Emergent properties of the optic tectum revealed by population analysis of direction and orientation selectivity

The Visual Systems of Zebrafish

The zebrafish visual system has become a paradigmatic preparation for behavioral and systems neuroscience. Around 40 types of retinal ganglion cells (RGCs) serve as matched filters for stimulus features, including light, optic flow, prey, and objects on a collision course. RGCs distribute their signals via axon collaterals to 12 retinorecipient areas in forebrain and midbrain. The major visuomotor hub, the optic tectum, harbors nine RGC input layers that combine information on multiple features. The retinotopic map in the tectum is locally adapted to visual scene statistics and visual subfield-specific behavioral demands. Tectal projections to premotor centers are topographically organized according to behavioral commands. The known connectivity in more than 20 processing streams allows us to dissect the cellular basis of elementary perceptual and cognitive functions. Visually evoked responses, such as prey capture or loom avoidance, are controlled by dedicated multistation pathways that-at least in the larva-resemble labeled lines. This architecture serves the neuronal code's purpose of driving adaptive behavior.

Pathways from the Superior Colliculus to the Basal Ganglia

The present work aims to review the structural organization of the mammalian superior colliculus (SC), the putative pathways connecting the SC and the basal ganglia, and their role in organizing complex behavioral output. First, we review how the complex intrinsic connections between the SC's laminae projections allow for the construction of spatially aligned, visual-multisensory maps of the surrounding environment. Moreover, we present a summary of the sensory-motor inputs of the SC, including a description of the integration of multi-sensory inputs relevant to behavioral control. We further examine the major descending outputs toward the brainstem and spinal cord. As the central piece of this review, we provide a thorough analysis covering the putative interactions between the SC and the basal ganglia. To this end, we explore the diverse thalamic routes by which information from the SC may reach the striatum, including the pathways through the lateral posterior, parafascicular, and rostral intralaminar thalamic nuclei. We also examine the interactions between the SC and subthalamic nucleus, representing an additional pathway for the tectal modulation of the basal ganglia. Moreover, we discuss how information from the SC might also be relayed to the basal ganglia through midbrain tectonigral and tectotegmental projections directed at the substantia nigra compacta and ventrotegmental area, respectively, influencing the dopaminergic outflow to the dorsal and ventral striatum. We highlight the vast interplay between the SC and the basal ganglia and raise several missing points that warrant being addressed in future studies.

Recurrent network interactions explain tectal response variability and experience-dependent behavior

Response variability is an essential and universal feature of sensory processing and behavior. It arises from fluctuations in the internal state of the brain, which modulate how sensory information is represented and transformed to guide behavioral actions. In part, brain state is shaped by recent network activity, fed back through recurrent connections to modulate neuronal excitability. However, the degree to which these interactions influence response variability and the spatial and temporal scales across which they operate, are poorly understood. Here, we combined population recordings and modeling to gain insights into how neuronal activity modulates network state and thereby impacts visually evoked activity and behavior. First, we performed cellular-resolution calcium imaging of the optic tectum to monitor ongoing activity, the pattern of which is both a cause and consequence of changes in network state. We developed a minimal network model incorporating fast, short range, recurrent excitation and long-lasting, activity-dependent suppression that reproduced a hallmark property of tectal activity - intermittent bursting. We next used the model to estimate the excitability state of tectal neurons based on recent activity history and found that this explained a portion of the trial-to-trial variability in visually evoked responses, as well as spatially selective response adaptation. Moreover, these dynamics also predicted behavioral trends such as selective habituation of visually evoked prey-catching. Overall, we demonstrate that a simple recurrent interaction motif can be used to estimate the effect of activity upon the incidental state of a neural network and account for experience-dependent effects on sensory encoding and visually guided behavior.

From perception to behavior: The neural circuits underlying prey hunting in larval zebrafish

A key challenge for neural systems is to extract relevant information from the environment and make appropriate behavioral responses. The larval zebrafish offers an exciting opportunity for studying these sensing processes and sensory-motor transformations. Prey hunting is an instinctual behavior of zebrafish that requires the brain to extract and combine different attributes of the sensory input and form appropriate motor outputs. Due to its small size and transparency the larval zebrafish brain allows optical recording of whole-brain activity to reveal the neural mechanisms involved in prey hunting and capture. In this review we discuss how the larval zebrafish brain processes visual information to identify and locate prey, the neural circuits governing the generation of motor commands in response to prey, how hunting behavior can be modulated by internal states and experience, and some outstanding questions for the field.

Optic flow in the natural habitats of zebrafish supports spatial biases in visual self-motion estimation

Animals benefit from knowing if and how they are moving. Across the animal kingdom, sensory information in the form of optic flow over the visual field is used to estimate self-motion. However, different species exhibit strong spatial biases in how they use optic flow. Here, we show computationally that noisy natural environments favor visual systems that extract spatially biased samples of optic flow when estimating self-motion. The performance associated with these biases, however, depends on interactions between the environment and the animal's brain and behavior. Using the larval zebrafish as a model, we recorded natural optic flow associated with swimming trajectories in the animal's habitat with an omnidirectional camera mounted on a mechanical arm. An analysis of these flow fields suggests that lateral regions of the lower visual field are most informative about swimming speed. This pattern is consistent with the recent findings that zebrafish optomotor responses are preferentially driven by optic flow in the lateral lower visual field, which we extend with behavioral results from a high-resolution spherical arena. Spatial biases in optic-flow sampling are likely pervasive because they are an effective strategy for determining self-motion in noisy natural environments.

Optic flow in the natural habitats of zebrafish supports spatial biases in visual self-motion estimation

Animals benefit from knowing if and how they are moving. Across the animal kingdom, sensory information in the form of optic flow over the visual field is used to estimate self-motion. However, different species exhibit strong spatial biases in how they use optic flow. Here, we show computationally that noisy natural environments favor visual systems that extract spatially biased samples of optic flow when estimating self-motion. The performance associated with these biases, however, depends on interactions between the environment and the animal's brain and behavior. Using the larval zebrafish as a model, we recorded natural optic flow associated with swimming trajectories in the animal's habitat with an omnidirectional camera mounted on a mechanical arm. An analysis of these flow fields suggests that lateral regions of the lower visual field are most informative about swimming speed. This pattern is consistent with the recent findings that zebrafish optomotor responses are preferentially driven by optic flow in the lateral lower visual field, which we extend with behavioral results from a high-resolution spherical arena. Spatial biases in optic-flow sampling are likely pervasive because they are an effective strategy for determining self-motion in noisy natural environments.

Optic flow in the natural habitats of zebrafish supports spatial biases in visual self-motion estimation

Animals benefit from knowing if and how they are moving. Across the animal kingdom, sensory information in the form of optic flow over the visual field is used to estimate self-motion. However, different species exhibit strong spatial biases in how they use optic flow. Here, we show computationally that noisy natural environments favor visual systems that extract spatially biased samples of optic flow when estimating self-motion. The performance associated with these biases, however, depends on interactions between the environment and the animal's brain and behavior. Using the larval zebrafish as a model, we recorded natural optic flow associated with swimming trajectories in the animal's habitat with an omnidirectional camera mounted on a mechanical arm. An analysis of these flow fields suggests that lateral regions of the lower visual field are most informative about swimming speed. This pattern is consistent with the recent findings that zebrafish optomotor responses are preferentially driven by optic flow in the lateral lower visual field, which we extend with behavioral results from a high-resolution spherical arena. Spatial biases in optic-flow sampling are likely pervasive because they are an effective strategy for determining self-motion in noisy natural environments.

A robust receptive field code for optic flow detection and decomposition during self-motion

The perception of optic flow is essential for any visually guided behavior of a moving animal. To mechanistically predict behavior and understand the emergence of self-motion perception in vertebrate brains, it is essential to systematically characterize the motion receptive fields (RFs) of optic-flow-processing neurons. Here, we present the fine-scale RFs of thousands of motion-sensitive neurons studied in the diencephalon and the midbrain of zebrafish. We found neurons that serve as linear filters and robustly encode directional and speed information of translation-induced optic flow. These neurons are topographically arranged in pretectum according to translation direction. The unambiguous encoding of translation enables the decomposition of translational and rotational self-motion information from mixed optic flow. In behavioral experiments, we successfully demonstrated the predicted decomposition in the optokinetic and optomotor responses. Together, our study reveals the algorithm and the neural implementation for self-motion estimation in a vertebrate visual system.

The Role of Estrogen and Thyroid Hormones in Zebrafish Visual System Function

Visual system development is a highly complex process involving coordination of environmental cues, cell pathways, and integration of functional circuits. Consequently, a change to any step, due to a mutation or chemical exposure, can lead to deleterious consequences. One class of chemicals known to have both overt and subtle effects on the visual system is endocrine disrupting compounds (EDCs). EDCs are environmental contaminants which alter hormonal signaling by either preventing compound synthesis or binding to postsynaptic receptors. Interestingly, recent work has identified neuronal and sensory systems, particularly vision, as targets for EDCs. In particular, estrogenic and thyroidogenic signaling have been identified as critical modulators of proper visual system development and function. Here, we summarize and review this work, from our lab and others, focusing on behavioral, physiological, and molecular data collected in zebrafish. We also discuss different exposure regimes used, including long-lasting effects of developmental exposure. Overall, zebrafish are a model of choice to examine the impact of EDCs and other compounds targeting estrogen and thyroid signaling and the consequences of exposure in visual system development and function.

Contributions of Luminance and Motion to Visual Escape and Habituation in Larval Zebrafish

Animals from insects to humans perform visual escape behavior in response to looming stimuli, and these responses habituate if looms are presented repeatedly without consequence. While the basic visual processing and motor pathways involved in this behavior have been described, many of the nuances of predator perception and sensorimotor gating have not. Here, we have performed both behavioral analyses and brain-wide cellular-resolution calcium imaging in larval zebrafish while presenting them with visual loom stimuli or stimuli that selectively deliver either the movement or the dimming properties of full loom stimuli. Behaviorally, we find that, while responses to repeated loom stimuli habituate, no such habituation occurs when repeated movement stimuli (in the absence of luminance changes) are presented. Dim stimuli seldom elicit escape responses, and therefore cannot habituate. Neither repeated movement stimuli nor repeated dimming stimuli habituate the responses to subsequent full loom stimuli, suggesting that full looms are required for habituation. Our calcium imaging reveals that motion-sensitive neurons are abundant in the brain, that dim-sensitive neurons are present but more rare, and that neurons responsive to both stimuli (and to full loom stimuli) are concentrated in the tectum. Neurons selective to full loom stimuli (but not to movement or dimming) were not evident. Finally, we explored whether movement- or dim-sensitive neurons have characteristic response profiles during habituation to full looms. Such functional links between baseline responsiveness and habituation rate could suggest a specific role in the brain-wide habituation network, but no such relationships were found in our data. Overall, our results suggest that, while both movement- and dim-sensitive neurons contribute to predator escape behavior, neither plays a specific role in brain-wide visual habituation networks or in behavioral habituation.

The tectum/superior colliculus as the vertebrate solution for spatial sensory integration and action

The superior colliculus, or tectum in the case of non-mammalian vertebrates, is a part of the brain that registers events in the surrounding space, often through vision and hearing, but also through electrosensation, infrared detection, and other sensory modalities in diverse vertebrate lineages. This information is used to form maps of the surrounding space and the positions of different salient stimuli in relation to the individual. The sensory maps are arranged in layers with visual input in the uppermost layer, other senses in deeper positions, and a spatially aligned motor map in the deepest layer. Here, we will review the organization and intrinsic function of the tectum/superior colliculus and the information that is processed within tectal circuits. We will also discuss tectal/superior colliculus outputs that are conveyed directly to downstream motor circuits or via the thalamus to cortical areas to control various aspects of behavior. The tectum/superior colliculus is evolutionarily conserved among all vertebrates, but tailored to the sensory specialties of each lineage, and its roles have shifted with the emergence of the cerebral cortex in mammals. We will illustrate both the conserved and divergent properties of the tectum/superior colliculus through vertebrate evolution by comparing tectal processing in lampreys belonging to the oldest group of extant vertebrates, larval zebrafish, rodents, and other vertebrates including primates.

Neurodegeneration, Neuroprotection and Regeneration in the Zebrafish Retina

Neurodegenerative retinal diseases, such as glaucoma and diabetic retinopathy, involve a gradual loss of neurons in the retina as the disease progresses. Central nervous system neurons are not able to regenerate in mammals, therefore, an often sought after course of treatment for neuronal loss follows a neuroprotective or regenerative strategy. Neuroprotection is the process of preserving the structure and function of the neurons that have survived a harmful insult; while regenerative approaches aim to replace or rewire the neurons and synaptic connections that were lost, or induce regrowth of damaged axons or dendrites. In order to test the neuroprotective effectiveness or the regenerative capacity of a particular agent, a robust experimental model of retinal neuronal damage is essential. Zebrafish are being used more often in this type of study because their eye structure and development is well-conserved between zebrafish and mammals. Zebrafish are robust genetic tools and are relatively inexpensive to maintain. The large array of functional and behavioral tests available in zebrafish makes them an attractive model for neuroprotection studies. Some common insults used to model retinal disease and study neuroprotection in zebrafish include intense light, chemical toxicity and mechanical damage. This review covers the existing retinal neuroprotection and regeneration literature in the zebrafish and highlights their potential for future studies.

Parallel Channels for Motion Feature Extraction in the Pretectum and Tectum of Larval Zebrafish

Non-cortical visual areas in vertebrate brains extract relevant stimulus features, such as motion, object size, and location, to support diverse behavioral tasks. The optic tectum and pretectum, two primary visual areas in zebrafish, are involved in motion processing, and yet their differential neural representation of behaviorally relevant visual features is unclear. Here, we characterize receptive fields (RFs) of motion-sensitive neurons in the diencephalon and midbrain. We show that RFs of many pretectal neurons are large and sample the lower visual field, whereas RFs of tectal neurons are mostly small-size selective and sample the upper nasal visual field more densely. Furthermore, optomotor swimming can reliably be evoked by presenting forward motion in the lower temporal visual field alone, matching the lower visual field bias of the pretectum. Thus, tectum and pretectum extract different visual features from distinct regions of visual space, which is likely a result of their adaptations to hunting and optomotor behavior, respectively.

Functional, molecular and morphological heterogeneity of superficial interneurons in the larval zebrafish tectum

The superficial interneurons, SINs, of the zebrafish tectum, have been implicated in a range of visual functions, including size discrimination, directional selectivity, and looming-evoked escape. This raises the question if SIN subpopulations, despite their morphological similarities and shared anatomical position in the retinotectal processing stream, carry out diverse, task-specific functions in visual processing, or if they have simple tuning properties in common. Here we have further characterized the SINs through functional imaging, electrophysiological recordings, and neurotransmitter typing in two transgenic lines, the widely used Gal4s1156t and the recently reported LCRRH2-RH2-2:GFP. We found that about a third of the SINs strongly responded to changes in whole-field light levels, with a strong preference for OFF over ON stimuli. Interestingly, individual SINs were selectively tuned to a diverse range of narrow luminance decrements. Overall responses to whole-field luminance steps did not vary with the position of the SIN cell body along the depth of the tectal neuropil or with the orientation of its neurites. We ruled out the possibility that intrinsic photosensitivity of Gal4s1156t+ SINs contribute to the measured visual responses. We found that, while most SINs express GABAergic markers, a substantial minority express an excitatory neuronal marker, the vesicular glutamate transporter, expanding the possible roles of SIN function in the tectal circuitry. In conclusion, SINs represent a molecularly, morphologically, and functionally heterogeneous class of interneurons, with subpopulations that detect a range of specific visual features, to which we have now added narrow luminance decrements.

An Optical Illusion Pinpoints an Essential Circuit Node for Global Motion Processing

Direction-selective (DS) neurons compute the direction of motion in a visual scene. Brain-wide imaging in larval zebrafish has revealed hundreds of DS neurons scattered throughout the brain. However, the exact population that causally drives motion-dependent behaviors-e.g., compensatory eye and body movements-remains largely unknown. To identify the behaviorally relevant population of DS neurons, here we employ the motion aftereffect (MAE), which causes the well-known "waterfall illusion." Together with region-specific optogenetic manipulations and cellular-resolution functional imaging, we found that MAE-responsive neurons represent merely a fraction of the entire population of DS cells in larval zebrafish. They are spatially clustered in a nucleus in the ventral lateral pretectal area and are necessary and sufficient to steer the entire cycle of optokinetic eye movements. Thus, our illusion-based behavioral paradigm, combined with optical imaging and optogenetics, identified key circuit elements of global motion processing in the vertebrate brain.

Calcium Imaging in the Zebrafish

The zebrafish (Danio rerio) has emerged as a widely used model system during the last four decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging, including calcium imaging of intracellular transients in many different tissues. While being a vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function in health and disease. Since the mid 90's, the development and neuronal function of the embryonic, larval, and later, adult zebrafish have been studied using calcium imaging methods. This updated chapter is reviewing the advances in methods and research findings of zebrafish calcium imaging during the last decade. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Synthetic calcium indicators, conjugated to dextrans and acetoxymethyl (AM) esters, are still used to label specific neuronal cell types in the hindbrain and the olfactory system. However, genetically encoded calcium indicators, such as aequorin and the GCaMP family of indicators, expressed in various tissues by the use of cell-specific promoters, are now the choice for most applications, including brain-wide imaging. Calcium imaging in the zebrafish has contributed greatly to our understanding of basic biological principles during development and adulthood, and the function of disease-related genes in a vertebrate system.

Superior Colliculus: A Vision for Orienting

The superior colliculus contains neurons sensitive to motion direction. New research shows that these neurons are anatomically clustered: those representing the region of visual space 'seen' by both eyes preferentially respond to nasal motion directions and others to opposite directions.

The Zebrafish Visual System: From Circuits to Behavior

Visual stimuli can evoke complex behavioral responses, but the underlying streams of neural activity in mammalian brains are difficult to follow because of their size. Here, I review the visual system of zebrafish larvae, highlighting where recent experimental evidence has localized the functional steps of visuomotor transformations to specific brain areas. The retina of a larva encodes behaviorally relevant visual information in neural activity distributed across feature-selective ganglion cells such that signals representing distinct stimulus properties arrive in different areas or layers of the brain. Motor centers in the hindbrain encode motor variables that are precisely tuned to behavioral needs within a given stimulus setting. Owing to rapid technological progress, larval zebrafish provide unique opportunities for obtaining a comprehensive understanding of the intermediate processing steps occurring between visual and motor centers, revealing how visuomotor transformations are implemented in a vertebrate brain.

High throughput, rapid receptive field estimation for global motion sensitive neurons using a contiguous motion noise stimulus

BACKGROUND

The systematic characterization of receptive fields (RF) is essential for understanding visual motion processing. The performance of RF estimation depends on the employed stimuli, the complexity of the encoded features, and the quality of the activity readout. Calcium imaging is an attractive readout method for high-throughput neuronal activity recordings. However, calcium recordings are oftentimes noisy and of low temporal resolution. The RF estimation of neurons sensitive to global motion is particularly challenging due to their potentially complex combination of preferred directions across visual field positions.

NEW METHOD

Here, we present a novel noise stimulus, which is enriched with spatiotemporally contiguous motion and thus triggers robust calcium responses. We combined this contiguous motion noise (CMN) stimulus with reverse correlation followed by a two-step nonparametric cluster-based bootstrapping test for efficient and reliable RF estimation.

RESULTS

The in silico evaluation of our approach showed that RF centre positions and preferred directions are reliably detected in most of the simulated neurons. Suppressive RF components were detected in 40% of the simulated neurons. We successfully applied our approach to estimate the RFs of 163 motion-sensitive neurons in vivo within 40 min in the pretectum of zebrafish. Many in vivo neurons were sensitive to elaborate directional flow fields in their RFs.

COMPARISON WITH EXISTING METHODS

Our approach outperforms white noise methods and others due to the optimized motion stimulus statistics and ascertainable fine RF structures.

CONCLUSIONS

The CMN method enables efficient, non-biased RF estimation and will benefit systematic high-throughput investigations of RFs using calcium imaging.

Functional Integration of Newborn Neurons in the Zebrafish Optic Tectum

Neurogenesis persists during adulthood in restricted parts of the vertebrate brain. In the optic tectum (OT) of the zebrafish larva, newborn neurons are continuously added and contribute to visual information processing. Recent studies have started to describe the functional development and fate of newborn neurons in the OT. Like the mammalian brain, newborn neurons in the OT require sensory inputs for their integration into local networks and survival. Recent findings suggest that the functional development of newborn neurons requires both activity-dependent and hard-wired mechanisms for proper circuit integration. Here, we review these findings and argue that the study of neurogenesis in non-mammalian species will help elucidate the general mechanisms of circuit assembly following neurogenesis.

Updated functional segregation of retinal ganglion cell projections in the tectum of a cyprinid fish-further elaboration based on microelectrode recordings

Single-unit responses of retinal ganglion cells (GCs) were recorded extracellularly from their axonal terminals in the tectum opticum (TO) of the intact fish (goldfish, carp). The depths of retinal units consecutively recorded along the track of the microelectrode were measured. At the depth of around 50 μm, the responses of six types of direction-selective (DS) GCs were regularly recorded. Responses of two types of orientation-selective (OS) GCs and detectors of white and black spots occurred approximately 50 μm deeper. Responses of GCs with dark- and light-sustained activity were recorded deeper than all others, at about 200 μm. The receptive fields of consecutively recorded units overlap, so they analyze the same fragment of the visual scene, focused by eye optic on the photoreceptor raster. The responses of pairs of DS GCs (ON and OFF units that preferred same direction of stimulus movement) and OS GCs (detectors of vertical and horizontal lines) were often simultaneously recorded at one position of the microelectrode. (The paired recordings of certain units amounted about fourth part of all recordings.) This suggests that their axonal arborizations are located close to each other in the tectal retinorecipient layer. Electrophysiological method, thus, allows to indirectly clarify and make precise the morphology of the retino-tectal connections and to establish a morpho-physiological correspondence.

Putative targets of direction-selective retinal ganglion cells in the tectum opticum of cyprinid fish

Responses of direction selective (DS) units of retinal and tectal origin were recorded extracellularly from the tectum opticum (TO) of immobilized fish. The data were collected from three cyprinid species - goldfish, carp and roach. Responses of the retinal DS ganglion cells (GCs) were recorded from their axon terminals in the superficial layers of TO. According to their preferred directions DS GCs, characterized by small receptive fields (3-8°), can be divided in three distinct groups, each group containing ON and OFF subtypes approximately in equal quantity. Conversely, direction-selective tectal neurons (DS TNs), recorded at two different tectal levels deeper than the zone of retinal DS afferents, are characterized by large receptive fields (up to 60°) and are indifferent to any sign of contrast i.e. can be considered as ON-OFF type units. Fish DS TNs unlike the retinal DS GCs, select four preferred directions. Three types of tectal DS units prefer practically the same directions as those already selected on the retinal level - caudo-rostral, dorso-ventral and ventro-dorsal. The fact that three preferred directions of DS GCs and DS TNs coincide allows us to assume that three types of DS GCs are input neurons for corresponding types of DS TNs. The fourth group of DS TNs has the emergent rostro-caudal preference not explicitly present in any of the DS GC inputs. These units are recorded in deep TO layers exclusively. Receptive fields of these DS neurons could be entirely formed on the tectal level. Possible interrelations between retinal and tectal DS units are discussed.

Selective processing of all rotational and translational optic flow directions in the zebrafish pretectum and tectum

Background

The processing of optic flow in the pretectum/accessory optic system allows animals to stabilize retinal images by executing compensatory optokinetic and optomotor behavior. The success of this behavior depends on the integration of information from both eyes to unequivocally identify all possible translational or rotational directions of motion. However, it is still unknown whether the precise direction of ego-motion is already identified in the zebrafish pretectum or later in downstream premotor areas.

Results

Here, we show that the zebrafish pretectum and tectum each contain four populations of motion-sensitive direction-selective (DS) neurons, with each population encoding a different preferred direction upon monocular stimulation. In contrast, binocular stimulation revealed the existence of pretectal and tectal neurons that are specifically tuned to only one of the many possible combinations of monocular motion, suggesting that further downstream sensory processing might not be needed to instruct appropriate optokinetic and optomotor behavior.

Conclusion

Our results suggest that local, task-specific pretectal circuits process DS retinal inputs and carry out the binocular sensory computations necessary for optokinetic and optomotor behavior.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0648-2) contains supplementary material, which is available to authorized users.

Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain

The vestibular system, which reports on motion and gravity, is essential to postural control, balance, and egocentric representations of movement and space. The motion needed to stimulate the vestibular system complicates studying its circuitry, so we previously developed a method for fictive vestibular stimulation in zebrafish, using optical trapping to apply physical forces to the otoliths. Here, we combine this approach with whole-brain calcium imaging at cellular resolution, delivering a comprehensive map of the brain regions and cellular responses involved in basic vestibular processing. We find responses broadly distributed across the brain, with unique profiles of cellular responses and topography in each region. The most widespread and abundant responses involve excitation that is graded to the stimulus strength. Other responses, localized to the telencephalon and habenulae, show excitation that is only weakly correlated to stimulus strength and that is sensitive to weak stimuli. Finally, numerous brain regions contain neurons that are inhibited by vestibular stimuli, and these neurons are often tightly localized spatially within their regions. By exerting separate control over the left and right otoliths, we explore the laterality of brain-wide vestibular processing, distinguishing between neurons with unilateral and bilateral vestibular sensitivity and revealing patterns whereby conflicting signals from the ears mutually cancel. Our results confirm previously identified vestibular responses in specific regions of the larval zebrafish brain while revealing a broader and more extensive network of vestibular responsive neurons than has previously been described. This provides a departure point for more targeted studies of the underlying functional circuits.

Optic tectal superficial interneurons detect motion in larval zebrafish

Detection of moving objects is an essential skill for animals to hunt prey, recognize conspecifics and avoid predators. The zebrafish, as a vertebrate model, primarily uses its elaborate visual system to distinguish moving objects against background scenes. The optic tectum (OT) receives and integrates inputs from various types of retinal ganglion cells (RGCs), including direction-selective (DS) RGCs and size-selective RGCs, and is required for both prey capture and predator avoidance. However, it remains largely unknown how motion information is processed within the OT. Here we performed in vivo whole-cell recording and calcium imaging to investigate the role of superficial interneurons (SINs), a specific type of optic tectal neurons, in motion detection of larval zebrafish. SINs mainly receive excitatory synaptic inputs, exhibit transient ON- or OFF-type of responses evoked by light flashes, and possess a large receptive field (RF). One fifth of SINs are DS and classified into two subsets with separate preferred directions. Furthermore, SINs show size-dependent responses to moving dots. They are efficiently activated by moving objects but not static ones, capable of showing sustained responses to moving objects and having less visual adaptation than periventricular neurons (PVNs), the principal tectal cells. Behaviorally, ablation of SINs impairs prey capture, which requires local motion detection, but not global looming-evoked escape. Finally, starvation enhances the gain of SINs' motion responses while maintaining their size tuning and DS. These results indicate that SINs serve as a motion detector for sensing and localizing sized moving objects in the visual field.

Retinotopic Separation of Nasal and Temporal Motion Selectivity in the Mouse Superior Colliculus

Sensory neurons often display an ordered spatial arrangement that enhances the encoding of specific features on different sides of natural borders in the visual field (for example, [1-3]). In central visual areas, one prominent natural border is formed by the confluence of information from the two eyes, the monocular-binocular border [4]. Here, we investigate whether receptive field properties of neurons in the mouse superior colliculus show any systematic organization about the monocular-binocular border. The superior colliculus is a layered midbrain structure that plays a significant role in the orienting responses of the eye, head, and body [5]. Its superficial layers receive direct input from the majority of retinal ganglion cells and are retinotopically organized [6, 7]. Using two-photon calcium imaging, we recorded the activity of collicular neurons from the superficial layers of awake mice and determined their direction selectivity, orientation selectivity, and retinotopic location. This revealed that nearby direction-selective neurons have a strong tendency to prefer the same motion direction. In retinotopic space, the local preference of direction-selective neurons shows a sharp transition in the preference for nasal versus temporal motion at the monocular-binocular border. The maps representing orientation and direction appear to be independent. These results illustrate the important coherence between the spatial organization of inputs and response properties within the visual system and suggest a re-analysis of the receptive field organization within the superior colliculus from an ecological perspective.

Luminance Changes Drive Directional Startle through a Thalamic Pathway

Looming visual stimuli result in escape responses that are conserved from insects to humans. Despite their importance for survival, the circuits mediating visual startle have only recently been explored in vertebrates. Here we show that the zebrafish thalamus is a luminance detector critical to visual escape. Thalamic projection neurons deliver dim-specific information to the optic tectum, and ablations of these projections disrupt normal tectal responses to looms. Without this information, larvae are less likely to escape from dark looming stimuli and lose the ability to escape away from the source of the loom. Remarkably, when paired with an isoluminant loom stimulus to the opposite eye, dimming is sufficient to increase startle probability and to reverse the direction of the escape so that it is toward the loom. We suggest that bilateral comparisons of luminance, relayed from the thalamus to the tectum, facilitate escape responses and are essential for their directionality.

Principles of Functional Circuit Connectivity: Insights From Spontaneous Activity in the Zebrafish Optic Tectum

The brain is continuously active, even in the absence of external stimulation. In the optic tectum of the zebrafish larva, this spontaneous activity is spatially organized and reflects the circuit's functional connectivity. The structure of the spontaneous activity displayed patterns associated with aspects of the larva's preferences when engaging in complex visuo-motor behaviors, suggesting that the tectal circuit is adapted for the circuit's functional role in detecting visual cues and generating adequate motor behaviors. Further studies in sensory deprived larvae suggest that the basic structure of the functional connectivity patterns emerges even in the absence of retinal inputs, but that its fine structure is affected by visual experience.

Integrative whole-brain neuroscience in larval zebrafish

Due to their small size and transparency, zebrafish larvae are amenable to a range of fluorescence microscopy techniques. With the development of sensitive genetically encoded calcium indicators, this has extended to the whole-brain imaging of neural activity with cellular resolution. This technique has been used to study brain-wide population dynamics accompanying sensory processing and sensorimotor transformations, and has spurred the development of innovative closed-loop behavioral paradigms in which stimulus-response relationships can be studied. More recently, microscopes have been developed that allow whole-brain calcium imaging in freely swimming and behaving larvae. In this review, we highlight the technologies underlying whole-brain functional imaging in zebrafish, provide examples of the sensory and motor processes that have been studied with this technique, and discuss the need to merge data from whole-brain functional imaging studies with neurochemical and anatomical information to develop holistic models of functional neural circuits.

Imaging Neuronal Activity in the Optic Tectum of Late Stage Larval Zebrafish

The zebrafish is an established model to study the development and function of visual neuronal circuits in vivo, largely due to their optical accessibility at embryonic and larval stages. In the past decade multiple experimental paradigms have been developed to study visually-driven behaviours, particularly those regulated by the optic tectum, the main visual centre in lower vertebrates. With few exceptions these techniques are limited to young larvae (7–9 days post-fertilisation, dpf). However, many forms of visually-driven behaviour, such as shoaling, emerge at later developmental stages. Consequently, there is a need for an experimental paradigm to image the visual system in zebrafish larvae beyond 9 dpf. Here, we show that using NBT:GCaMP3 line allows for imaging neuronal activity in the optic tectum in late stage larvae until at least 21 dpf. Utilising this line, we have characterised the receptive field properties of tectal neurons of the 2–3 weeks old fish in the cell bodies and the neuropil. The NBT:GCaMP3 line provides a complementary approach and additional opportunities to study neuronal activity in late stage zebrafish larvae.

Orientation-Selective Retinal Circuits in Vertebrates

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

Sustained Rhythmic Brain Activity Underlies Visual Motion Perception in Zebrafish

Following moving visual stimuli (conditioning stimuli, CS), many organisms perceive, in the absence of physical stimuli, illusory motion in the opposite direction. This phenomenon is known as the motion aftereffect (MAE). Here, we use MAE as a tool to study the neuronal basis of visual motion perception in zebrafish larvae. Using zebrafish eye movements as an indicator of visual motion perception, we find that larvae perceive MAE. Blocking eye movements using optogenetics during CS presentation did not affect MAE, but tectal ablation significantly weakened it. Using two-photon calcium imaging of behaving GCaMP3 larvae, we find post-stimulation sustained rhythmic activity among direction-selective tectal neurons associated with the perception of MAE. In addition, tectal neurons tuned to the CS direction habituated, but neurons in the retina did not. Finally, a model based on competition between direction-selective neurons reproduced MAE, suggesting a neuronal circuit capable of generating perception of visual motion.

A Three-Layer Network Model of Direction Selective Circuits in the Optic Tectum

The circuit mechanisms that give rise to direction selectivity in the retina have been studied extensively but how direction selectivity is established in retinorecipient areas of the brain is less well understood. Using functional imaging in larval zebrafish we examine how the direction of motion is encoded by populations of neurons at three layers of the optic tectum; retinal ganglion cell axons (RGCs), a layer of superficial inhibitory interneurons (SINs), and periventricular neurons (PVNs), which constitute the majority of neurons in the tectum. We show that the representation of motion direction is transformed at each layer. At the level of RGCs and SINs the direction of motion is encoded by three direction-selective (DS) subtypes tuned to upward, downward, and caudal-to-rostral motion. However, the tuning of SINs is significantly narrower and this leads to a conspicuous gap in the representation of motion in the rostral-to-caudal direction at the level of SINs. Consistent with previous findings we demonstrate that, at the level of PVNs the direction of motion is encoded by four DS cell types which include an additional DS PVN cell type tuned to rostral-to-caudal motion. Strikingly, the tuning profile of this emergent cell type overlaps with the gap in the representation of rostral-to-caudal motion at the level of SINs. Using our functional imaging data we constructed a simple computational model that demonstrates how the emergent population of PVNs is generated by the interactions of cells at each layer of the tectal network. The model predicts that PVNs tuned to rostral-to-caudal motion can be generated via convergence of DS RGCs tuned to upward and downward motion and feedforward tuned inhibition via SINs which suppresses responses to non-preferred directions. Thus, by reshaping directional tuning that is inherited from the retina inhibitory inputs from SINs can generate a novel subtype of DS PVN and in so doing enhance the encoding of directional stimuli.

Prenatal cocaine exposure disrupts the dopaminergic system and its postnatal responses to cocaine

Impaired attention is the hallmark consequence of prenatal cocaine exposure (PCE), affecting brain development, learning, memory and social adaptation starting at an early age. To date, little is known about the brain structures and neurochemical processes involved in this effect. Through focusing on the visual system and employing zebrafish as a model, we show that PCE reduces expression of dopamine receptor Drd1, with levels reduced in the optic tectum and other brain regions, but not the telencephalon. Organism-wide, PCE results in a 1.7-fold reduction in the expression of the dopamine transporter (dat), at baseline. Acute cocaine administration leads to a 2-fold reduction in dat in drug-naive larvae but not PCE fish. PCE sensitizes animals to an anxiogenic-like behavioral effect of acute cocaine, bottom-dwelling, while loss of DAT due to genetic knockout (DATKO) leads to bottom-dwelling behavior at baseline. Neuronal calcium responses to visual stimuli in both PCE and DATKO fish show tolerance to acute cocaine in the principal regions of visual attention, the telencephalon and optic tectum. The zebrafish model can provide a sensitive assay by which to elucidate the molecular mechanisms and brain region-specific consequences of PCE, and facilitate the search for effective therapeutic solutions.

Active Dendritic Properties and Local Inhibitory Input Enable Selectivity for Object Motion in Mouse Superior Colliculus Neurons

Neurons respond to specific features of sensory stimuli. In the visual system, for example, some neurons respond to motion of small but not large objects, whereas other neurons prefer motion of the entire visual field. Separate neurons respond equally to local and global motion but selectively to additional features of visual stimuli. How and where does response selectivity emerge? Here, we show that wide-field (WF) cells in retino-recipient layers of the mouse superior colliculus (SC) respond selectively to small moving objects. Moreover, we identify two mechanisms that contribute to this selectivity. First, we show that input restricted to a small portion of the broad dendritic arbor of WF cells is sufficient to trigger dendritic spikes that reliably propagate to the soma/axon. In vivo whole-cell recordings reveal that nearly every action potential evoked by visual stimuli has characteristics of spikes initiated in dendrites. Second, inhibitory input from a different class of SC neuron, horizontal cells, constrains the range of stimuli to which WF cells respond. Horizontal cells respond preferentially to the sudden appearance or rapid movement of large stimuli. Optogenetic reduction of their activity reduces movement selectivity and broadens size tuning in WF cells by increasing the relative strength of responses to stimuli that appear suddenly or cover a large region of space. Therefore, strongly propagating dendritic spikes enable small stimuli to drive spike output in WF cells and local inhibition helps restrict responses to stimuli that are both small and moving.

SIGNIFICANCE STATEMENT

How do neurons respond selectively to some sensory stimuli but not others? In the visual system, a particularly relevant stimulus feature is object motion, which often reveals other animals. Here, we show how specific cells in the superior colliculus, one synapse downstream of the retina, respond selectively to object motion. These wide-field (WF) cells respond strongly to small objects that move slowly anywhere through a large region of space, but not to stationary objects or full-field motion. Action potential initiation in dendrites enables small stimuli to trigger visual responses and inhibitory input from cells that prefer large, suddenly appearing, or quickly moving stimuli restricts responses of WF cells to objects that are small and moving.

Characterisation of sensitivity and orientation tuning for visually responsive ensembles in the zebrafish tectum

Sensory coding relies on ensembles of co-active neurons, but these ensembles change from trial to trial of the same stimulus. This is due in part to wide variability in the responsiveness of neurons within these ensembles, with some neurons responding regularly to a stimulus while others respond inconsistently. The specific functional properties that cause neurons to respond more or less consistently have not been thoroughly explored. Here, we have examined neuronal ensembles in the zebrafish tectum responsive to repeated presentations of a visual stimulus, and have explored how these populations change when the orientation or brightness of the stimulus is altered. We found a continuum of response probabilities across the neurons in the visual ensembles, with the most responsive neurons focused toward the spatial centre of the ensemble. As the visual stimulus was made dimmer, these neurons remained active, suggesting higher overall responsiveness. However, these cells appeared to represent the most consistent end of a continuum, rather than a functionally distinct “core” of highly responsive neurons. Reliably responsive cells were broadly tuned to a range of stimulus orientations suggesting that, at least for this stimulus property, tight stimulus tuning was not responsible for their consistent responses.

From the optic tectum to the primary visual cortex: migration through evolution of the saliency map for exogenous attentional guidance

Recent data have supported the hypothesis that, in primates, the primary visual cortex (V1) creates a saliency map from visual input. The exogenous guidance of attention is then realized by means of monosynaptic projections to the superior colliculus, which can select the most salient location as the target of a gaze shift. V1 is less prominent, or is even absent in lower vertebrates such as fish; whereas the superior colliculus, called optic tectum in lower vertebrates, also receives retinal input. I review the literature and propose that the saliency map has migrated from the tectum to V1 over evolution. In addition, attentional benefits manifested as cueing effects in humans should also be present in lower vertebrates.

Neural Mechanisms Generating Orientation Selectivity in the Retina

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

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

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

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Orientation tuning is present also among some bipolar cell presynaptic terminals

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

Functional Profiles of Visual-, Auditory-, and Water Flow-Responsive Neurons in the Zebrafish Tectum

The tectum has long been known as a hub of visual processing, and recent studies have elucidated many of the circuit-level mechanisms by which tectal neurons filter visual information. Here, we use population-scale imaging of tectal neurons expressing a genetically encoded calcium indicator to characterize tectal responses to non-visual stimuli in zebrafish. We identify ensembles of neurons responsive to stimuli for each of three sensory modalities: vision, audition, and water flow sensation. These ensembles display consistently represented response profiles to our stimuli, and each has a preferred stimulus and salient feature to which it is most responsive. Each sensory modality drives a unique spatial profile of activity in the tectal neuropil, suggesting that the neuropil's laminar structure functionally subserves multiple modalities. The positions of the responsive neurons in the periventricular layer are also distinct across modalities, and very few neurons are responsive to multiple modalities. The cells contributing to each ensemble are highly variable from trial to trial, but ensembles contain "cores" of reliably responsive cells, suggesting a mechanism whereby they could maintain consistency in reporting salient stimulus features while retaining flexibility to report on similar stimuli. Finally, we find that co-presentation of auditory or water flow stimuli suppress visual responses in the tectum.

Neural Circuits Underlying Visually Evoked Escapes in Larval Zebrafish

Escape behaviors deliver organisms away from imminent catastrophe. Here, we characterize behavioral responses of freely swimming larval zebrafish to looming visual stimuli simulating predators. We report that the visual system alone can recruit lateralized, rapid escape motor programs, similar to those elicited by mechanosensory modalities. Two-photon calcium imaging of retino-recipient midbrain regions isolated the optic tectum as an important center processing looming stimuli, with ensemble activity encoding the critical image size determining escape latency. Furthermore, we describe activity in retinal ganglion cell terminals and superficial inhibitory interneurons in the tectum during looming and propose a model for how temporal dynamics in tectal periventricular neurons might arise from computations between these two fundamental constituents. Finally, laser ablations of hindbrain circuitry confirmed that visual and mechanosensory modalities share the same premotor output network. We establish a circuit for the processing of aversive stimuli in the context of an innate visual behavior.

Direction-selective units in goldfish retina and tectum opticum - review and new aspects

The output units of fish retina, i.e., the retinal ganglion cells (detectors), send highly processed information to the primary visual centers of the brain, settled in the midbrain formation tectum opticum (TO). Axons of different fish motion detectors terminate in different tectal levels. In the superficial layer of TO, axons of direction-selective ganglion cells (DS GCs) are terminated. Single unit responses of the DS GCs were recorded in intact fish from their axon terminals in TO. Goldfish DS GCs projecting to TO were shown to comprise six physiological types according to their selectivity to sign of stimulus contrast (ON and OFF units) and their preferred directions: three directions separated by 120[Formula: see text]. These units, characterized by relatively small receptive fields and remarkable spatial resolution should be classified as local motion detectors. In addition to the retinal DS GCs, other kinds of DS units were extracellularly recorded in the superficial and deep sublaminae of tectum. Some features of their responses suggested that they originated from tectal neurons (TNs). Contrary to DS GCs which are characterized by small RFs and use separate ON and OFF channels, DS TNs have extra-large RFs and ON-OFF type responses. DS TNs were shown to select four preferred directions. Three of them are compatible with those already selected on the retinal level. Complementary to them, the fourth DS TN type with rostro-caudal preference (lacking in the retina) has been revealed. Possible functional interrelations between DS GCs and DS TNs are discussed.

Lamination Speeds the Functional Development of Visual Circuits

A common feature of the brain is the arrangement of synapses in layers. To examine the significance of this organizational feature, we studied the functional development of direction-selective (DS) circuits in the tectum of astray mutant zebrafish in which lamination of retinal ganglion cell (RGC) axons is lost. We show that although never laminar, the tuning of DS-RGC axons targeting the mutant tectum is normal. Analysis of mutant tectal neurons at late developmental stages reveals that directional tuning is indistinguishable from wild-type larvae. Furthermore, we show that structural plasticity of tectal dendrites and RGC axons compensates for the loss of lamination, establishing connectivity between DS-RGCs and their normal tectal targets. However, tectal direction selectivity is severely perturbed at earlier developmental stages. Thus, the formation of synaptic laminae is ultimately dispensable for the correct wiring of direction-selective tectal circuits, but it is crucial for the rapid assembly of these networks.

Video Abstract

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Robo2 directs lamination of direction-selective retinal axons and tectal dendrites

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Tectal lamination is required for rapid assembly of direction-selective circuits

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Functional direction-selective circuits eventually form when lamination is lost

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Structural plasticity compensates for the loss of tectal lamination

Adaptation-induced modification of motion selectivity tuning in visual tectal neurons of adult zebrafish

In the developing brain, training-induced emergence of direction selectivity and plasticity of orientation tuning appear to be widespread phenomena. These are found in the visual pathway across different classes of vertebrates. Moreover, short-term plasticity of orientation tuning in the adult brain has been demonstrated in several species of mammals. However, it is unclear whether neuronal orientation and direction selectivity in nonmammalian species remains modifiable through short-term plasticity in the fully developed brain. To address this question, we analyzed motion tuning of neurons in the optic tectum of adult zebrafish by calcium imaging. In total, orientation and direction selectivity was enhanced by adaptation, responses of previously orientation-selective neurons were sharpened, and even adaptation-induced emergence of selectivity in previously nonselective neurons was observed in some cases. The different observed effects are mainly based on the relative distance between the previously preferred and the adaptation direction. In those neurons in which a shift of the preferred orientation or direction was induced by adaptation, repulsive shifts (i.e., away from the adapter) were more prevalent than attractive shifts. A further novel finding for visually induced adaptation that emerged from our study was that repulsive and attractive shifts can occur within one brain area, even with uniform stimuli. The type of shift being induced also depends on the difference between the adapting and the initially preferred stimulus direction. Our data indicate that, even within the fully developed optic tectum, short-term plasticity might have an important role in adjusting neuronal tuning functions to current stimulus conditions.

Neurons in the most superficial lamina of the mouse superior colliculus are highly selective for stimulus direction

The superior colliculus (SC) is a layered midbrain structure important for multimodal integration and sensorimotor transformation. Its superficial layers are purely visual and receive depth-specific projections from distinct subtypes of retinal ganglion cells. Here we use two-photon calcium imaging to characterize the response properties of neurons in the most superficial lamina of the mouse SC, an undersampled population with electrophysiology. We find that these neurons have compact receptive fields with primarily overlapping ON and OFF subregions and are highly direction selective. The high selectivity is observed in both excitatory and inhibitory neurons. These neurons do not cluster according to their direction preference and lack orientation selectivity. In addition, we perform single-unit recordings and show that direction selectivity declines with depth in the SC. Together, our experiments reveal for the first time a highly specialized lamina in the most superficial SC for movement direction, a finding that has important implications for understanding signal transformation in the early visual system.

Deletion of a kinesin I motor unmasks a mechanism of homeostatic branching control by neurotrophin-3

Development and function of highly polarized cells such as neurons depend on microtubule-associated intracellular transport, but little is known about contributions of specific molecular motors to the establishment of synaptic connections. In this study, we investigated the function of the Kinesin I heavy chain Kif5aa during retinotectal circuit formation in zebrafish. Targeted disruption of Kif5aa does not affect retinal ganglion cell differentiation, and retinal axons reach their topographically correct targets in the tectum, albeit with a delay. In vivo dynamic imaging showed that anterograde transport of mitochondria is impaired, as is synaptic transmission. Strikingly, disruption of presynaptic activity elicits upregulation of Neurotrophin-3 (Ntf3) in postsynaptic tectal cells. This in turn promotes exuberant branching of retinal axons by signaling through the TrkC receptor (Ntrk3). Thus, our study has uncovered an activity-dependent, retrograde signaling pathway that homeostatically controls axonal branching.

Preference for concentric orientations in the mouse superior colliculus

The superior colliculus is a layered structure important for body- and gaze-orienting responses. Its superficial layer is, next to the lateral geniculate nucleus, the second major target of retinal ganglion axons and is retinotopically organized. Here we show that in the mouse there is also a precise organization of orientation preference. In columns perpendicular to the tectal surface, neurons respond to the same visual location and prefer gratings of the same orientation. Calcium imaging and extracellular recording revealed that the preferred grating varies with retinotopic location, and is oriented parallel to the concentric circle around the centre of vision through the receptive field. This implies that not all orientations are equally represented across the visual field. This makes the superior colliculus different from visual cortex and unsuitable for translation-invariant object recognition and suggests that visual stimuli might have different behavioural consequences depending on their retinotopic location.

The mammalian superior colliculus (SC) processes visual stimuli but little is known about the spatial organization of the response preferences for specific visual features. Here the authors show that the mouse SC contains a map for orientation preference such that preferred grating orientation is aligned to concentric circles around the centre of the visual field.

Visuomotor transformations underlying hunting behavior in zebrafish

Visuomotor circuits filter visual information and determine whether or not to engage downstream motor modules to produce behavioral outputs. However, the circuit mechanisms that mediate and link perception of salient stimuli to execution of an adaptive response are poorly understood. We combined a virtual hunting assay for tethered larval zebrafish with two-photon functional calcium imaging to simultaneously monitor neuronal activity in the optic tectum during naturalistic behavior. Hunting responses showed mixed selectivity for combinations of visual features, specifically stimulus size, speed, and contrast polarity. We identified a subset of tectal neurons with similar highly selective tuning, which show non-linear mixed selectivity for visual features and are likely to mediate the perceptual recognition of prey. By comparing neural dynamics in the optic tectum during response versus non-response trials, we discovered premotor population activity that specifically preceded initiation of hunting behavior and exhibited anatomical localization that correlated with motor variables. In summary, the optic tectum contains non-linear mixed selectivity neurons that are likely to mediate reliable detection of ethologically relevant sensory stimuli. Recruitment of small tectal assemblies appears to link perception to action by providing the premotor commands that release hunting responses. These findings allow us to propose a model circuit for the visuomotor transformations underlying a natural behavior.

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Zebrafish hunting responses are triggered by conjunctions of visual features

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Tectal neurons show non-linear mixed selectivity for prey-like visual stimuli

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Tectal assemblies show premotor activity specifically preceding hunting responses