribute over the basolateral surface of type 1 vestibular hair cell and/or inner surface of calyx and heminode of the vestibular nerve connected to the type 1 vestibular hair cells of the inner ear. However, the precise localization of KCNQ4 is still controversial and little is known about the vestibular phenotypes caused by KCNQ4 dysfunction or the specific role of KCNQ4 in the vestibular organs. To investigate the role of KCNQ4 in the vestibular organ, 6-g hypergravity stimulation for 24 h, which represents excessive mechanical stimulation of the sensory epithelium, was applied to p.W277S Kcnq4 transgenic mice. KCNQ4 was detected on the inner surface of calyx of the vestibular afferent in transmission electron microscope images with immunogold labelling. Vestibular function decrease was more severe in the Kcnq4p.W277S/p.W277S mice than in the Kcnq4+/+ and Kcnq4+/p.W277S mice after the stimulation. The vestibular function loss was resulted from the loss of type 1 vestibular hair cells, which was possibly caused by increased depolarization duration. Retigabine, a KCNQ activator, prevented hypergravity-induced vestibular dysfunction and hair cell loss. Patients with KCNQ4 mutations also showed abnormal clinical vestibular function tests. These findings suggest that KCNQ4 plays an essential role in calyx and afferent of type 1 vestibular hair cell preserving vestibular function against excessive mechanical stimulation.

Behind mouse eyes: The function and control of eye movements in mice

The mouse visual system has become the most popular model to study the cellular and circuit mechanisms of sensory processing. However, the importance of eye movements only started to be appreciated recently. Eye movements provide a basis for predictive sensing and deliver insights into various brain functions and dysfunctions. A plethora of knowledge on the central control of eye movements and their role in perception and behaviour arose from work on primates. However, an overview of various eye movements in mice and a comparison to primates is missing. Here, we review the eye movement types described to date in mice and compare them to those observed in primates. We discuss the central neuronal mechanisms for their generation and control. Furthermore, we review the mounting literature on eye movements in mice during head-fixed and freely moving behaviours. Finally, we highlight gaps in our understanding and suggest future directions for research.

Necl-1/CADM3 regulates cone synapse formation in the mouse retina

In vertebrates, retinal neural circuitry for visual perception is organized in specific layers. The outer plexiform layer is the first synaptic region in the visual pathway, where photoreceptor synaptic terminals connect with bipolar and horizontal cell processes. However, molecular mechanisms underlying cone synapse formation to mediate OFF pathways remain unknown. This study reveals that Necl-1/CADM3 is localized at S- and S/M-opsin-containing cones and dendrites of type 4 OFF cone bipolar cells (CBCs). In Necl-1-/- mouse retina, synapses between cones and type 4 OFF CBCs were dislocated, horizontal cell distribution became abnormal, and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors were dislocated. Necl-1-/- mice exhibited aberrant short-wavelength-light-elicited signal transmission from cones to OFF CBCs, which was rescued by AMPA receptor potentiator. Additionally, Necl-1-/- mice showed impaired optokinetic responses. These findings suggest that Necl-1 regulates cone synapse formation to mediate OFF cone pathways elicited by short-wavelength light in mouse retina.

A semi-automated method for quantifying optokinetic reflex tracking acuity

The study of murine behavioral responses to visual stimuli is a key component of understanding mammalian visual circuitry. One notable response is the optokinetic reflex (OKR), a highly conserved innate behavior necessary for image stabilization on the retina. The OKR provides a robust readout of image tracking ability and has been extensively studied to understand the logic of visual system circuitry and function in mice from different genetic backgrounds. The OKR consists of two phases: a slow tracking phase as the eye follows a stimulus to the edge of the visual plane, and a compensatory fast phase saccade that maintains the image within the visual field. Assessment of the OKR has previously relied on counting individual compensatory eye saccades to estimate tracking speed. To obtain a more direct quantification of tracking ability, we have developed a novel, semi-automated analysis program that allows for rapid and reproducible quantification of unidirectional tracking gains, in addition to being adaptable to any video-oculography equipment. Our analysis program allows for the selection of slow tracking phases, modeling of the vertical and horizontal eye vectors, quantification of eye movement relative to the stimulus, and organization of resultant data into a usable spreadsheet for statistical and graphical comparisons. This quantitative and streamlined analysis pipeline provides a faster and more direct measurement of OKR responses, thereby facilitating further study of visual behavior responses.

SUMMARY

We describe here a semi-automated quantitative analysis method that directly measures eye tracking resulting from murine visual system responses to two-dimensional image motion. A Python-based user interface and analysis algorithm allows for higher throughput and more quantitative measurements of eye tracking parameters than previous methods.

Purkinje cell microzones mediate distinct kinematics of a single movement

The classification of neuronal subpopulations has significantly advanced, yet its relevance for behavior remains unclear. The highly organized flocculus of the cerebellum, known to fine-tune multi-axial eye movements, is an ideal substrate for the study of potential functions of neuronal subpopulations. Here, we demonstrate that its recently identified subpopulations of 9+ and 9- Purkinje cells exhibit an intermediate Aldolase C expression and electrophysiological profile, providing evidence for a graded continuum of intrinsic properties among PC subpopulations. By identifying and utilizing two Cre-lines that genetically target these floccular domains, we show with high spatial specificity that these subpopulations of Purkinje cells participate in separate micromodules with topographically organized connections. Finally, optogenetic excitation of the respective subpopulations results in movements around the same axis in space, yet with distinct kinematic profiles. These results indicate that Purkinje cell subpopulations integrate in discrete circuits and mediate particular parameters of single movements.

A Dhdds K42E knock-in RP59 mouse model shows inner retina pathology and defective synaptic transmission

Retinitis pigmentosa (RP) defines a group of hereditary progressive rod-cone degenerations that exhibit a common phenotype caused by variants in over 70 genes. While most variants in the dehydrodolichyl diphosphate synthase (DHDDS) gene result in syndromic abnormalities, some variants cause non-syndromic RP (RP59). DHDDS encodes one subunit of the enzyme cis-prenyltransferase (CPT), which is required for the synthesis of dolichol (Dol), that is a necessary protein glycosylation cofactor. We previously reported the creation and initial characterization of a knock-in (KI) mouse model harboring the most prevalent RP59-associated DHDDS variant (K42E) to understand how defects in DHDDS lead to retina-specific pathology. This model exhibited no profound retinal degeneration, nor protein N-glycosylation defects. Here, we report that the Dol isoprenylogue species in retina, liver, and brain of the K42E mouse model are statistically shorter than in the corresponding tissues of age-matched controls, as reported in blood and urine of RP59 patients. Retinal transcriptome analysis demonstrated elevation of many genes encoding proteins involved in synaptogenesis and synaptic function. Quantitative retinal cell layer thickness measurements demonstrated a significant reduction in the inner nuclear layer (INL) and total retinal thickness (TRT) beginning at postnatal (PN) ∼2 months, progressively increasing to PN 18-mo. Histological analysis revealed cell loss in the INL, outer plexiform layer (OPL) disruption, and ectopic localization of outer nuclear layer (ONL) nuclei into the OPL of K42E mutant retinas, relative to controls. Electroretinograms (ERGs) of mutant mice exhibited reduced b-wave amplitudes beginning at PN 1-mo, progressively declining through PN 18-mo, without appreciable a-wave attenuation, relative to controls. Our results suggest that the underlying cause of DHDDS K42E variant driven RP59 retinal pathology is defective synaptic transmission from outer to inner retina.

Cortical Integration of Vestibular and Visual Cues for Navigation, Visual Processing, and Perception

Despite increasing evidence of its involvement in several key functions of the cerebral cortex, the vestibular sense rarely enters our consciousness. Indeed, the extent to which these internal signals are incorporated within cortical sensory representation and how they might be relied upon for sensory-driven decision-making, during, for example, spatial navigation, is yet to be understood. Recent novel experimental approaches in rodents have probed both the physiological and behavioral significance of vestibular signals and indicate that their widespread integration with vision improves both the cortical representation and perceptual accuracy of self-motion and orientation. Here, we summarize these recent findings with a focus on cortical circuits involved in visual perception and spatial navigation and highlight the major remaining knowledge gaps. We suggest that vestibulo-visual integration reflects a process of constant updating regarding the status of self-motion, and access to such information by the cortex is used for sensory perception and predictions that may be implemented for rapid, navigation-related decision-making.

Effect of Green Tea and Tea Catechin on the Visual Motion Processing for Optokinetic Responses in Mice

In general, catechins contained in green tea are believed to have positive effects on the human body and mental health. The intake of epigallocatechin gallate (EGCG), a major polyphenol in green tea, is known to be effective for retinal protection; however, whether green tea and/or EGCG affect visual function remains unknown. In the present study, we investigated the effect of green tea and EGCG on visual motion processing by measuring optokinetic responses (OKRs) in young adult and aging mice. Young and aging mice (C57BL6/J) were fed a control diet (control) or the test diet, which contained matcha green tea powder or green tea extract (dried sencha green tea infusion), for 1 month, and their OKRs were measured. They were then intraperitoneally administered saline (control) or EGCG, and OKRs were measured. We found that the OKRs of young and aging mice after green tea intake and after EGCG administration showed higher temporal sensitivity than those of control mice. The visual ability to detect moving objects was enhanced in young and aging mice upon intake of green tea or EGCG. From the above results, the visual motion processing for optokinetic responses by ingesting green tea was enhanced, which may be related to the effect of EGCG.

Low Gain Values of the Vestibulo-Ocular Reflex Can Optimize Retinal Image Slip

The angular vestibulo-ocular reflex (aVOR) stabilizes retinal images by counter-rotating the eyes during head rotations. Perfect compensatory movements would thus rotate the eyes exactly opposite to the head, that is, eyes vs. head would exhibit a unity gain. However, in many species, but also in elderly humans or patients with a history of vestibular damage, the aVOR is far from compensatory with gains that are in part considerably lower than unity. The reason for this apparent suboptimality is unknown. Here, we propose that low VOR gain values reflect an optimal adaptation to sensory and motor signal variability. According to this hypothesis, gaze stabilization mechanisms that aim at minimizing the overall retinal image slip must consider the effects of (1) sensory and motor noise and (2) dynamic constraints of peripheral and central nervous processing. We demonstrate that a computational model for optimizing retinal image slip in the presence of such constraints of signal processing in fact predicts gain values smaller than unity. We further show specifically for tadpoles of the clawed toad, Xenopus laevis with particularly low gain values that previously reported VOR gains quantitatively correspond to the observed variability of eye movements and thus constitute an optimal adaptation mechanism. We thus hypothesize that lower VOR gain values in elderly human subjects or recovered patients with a history of vestibular damage may be the sign of an optimization given higher noise levels rather than a direct consequence of the damage, such as an inability of executing fast compensatory eye movements.

Impact of walking speed and motion adaptation on optokinetic nystagmus-like head movements in the blowfly Calliphora

The optokinetic nystagmus is a gaze-stabilizing mechanism reducing motion blur by rapid eye rotations against the direction of visual motion, followed by slower syndirectional eye movements minimizing retinal slip speed. Flies control their gaze through head turns controlled by neck motor neurons receiving input directly, or via descending neurons, from well-characterized directional-selective interneurons sensitive to visual wide-field motion. Locomotion increases the gain and speed sensitivity of these interneurons, while visual motion adaptation in walking animals has the opposite effects. To find out whether flies perform an optokinetic nystagmus, and how it may be affected by locomotion and visual motion adaptation, we recorded head movements of blowflies on a trackball stimulated by progressive and rotational visual motion. Flies flexibly responded to rotational stimuli with optokinetic nystagmus-like head movements, independent of their locomotor state. The temporal frequency tuning of these movements, though matching that of the upstream directional-selective interneurons, was only mildly modulated by walking speed or visual motion adaptation. Our results suggest flies flexibly control their gaze to compensate for rotational wide-field motion by a mechanism similar to an optokinetic nystagmus. Surprisingly, the mechanism is less state-dependent than the response properties of directional-selective interneurons providing input to the neck motor system.

Evaluation of Spatially Targeted Scleral Stiffening on Neuroprotection in a Rat Model of Glaucoma

Purpose

Scleral stiffening may protect against glaucomatous retinal ganglion cell (RGC) loss or dysfunction associated with ocular hypertension. Here, we assess the potential neuroprotective effects of two treatments designed to stiffen either the entire posterior sclera or only the sclera adjacent to the peripapillary sclera in an experimental model of glaucoma.

Methods

Rat sclerae were stiffened in vivo using either genipin (crosslinking the entire posterior sclera) or a regionally selective photosensitizer, methylene blue (stiffening only the juxtaperipapillary region surrounding the optic nerve). Ocular hypertension was induced using magnetic microbeads delivered to the anterior chamber. Morphological and functional outcomes, including optic nerve axon count and appearance, retinal thickness measured by optical coherence tomography, optomotor response, and electroretinography traces, were assessed.

Results

Both local (juxtaperipapillary) and global (whole posterior) scleral stiffening treatments were successful at increasing scleral stiffness, but neither provided demonstrable neuroprotection in hypertensive eyes as assessed by RGC axon counts and appearance, optomotor response, or electroretinography. There was a weak indication that scleral crosslinking protected against retinal thinning as assessed by optical coherence tomography.

Conclusions

Scleral stiffening was not demonstrated to be neuroprotective in ocular hypertensive rats. We hypothesize that the absence of benefit may in part be due to RGC loss associated with the scleral stiffening agents themselves (mild in the case of genipin, and moderate in the case of methylene blue), negating any potential benefit of scleral stiffening.

Translational Relevance

The development of scleral stiffening as a neuroprotective treatment will require the identification of better tolerated stiffening protocols and further preclinical testing.

Vestibulo-Ocular Reflex Short-Term Adaptation Is Halved After Compensation for Unilateral Labyrinthectomy

Several prior studies, including those from this laboratory, have suggested that vestibulo-ocular reflex (VOR) adaptation and compensation are two neurologically related mechanisms. We therefore hypothesised that adaptation would be affected by compensation, depending on the amount of overlap between these two mechanisms. To better understand this overlap, we examined the effect of gain-increase (gain = eye velocity/head velocity) adaptation training on the VOR in compensated mice since both adaptation and compensation mechanisms are presumably driving the gain to increase. We tested 11 cba129 controls and 6 α9-knockout mice, which have a compromised efferent vestibular system (EVS) known to affect both adaptation and compensation mechanisms. Baseline VOR gains across frequencies (0.2 to 10 Hz) and velocities (20 to 100°/s) were measured on day 28 after unilateral labyrinthectomy (UL) and post-adaptation gains were measured after gain-increase training on day 31 post-UL. Our findings showed that after chronic compensation gain-increase adaptation, as a percentage of baseline, in both strains of mice (~14%), was about half compared to their previously reported healthy, non-operated counterparts (~32%). Surprisingly, there was no difference in gain-increase adaptation between control and α9-knockout mice. These data support the notion that adaptation and compensation are separate but overlapping processes. They also suggest that half of the original adaptation capacity remained in chronically compensated mice, regardless of EVS compromise associated with α9-knockout mice, and strongly suggest VOR adaptation training is a viable treatment strategy for vestibular rehabilitation therapy and, importantly, augments the compensatory process.

Mapping vestibular and visual contributions to angular head velocity tuning in the cortex

Neurons that signal the angular velocity of head movements (AHV cells) are important for processing visual and spatial information. However, it has been challenging to isolate the sensory modality that drives them and to map their cortical distribution. To address this, we develop a method that enables rotating awake, head-fixed mice under a two-photon microscope in a visual environment. Starting in layer 2/3 of the retrosplenial cortex, a key area for vision and navigation, we find that 10% of neurons report angular head velocity (AHV). Their tuning properties depend on vestibular input with a smaller contribution of vision at lower speeds. Mapping the spatial extent, we find AHV cells in all cortical areas that we explored, including motor, somatosensory, visual, and posterior parietal cortex. Notably, the vestibular and visual contributions to AHV are area dependent. Thus, many cortical circuits have access to AHV, enabling a diverse integration with sensorimotor and cognitive information.

Multisensory coding of angular head velocity in the retrosplenial cortex

To successfully navigate the environment, animals depend on their ability to continuously track their heading direction and speed. Neurons that encode angular head velocity (AHV) are fundamental to this process, yet the contribution of various motion signals to AHV coding in the cortex remains elusive. By performing chronic single-unit recordings in the retrosplenial cortex (RSP) of the mouse and tracking the activity of individual AHV cells between freely moving and head-restrained conditions, we find that vestibular inputs dominate AHV signaling. Moreover, the addition of visual inputs onto these neurons increases the gain and signal-to-noise ratio of their tuning during active exploration. Psychophysical experiments and neural decoding further reveal that vestibular-visual integration increases the perceptual accuracy of angular self-motion and the fidelity of its representation by RSP ensembles. We conclude that while cortical AHV coding requires vestibular input, where possible, it also uses vision to optimize heading estimation during navigation.

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Angular head velocity (AHV) coding is widespread in the retrosplenial cortex (RSP)

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AHV cells maintain their tuning during passive motion and require vestibular input

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The perception of angular self-motion is improved when visual cues are present

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AHV coding is similarly improved when both vestibular and visual stimuli are used

Rats spontaneously perceive global motion direction of drifting plaids

Computing global motion direction of extended visual objects is a hallmark of primate high-level vision. Although neurons selective for global motion have also been found in mouse visual cortex, it remains unknown whether rodents can combine multiple motion signals into global, integrated percepts. To address this question, we trained two groups of rats to discriminate either gratings (G group) or plaids (i.e., superpositions of gratings with different orientations; P group) drifting horizontally along opposite directions. After the animals learned the task, we applied a visual priming paradigm, where presentation of the target stimulus was preceded by the brief presentation of either a grating or a plaid. The extent to which rat responses to the targets were biased by such prime stimuli provided a measure of the spontaneous, perceived similarity between primes and targets. We found that gratings and plaids, when used as primes, were equally effective at biasing the perception of plaid direction for the rats of the P group. Conversely, for the G group, only the gratings acted as effective prime stimuli, while the plaids failed to alter the perception of grating direction. To interpret these observations, we simulated a decision neuron reading out the representations of gratings and plaids, as conveyed by populations of either component or pattern cells (i.e., local or global motion detectors). We concluded that the findings for the P group are highly consistent with the existence of a population of pattern cells, playing a functional role similar to that demonstrated in primates. We also explored different scenarios that could explain the failure of the plaid stimuli to elicit a sizable priming magnitude for the G group. These simulations yielded testable predictions about the properties of motion representations in rodent visual cortex at the single-cell and circuitry level, thus paving the way to future neurophysiology experiments.

Deficiency of the neurodevelopmental disorder-associated gene Cyfip2 alters the retinal ganglion cell properties and visual acuity

Intellectual disability (ID) is a neurodevelopmental disorder affecting approximately 0.5–3% of the population in the developed world. Individuals with ID exhibit deficits in intelligence, impaired adaptive behavior and often visual impairments. Cytoplasmic fragile X mental retardation 1 (FMR1)-interacting protein 2 (CYFIP2) is an interacting partner of the FMR protein, whose loss results in fragile X syndrome, the most common inherited cause of ID. Recently, CYFIP2 variants have been found in patients with early-onset epileptic encephalopathy, developmental delay and ID. Such individuals often exhibit visual impairments; however, the underlying mechanism is poorly understood. In the present study, we investigated the role of Cyfip2 in retinal and visual functions by generating and analyzing Cyfip2 conditional knockout (CKO) mice. While we found no major differences in the layer structures and cell compositions between the control and Cyfip2 CKO retinas, a subset of genes associated with the transporter and channel activities was differentially expressed in Cyfip2 CKO retinas than in the controls. Multi-electrode array recordings showed more sustained and stronger responses to positive flashes of the ON ganglion cells in the Cyfip2 CKO retina than in the controls, although electroretinogram analysis revealed that Cyfip2 deficiency unaffected the photoreceptor and ON bipolar cell functions. Furthermore, analysis of initial and late phase optokinetic responses demonstrated that Cyfip2 deficiency impaired the visual function at the organismal level. Together, our results shed light on the molecular mechanism underlying the visual impairments observed in individuals with CYFIP2 variants and, more generally, in patients with neurodevelopmental disorders, including ID.

Development of the vertebrate retinal direction-selective circuit

The vertebrate retina contains an array of neural circuits that detect distinct features in visual space. Direction-selective (DS) circuits are an evolutionarily conserved retinal circuit motif - from zebrafish to rodents to primates - specialized for motion detection. During retinal development, neuronal subtypes that wire DS circuits form exquisitely precise connections with each other to shape the output of retinal ganglion cells tuned for specific speeds and directions of motion. In this review, we follow the chronology of DS circuit development in the vertebrate retina, including the cellular, molecular, and activity-dependent mechanisms that regulate the formation of DS circuits, from cell birth and migration to synapse formation and refinement. We highlight recent findings that identify genetic programs critical for specifying neuronal subtypes within DS circuits and molecular interactions essential for responses along the cardinal axes of motion. Finally, we discuss the roles of DS circuits in visual behavior and in certain human visual disease conditions. As one of the best-characterized circuits in the vertebrate retina, DS circuits represent an ideal model system for studying the development of neural connectivity at the level of individual genes, cells, and behavior.

Differential spatiotemporal development of Purkinje cell populations and cerebellum-dependent sensorimotor behaviors

Distinct populations of Purkinje cells (PCs) with unique molecular and connectivity features are at the core of the modular organization of the cerebellum. Previously, we showed that firing activity of PCs differs between ZebrinII-positive and ZebrinII-negative cerebellar modules (Zhou et al., 2014; Wu et al., 2019). Here, we investigate the timing and extent of PC differentiation during development in mice. We found that several features of PCs, including activity levels, dendritic arborization, axonal shape and climbing fiber input, develop differentially between nodular and anterior PC populations. Although all PCs show a particularly rapid development in the second postnatal week, anterior PCs typically have a prolonged physiological and dendritic maturation. In line herewith, younger mice exhibit attenuated anterior-dependent eyeblink conditioning, but faster nodular-dependent compensatory eye movement adaptation. Our results indicate that specific cerebellar regions have unique developmental timelines which match with their related, specific forms of cerebellum-dependent behaviors.

Mouse entorhinal cortex encodes a diverse repertoire of self-motion signals

Neural circuits generate representations of the external world from multiple information streams. The navigation system provides an exceptional lens through which we may gain insights about how such computations are implemented. Neural circuits in the medial temporal lobe construct a map-like representation of space that supports navigation. This computation integrates multiple sensory cues, and, in addition, is thought to require cues related to the individual's movement through the environment. Here, we identify multiple self-motion signals, related to the position and velocity of the head and eyes, encoded by neurons in a key node of the navigation circuitry of mice, the medial entorhinal cortex (MEC). The representation of these signals is highly integrated with other cues in individual neurons. Such information could be used to compute the allocentric location of landmarks from visual cues and to generate internal representations of space.

Influence of Aging on the Retina and Visual Motion Processing for Optokinetic Responses in Mice

The decline in visual function due to normal aging impacts various aspects of our daily lives. Previous reports suggest that the aging retina exhibits mislocalization of photoreceptor terminals and reduced amplitudes of scotopic and photopic electroretinogram (ERG) responses in mice. These abnormalities are thought to contribute to age-related visual impairment; however, the extent to which visual function is impaired by aging at the organismal level is unclear. In the present study, we focus on the age-related changes of the optokinetic responses (OKRs) in visual processing. Moreover, we investigated the initial and late phases of the OKRs in young adult (2-3 months old) and aging mice (21-24 months old). The initial phase was evaluated by measuring the open-loop eye velocity of OKRs using sinusoidal grating patterns of various spatial frequencies (SFs) and moving at various temporal frequencies (TFs) for 0.5 s. The aging mice exhibited initial OKRs with a spatiotemporal frequency tuning that was slightly different from those in young adult mice. The late-phase OKRs were investigated by measuring the slow-phase velocity of the optokinetic nystagmus evoked by sinusoidal gratings of various spatiotemporal frequencies moving for 30 s. We found that optimal SF and TF in the normal aging mice are both reduced compared with those in young adult mice. In addition, we measured the OKRs of 4.1G-null (4.1G ^-/-) mice, in which mislocalization of photoreceptor terminals is observed even at the young adult stage. We found that the late phase OKR was significantly impaired in 4.1G ^{- / -} mice, which exhibit significantly reduced SF and TF compared with control mice. These OKR abnormalities observed in 4.1G ^{- / -} mice resemble the abnormalities found in normal aging mice. This finding suggests that these mice can be useful mouse models for studying the aging of the retinal tissue and declining visual function. Taken together, the current study demonstrates that normal aging deteriorates to visual motion processing for both the initial and late phases of OKRs. Moreover, it implies that the abnormalities of the visual function in the normal aging mice are at least partly due to mislocalization of photoreceptor synapses.

Long term visuo-vestibular mismatch in freely behaving mice differentially affects gaze stabilizing reflexes

The vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) work synergistically to stabilize gaze in response to head movements. We previously demonstrated that a 14-day visuo-vestibular mismatch (VVM) protocol applied in freely behaving mice decreased the VOR gain. Here, we show for the first time that the OKR gain is also reduced and report on the recovery dynamics of both VOR and OKR after the end of the VVM protocol. Using sinusoidally-modulated stimulations, the decreases in VOR and OKR were found to be frequency-selective with larger reductions for frequencies < 0.5 Hz. Constant-velocity OKR stimulation tests demonstrated that the persistent components of the OKR were not modified while the transient, initial responses were. To identify the signals driving VOR and OKR reductions, we compared the responses of mice exposed to a high-contrast and no-contrast VVM. Despite being more robust in the high-contrast conditions, reductions were largely comparable and recovered with a similar time course. An analysis that directly compared VOR and OKR responses revealed that, alterations in the VOR were of significantly larger amplitude with significantly slower dynamics of recovery. Our findings are evidence for a frequency-selective influence of visual signals in the tuning of gaze stabilizing reflexes in normal mice.

Dynamics of gaze control during prey capture in freely moving mice

Many studies of visual processing are conducted in constrained conditions such as head- and gaze-fixation, and therefore less is known about how animals actively acquire visual information in natural contexts. To determine how mice target their gaze during natural behavior, we measured head and bilateral eye movements in mice performing prey capture, an ethological behavior that engages vision. We found that the majority of eye movements are compensatory for head movements, thereby serving to stabilize the visual scene. During movement, however, periods of stabilization are interspersed with non-compensatory saccades that abruptly shift gaze position. Notably, these saccades do not preferentially target the prey location. Rather, orienting movements are driven by the head, with the eyes following in coordination to sequentially stabilize and recenter the gaze. These findings relate eye movements in the mouse to other species, and provide a foundation for studying active vision during ethological behaviors in the mouse.

Two Distinct Types of Eye-Head Coupling in Freely Moving Mice

Animals actively interact with their environment to gather sensory information. There is conflicting evidence about how mice use vision to sample their environment. During head restraint, mice make rapid eye movements coupled between the eyes, similar to conjugate saccadic eye movements in humans. However, when mice are free to move their heads, eye movements are more complex and often non-conjugate, with the eyes moving in opposite directions. We combined head and eye tracking in freely moving mice and found both observations are explained by two eye-head coupling types, associated with vestibular mechanisms. The first type comprised non-conjugate eye movements, which compensate for head tilt changes to maintain a similar visual field relative to the horizontal ground plane. The second type of eye movements was conjugate and coupled to head yaw rotation to produce a "saccade and fixate" gaze pattern. During head-initiated saccades, the eyes moved together in the head direction but during subsequent fixation moved in the opposite direction to the head to compensate for head rotation. This saccade and fixate pattern is similar to humans who use eye movements (with or without head movement) to rapidly shift gaze but in mice relies on combined head and eye movements. Both couplings were maintained during social interactions and visually guided object tracking. Even in head-restrained mice, eye movements were invariably associated with attempted head motion. Our results reveal that mice combine head and eye movements to sample their environment and highlight similarities and differences between eye movements in mice and humans.

Inducible rodent models of glaucoma

Glaucoma is one of the leading causes of vision impairment worldwide. In order to further understand the molecular pathobiology of this disease and to develop better therapies, clinically relevant animal models are necessary. In recent years, both the rat and mouse have become popular models in glaucoma research. Key reasons are: many important biological similarities shared among rodent eyes and the human eye; development of improved methods to induce glaucoma and to evaluate glaucomatous damage; availability of genetic tools in the mouse; as well as the relatively low cost of rodent studies. Commonly studied rat and mouse glaucoma models include intraocular pressure (IOP)-dependent and pressure-independent models. The pressure-dependent models address the most important risk factor of elevated IOP, whereas the pressure-independent models assess "normal tension" glaucoma and other "non-IOP" related factors associated with glaucomatous damage. The current article provides descriptions of these models, their characterizations, specific techniques to induce glaucoma, mechanisms of injury, advantages, and limitations.

Monocular and binocular opto-locomotor reflex biases for random dot motion in mice

We investigated the relationship between eyes receiving visual input of large field translating random dot motion and subsequent reflexive changes in running direction in mice. The animals were head-fixed running on a Styrofoam ball and the opto-locomotor reflex (OLR) was measured in response to 2 s of dots patterns moving horizontally to the left or right. We measured the OLR in conditions with both eyes open (binocular) and one eye closed (monocular). When we covered the right or left eye in the monocular condition, we found reflexive behavior to be delayed for a few hundred milliseconds to leftward or rightward motion, respectively. After this delay, the bias disappeared and reflexive behavior was similar to responses to motion under binocular conditions. These results might be explained by different contributions of subcortical and cortical visual motion processing pathways to the OLR. Furthermore, we found no evidence for nonlinear interactions between the two eyes, because the sum of the OLR of the two monocular conditions was equal in amplitude and temporal characteristics to the OLR under binocular conditions.

Nondestructive and objective assessment of the vestibular function in rodent models: A review

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

Retinoic acid degradation shapes zonal development of vestibular organs and sensitivity to transient linear accelerations

Each vestibular sensory epithelium in the inner ear is divided morphologically and physiologically into two zones, called the striola and extrastriola in otolith organ maculae, and the central and peripheral zones in semicircular canal cristae. We found that formation of striolar/central zones during embryogenesis requires Cytochrome P450 26b1 (Cyp26b1)-mediated degradation of retinoic acid (RA). In Cyp26b1 conditional knockout mice, formation of striolar/central zones is compromised, such that they resemble extrastriolar/peripheral zones in multiple features. Mutants have deficient vestibular evoked potential (VsEP) responses to jerk stimuli, head tremor and deficits in balance beam tests that are consistent with abnormal vestibular input, but normal vestibulo-ocular reflexes and apparently normal motor performance during swimming. Thus, degradation of RA during embryogenesis is required for formation of highly specialized regions of the vestibular sensory epithelia with specific functions in detecting head motions.

Understanding the circuit basis of cognitive functions using mouse models

Understanding how cognitive functions arise from computations occurring in the brain requires the ability to measure and perturb neural activity while the relevant circuits are engaged for specific cognitive processes. Rapid technical advances have led to the development of new approaches to transiently activate and suppress neuronal activity as well as to record simultaneously from hundreds to thousands of neurons across multiple brain regions during behavior. To realize the full potential of these approaches for understanding cognition, however, it is critical that behavioral conditions and stimuli are effectively designed to engage the relevant brain networks. Here, we highlight recent innovations that enable this combined approach. In particular, we focus on how to design behavioral experiments that leverage the ever-growing arsenal of technologies for controlling and measuring neural activity in order to understand cognitive functions.

Negative optokinetic afternystagmus in larval zebrafish demonstrates set-point adaptation

Motor learning is essential to maintain accurate behavioral responses. We used a larval zebrafish model to study ocular motor learning behaviors. During a sustained period of optokinetic stimulation in 5-day-old wild-type zebrafish larvae the slow-phase eye velocity decreased over time. Then interestingly, a long-lasting and robust negative optokinetic afternystagmus (OKAN) was evoked upon light extinction. The slow-phase velocity, the quick-phase frequency, and the decay time constant of the negative OKAN were dependent on the stimulus duration and the adaptation to the preceding optokinetic stimulation. Based on these results, we propose a sensory adaptation process during continued optokinetic stimulation, which, when the stimulus is removed, leads to a negative OKAN as the result of a changed retinal slip velocity set point, and thus, a sensorimotor memory. The pronounced negative OKAN in larval zebrafish not only provides a practical solution to the hitherto unsolved problems of observing negative OKAN, but also, and most importantly, can be readily applied as a powerful model for studying sensorimotor learning and memory in vertebrates.

Paradoxical Rules of Spike Train Decoding Revealed at the Sensitivity Limit of Vision

All sensory information is encoded in neural spike trains. It is unknown how the brain utilizes this neural code to drive behavior. Here, we unravel the decoding rules of the brain at the most elementary level by linking behavioral decisions to retinal output signals in a single-photon detection task. A transgenic mouse line allowed us to separate the two primary retinal outputs, ON and OFF pathways, carrying information about photon absorptions as increases and decreases in spiking, respectively. We measured the sensitivity limit of rods and the most sensitive ON and OFF ganglion cells and correlated these results with visually guided behavior using markerless head and eye tracking. We show that behavior relies only on the ON pathway even when the OFF pathway would allow higher sensitivity. Paradoxically, behavior does not rely on the spike code with maximal information but instead relies on a decoding strategy based on increases in spiking.

TRPC3 is a major contributor to functional heterogeneity of cerebellar Purkinje cells

Despite the canonical homogeneous character of its organization, the cerebellum plays differential computational roles in distinct sensorimotor behaviors. Previously, we showed that Purkinje cell (PC) activity differs between zebrin-negative (Z–) and zebrin-positive (Z+) modules (Zhou et al., 2014). Here, using gain-of-function and loss-of-function mouse models, we show that transient receptor potential cation channel C3 (TRPC3) controls the simple spike activity of Z–, but not Z+ PCs. In addition, TRPC3 regulates complex spike rate and their interaction with simple spikes, exclusively in Z– PCs. At the behavioral level, TRPC3 loss-of-function mice show impaired eyeblink conditioning, which is related to Z– modules, whereas compensatory eye movement adaptation, linked to Z+ modules, is intact. Together, our results indicate that TRPC3 is a major contributor to the cellular heterogeneity that introduces distinct physiological properties in PCs, conjuring functional heterogeneity in cerebellar sensorimotor integration.

Mice Discriminate Stereoscopic Surfaces Without Fixating in Depth

Stereopsis is a ubiquitous feature of primate mammalian vision, but little is known about if and how rodents such as mice use stereoscopic vision. We used random dot stereograms to test for stereopsis in male and female mice, and they were able to discriminate near from far surfaces over a range of disparities, with diminishing performance for small and large binocular disparities. Based on two-photon measurements of disparity tuning, the range of disparities represented in the visual cortex aligns with the behavior and covers a broad range of disparities. When we examined their binocular eye movements, we found that, unlike primates, mice did not systematically vary relative eye positions or use vergence eye movements when presented with different disparities. Nonetheless, the representation of disparity tuning was wide enough to capture stereoscopic information over a range of potential vergence angles. Although mice share fundamental characteristics of stereoscopic vision with primates and carnivores, their lack of disparity-dependent vergence eye movements and wide neuronal representation suggests that they may use a distinct strategy for stereopsis.SIGNIFICANCE STATEMENT Binocular vision allows us to derive depth information by comparing right and left eye information. We characterized binocular integration in mice because tools exist in these animals to dissect the underlying neural circuitry for binocular vision. Using random dot stereograms, we find that behavior and disparity tuning in the visual cortex share fundamental characteristics with primates, but we did not observe any evidence of disparity-dependent changes in vergence angle. We propose that mice use a distinct strategy of stereopsis compared with primates by using a broad range of disparities to encode depth over a large field of view and to compensate for nonstereoscopic changes in vergence angle that arise during natural behavior.

Spontaneous mutations of the Zpld1 gene in mice cause semicircular canal dysfunction but do not impair gravity receptor or hearing functions

The cupula is a gelatinous membrane overlying the crista ampullaris of the semicircular canal, important for sensing rotation of the head and critical for normal balance. Recently the zona pellucida like domain containing 1 protein (ZPLD1, also known as cupulin) was identified in the cupula of fish. Here, we describe two new spontaneous mutations in the mouse Zpld1 gene, which were discovered by the circling behavior of mutant mice, an indicator of balance dysfunction. The Zpld1 mutant mice exhibited normal hearing function as assessed by auditory brainstem response (ABR) measurements, and their otolithic organs appeared normal. In the inner ear, Zpld1 mRNA expression was detected only in the hair cells and supporting cells of the crista ampullaris. Normal vestibular sensory evoked potential (VsEP) responses and abnormal vestibulo-ocular reflex (VOR) responses demonstrated that the vestibular dysfunction of the Zpld1 mutant mice is caused by loss of sensory input for rotary head movements (detected by cristae ampullaris) and not by loss of input for linear head translations (detected by maculae of the utricle and saccule). Taken together, these results are consistent with ZPLD1 being an important functional component of the cupula, but not tectorial or otoconial membranes.

A Head-Mounted Camera System Integrates Detailed Behavioral Monitoring with Multichannel Electrophysiology in Freely Moving Mice

Breakthroughs in understanding the neural basis of natural behavior require neural recording and intervention to be paired with high-fidelity multimodal behavioral monitoring. An extensive genetic toolkit for neural circuit dissection, and well-developed neural recording technology, make the mouse a powerful model organism for systems neuroscience. However, most methods for high-bandwidth acquisition of behavioral data in mice rely upon fixed-position cameras and other off-animal devices, complicating the monitoring of animals freely engaged in natural behaviors. Here, we report the development of a lightweight head-mounted camera system combined with head-movement sensors to simultaneously monitor eye position, pupil dilation, whisking, and pinna movements along with head motion in unrestrained, freely behaving mice. The power of the combined technology is demonstrated by observations linking eye position to head orientation; whisking to non-tactile stimulation; and, in electrophysiological experiments, visual cortical activity to volitional head movements.

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Eyes, whiskers, head, and neural activity monitored in freely moving mice

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System generates stable video output and leaves mouse behavior largely unchanged

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Close link between eye and head movements at both slow and fast timescales

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Active head movements in the dark strongly modulate primary visual cortex activity

Measuring vision using innate behaviours in mice with intact and impaired retina function

Measuring vision in rodents is a critical step for understanding vision, improving models of human disease, and developing therapies. Established behavioural tests for perceptual vision, such as the visual water task, rely on learning. The learning process, while effective for sighted animals, can be laborious and stressful in animals with impaired vision, requiring long periods of training. Current tests that that do not require training are based on sub-conscious, reflex responses (e.g. optokinetic nystagmus) that don't require involvement of visual cortex and higher order thalamic nuclei. A potential alternative for measuring vision relies on using visually guided innate defensive responses, such as escape or freeze, that involve cortical and thalamic circuits. In this study we address this possibility in mice with intact and degenerate retinas. We first develop automatic methods to detect behavioural responses based on high dimensional tracking and changepoint detection of behavioural time series. Using those methods, we show that visually guided innate responses can be elicited using parametisable stimuli, and applied to describing the limits of visual acuity in healthy animals and discriminating degrees of visual dysfunction in mouse models of retinal degeneration.

Extraocular muscle function is impaired in ryr3 -/- mice

Extraocular muscles are specialized skeletal muscles expressing a particular set of proteins involved in calcium homeostasis, including RYR3. Eckhardt et al. investigate extraocular muscle function in ryr3^−/− mice and show that ablation of RYR3 significantly impacts vision.

Calcium is an ubiquitous second messenger mediating numerous physiological processes, including muscle contraction and neuronal excitability. Ca²⁺ is stored in the ER/SR and is released into the cytoplasm via the opening of intracellular inositol trisphosphate receptor and ryanodine receptor calcium channels. Whereas in skeletal muscle, isoform 1 of the RYR is the main channel mediating calcium release from the SR leading to muscle contraction, the function of ubiquitously expressed ryanodine receptor 3 (RYR3) is far from clear; it is not known whether RYR3 plays a role in excitation–contraction coupling. We recently reported that human extraocular muscles express high levels of RYR3, suggesting that such muscles may be useful to study the function of this isoform of the Ca²⁺ channel. In the present investigation, we characterize the visual function of ryr3^−/− mice. We observe that ablation of RYR3 affects both mechanical properties and calcium homeostasis in extraocular muscles. These changes significantly impact vision. Our results reveal for the first time an important role for RYR3 in extraocular muscle function.

Mouse Extraocular Muscles and the Musculotopic Organization of Their Innervation

The organization of extraocular muscles (EOMs) and their motor nuclei was investigated in the mouse due to the increased importance of this model for oculomotor research. Mice showed a standard EOM organization pattern, although their eyes are set at the side of the head. They do have more prominent oblique muscles, whose insertion points differ from those of frontal-eyed species. Retrograde tracers revealed that the motoneuron layout aligns with the general vertebrate plan with respect to nuclei and laterality. The mouse departed in some significant respects from previously studied species. First, more overlap between the distributions of muscle-specific motoneuronal pools was present in the oculomotor nucleus (III). Furthermore, motoneuron dendrites for each pool filled the entire III and extended beyond the edge of the abducens nucleus (VI). This suggests mouse extraocular motoneuron afferents must target specific pools based on features other than dendritic distribution and nuclear borders. Second, abducens internuclear neurons are located outside the VI. We concluded this because no unlabeled abducens internuclear neurons were observed following lateral rectus muscle injections and because retrograde tracer injections into the III labeled cells immediately ventral and ventrolateral to the VI, not within it. This may provide an anatomical substrate for differential input to motoneurons and internuclear neurons that allows rodents to move their eyes more independently. Finally, while soma size measurements suggested motoneuron subpopulations supplying multiply and singly innervated muscle fibers are present, markers for neurofilaments and perineuronal nets indicated overlap in the size distributions of the two populations. Anat Rec, 302:1865-1885, 2019. © 2019 American Association for Anatomy.

P2X2 Receptor Deficiency in Mouse Vestibular End Organs Attenuates Vestibular Function

P2X₂ receptors are ligand-gated cation channels activated by extracellular ATP that modulate neural transmission in various neuronal systems. Although the function and distribution of P2X₂ receptors in the cochlea portion of the inner ear are well established, their physiological role in the vestibular portion is still not understood. Therefore, we investigated P2X₂ receptor localization in the peripheral vestibular portion, and assessed their physiological function in vivo using P2X₂ receptor knock out (P2X₂-KO) mice. Histological analysis revealed that P2X₂ receptors were localized on the epithelial surface of supporting and transitional cells of the vestibular end organs. To examine vestibular function in P2X₂-KO mice, we conducted behavioral tests and tested the vestibulo-ocular reflex (VOR) during sinusoidal rotations. P2X₂-KO mice exhibited significant motor balance impairment in the balance beam test. VOR gain in P2X₂-KO mice was significantly reduced, with no decrease in the optokinetic response. In conclusion, we showed that P2X₂ receptors are mainly localized in the supporting cells of the vestibular inner ear, and the loss of P2X₂ receptors causes mild vestibular dysfunction. Taken together, our findings suggest that the P2X₂ receptor plays a modulatory role in vestibular function.

A Role for Mouse Primary Visual Cortex in Motion Perception

Visual motion is an ethologically important stimulus throughout the animal kingdom. In primates, motion perception relies on specific higher-order cortical regions. Although mouse primary visual cortex (V1) and higher-order visual areas show direction-selective (DS) responses, their role in motion perception remains unknown. Here, we tested whether V1 is involved in motion perception in mice. We developed a head-fixed discrimination task in which mice must report their perceived direction of motion from random dot kinematograms (RDKs). After training, mice made around 90% correct choices for stimuli with high coherence and performed significantly above chance for 16% coherent RDKs. Accuracy increased with both stimulus duration and visual field coverage of the stimulus, suggesting that mice in this task integrate motion information in time and space. Retinal recordings showed that thalamically projecting On-Off DS ganglion cells display DS responses when stimulated with RDKs. Two-photon calcium imaging revealed that neurons in layer (L) 2/3 of V1 display strong DS tuning in response to this stimulus. Thus, RDKs engage motion-sensitive retinal circuits as well as downstream visual cortical areas. Contralateral V1 activity played a key role in this motion direction discrimination task because its reversible inactivation with muscimol led to a significant reduction in performance. Neurometric-psychometric comparisons showed that an ideal observer could solve the task with the information encoded in DS L2/3 neurons. Motion discrimination of RDKs presents a powerful behavioral tool for dissecting the role of retino-forebrain circuits in motion processing.

The opto-locomotor reflex as a tool to measure sensitivity to moving random dot patterns in mice

We designed a method to quantify mice visual function by measuring reflexive opto-locomotor responses. Mice were placed on a Styrofoam ball at the center of a large dome on the inside of which we projected moving random dot patterns. Because we fixed the heads of the mice in space and the ball was floating on pressurized air, locomotion of the mice was translated to rotation of the ball, which we registered. Sudden onsets of rightward or leftward moving patterns caused the mice to reflexively change their running direction. We quantified the opto-locomotor responses to different pattern speeds, luminance contrasts, and dot sizes. We show that the method is fast and reliable and the magnitude of the reflex is stable within sessions. We conclude that this opto-locomotor reflex method is suitable to quantify visual function in mice.

Modulation of the zebrafish optokinetic reflex by pharmacologic agents targeting GABAA receptors

Optokinetic reflex (OKR) responses provide a convenient means to evaluate visual, integrative and oculomotor function in larval zebrafish. We measured multiple aspects of the OKR response in zebrafish exposed systemically to compounds altering signaling at GABA_A receptors in order to derive quantitative concentration-response relationships. The GABA_A antagonist picrotoxin caused concentration-dependent decreases in reflex gain, saccade velocity, saccade amplitude, interocular concordance and interocular gain. Conversely, the GABA_A agonist gaboxadol provoked increases in reflex gain, saccade velocity, saccade amplitude and ocular range at low concentrations, and decreases in some of these parameters at higher concentrations. These data show that GABA_A signaling influences multiple aspects of the OKR (including gain, generation of saccades, and coordination between the two eyes) and provide proof of concept that quantitative OKR analysis can be used as a tool for chemical biology and neuropharmacology applications.

Pupil Size Coupling to Cortical States Protects the Stability of Deep Sleep via Parasympathetic Modulation

During wakefulness, pupil diameter can reflect changes in attention, vigilance, and cortical states. How pupil size relates to cortical activity during sleep, however, remains unknown. Pupillometry during natural sleep is inherently challenging since the eyelids are usually closed. Here, we present a novel head-fixed sleep paradigm in combination with infrared back-illumination pupillometry (iBip) allowing robust tracking of pupil diameter in sleeping mice. We found that pupil size can be used as a reliable indicator of sleep states and that cortical activity becomes tightly coupled to pupil size fluctuations during non-rapid eye movement (NREM) sleep. Pharmacological blocking experiments indicate that the observed pupil size changes during sleep are mediated via the parasympathetic system. We furthermore found that constrictions of the pupil during NREM episodes might play a protective role for stability of sleep depth. These findings reveal a fundamental relationship between cortical activity and pupil size, which has so far been hidden behind closed eyelids.

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Infrared back-illumination allows accurate pupillometry in sleeping mice

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Brain activity and pupil diameter are tightly coupled during sleep

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The parasympathetic system is the main driver of pupillary changes during NREM sleep

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Pupillary constrictions might have a protective function to stabilize deep sleep

Vision in laboratory rodents-Tools to measure it and implications for behavioral research

Mice and rats are nocturnal mammals and their vision is specialized for detection of motion and contrast in dim light conditions. These species possess a large proportion of UV-sensitive cones in their retinas and the majority of their optic nerve axons target superior colliculus rather than visual cortex. Therefore, it was a widely held belief that laboratory rodents hardly utilize vision during day-time behavior. This dogma is being questioned as accumulating evidence suggests that laboratory rodents are able to perform complex visual functions, such as perceiving subjective contours, and that declined vision may affect their performance in many behavioral tasks. For instance, genetic engineering may have unexpected consequences on vision as mouse models of Alzheimer's and Huntington's diseases have declined visual function. Rodent vision can be tested in numerous ways using operant training or reflex-based behavioral tasks, or alternatively using electrophysiological recordings. In this article, we will first provide a summary of visual system and explain its characteristics unique to rodents. Then, we present well-established techniques to test rodent vision, with an emphasis on pattern vision: visual water test, optomotor reflex test, pattern electroretinography and pattern visual evoked potentials. Finally, we highlight the importance of visual phenotyping in rodents. As the number of genetically engineered rodent models and volume of behavioral testing increase simultaneously, the possibility of visual dysfunctions needs to be addressed. Neglect in this matter potentially leads to crude biases in the field of neuroscience and beyond.

Versatile functional roles of horizontal cells in the retinal circuit

In the retinal circuit, environmental light signals are converted into electrical signals that can be decoded properly by the brain. At the first synapse of the visual system, information flow from photoreceptors to bipolar cells is modulated by horizontal cells (HCs), however, their functional contribution to retinal output and individual visual function is not fully understood. In the current study, we investigated functional roles for HCs in retinal ganglion cell (RGC) response properties and optokinetic responses by establishing a HC-depleted mouse line. We observed that HC depletion impairs the antagonistic center-surround receptive field formation of RGCs, supporting a previously reported HC function revealed by pharmacological approaches. In addition, we found that HC loss reduces both the ON and OFF response diversities of RGCs, impairs adjustment of the sensitivity to ambient light at the retinal output level, and alters spatial frequency tuning at an individual level. Taken together, our current study suggests multiple functional aspects of HCs crucial for visual processing.

Comparison of optomotor and optokinetic reflexes in mice

During animal locomotion or position adjustments, the visual system uses image stabilization reflexes to compensate for global shifts in the visual scene. These reflexes elicit compensatory head movements (optomotor response, OMR) in unrestrained animals or compensatory eye movements (optokinetic response, OKR) in head-fixed or unrestrained animals exposed to globally rotating striped patterns. In mice, OMR are relatively easy to observe and find broad use in the rapid evaluation of visual function. OKR determinations are more involved experimentally but yield more stereotypical, easily quantifiable results. The relative contributions of head and eye movements to image stabilization in mice have not been investigated. We are using newly developed software and apparatus to accurately quantitate mouse head movements during OMR, quantitate eye movements during OKR, and determine eye movements in freely behaving mice. We provide the first direct comparison of OMR and OKR gains (head or eye velocity/stimulus velocity) and find that the two reflexes have comparable dependencies on stimulus luminance, contrast, spatial frequency, and velocity. OMR and OKR are similarly affected in genetically modified mice with defects in retinal ganglion cells (RGC) compared with wild-type, suggesting they are driven by the same sensory input (RGC type). OKR eye movements have much higher gains than the OMR head movements, but neither can fully compensate global visual shifts. However, combined eye and head movements can be detected in unrestrained mice performing OMR, suggesting they can cooperate to achieve image stabilization, as previously described for other species.NEW & NOTEWORTHY We provide the first quantitation of head gain during optomotor response in mice and show that optomotor and optokinetic responses have similar psychometric curves. Head gains are far smaller than eye gains. Unrestrained mice combine head and eye movements to respond to visual stimuli, and both monocular and binocular fields are used during optokinetic responses. Mouse OMR and OKR movements are heterogeneous under optimal and suboptimal stimulation and are affected in mice lacking ON direction-selective retinal ganglion cells.

Long-Lasting Visuo-Vestibular Mismatch in Freely-Behaving Mice Reduces the Vestibulo-Ocular Reflex and Leads to Neural Changes in the Direct Vestibular Pathway

Calibration of the vestibulo-ocular reflex (VOR) depends on the presence of visual feedback. However, the cellular mechanisms associated with VOR modifications at the level of the brainstem remain largely unknown. A new protocol was designed to expose freely behaving mice to a visuo-vestibular mismatch during a 2-week period. This protocol induced a 50% reduction of the VOR. In vivo pharmacological experiments demonstrated that the VOR reduction depends on changes located outside the flocculus/paraflocculus complex. The cellular mechanisms associated with the VOR reduction were then studied in vitro on brainstem slices through a combination of vestibular afferent stimulation and patch-clamp recordings of central vestibular neurons. The evoked synaptic activity demonstrated that the efficacy of the synapses between vestibular afferents and central vestibular neurons was decreased. In addition, a long-term depression protocol failed to further decrease the synapse efficacy, suggesting that the VOR reduction might have occurred through depression-like mechanisms. Analysis of the intrinsic membrane properties of central vestibular neurons revealed that the synaptic changes were supplemented by a decrease in the spontaneous discharge and excitability of a subpopulation of neurons. Our results provide evidence that a long-lasting visuo-vestibular mismatch leads to changes in synaptic transmission and intrinsic properties of central vestibular neurons in the direct VOR pathway. Overall, these results open new avenues for future studies on visual and vestibular interactions conducted in vivo and in vitro.