Distinct cell types in the superficial superior colliculus project to the dorsal lateral geniculate and lateral posterior thalamic nuclei

Excitatory Projections of Wide Field Collicular Neurons to the Nucleus of the Optic Tract in the Rat

The superficial layers of the mammalian superior colliculus (SC) contain neurons that are generally responsive to visual stimuli but can differ considerably in morphology and response properties. To elucidate the structure and function of these neurons, we combined extracellular recording and juxtacellular labeling, detailed anatomical reconstruction, and ultrastructural analysis of the synaptic contacts of labeled neurons, using transmission electron microscopy. Our labeled neurons project to different brainstem nuclei. Of particular importance are neurons that fit the morphological criteria of the wide field (WF) neurons and whose dendrites are horizontally oriented. They display a rather characteristic axonal projection pattern to the nucleus of optic tract (NOT); thus, we call them superior collicular WF projecting to the NOT (SCWFNOT) neurons. We corroborated the morphological characterization of this neuronal type as a distinct neuronal class with the help of unsupervised hierarchical cluster analysis. Our ultrastructural data demonstrate that SCWFNOT neurons establish excitatory connections with their targets in the NOT. Although, in rodents, the literature about the WF neurons has focused on their extensive projection to the lateral posterior nucleus of the thalamus, as a conduit for information to reach the visual association areas of the cortex, our data suggest that this subclass of WF neurons may participate in the optokinetic nystagmus.

Stimulus-dependent differences in cortical versus subcortical contributions to visual detection in mice

The primary visual cortex (V1) and the superior colliculus (SC) both occupy stations early in the processing of visual information. They have long been thought to perform distinct functions, with the V1 supporting the perception of visual features and the SC regulating orienting to visual inputs. However, growing evidence suggests that the SC supports the perception of many of the same visual features traditionally associated with the V1. To distinguish V1 and SC contributions to visual processing, it is critical to determine whether both areas causally contribute to the detection of specific visual stimuli. Here, mice reported changes in visual contrast or luminance near their perceptual threshold while white noise patterns of optogenetic stimulation were delivered to V1 or SC inhibitory neurons. We then performed a reverse correlation analysis on the optogenetic stimuli to estimate a neuronal-behavioral kernel (NBK), a moment-to-moment estimate of the impact of V1 or SC inhibition on stimulus detection. We show that the earliest moments of stimulus-evoked activity in the SC are critical for the detection of both luminance and contrast changes. Strikingly, there was a robust stimulus-aligned modulation in the V1 contrast-detection NBK but no sign of a comparable modulation for luminance detection. The data suggest that behavioral detection of visual contrast depends on both V1 and SC spiking, whereas mice preferentially use SC activity to detect changes in luminance. Electrophysiological recordings showed that neurons in both the SC and V1 responded strongly to both visual stimulus types, while the reverse correlation analysis reveals when these neuronal signals actually contribute to visually guided behaviors.

Retinal origin of orientation but not direction selective maps in the superior colliculus

Neurons in the mouse superior colliculus ("colliculus") are arranged in ordered spatial maps. While orientation-selective (OS) neurons form a concentric map aligned to the center of vision, direction-selective (DS) neurons are arranged in patches with changing preferences across the visual field. It remains unclear whether these maps are a consequence of feedforward input from the retina or local computations in the colliculus. To determine whether these maps originate in the retina, we mapped the local and global distribution of OS and DS retinal ganglion cell axon boutons using in vivo two-photon calcium imaging. We found that OS boutons formed patches that matched the distribution of OS neurons within the colliculus. DS boutons displayed fewer regional specializations, better reflecting the organization of DS neurons in the retina. Both eyes convey similar orientation but different DS inputs to the colliculus, as shown in recordings from retinal explants. These data demonstrate that orientation and direction maps within the colliculus are independent, where orientation maps are likely inherited from the retina, but direction maps require additional computations.

Cadherin-13 Maintains Retinotectal Synapses via Transneuronal Interactions

Maintaining precise synaptic contacts between neuronal partners is critical to ensure the proper functioning of the mammalian central nervous system (CNS). Diverse cell recognition molecules, such as classic cadherins (Cdhs), are part of the molecular machinery mediating synaptic choices during development and synaptic maintenance. Yet, the principles governing neuron-neuron wiring across diverse CNS neuron types remain largely unknown. The retinotectal synapses, connections from the retinal ganglion cells (RGCs) to the superior collicular (SC) neurons, offer an ideal experimental system to reveal molecular logic underlying synaptic choices and formation. This is due to the retina's unidirectional and laminar-restricted projections to the SC and the large databases of presynaptic RGC subtypes and postsynaptic SC neuronal types. Here, we focused on determining the role of Type II Cdhs in wiring the retinotectal synapses. We surveyed Cdhs expression patterns at neuronal resolution and revealed that Cdh13 is enriched in the wide-field neurons in the superficial SC (sSC). In either the Cdh13 null mutant or selective adult deletion within the wide-field neurons, there is a significant reduction of spine densities in the distal dendrites of these neurons in both sexes. Additionally, Cdh13 removal from presynaptic RGCs reduced dendritic spines in the postsynaptic wide-field neurons. Cdh13-expressing RGCs use differential mechanisms than αRGCs and On-Off Direction-Selective Ganglion Cells (ooDSGCs) to form specific retinotectal synapses. The results revealed a selective transneuronal interaction mediated by Cdh13 to maintain proper retinotectal synapses in vivo.

Enhancement and contextual modulation of visuospatial processing by thalamocollicular projections from ventral lateral geniculate nucleus

In the mammalian visual system, the ventral lateral geniculate nucleus (vLGN) of the thalamus receives salient visual input from the retina and sends prominent GABAergic axons to the superior colliculus (SC). However, whether and how vLGN contributes to fundamental visual information processing remains largely unclear. Here, we report in mice that vLGN facilitates visually-guided approaching behavior mediated by the lateral SC and enhances the sensitivity of visual object detection. This can be attributed to the extremely broad spatial integration of vLGN neurons, as reflected in their much lower preferred spatial frequencies and broader spatial receptive fields than SC neurons. Through GABAergic thalamocollicular projections, vLGN specifically exerts prominent surround suppression of visuospatial processing in SC, leading to a fine tuning of SC preferences to higher spatial frequencies and smaller objects in a context-dependent manner. Thus, as an essential component of the central visual processing pathway, vLGN serves to refine and contextually modulate visuospatial processing in SC-mediated visuomotor behaviors via visually-driven long-range feedforward inhibition.

Characterization of primary visual cortex input to specific cell types in the superior colliculus

The superior colliculus is a critical brain region involved in processing visual information. It receives visual input directly from the retina, as well as via a projection from primary visual cortex. Here we determine which cell types in the superficial superior colliculus receive visual input from primary visual cortex in mice. Neurons in the superficial layers of the superior colliculus were classified into four groups - Wide-field, narrow-field, horizontal and stellate - based on their morphological and electrophysiological properties. To determine functional connections between V1 and these four different cell types we expressed Channelrhodopsin2 in primary visual cortex and then optically stimulated these axons while recording from different neurons in the superficial superior colliculus using whole-cell patch-clamp recording in vitro. We found that all four cell types in the superficial layers of the superior colliculus received monosynaptic (direct) input from V1. Wide-field neurons were more likely than other cell types to receive primary visual cortex input. Our results provide information on the cell specificity of the primary visual cortex to superior colliculus projection, increasing our understanding of how visual information is processed in the superior colliculus at the single cell level.

Brn3b regulates the formation of fear-related midbrain circuits and defensive responses to visual threat

Defensive responses to visually threatening stimuli represent an essential fear-related survival instinct, widely detected across species. The neural circuitry mediating visually triggered defensive responses has been delineated in the midbrain. However, the molecular mechanisms regulating the development and function of these circuits remain unresolved. Here, we show that midbrain-specific deletion of the transcription factor Brn3b causes a loss of neurons projecting to the lateral posterior nucleus of the thalamus. Brn3b deletion also down-regulates the expression of the neuropeptide tachykinin 2 (Tac2). Furthermore, Brn3b mutant mice display impaired defensive freezing responses to visual threat precipitated by social isolation. This behavioral phenotype could be ameliorated by overexpressing Tac2, suggesting that Tac2 acts downstream of Brn3b in regulating defensive responses to threat. Together, our experiments identify specific genetic components critical for the functional organization of midbrain fear-related visual circuits. Similar mechanisms may contribute to the development and function of additional long-range brain circuits underlying fear-associated behavior.

Single-nucleus RNA sequencing of developing superior colliculus identifies neuronal diversity and candidate mediators of circuit assembly

The superior colliculus (SC) is a sensorimotor structure in the midbrain that integrates input from multiple sensory modalities to initiate motor commands. It undergoes well-characterized steps of circuit assembly during development, rendering the mouse SC a popular model to study establishment of neural connectivity. Here we perform single-nucleus RNA-sequencing analysis of the mouse SC isolated at various developmental time points. Our study provides a transcriptomic landscape of the cell types that comprise the SC across murine development with particular emphasis on neuronal heterogeneity. We report a repertoire of genes differentially expressed across the different postnatal ages, many of which are known to regulate axon guidance and synapse formation. Using these data, we find that Pax7 expression is restricted to a subset of GABAergic neurons. Our data provide a valuable resource for interrogating the mechanisms of circuit development and identifying markers for manipulating specific SC neuronal populations and circuits.

Retinal input integration in excitatory and inhibitory neurons in the mouse superior colliculus in vivo

The superior colliculus (SC) is a midbrain structure that receives inputs from retinal ganglion cells (RGCs). The SC contains one of the highest densities of inhibitory neurons in the brain but whether excitatory and inhibitory SC neurons differentially integrate retinal activity in vivo is still largely unknown. We recently established a recording approach to measure the activity of RGCs simultaneously with their postsynaptic SC targets in vivo, to study how SC neurons integrate RGC activity. Here, we employ this method to investigate the functional properties that govern retinocollicular signaling in a cell type-specific manner by identifying GABAergic SC neurons using optotagging in VGAT-ChR2 mice. Our results demonstrate that both excitatory and inhibitory SC neurons receive comparably strong RGC inputs and similar wiring rules apply for RGCs innervation of both SC cell types, unlike the cell type-specific connectivity in the thalamocortical system. Moreover, retinal activity contributed more to the spiking activity of postsynaptic excitatory compared to inhibitory SC neurons. This study deepens our understanding of cell type-specific retinocollicular functional connectivity and emphasizes that the two major brain areas for visual processing, the visual cortex and the SC, differently integrate sensory afferent inputs.

Pathway-specific inputs to the superior colliculus support flexible responses to visual threat

Behavioral flexibility requires directing feedforward sensory information to appropriate targets. In the superior colliculus, divergent outputs orchestrate different responses to visual threats, but the circuit organization enabling the flexible routing of sensory information remains unknown. To determine this structure, we focused on inhibitory projection (Gad2) neurons. Trans-synaptic tracing and neuronal recordings revealed that Gad2 neurons projecting to the lateral geniculate nucleus (LGN) and the parabigeminal nucleus (PBG) form two separate populations, each receiving a different set of non-retinal inputs. Inhibiting the LGN- or PBG-projecting Gad2 neurons resulted in opposing effects on behavior; increasing freezing or escape probability to visual looming, respectively. Optogenetic activation of selected inputs to the LGN- and PBG-projecting Gad2 cells predictably regulated responses to visual threat. These data suggest that projection-specific sampling of brain-wide inputs provides a circuit design principle that enables visual inputs to be selectively routed to produce context-specific behavior.

Temporal weighting of cortical and subcortical spikes reveals stimulus dependent differences in their contributions to behavior

The primary visual cortex (V1) and the superior colliculus (SC) both occupy stations early in the processing of visual information. They have long been thought to perform distinct functions, with V1 supporting perception of visual features and the SC regulating orienting to visual inputs. However, growing evidence suggests that the SC supports perception of many of the same visual features traditionally associated with V1. To distinguish V1 and SC contributions to visual processing, it is critical to determine whether both areas causally contribute to perception of specific visual stimuli. Here, mice reported changes in visual contrast or luminance near perceptual threshold while we presented white noise patterns of optogenetic stimulation to V1 or SC inhibitory neurons. We then performed a reverse correlation analysis on the optogenetic stimuli to estimate a neuronal-behavioral kernel (NBK), a moment-to-moment estimate of the impact of V1 or SC inhibition on stimulus detection. We show that the earliest moments of stimulus-evoked activity in SC are critical for detection of both luminance or contrast changes. Strikingly, there was a robust stimulus-aligned modulation in the V1 contrast-detection NBK, but no sign of a comparable modulation for luminance detection. The data suggest that perception of visual contrast depends on both V1 and SC spiking, whereas mice preferentially use SC activity to detect changes in luminance. Electrophysiological recordings showed that neurons in both SC and V1 responded strongly to both visual stimulus types, while the reverse correlation analysis reveals when these neuronal signals actually contribute to visually-guided behaviors.

The neural basis of defensive behaviour evolution in Peromyscus mice

Evading imminent predator threat is critical for survival. Effective defensive strategies can vary, even between closely related species. However, the neural basis of such species-specific behaviours is still poorly understood. Here we find that two sister species of deer mice (genus Peromyscus) show different responses to the same looming stimulus: P. maniculatus, which occupy densely vegetated habitats, predominantly dart to escape, while the open field specialist, P. polionotus, pause their movement. This difference arises from species-specific escape thresholds, is largely context-independent, and can be triggered by both visual and auditory threat stimuli. Using immunohistochemistry and electrophysiological recordings, we find that although visual threat activates the superior colliculus in both species, the role of the dorsal periaqueductal gray (dPAG) in driving behaviour differs. While dPAG activity scales with running speed and involves both excitatory and inhibitory neurons in P. maniculatus, the dPAG is largely silent in P. polionotus, even when darting is triggered. Moreover, optogenetic activation of excitatory dPAG neurons reliably elicits darting behaviour in P. maniculatus but not P. polionotus. Together, we trace the evolution of species-specific escape thresholds to a central circuit node, downstream of peripheral sensory neurons, localizing an ecologically relevant behavioural difference to a specific region of the complex mammalian brain.

Mapping visual functions onto molecular cell types in the mouse superior colliculus

The superficial superior colliculus (sSC) carries out diverse roles in visual processing and behaviors, but how these functions are delegated among collicular neurons remains unclear. Here, using single-cell transcriptomics, we identified 28 neuron subtypes and subtype-enriched marker genes from tens of thousands of adult mouse sSC neurons. We then asked whether the sSC's molecular subtypes are tuned to different visual stimuli. Specifically, we imaged calcium dynamics in single sSC neurons in vivo during visual stimulation and then mapped marker gene transcripts onto the same neurons ex vivo. Our results identify a molecular subtype of inhibitory neuron accounting for ∼50% of the sSC's direction-selective cells, suggesting a genetic logic for the functional organization of the sSC. In addition, our studies provide a comprehensive molecular atlas of sSC neuron subtypes and a multimodal mapping method that will facilitate investigation of their respective functions, connectivity, and development.

Single-Nucleus RNA Sequencing of Developing and Mature Superior Colliculus Identifies Neuronal Diversity and Candidate Mediators of Circuit Assembly

The superior colliculus (SC) is a sensorimotor structure in the midbrain that integrates input from multiple sensory modalities to initiate motor commands. It undergoes well-characterized steps of circuit assembly during development, rendering the mouse SC a popular model to study establishment and refinement of neural connectivity. Here we performed single nucleus RNA-sequencing analysis of the mouse SC isolated at various developmental time points. Our study provides a transcriptomic landscape of the cell types that comprise the SC across murine development with particular emphasis on neuronal heterogeneity. We used these data to identify Pax7 as a marker for an anatomically homogeneous population of GABAergic neurons. Lastly, we report a repertoire of genes differentially expressed across the different postnatal ages, many of which are known to regulate axon guidance and synapse formation. Our data provide a valuable resource for interrogating the mechanisms of circuit development, and identifying markers for manipulating specific SC neuronal populations and circuits.

Synaptic properties of mouse tecto-parabigeminal pathways

The superior colliculus (SC) is a critical hub for the generation of visually-evoked orienting and defensive behaviors. Among the SC's myriad downstream targets is the parabigeminal nucleus (PBG), the mammalian homolog of the nucleus isthmi, which has been implicated in motion processing and the production of defensive behaviors. The inputs to the PBG are thought to arise exclusively from the SC but little is known regarding the precise synaptic relationships linking the SC to the PBG. In the current study, we use optogenetics as well as viral tracing and electron microscopy in mice to better characterize the anatomical and functional properties of the SC-PBG circuit, as well as the morphological and ultrastructural characteristics of neurons residing in the PBG. We characterized GABAergic SC-PBG projections (that do not contain parvalbumin) and glutamatergic SC-PBG projections (which include neurons that contain parvalbumin). These two terminal populations were found to converge on different morphological populations of PBG neurons and elicit opposing postsynaptic effects. Additionally, we identified a population of non-tectal GABAergic terminals in the PBG that partially arise from neurons in the surrounding tegmentum, as well as several organizing principles that divide the nucleus into anatomically distinct regions and preserve a coarse retinotopy inherited from its SC-derived inputs. These studies provide an essential first step toward understanding how PBG circuits contribute to the initiation of behavior in response to visual signals.

Retinal waves align the concentric orientation map in mouse superior colliculus to the center of vision

Neurons in the mouse superior colliculus (SC) are arranged in a concentric orientation map, which is aligned to the center of vision and the optic flow experienced by the mouse. The origin of this map remains unclear. Here, we propose that spontaneous retinal waves during development provide a scaffold to establish the concentric orientation map within the SC and its alignment to the optic flow. We test this hypothesis by modeling the orientation-tuned SC neurons that receive ON/OFF retinal inputs. Our model suggests that the propagation direction bias of stage III retinal waves, together with OFF-delayed responses, shapes the spatial organization of the orientation map. The OFF delay establishes orientation-tuned neurons by segregating their ON/OFF receptive subfields, the wave-like activities form the concentric pattern, and the direction biases align the map to the center of vision. Together, retinal waves may play an instructive role in establishing functional properties of single SC neurons and their spatial organization within maps.

In vivo whole-cell recording from morphologically identified mouse superior colliculus neurons

In vivo whole-cell recording, when combined with morphological characterization after biocytin labeling, is a powerful technique to study subthreshold synaptic processing in cell-type-identified neuronal populations. Here, we describe steps for performing whole-cell recordings in the superior colliculus of urethane-anesthetized mice, a major visual processing region in the rodent brain. We detail two types of visual stimulation techniques: full-field light-emitting diode (LED) flashes and visual stimuli shown on monitors. While we focus on superior colliculus, this protocol is applicable to other brain areas.

High-density electrode recordings reveal strong and specific connections between retinal ganglion cells and midbrain neurons

The superior colliculus is a midbrain structure that plays important roles in visually guided behaviors in mammals. Neurons in the superior colliculus receive inputs from retinal ganglion cells but how these inputs are integrated in vivo is unknown. Here, we discovered that high-density electrodes simultaneously capture the activity of retinal axons and their postsynaptic target neurons in the superior colliculus, in vivo. We show that retinal ganglion cell axons in the mouse provide a single cell precise representation of the retina as input to superior colliculus. This isomorphic mapping builds the scaffold for precise retinotopic wiring and functionally specific connection strength. Our methods are broadly applicable, which we demonstrate by recording retinal inputs in the optic tectum in zebra finches. We find common wiring rules in mice and zebra finches that provide a precise representation of the visual world encoded in retinal ganglion cells connections to neurons in retinorecipient areas.

The superior colliculus receives visual information from retinal ganglion cells, but it remains unclear how this information is organized and integrated in vivo. Here the authors describe how high-density electrodes can simultaneously capture the activity of incoming axons and target neurons in the superior colliculus, and demonstrate isomorphic mapping and strong and specific connections in mice and zebrafinches.

The “Primitive Brain Dysfunction” Theory of Autism: The Superior Colliculus Role

A better understanding of the pathogenesis of autism will help clarify our conception of the complexity of normal brain development. The crucial deficit may lie in the postnatal changes that vision produces in the brainstem nuclei during early life. The superior colliculus is the primary brainstem visual center. Although difficult to examine in humans with present techniques, it is known to support behaviors essential for every vertebrate to survive, such as the ability to pay attention to relevant stimuli and to produce automatic motor responses based on sensory input. From birth to death, it acts as a brain sentinel that influences basic aspects of our behavior. It is the main brainstem hub that lies between the environment and the rest of the higher neural system, making continuous, implicit decisions about where to direct our attention. The conserved cortex-like organization of the superior colliculus in all vertebrates allows the early appearance of primitive emotionally-related behaviors essential for survival. It contains first-line specialized neurons enabling the detection and tracking of faces and movements from birth. During development, it also sends the appropriate impulses to help shape brain areas necessary for social-communicative abilities. These abilities require the analysis of numerous variables, such as the simultaneous evaluation of incoming information sustained by separate brain networks (visual, auditory and sensory-motor, social, emotional, etc.), and predictive capabilities which compare present events to previous experiences and possible responses. These critical aspects of decision-making allow us to evaluate the impact that our response or behavior may provoke in others. The purpose of this review is to show that several enigmas about the complexity of autism might be explained by disruptions of collicular and brainstem functions. The results of two separate lines of investigation: 1. the cognitive, etiologic, and pathogenic aspects of autism on one hand, and two. the functional anatomy of the colliculus on the other, are considered in order to bridge the gap between basic brain science and clinical studies and to promote future research in this unexplored area.

Tangential high-density electrode insertions allow to simultaneously measure neuronal activity across an extended region of the visual field in mouse superior colliculus

BACKGROUND

The superior colliculus (SC) is a midbrain structure that plays a central role in visual processing. Although we have learned a considerable amount about the function of single SC neurons, the way in which sensory information is represented and processed on the population level in awake behaving animals and across a large region of the retinotopic map is still largely unknown. Partially because the SC is anatomically located below the cortical sheet and the transverse sinus, which render the measure of neuronal activity from a large population of neurons in the SC technically difficult to perform.

NEW METHOD

To address this, we propose a tangential recording configuration using high-density electrode probes (Neuropixels) in mouse SC in vivo. This method permits a large number of recording sites (~200) inside the SC circuitry allowing to record from a large population of SC neurons along a vast area of retinotopic space.

RESULTS

This approach provides a unique opportunity to measure the activity of SC neuronal populations over up to ~2mm of SC tissue reporting for the first time the continuous receptive fields coverage of almost the entire SC retinotopy. Here we describe how to perform targeted tangential recordings along the anterior-posterior and the medio-lateral axis of the mouse SC in vivo in the upper visual layers. Furthermore, we describe how to combine this approach with optogenetic tools for cell-type identification on the population level.

COMPARISON WITH EXISTING METHODS

Vertical insertion has been a standard way to record visual responses in the SC. Inserting multi-shank probes vertically allows to cover a larger region of the SC but misses both the complete extent of the available retinotopy and the continuous measure allowed by the high density of recording sites on Neuropixels probes.

CONCLUSION

Altogether tangential insertions in the upper visual layers of the mouse SC using Neuropixels permit for the first time to access a majority of the retinotopically organized visual representation of the world at an unprecedented precision.

Trans-Seq maps a selective mammalian retinotectal synapse instructed by Nephronectin

The mouse visual system serves as an accessible model to understand mammalian circuit wiring. Despite rich knowledge in retinal circuits, the long-range connectivity map from distinct retinal ganglion cell (RGC) types to diverse brain neuron types remains unknown. In this study, we developed an integrated approach, called Trans-Seq, to map RGCs to superior collicular (SC) circuits. Trans-Seq combines a fluorescent anterograde trans-synaptic tracer, consisting of codon-optimized wheat germ agglutinin fused to mCherry, with single-cell RNA sequencing. We used Trans-Seq to classify SC neuron types innervated by genetically defined RGC types and predicted a neuronal pair from αRGCs to Nephronectin-positive wide-field neurons (NPWFs). We validated this connection using genetic labeling, electrophysiology and retrograde tracing. We then used transcriptomic data from Trans-Seq to identify Nephronectin as a determinant for selective synaptic choice from αRGC to NPWFs via binding to Integrin α8β1. The Trans-Seq approach can be broadly applied for post-synaptic circuit discovery from genetically defined pre-synaptic neurons.

Functional Organisation of the Mouse Superior Colliculus

The superior colliculus (SC) is a highly conserved area of the mammalian midbrain that is widely implicated in the organisation and control of behaviour. SC receives input from a large number of brain areas, and provides outputs to a large number of areas. The convergence and divergence of anatomical connections with different areas and systems provides challenges for understanding how SC contributes to behaviour. Recent work in mouse has provided large anatomical datasets, and a wealth of new data from experiments that identify and manipulate different cells within SC, and their inputs and outputs, during simple behaviours. These data offer an opportunity to better understand the roles that SC plays in these behaviours. However, some of the observations appear, at first sight, to be contradictory. Here we review this recent work and hypothesise a simple framework which can capture the observations, that requires only a small change to previous models. Specifically, the functional organisation of SC can be explained by supposing that three largely distinct circuits support three largely distinct classes of simple behaviours-arrest, turning towards, and the triggering of escape or capture. These behaviours are hypothesised to be supported by the optic, intermediate and deep layers, respectively.

Effects of Light Isoflurane Anesthesia on Organization of Direction and Orientation Selectivity in the Superficial Layer of the Mouse Superior Colliculus

The superior colliculus (SC) is the midbrain center for integrating visual and multimodal sensory information. Neurons in the SC exhibit direction and orientation selectivity. Recent studies reported that neurons with similar preferences formed clusters in the mouse SC (Ahmadlou and Heimel, 2015; Feinberg and Meister, 2015; de Malmazet et al., 2018; Li et al., 2020). However, it remains controversial as to how these clusters are organized within the SC (Inayat et al., 2015; Chen et al., 2021). Here, we found that different brain states (i.e., awake or anesthetized with isoflurane) changed the selectivity of individual SC neurons and organizations of the neuronal population in both male and female mice. Using two-photon Ca2+ imaging, we examined both individual neuronal responses and the spatial patterns of their population responses. Under isoflurane anesthesia, orientation selectivity increased and a larger number of orientation-selective cells were observed when compared with the awake condition, whereas the proportions of direction-selective cells were similar in both conditions. Furthermore, direction- and orientation-selective cells located at closer positions showed more similar preferences, and cluster-like spatial patterns were enhanced. Inhibitory responses of direction-selective neurons were also reduced under isoflurane anesthesia. Thus, the changes in the spatial organization of response patterns were considered to be because of changes in the balance of excitation and inhibition, with excitation dominance, in the local circuits. These results provide new insights into the possibility that the functional organization of feature selectivity in the brain is affected by brain state.SIGNIFICANCE STATEMENT Recent large-scale recording studies are changing our view of visual maps in the superior colliculus (SC), including findings of cluster-like localizations of direction- and orientation-selective neurons. However, results from several laboratories are conflicting regarding the presence of cluster-like organization. Here, we demonstrated that light isoflurane anesthesia affected the direction- and orientation-tuning properties in the mouse superficial SC and that their cluster-like localization pattern was enhanced by the anesthesia. Furthermore, the effect of anesthesia on direction selectivity appeared to be different in the excitatory and inhibitory populations in the SC. Our results suggest that the functional organization of direction and orientation selectivity might be regulated by the excitation-inhibition balance that depends on the brain state.

Mechanisms of Plasticity in Subcortical Visual Areas

Visual plasticity is classically considered to occur essentially in the primary and secondary cortical areas. Subcortical visual areas such as the dorsal lateral geniculate nucleus (dLGN) or the superior colliculus (SC) have long been held as basic structures responsible for a stable and defined function. In this model, the dLGN was considered as a relay of visual information travelling from the retina to cortical areas and the SC as a sensory integrator orienting body movements towards visual targets. However, recent findings suggest that both dLGN and SC neurons express functional plasticity, adding unexplored layers of complexity to their previously attributed functions. The existence of neuronal plasticity at the level of visual subcortical areas redefines our approach of the visual system. The aim of this paper is therefore to review the cellular and molecular mechanisms for activity-dependent plasticity of both synaptic transmission and cellular properties in subcortical visual areas.

Optogenetic fUSI for brain-wide mapping of neural activity mediating collicular-dependent behaviors

Neuronal cell types are arranged in brain-wide circuits that guide behavior. In mice, the superior colliculus innervates a set of targets that direct orienting and defensive actions. We combined functional ultrasound imaging (fUSI) with optogenetics to reveal the network of brain regions functionally activated by four collicular cell types. Stimulating each neuronal group triggered different behaviors and activated distinct sets of brain nuclei. This included regions not previously thought to mediate defensive behaviors, for example, the posterior paralaminar nuclei of the thalamus (PPnT), which we show to play a role in suppressing habituation. Neuronal recordings with Neuropixels probes show that (1) patterns of spiking activity and fUSI signals correlate well in space and (2) neurons in downstream nuclei preferentially respond to innately threatening visual stimuli. This work provides insight into the functional organization of the networks governing innate behaviors and demonstrates an experimental approach to explore the whole-brain neuronal activity downstream of targeted cell types.

Unraveling circuits of visual perception and cognition through the superior colliculus

The superior colliculus is a conserved sensorimotor structure that integrates visual and other sensory information to drive reflexive behaviors. Although the evidence for this is strong and compelling, a number of experiments reveal a role for the superior colliculus in behaviors usually associated with the cerebral cortex, such as attention and decision-making. Indeed, in addition to collicular outputs targeting brainstem regions controlling movements, the superior colliculus also has ascending projections linking it to forebrain structures including the basal ganglia and amygdala, highlighting the fact that the superior colliculus, with its vast inputs and outputs, can influence processing throughout the neuraxis. Today, modern molecular and genetic methods combined with sophisticated behavioral assessments have the potential to make significant breakthroughs in our understanding of the evolution and conservation of neuronal cell types and circuits in the superior colliculus that give rise to simple and complex behaviors.

Retinal ganglion cell defects cause decision shifts in visually evoked defense responses

A variety of visual cues can trigger defensive reactions in mice and other species. In mice, looming stimuli that mimic an approaching aerial predator elicit flight or freezing reactions, while sweeping stimuli that mimic an aerial predator flying parallel to the ground typically elicit freezing. The retinal ganglion cell (RGC) types involved in these circuits are largely unknown. We previously discovered that loss of RGC subpopulations in Brn3b knockout mice results in distinct visual response deficits. Here, we report that retinal or global loss of Brn3b selectively ablates the fleeing response to looming stimuli while leaving the freeze response intact. In contrast, freezing responses to sweeping stimuli are significantly affected. Genetic manipulations removing three RGC subpopulations (Brn3a⁺ betta RGCs, Opn4⁺Brn3b⁺, and Brn3c⁺Brn3b⁺ RGCs) result in milder phenocopies of Brn3b knockout response deficits. These findings show that flight and freezing responses to distinct visual cues are mediated by circuits that can already be separated at the level of the retina, potentially by enlisting dedicated RGC types.NEW & NOTEWORTHY Flight and freezing response choices evoked by visual stimuli are controlled by brain stem and thalamic circuits. Genetically modified mice with loss of specific retinal ganglion cell (RGC) subpopulations have altered flight versus freezing choices in response to some but not other visual stimuli. This finding suggests that "threatening" visual stimuli may be computed already at the level of the retina and communicated via dedicated pathways (RGCs) to the brain.

Anatomical and electrophysiological analysis of cholinergic inputs from the parabigeminal nucleus to the superficial superior colliculus

Superior colliculus (SC) is a midbrain structure that integrates sensory inputs and generates motor commands to initiate innate motor behaviors. Its retinorecipient superficial layers (sSC) receive dense cholinergic projections from the parabigeminal nucleus (PBN). Our previous in vitro study revealed that acetylcholine induces fast inward current followed by prominent GABAergic inhibition within the sSC circuits (Endo T, Yanagawa Y, Obata K, Isa T. J Neurophysiol 94: 3893-3902, 2005). Acetylcholine-mediated facilitation of GABAergic inhibition may play an important role in visual signal processing in the sSC; however, both the anatomical and physiological properties of cholinergic inputs from PBN have not been studied in detail in vivo. In this study, we specifically visualized and optogenetically manipulated the cholinergic neurons in the PBN after focal injections of Cre-dependent viral vectors in mice that express Cre in cholinergic neurons. We revealed that the cholinergic projections terminated densely in the medial part of the mouse sSC. This suggests that the cholinergic inputs mediate visual processing in the upper visual field, which would be critical for predator detection. We further analyzed the physiological roles of the cholinergic inputs by recording looming-evoked visual responses from sSC neurons during optogenetic activation or inactivation of PBN cholinergic neurons in anesthetized mice. We found that optogenetic manipulations in either direction induced response suppression in most neurons, whereas response facilitation was observed in a few neurons after the optogenetic activation. These results support a circuit model that suggests that the PBN cholinergic inputs enhance functions of the sSC in detecting visual targets by facilitating the center excitation-surround inhibition.NEW & NOTEWORTHY The modulatory role of the cholinergic inputs from the parabigeminal nucleus in the visual responses in the superficial superior colliculus (sSC) remains unknown. Here we report that the cholinergic projections terminate densely in the medial sSC and optogenetic manipulations of the cholinergic inputs affect the looming-evoked response and enhance surround inhibition in the sSC. Our data suggest that cholinergic inputs to the sSC contribute to the visual detection of predators.

Context-dependent modulation of natural approach behaviour in mice

Specific features of visual objects innately draw approach responses in animals, and provide natural signals of potential reward. However, visual sampling behaviours and the detection of salient, rewarding stimuli are context and behavioural state-dependent and it remains unclear how visual perception and orienting responses change with specific expectations. To start to address this question, we employed a virtual stimulus orienting paradigm based on prey capture to quantify the conditional expression of visual stimulus-evoked innate approaches in freely moving mice. We found that specific combinations of stimulus features selectively evoked innate approach or freezing responses when stimuli were unexpected. We discovered that prey capture experience, and therefore the expectation of prey in the environment, selectively modified approach frequency, as well as altered those visual features that evoked approach. Thus, we found that mice exhibit robust and selective orienting responses to parameterized visual stimuli that can be robustly and specifically modified via natural experience. This work provides critical insight into how natural appetitive behaviours are driven by both specific features of visual motion and internal states that alter stimulus salience.

Visual Response Characteristics in Lateral and Medial Subdivisions of the Rat Pulvinar

The pulvinar is a higher-order thalamic relay and a central component of the extrageniculate visual pathway, with input from the superior colliculus and visual cortex and output to all of visual cortex. Rodent pulvinar, more commonly called the lateral posterior nucleus (LP), consists of three highly-conserved subdivisions, and offers the advantage of simplicity in its study compared to more subdivided primate pulvinar. Little is known about receptive field properties of LP, let alone whether functional differences exist between different LP subdivisions, making it difficult to understand what visual information is relayed and what kinds of computations the pulvinar might support. Here, we characterized single-cell response properties in two V1 recipient subdivisions of rat pulvinar, the rostromedial (LPrm) and lateral (LPl), and found that a fourth of the cells were selective for orientation, compared to half in V1, and that LP tuning widths were significantly broader. Response latencies were also significantly longer and preferred size more than three times larger on average than in V1; the latter suggesting pulvinar as a source of spatial context to V1. Between subdivisons, LPl cells preferred higher temporal frequencies, whereas LPrm showed a greater degree of direction selectivity and pattern motion detection. Taken together with known differences in connectivity patterns, these results suggest two separate visual feature processing channels in the pulvinar, one in LPl related to higher speed processing which likely derives from superior colliculus input, and the other in LPrm for motion processing derived through input from visual cortex. SIGNIFICANCE STATEMENT: The pulvinar has a perplexing role in visual cognition as no clear link has been found between the functional properties of its neurons and behavioral deficits that arise when it is damaged. The pulvinar, called the lateral posterior nucleus (LP) in rats, is a higher order thalamic relay with input from the superior colliculus and visual cortex and output to all of visual cortex. By characterizing single-cell response properties in anatomically distinct subdivisions we found two separate visual feature processing channels in the pulvinar, one in lateral LP related to higher speed processing which likely derives from superior colliculus input, and the other in rostromedial LP for motion processing derived through input from visual cortex.

A Causal Role for Mouse Superior Colliculus in Visual Perceptual Decision-Making

The superior colliculus (SC) is arguably the most important visual structure in the mouse brain and is well known for its involvement in innate responses to visual threats and prey items. In other species, the SC plays a central role in voluntary as well as innate visual functions, including crucial contributions to selective attention and perceptual decision-making. In the mouse, the possible role of the SC in voluntary visual choice behaviors has not been established. Here, we demonstrate that the mouse SC of both sexes plays a causal role in visual perceptual decision-making by transiently inhibiting SC activity during an orientation change detection task. First, unilateral SC inhibition-induced spatially specific deficits in detection. Hit rates were reduced, and reaction times increased for orientation changes in the contralateral but not ipsilateral visual field. Second, the deficits caused by SC inhibition were specific to a temporal epoch coincident with early visual burst responses in the SC. Inhibiting SC during this 100-ms period caused a contralateral detection deficit, whereas inhibition immediately before or after did not. Third, SC inhibition reduced visual detection sensitivity. Psychometric analysis revealed that inhibiting SC visual activity significantly increased detection thresholds for contralateral orientation changes. In addition, effects on detection thresholds and lapse rates caused by SC inhibition were larger in the presence of a competing visual stimulus, indicating a role for the mouse SC in visual target selection. Together, our results demonstrate that the mouse SC is necessary for the normal performance of voluntary visual choice behaviors.SIGNIFICANCE STATEMENT The mouse superior colliculus (SC) has become a popular model for studying the circuit organization and development of the visual system. Although the SC is a fundamental component of the visual pathways in mice, its role in visual perceptual decision-making is not clear. By investigating how temporally precise SC inhibition influenced behavioral performance during a visually guided orientation change detection task, we identified a 100-ms temporal epoch of SC visual activity that is crucial for the ability of mice to detect behaviorally relevant visual changes. In addition, we found that SC inhibition also caused deficits in visual target selection. Thus, our findings highlight the importance of the SC for visual perceptual choice behavior in the mouse.

The Evolution of the Pulvinar Complex in Primates and Its Role in the Dorsal and Ventral Streams of Cortical Processing

Current evidence supports the view that the visual pulvinar of primates consists of at least five nuclei, with two large nuclei, lateral pulvinar ventrolateral (PLvl) and central lateral nucleus of the inferior pulvinar (PIcl), contributing mainly to the ventral stream of cortical processing for perception, and three smaller nuclei, posterior nucleus of the inferior pulvinar (PIp), medial nucleus of the inferior pulvinar (PIm), and central medial nucleus of the inferior pulvinar (PIcm), projecting to dorsal stream visual areas for visually directed actions. In primates, both cortical streams are highly dependent on visual information distributed from primary visual cortex (V1). This area is so vital to vision that patients with V1 lesions are considered "cortically blind". When the V1 inputs to dorsal stream area middle temporal visual area (MT) are absent, other dorsal stream areas receive visual information relayed from the superior colliculus via PIp and PIcm, thereby preserving some dorsal stream functions, a phenomenon called "blind sight". Non-primate mammals do not have a dorsal stream area MT with V1 inputs, but superior colliculus inputs to temporal cortex can be more significant and more visual functions are preserved when V1 input is disrupted. The current review will discuss how the different visual streams, especially the dorsal stream, have changed during primate evolution and we propose which features are retained from the common ancestor of primates and their close relatives.

A projection specific logic to sampling visual inputs in mouse superior colliculus

Using sensory information to trigger different behaviors relies on circuits that pass through brain regions. The rules by which parallel inputs are routed to downstream targets are poorly understood. The superior colliculus mediates a set of innate behaviors, receiving input from >30 retinal ganglion cell types and projecting to behaviorally important targets including the pulvinar and parabigeminal nucleus. Combining transsynaptic circuit tracing with in vivo and ex vivo electrophysiological recordings, we observed a projection-specific logic where each collicular output pathway sampled a distinct set of retinal inputs. Neurons projecting to the pulvinar or the parabigeminal nucleus showed strongly biased sampling from four cell types each, while six others innervated both pathways. The visual response properties of retinal ganglion cells correlated well with those of their disynaptic targets. These findings open the possibility that projection-specific sampling of retinal inputs forms a basis for the selective triggering of behaviors by the superior colliculus.

GABAergic cell types in the superficial layers of the mouse superior colliculus

To begin to unravel the complexities of GABAergic circuits in the superior colliculus (SC), we utilized mouse lines that express green fluorescent protein (GFP) in cells that contain the 67 kDa isoform of glutamic acid decarboxylase (GAD67-GFP), or Cre-recombinase in cells that contain glutamic acid decarboxylase (GAD; GAD2-cre). We used Cre-dependent virus injections in GAD2-Cre mice and tracer injections in GAD67-GFP mice, as well as immunocytochemical staining for gamma amino butyric acid (GABA) and parvalbumin (PV) to characterize GABAergic cells that project to the pretectum (PT), ventral lateral geniculate nucleus (vLGN) or parabigeminal nucleus (PBG), and interneurons in the stratum griseum superficiale (SGS) that do not project outside the SC. We found that approximately 30% of SGS neurons in the mouse are GABAergic. Of these GABAergic neurons, we identified three categories of potential interneurons in the GAD67-GFP line (GABA+GFP ~45%, GABA+GFP + PV ~15%, and GABA+PV ~10%). GABAergic cells that did not contain GFP or PV were identified as potential projection neurons (GABA only ~30%). We found that GABAergic neurons that project to the PBG are primarily located in the SGS and exhibit narrow field vertical, stellate, and horizontal dendritic morphologies, while GABAergic neurons that project to the PT and vLGN are primarily located in layers ventral to the SGS. In addition, we examined GABA and GAD67-containing elements of the mouse SGS using electron microscopy to further delineate the relationship between GABAergic circuits and retinotectal input. Approximately 30% of retinotectal synaptic targets are the presynaptic dendrites of GABAergic interneurons, and GAD67-GFP interneurons are a source of these presynaptic dendrites.

Functional modulation of primary visual cortex by the superior colliculus in the mouse

The largest targets of retinal input in mammals are the dorsal lateral geniculate nucleus (dLGN), a relay to the primary visual cortex (V1), and the superior colliculus. V1 innervates and influences the superior colliculus. Here, we find that, in turn, superior colliculus modulates responses in mouse V1. Optogenetically inhibiting the superior colliculus reduces responses in V1 to optimally sized stimuli. Superior colliculus could influence V1 via its strong projection to the lateral posterior nucleus (LP/Pulvinar) or its weaker projection to the dLGN. Inhibiting superior colliculus strongly reduces activity in LP. Pharmacologically silencing LP itself, however, does not remove collicular modulation of V1. The modulation is instead due to a collicular gain modulation of the dLGN. Surround suppression operating in V1 explains the different effects for differently sized stimuli. Computations of visual saliency in the superior colliculus can thus influence tuning in the visual cortex via a tectogeniculate pathway.

The role of the superior colliculus (SC) in modulating V1 cortical activity is not clear. Here, the authors demonstrate the functional role of SC in modulating V1 responses through an excitatory pathway via the dorsal lateral geniculate nucleus.

The Mouse Superior Colliculus as a Model System for Investigating Cell Type-Based Mechanisms of Visual Motor Transformation

The mouse superior colliculus (SC) is a laminar midbrain structure involved in processing and transforming multimodal sensory stimuli into ethologically relevant behaviors such as escape, defense, and orienting movements. The SC is unique in that the sensory (visual, auditory, and somatosensory) and motor maps are overlaid. In the mouse, the SC receives inputs from more retinal ganglion cells than any other visual area. This makes the mouse SC an ideal model system for understanding how visual signals processed by retinal circuits are used to mediate visually guided behaviors. This Perspective provides an overview of the current understanding of visual motor transformations operated by the mouse SC and discusses the challenges to be overcome when investigating the input–output relationships in single collicular cell types.