Segregated anatomical input to sub-regions of the rodent superior colliculus associated with approach and defense

Neural Circuits of Fear and Anxiety: Insights from a Neuroethological Perspective

The predatory imminence continuum (PIC) of antipredator defensive behavior has been a helpful strategy for modeling anxiety and fear-related disorders in nonclinical research. The PIC is divided into three different sequential stages that reflect defensive behavioral strategy in response to predatory imminence. However, the PIC was experimentally addressed with a series of shock-based fear conditioning experiments rather than predatory threats. In this article, we consider the PIC in a more naturalistic behavioral setting, focusing on analyzing the neural systems of animals responding to terrestrial and aerial predators. Of relevance, there is a sequential engagement of the distinct neural circuits along each phase of the PIC. In the preencounter phase, prefrontal cortical networks are particularly involved in planning and organizing behavioral responses to ambiguous threats. As the predatory cues or the real predator is detected, there is an engagement of amygdalar and hippocampal > hypothalamic pathways in conjunction with the periaqueductal gray, which organize fear responses. This dynamic particularly reveals how specific neural circuits are set into action to subserve distinct defensive responses. Moreover, we further explore the neural circuits governing other fearful situations outside the context of the PIC, including agonistic social encounters and interoceptive challenges. This analysis reveals an interesting overlap between the neural systems responding to these threats and those involved in response to predatory threats. The present review clarifies how defensive circuits respond to natural threats and provides a more realistic view of the neural systems underlying anxiety and fear responses.

Superior Colliculus Controls the Activity of the Substantia Nigra Pars Compacta and Ventral Tegmental Area in an Asymmetrical Manner

Dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) play a crucial role in controlling animals' orienting and approach behaviors toward relevant environmental stimuli. The ventral midbrain receives sensory input from the superior colliculus (SC), a tectal region that processes information from contralateral receptive fields of various modalities. Given the significant influence of dopamine release imbalance in the left and right striatum on animals' movement direction, our study aimed to investigate the lateralization of the connection between the lateral SC and the midbrain DA system in male rats. We explored the circuit's anatomy using transsynaptic viral tract-tracing and its physiology using in vivo single-unit and ex vivo multi-electrode array recordings of SNc and VTA neuronal activity combined with optogenetic stimulation of either the ipsilateral or contralateral SC or its terminals. During the experiments, DA neurons were identified optogenetically (in vivo recordings) or pharmacologically (ex vivo recordings). Anatomical findings revealed a bilateral innervation pattern of the lateral SC to the ventral midbrain, with a significantly stronger ipsilateral connection, particularly evident in the SNc, involving both DA and non-DA neurons. This anatomical asymmetry was also expressed during in vivo and ex vivo recordings, which showed a predominance of ipsilateral connections, especially within the SNc. Ex vivo recordings also confirmed that this lateralized pathway is direct. The described features of the SC→VTA/SNc neuronal circuit, particularly its anatomical and physiological asymmetry, suggest its involvement in orienting and approach behaviors guided by the direction of incoming sensory stimuli.

Brainwide Projections of Mouse Dopaminergic Zona Incerta Neurons

The zona incerta (ZI) supports diverse behaviors including binge feeding, sleep-wake cycles, nociception, and hunting. Diverse ZI functions can be attributed to its heterogeneous neurochemical characterization, cytoarchitecture, and efferent connections. The ZI is predominantly GABAergic, but we recently identified a subset of medial ZI GABA cells that are marked by the enzyme tyrosine hydroxylase (TH) and produce dopamine (DA). While the role of GABA within the ZI is well studied, less is known about the functions of ZI DA cells. To identify potential roles of ZI DA cells, we further phenotyped them and mapped their efferent fiber projections. We showed that wild-type TH-immunoreactive (-ir) ZI cells did not express somatostatin or calretinin immunoreactivity. We next validated a Th-cre;L10-Egfp mouse line and found that medial Egfp ZI cells were more likely to be TH-ir. We therefore delivered a Cre-dependent virus into the medial ZI of Th-cre or Th-cre;L10-Egfp mice and selected two injection cases for full brain mapping, namely, cases with the lowest and highest colocalization between TH-ir and virally transduced, DsRed-labeled cells, to identify common target sites. Overall, DsRed-labeled fibers were distributed brainwide and were most prominent within the motor-related midbrain (MBmot), notably the periaqueductal gray area and superior colliculus. We also observed numerous DsRed-labeled fibers within the polymodal association cortex-related thalamus (DORpm), like paraventricular thalamic nucleus and nucleus of reunions, that processes external and internal sensory input. Overall, ZI DA cells displayed a similar fiber profile to ZI GABA cells and may integrate sensory input to coordinate motor output at their target sites.

Functional segregation and dynamic integration of the corticotectal descending signal in rat

The superior colliculus (SC) receives inputs from various brain regions in a layer- and radial subregion-specific manner, but whether the SC exhibits subregion-specific dynamics remains unclear. To address this issue, we recorded the spiking activity of single SC neurons while photoactivating cortical areas in awake head-fixed Thy1-ChR2 rats. We classified 309 neurons that responded significantly into 8 clusters according to the response dynamics. Among them, neurons with monophasic excitatory responses (7-12 ms latency) that returned to baseline within 20 ms were commonly observed in the optic and intermediate gray layers of centromedial and centrolateral SC. In contrast, neurons with complex polyphasic responses were commonly observed in the deep layers of the anterolateral SC. Cross-correlation analysis suggested that the complex pattern could be only partly explained by an internal circuit of the deep gray layer. Our results indicate that medial to centrolateral SC neurons simply relay cortical activity, whereas neurons in the deep layers of the anterolateral SC dynamically integrate inputs from the cortex, SNr, CN, and local circuits. These findings suggest a spatial gradient in SC integration, with a division of labor between simple relay circuits and those integrating complex dynamics.

Genetically defined neuron types underlying visuomotor transformation in the superior colliculus

The superior colliculus (SC) is a conserved midbrain structure that is important for transforming visual and other sensory information into motor actions. Decades of investigations in numerous species have made the SC and its nonmammalian homologue, the optic tectum, one of the best studied structures in the brain, with rich information now available regarding its anatomical organization, its extensive inputs and outputs and its important functions in many reflexive and cognitive behaviours. Excitingly, recent studies using modern genomic and physiological approaches have begun to reveal the diverse neuronal subtypes in the SC, as well as their unique functions in visuomotor transformation. Studies have also started to uncover how subtypes of SC neurons form intricate circuits to mediate visual processing and visually guided behaviours. Here, we review these recent discoveries on the cell types and neuronal circuits underlying visuomotor transformations mediated by the SC. We also highlight the important future directions made possible by these new developments.

Brain-wide projections of mouse dopaminergic zona incerta neurons

The zona incerta (ZI) supports diverse behaviors including binge feeding, sleep/wake cycles, nociception, and hunting. This diversity of functions can be attributed to the heterogenous neurochemicals, cytoarchitecture, and efferent connections that characterize the ZI. The ZI is predominantly GABAergic, but we recently identified a subset of medial ZI GABA cells that co-express dopamine (DA), as marked by the enzyme tyrosine hydroxylase (TH). While the role of GABA within the ZI is well studied, little is understood about the function of ZI DA cells. To identify potential roles of ZI DA cells we mapped the efferent fiber projections from Th-cre ZI cells. We first validated a Th-cre;L10-Egfp mouse line and found that medial Egfp ZI cells were more likely to co-express TH-immunoreactivity (TH-ir). We thus delivered a cre-dependent virus into the medial ZI of Th-cre or Th-cre;L10-Egfp mice and selected two injection cases for full brain mapping. We selected the cases with the lowest (17%) and highest (53%) percentage of colocalization between TH-ir and virus transfected cells labelled with DsRed. Overall, DsRed-labelled fibers were observed throughout the brain and were most prominent within motor-related regions of the midbrain (MBmot), notably the periaqueductal grey area and superior colliculus. We also observed considerable DsRed-labelled fibers within the polymodal cortex associated regions of the thalamus (DORpm), including the paraventricular thalamic nucleus and nucleus of reunions. Overall, ZI DA cells displayed a similar connectivity profile to ZI GABA cells, suggesting that ZI DA cells may perform synergistic or opposing functions at the same target sites.

Understanding subcortical projections to the lateral posterior thalamic nucleus and its subregions using retrograde neural tracing

The rat lateral posterior thalamic nucleus (LP) is composed of the rostromedial (LPrm), lateral (LPl), and caudomedial parts, with LPrm and LPl being areas involved in information processing within the visual cortex. Nevertheless, the specific differences in the subcortical projections to the LPrm and LPl remain elusive. In this study, we aimed to reveal the subcortical regions that project axon fibers to the LPl and LPrm using a retrograde neural tracer, Fluorogold (FG). After FG injection into the LPrm or LPl, the area was visualized immunohistochemically. Retrogradely labeled neurons from the LPrm were distributed in the retina and the region from the diencephalon to the medulla oblongata. Diencephalic labeling was found in the reticular thalamic nucleus (Rt), zona incerta (ZI), ventral lateral geniculate nucleus (LGv), intergeniculate leaflet (IGL), and hypothalamus. In the midbrain, prominent labeling was found in the periaqueductal gray (PAG) and deep layers of the superior colliculus. Additionally, retrograde labeling was observed in the cerebellar and trigeminal nuclei. When injected into the LPl, several cell bodies were labeled in the visual-related regions, including the retina, LGv, IGL, and olivary pretectal nucleus (OPT), as well as in the Rt and anterior pretectal nucleus (APT). Less labeling was found in the cerebellum and medulla oblongata. When the number of retrogradely labeled neurons from the LPrm or LPl was compared as a percentage of total subcortical labeling, a larger percentage of subcortical inputs to the LPl included projections from the APT, OPT, and Rt, whereas a large proportion of subcortical inputs to the LPrm originated from the ZI, reticular formation, and PAG. These results suggest that LPrm not only has visual but also multiple sensory-and motor-related functions, whereas the LPl takes part in a more visual-specific role. This study enhances our understanding of subcortical neural circuits in the thalamus and may contribute to our exploration of the mechanisms and disorders related to sensory perception and sensory-motor integration.

Toward a neuroscience of natural behavior

One of the most exciting new developments in systems neuroscience is the progress being made toward neurophysiological experiments that move beyond simplified laboratory settings and address the richness of natural behavior. This is enabled by technological advances such as wireless recording in freely moving animals, automated quantification of behavior, and new methods for analyzing large data sets. Beyond new empirical methods and data, however, there is also a need for new theories and concepts to interpret that data. Such theories need to address the particular challenges of natural behavior, which often differ significantly from the scenarios studied in traditional laboratory settings. Here, we discuss some strategies for developing such novel theories and concepts and some example hypotheses being proposed.

Disentangling the identity of the zona incerta: a review of the known connections and latest implications

The zona incerta (ZI) is a subthalamic region composed by loosely packed neurochemically mixed neurons, juxtaposed to the main ascending and descending bundles. The extreme neurochemical diversity that characterizes this area, together with the diffuseness of its connections with the entire neuraxis and its hard-to-reach positioning in the brain caused the ZI to keep its halo of mystery for over a century. However, in the last decades, a rich albeit fragmentary body of knowledge regarding both the incertal anatomical connections and functional implications has been built mostly based on rodent studies and its lack of cohesion makes difficult to depict an integrated, exhaustive picture regarding the ZI and its roles. This review aims to provide a unified resource that summarizes the current knowledge regarding the anatomical profile of interactions of the ZI in rodents and non-human primates and the functional significance of its connections, highlighting the aspects still unbeknown to research.

Separation of Channels Subserving Approach and Avoidance/Escape at the Level of the Basal Ganglia and Related Brainstem Structures

The basal ganglia have the key function of directing our behavior in the context of events from our environment and/or our internal state. This function relies on afferents targeting the main input structures of the basal ganglia, entering bids for action selection at the level of the striatum or signals for behavioral interruption at the level of the subthalamic nucleus, with behavioral reselection facilitated by dopamine signaling. Numerous experiments have studied action selection in relation to inputs from the cerebral cortex. However, less is known about the anatomical and functional link between the basal ganglia and the brainstem. In this review, we describe how brainstem structures also project to the main input structures of the basal ganglia, namely the striatum, the subthalamic nucleus and midbrain dopaminergic neurons, in the context of approach and avoidance (including escape from threat), two fundamental, mutually exclusive behavioral choices in an animal's repertoire in which the brainstem is strongly involved. We focus on three particularly well-described loci involved in approach and avoidance, namely the superior colliculus, the parabrachial nucleus and the periaqueductal grey nucleus. We consider what is known about how these structures are related to the basal ganglia, focusing on their projections toward the striatum, dopaminergic neurons and subthalamic nucleus, and explore the functional consequences of those interactions.

Multivariate investigation of aging in mouse models expressing the Alzheimer's protective APOE2 allele: integrating cognitive metrics, brain imaging, and blood transcriptomics

APOE allelic variation is critical in brain aging and Alzheimer's disease (AD). The APOE2 allele associated with cognitive resilience and neuroprotection against AD remains understudied. We employed a multipronged approach to characterize the transition from middle to old age in mice with APOE2 allele, using behavioral assessments, image-derived morphometry and diffusion metrics, structural connectomics, and blood transcriptomics. We used sparse multiple canonical correlation analyses (SMCCA) for integrative modeling, and graph neural network predictions. Our results revealed brain sub-networks associated with biological traits, cognitive markers, and gene expression. The cingulate cortex emerged as a critical region, demonstrating age-associated atrophy and diffusion changes, with higher fractional anisotropy in males and middle-aged subjects. Somatosensory and olfactory regions were consistently highlighted, indicating age-related atrophy and sex differences. The hippocampus exhibited significant volumetric changes with age, with differences between males and females in CA3 and CA1 regions. SMCCA underscored changes in the cingulate cortex, somatosensory cortex, olfactory regions, and hippocampus in relation to cognition and blood-based gene expression. Our integrative modeling in aging APOE2 carriers revealed a central role for changes in gene pathways involved in localization and the negative regulation of cellular processes. Our results support an important role of the immune system and response to stress. This integrative approach offers novel insights into the complex interplay among brain connectivity, aging, and sex. Our study provides a foundation for understanding the impact of APOE2 allele on brain aging, the potential for detecting associated changes in blood markers, and revealing novel therapeutic intervention targets.

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.

Superior colliculus bidirectionally modulates choice activity in frontal cortex

Action selection occurs through competition between potential choice options. Neural correlates of choice competition are observed across frontal cortex and downstream superior colliculus (SC) during decision-making, yet how these regions interact to mediate choice competition remains unresolved. Here we report that SC can bidirectionally modulate choice competition and drive choice activity in frontal cortex. In the mouse, topographically matched regions of frontal cortex and SC formed a descending motor pathway for directional licking and a re-entrant loop via the thalamus. During decision-making, distinct neuronal populations in both frontal cortex and SC encoded opposing lick directions and exhibited competitive interactions. SC GABAergic neurons encoded ipsilateral choice and locally inhibited glutamatergic neurons that encoded contralateral choice. Activating or suppressing these cell types could bidirectionally drive choice activity in frontal cortex. These results thus identify SC as a major locus to modulate choice competition within the broader action selection network.

Optogenetic stimulation of the superior colliculus suppresses genetic absence seizures

While anti-seizure medications are effective for many patients, nearly one-third of individuals have seizures that are refractory to pharmacotherapy. Prior studies using evoked preclinical seizure models have shown that pharmacological activation or excitatory optogenetic stimulation of the deep and intermediate layers of the superior colliculus (DLSC) display multi-potent anti-seizure effects. Here we monitored and modulated DLSC activity to suppress spontaneous seizures in the WAG/Rij genetic model of absence epilepsy. Female and male WAG/Rij adult rats were employed as study subjects. For electrophysiology studies, we recorded single unit activity from microwire arrays placed within the DLSC. For optogenetic experiments, animals were injected with virus coding for channelrhodopsin-2 or a control vector, and we compared the efficacy of continuous neuromodulation to that of closed-loop neuromodulation paradigms. For each, we compared three stimulation frequencies on a within-subject basis (5, 20, 100 Hz). For closed-loop stimulation, we detected seizures in real time based on the EEG power within the characteristic frequency band of spike-and-wave discharges (SWDs). We quantified the number and duration of each SWD during each 2 h-observation period. Following completion of the experiment, virus expression and fibre-optic placement was confirmed. We found that single-unit activity within the DLSC decreased seconds prior to SWD onset and increased during and after seizures. Nearly 40% of neurons displayed suppression of firing in response to the start of SWDs. Continuous optogenetic stimulation of the DLSC (at each of the three frequencies) resulted in a significant reduction of SWDs in males and was without effect in females. In contrast, closed-loop neuromodulation was effective in both females and males at all three frequencies. These data demonstrate that activity within the DLSC is suppressed prior to SWD onset, increases at SWD onset, and that excitatory optogenetic stimulation of the DLSC exerts anti-seizure effects against absence seizures. The striking difference between open- and closed-loop neuromodulation approaches underscores the importance of the stimulation paradigm in determining therapeutic effects.

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.

Neural Circuit Mechanisms Involved in Animals' Detection of and Response to Visual Threats

Evading or escaping from predators is one of the most crucial issues for survival across the animal kingdom. The timely detection of predators and the initiation of appropriate fight-or-flight responses are innate capabilities of the nervous system. Here we review recent progress in our understanding of innate visually-triggered defensive behaviors and the underlying neural circuit mechanisms, and a comparison among vinegar flies, zebrafish, and mice is included. This overview covers the anatomical and functional aspects of the neural circuits involved in this process, including visual threat processing and identification, the selection of appropriate behavioral responses, and the initiation of these innate defensive behaviors. The emphasis of this review is on the early stages of this pathway, namely, threat identification from complex visual inputs and how behavioral choices are influenced by differences in visual threats. We also briefly cover how the innate defensive response is processed centrally. Based on these summaries, we discuss coding strategies for visual threats and propose a common prototypical pathway for rapid innate defensive responses.

Description and comparison of brain distribution of oxytocin receptors in Rhabdomys pumillio and Rhabdomys dilectus

Oxytocin receptor (OXTR) distribution in the brain has been associated with different reproductive and social strategies of species. Rhabdomys pumilio (R. pumilio) and Rhabdomys dilectus (R. dilectus) are two sister rodent species that live in large/medium (but flexible) or small (mostly solitary) social groups respectively. In this study, we describe and compare the distribution of OXTR in these two species. OXTR binding in the brain of R. pumilio (8 females and 5 males) and R. dilectus (8 females and 5 males) adults was determined using autoradiography. Our results revealed significant differences in the nucleus accumbens, diagonal band, medial preoptic area, lateral habenula, superior colliculus, periaqueductal area and anterior paraventricular nucleus (higher in R. dilectus), and the dorsal lateral septum and anterior bed nucleus of the stria terminalis (higher in R. pumilio). OXTR density in other brain regions, such as the amygdala nuclei and hippocampus, did not differ between the two species. Sex differences were found in the medial preoptic area and ventral region of the lateral septum in R. pumilio (OXTR density higher in males) and in the anterior paraventricular thalamic nucleus, ventromedial nucleus of the hypothalamus and basolateral amygdala of R. dilectus (OXTR density higher in females). A sex difference in the density of OXTR was also found in the posterior region of the bed nucleus of the stria terminalis, where it was higher in males than in females of both species. This study shows species-specific brain distribution of OXTR in R. pumilio and R. dilectus that are unique, but with similarities with other polygynous or promiscuous rodent species that live in variable size groups, such as R. norvergicus, C. sociabilis, S. teguina and M. musculus.

Superior colliculus cell types bidirectionally modulate choice activity in frontal cortex

Action selection occurs through competition between potential choice options. Neural correlates of choice competition are observed across frontal cortex and downstream superior colliculus (SC) during decision-making, yet how these regions interact to mediate choice competition remains unresolved. Here we report that cell types within SC can bidirectionally modulate choice competition and drive choice activity in frontal cortex. In the mouse, topographically matched regions of frontal cortex and SC formed a descending motor pathway for directional licking and a re-entrant loop via the thalamus. During decision-making, distinct neuronal populations in both frontal cortex and SC encoded opposing lick directions and exhibited push-pull dynamics. SC GABAergic neurons encoded ipsilateral choice and glutamatergic neurons encoded contralateral choice, and activating or suppressing these cell types could bidirectionally drive push-pull choice activity in frontal cortex. These results thus identify SC as a major locus to modulate choice competition within the broader action selection network.

Common neural correlates of vestibular stimulation and fear learning: an fMRI meta-analysis

BACKGROUND

A bidirectional functional link between vestibular and fear-related disorders has been previously suggested.

OBJECTIVE

To test a potential overlap of vestibular and fear systems with regard to their brain imaging representation maps.

METHODS

By use of voxel-based mapping permutation of subject images, we conducted a meta-analysis of earlier functional magnetic resonance imaging (fMRI) studies applying vestibular stimulation and fear conditioning in healthy volunteers.

RESULTS

Common clusters of concordance of vestibular stimulation and fear conditioning were found in the bilateral anterior insula cortex, ventrolateral prefrontal cortex and the right temporal pole, bilaterally in the adjacent ventrolateral prefrontal cortex, cingulate gyrus, secondary somatosensory cortex, superior temporal and intraparietal lobe, supplementary motor area and premotor cortex, as well as subcortical areas, such as the bilateral thalamus, mesencephalic brainstem including the collicular complex, pons, cerebellar vermis and bilateral cerebellar hemispheres. Peak areas of high concordance for activations during vestibular stimulation but deactivations during fear conditioning were centered on the posterior insula and S2.

CONCLUSIONS

The structural overlap of both networks allows the following functional interpretations: first, the amygdala, superior colliculi, and antero-medial thalamus might represent a release of preprogramed sensorimotor patterns of approach or avoidance. Second, the activation (vestibular system) and deactivation (fear system) of the bilateral posterior insula is compatible with the view that downregulation of the fear network by acute vestibular disorders or unfamiliar vestibular stimulation makes unpleasant perceived body accelerations less distressing. This also fits the clinical observation that patients with bilateral vestibular loss suffer from less vertigo-related anxiety.

Different strategies in pointing tasks and their impact on clinical bedside tests of spatial orientation

Deficits in spatial memory, orientation, and navigation are often early or neglected signs of degenerative and vestibular neurological disorders. A simple and reliable bedside test of these functions would be extremely relevant for diagnostic routine. Pointing at targets in the 3D environment is a basic well-trained common sensorimotor ability that provides a suitable measure. We here describe a smartphone-based pointing device using the built-in inertial sensors for analysis of pointing performance in azimuth and polar spatial coordinates. Interpretation of the vectors measured in this way is not trivial, since the individuals tested may use at least two different strategies: first, they may perform the task in an egocentric eye-based reference system by aligning the fingertip with the target retinotopically or second, by aligning the stretched arm and the index finger with the visual line of sight in allocentric world-based coordinates similar to using a rifle. The two strategies result in considerable differences of target coordinates. A pilot test with a further developed design of the device and an app for a standardized bedside utilization in five healthy volunteers revealed an overall mean deviation of less than 5° between the measured and the true coordinates. Future investigations of neurological patients comparing their performance before and after changes in body position (chair rotation) may allow differentiation of distinct orientational deficits in peripheral (vestibulopathy) or central (hippocampal or cortical) disorders.

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.

The Mesencephalic Locomotor Region: Beyond Locomotor Control

The mesencephalic locomotor region (MLR) was discovered several decades ago in the cat. It was functionally defined based on the ability of low threshold electrical stimuli within a region comprising the cuneiform and pedunculopontine nucleus to evoke locomotion. Since then, similar regions have been found in diverse vertebrate species, including the lamprey, skate, rodent, pig, monkey, and human. The MLR, while often viewed under the lens of locomotion, is involved in diverse processes involving the autonomic nervous system, respiratory system, and the state-dependent activation of motor systems. This review will discuss the pedunculopontine nucleus and cuneiform nucleus that comprises the MLR and examine their respective connectomes from both an anatomical and functional angle. From a functional perspective, the MLR primes the cardiovascular and respiratory systems before the locomotor activity occurs. Inputs from a variety of higher structures, and direct outputs to the monoaminergic nuclei, allow the MLR to be able to respond appropriately to state-dependent locomotion. These state-dependent effects are roughly divided into escape and exploratory behavior, and the MLR also can reinforce the selection of these locomotor behaviors through projections to adjacent structures such as the periaqueductal gray or to limbic and cortical regions. Findings from the rat, mouse, pig, and cat will be discussed to highlight similarities and differences among diverse species.

The PV2 cluster of parvalbumin neurons in the murine periaqueductal gray: connections and gene expression

The PV2 (Celio 1990), a cluster of parvalbumin-positive neurons located in the ventromedial region of the distal periaqueductal gray (PAG) has not been previously described as its own entity, leading us to study its extent, connections, and gene expression. It is an oval, bilateral, elongated cluster composed of approximately 475 parvalbumin-expressing neurons in a single mouse hemisphere. In its anterior portion it impinges upon the paratrochlear nucleus (Par4) and in its distal portion it is harbored in the posterodorsal raphe nucleus (PDR). It is known to receive inputs from the orbitofrontal cortex and from the parvafox nucleus in the ventrolateral hypothalamus. Using anterograde tracing methods in parvalbumin-Cre mice, the main projections of the PV2 cluster innervate the supraoculomotor periaqueductal gray (Su3) of the PAG, the parvafox nucleus of the lateral hypothalamus, the gemini nuclei of the posterior hypothalamus, the septal regions, and the diagonal band in the forebrain, as well as various nuclei within the reticular formation in the midbrain and brainstem. Within the brainstem, projections were discrete, but involved areas implicated in autonomic control. The PV2 cluster expressed various peptides and receptors, including the receptor for Adcyap1, a peptide secreted by one of its main afferences, namely, the parvafox nucleus. The expression of GAD1 and GAD2 in the region of the PV2, the presence of Vgat-1 in a subpopulation of PV2-neurons as well as the coexistence of GAD67 immunoreactivity with parvalbumin in terminal endings indicates the inhibitory nature of a subpopulation of PV2-neurons. The PV2 cluster may be part of a feedback controlling the activity of the hypothalamic parvafox and the Su3 nuclei in the periaqueductal gray.

The Superior Colliculus: Cell Types, Connectivity, and Behavior

The superior colliculus (SC), one of the most well-characterized midbrain sensorimotor structures where visual, auditory, and somatosensory information are integrated to initiate motor commands, is highly conserved across vertebrate evolution. Moreover, cell-type-specific SC neurons integrate afferent signals within local networks to generate defined output related to innate and cognitive behaviors. This review focuses on the recent progress in understanding of phenotypic diversity amongst SC neurons and their intrinsic circuits and long-projection targets. We further describe relevant neural circuits and specific cell types in relation to behavioral outputs and cognitive functions. The systematic delineation of SC organization, cell types, and neural connections is further put into context across species as these depend upon laminar architecture. Moreover, we focus on SC neural circuitry involving saccadic eye movement, and cognitive and innate behaviors. Overall, the review provides insight into SC functioning and represents a basis for further understanding of the pathology associated with SC dysfunction.

Evolution of behavioural control from chordates to primates

This article outlines a hypothetical sequence of evolutionary innovations, along the lineage that produced humans, which extended behavioural control from simple feedback loops to sophisticated control of diverse species-typical actions. I begin with basic feedback mechanisms of ancient mobile animals and follow the major niche transitions from aquatic to terrestrial life, the retreat into nocturnality in early mammals, the transition to arboreal life and the return to diurnality. Along the way, I propose a sequence of elaboration and diversification of the behavioural repertoire and associated neuroanatomical substrates. This includes midbrain control of approach versus escape actions, telencephalic control of local versus long-range foraging, detection of affordances by the dorsal pallium, diversified control of nocturnal foraging in the mammalian neocortex and expansion of primate frontal, temporal and parietal cortex to support a wide variety of primate-specific behavioural strategies. The result is a proposed functional architecture consisting of parallel control systems, each dedicated to specifying the affordances for guiding particular species-typical actions, which compete against each other through a hierarchy of selection mechanisms. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Superior Colliculus to VTA pathway controls orienting response and influences social interaction in mice

Social behaviours characterize cooperative, mutualistic, aggressive or parental interactions that occur among conspecifics. Although the Ventral Tegmental Area (VTA) has been identified as a key substrate for social behaviours, the input and output pathways dedicated to specific aspects of conspecific interaction remain understudied. Here, in male mice, we investigated the activity and function of two distinct VTA inputs from superior colliculus (SC-VTA) and medial prefrontal cortex (mPFC-VTA). We observed that SC-VTA neurons display social interaction anticipatory calcium activity, which correlates with orienting responses towards an unfamiliar conspecific. In contrast, mPFC-VTA neuron population activity increases after initiation of the social contact. While protracted phasic stimulation of SC-VTA pathway promotes head/body movements and decreases social interaction, inhibition of this pathway increases social interaction. Here, we found that SC afferents mainly target a subpopulation of dorsolateral striatum (DLS)-projecting VTA dopamine (DA) neurons (VTA^DA-DLS). While, VTA^DA-DLS pathway stimulation decreases social interaction, VTA^DA-Nucleus Accumbens stimulation promotes it. Altogether, these data support a model by which at least two largely anatomically distinct VTA sub-circuits oppositely control distinct aspects of social behaviour.

Solié, Contestabile et al. show that the superior colliculus to ventral tegmental area (VTA) pathway encodes orienting behavior toward conspecifics, and modulates VTA dopamine neurons projecting onto dorsolateral striatum perturbing social interaction.

Whole-Brain Afferent Inputs to the Caudate Nucleus, Putamen, and Accumbens Nucleus in the Tree Shrew Striatum

Day-active tree shrews have a well-developed internal capsule (ic) that clearly separates the caudate nucleus (Cd) and putamen (Pu). The striatum consists of the Cd, ic, Pu, and accumbens nucleus (Acb). Here, we characterized the cytoarchitecture of the striatum and the whole-brain inputs to the Cd, Pu, and Acb in tree shrews by using immunohistochemistry and the retrograde tracer Fluoro-Gold (FG). Our data show the distribution patterns of parvalbumin (PV), nitric oxide synthase (NOS), calretinin (CR), and tyrosine hydroxylase (TH) immunoreactivity in the striatum of tree shrews, which were different from those observed in rats. The Cd and Pu mainly received inputs from the thalamus, motor cortex, somatosensory cortex, subthalamic nucleus, substantia nigra, and other cortical and subcortical regions, whereas the Acb primarily received inputs from the anterior olfactory nucleus, claustrum, infralimbic cortex, thalamus, raphe nucleus, parabrachial nucleus, ventral tegmental area, and so on. The Cd, Pu, and Acb received inputs from different neuronal populations in the ipsilateral (60, 67, and 63 brain regions, respectively) and contralateral (23, 20, and 36 brain regions, respectively) brain hemispheres. Overall, we demonstrate that there are species differences between tree shrews and rats in the density of PV, NOS, CR, and TH immunoreactivity in the striatum. Additionally, we mapped for the first time the distribution of whole-brain input neurons projecting to the striatum of tree shrews with FG injected into the Cd, Pu, and Acb. The similarities and differences in their brain-wide input patterns may provide new insights into the diverse functions of the striatal subregions.

Superior colliculus projections to target populations in the supraoculomotor area of the macaque monkey

A projection by the superior colliculus to the supraoculomotor area (SOA) located dorsal to the oculomotor complex was first described in 1978. This projection’s targets have yet to be identified, although the initial study suggested that vertical gaze motoneuron dendrites might receive this input. Defining the tectal targets is complicated by the fact the SOA contains a number of different cell populations. In the present study, we used anterograde tracers to characterize collicular axonal arbors and retrograde tracers to label prospective SOA target populations in macaque monkeys. Close associations were not found with either superior or medial rectus motoneurons whose axons supply singly innervated muscle fibers. S-group motoneurons, which supply superior rectus multiply innervated muscle fibers, appeared to receive a very minor input, but C-group motoneurons, which supply medial rectus multiply innervated muscle fibers, received no input. A number of labeled boutons were observed in close association with SOA neurons projecting to the spinal cord, or the reticular formation in the pons and medulla. These descending output neurons are presumed to be peptidergic cells within the centrally projecting Edinger–Westphal population. It is possible the collicular input provides a signaling function for neurons in this population that serve roles in either stress responses, or in eating and drinking behavior. Finally, a number of close associations were observed between tectal terminals and levator palpebrae superioris motoneurons, suggesting the possibility that the superior colliculus provides a modest direct input for raising the eyelids during upward saccades.

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

Context dependence of head bobs in gerbils and potential neural contributions

Headbobs are up-down movements of the cranium associated with the use of motion parallax for depth perception. Mongolian gerbils (aka jirds; Meriones unguiculatus) often execute a series of headbobs prior to jumping between surfaces. Gerbils were tested in a jumping stand task and headbobs videotaped under three light levels approximating low daylight, dawn/dusk, and moonlight across a range of distances to target. Headbobs per trial increased linearly with increasing distance to the target platform, whereas headbob frequency (rate of headbobbing pre-jump on the start platform) increased with gap distance up to an intermediate level and then decreased. Overall, gerbils made the most headbobs per trial under the darkest conditions, whereas their headbobbing rate was highest for medium illumination, especially for medium-long gap distances. There was a positive correlation between headbob frequency and volume of the superior colliculus (SC), but no relationship between headbobs and relative size of the temporo-posterior (TP) visual cortex. The results suggest that gerbils employ a specific visuomotor strategy for depth perception differentially under different conditions. We suggest that the deployment of headbobs under specific conditions may be part of an SC-driven vigilant state, of which more rapid sampling of the visual environment using headbobs for depth estimation is one component. Moreover, the findings highlight the importance of considering ecological factors in designing studies of visual behavior and its underpinnings in rodents.

Dense cholinergic projections to auditory and multisensory nuclei of the intercollicular midbrain

Cholinergic axons from the pedunculopontine tegmental nucleus (PPT) innervate the inferior colliculus where they are positioned to modulate both excitatory and inhibitory circuits across the central nucleus and adjacent cortical regions. More rostral regions of the auditory midbrain include the nucleus of the brachium of the inferior colliculus (NBIC), the intercollicular tegmentum (ICt) and the rostral pole of the inferior colliculus (ICrp). These regions appear especially important for multisensory integration and contribute to orienting behavior and many aspects of auditory perception. These regions appear to receive cholinergic innervation but little is known about the distribution of cholinergic axons in these regions or the cells that they contact. The present study used immunostaining to examine the distribution of cholinergic axons and then used chemically-specific viral tracing to examine cholinergic projections from the PPT to the intercollicular areas in male and female transgenic rats. Staining with antibodies against vesicular acetylcholine transporter revealed dense cholinergic innervation throughout the NBIC, ICt and ICrp. Deposits of viral vector into the PPT labeled cholinergic axons bilaterally in the NBIC, ICt and ICrp. In each area, the projections were denser on the ipsilateral side. The axons appeared morphologically similar across the three areas. In each area, en passant and terminal boutons from these axons appeared in the neuropil and also in close apposition to cell bodies. Immunostaining with a marker for GABAergic cells suggested that the cholinergic axons likely contact both GABAergic and non-GABAergic cells in the NBIC, ICt and ICrp. Thus, the cholinergic axons could affect multisensory processing by modulating excitatory and inhibitory circuits in the NBIC, ICt and ICrp. The similarity of axons and their targets suggests there may be a common function for cholinergic innervation across the three areas. Given what is known about the PPT, such functions could be associated with arousal, sleep-wake cycle, reward and plasticity.

Throwing open the doors of perception: The role of dopamine in visual processing

Animals form associations between visual cues and behaviours. Although dopamine is known to be critical in many areas of the brain to bind sensory information with appropriate responses, dopamine's role in the visual system is less well understood. Visual signals, which indicate the likely occurrence of a rewarding or aversive stimulus or indicate the context within which such stimuli may arrive, modulate activity in the superior colliculus and alter behaviour. However, such signals primarily originate in cortical and basal ganglia circuits, and evidence of direct signalling from midbrain dopamine neurons to superior colliculus is lacking. Instead, hypothalamic A13 dopamine neurons innervate the superior colliculus, and dopamine receptors are differentially expressed in the superior colliculus, with D1 receptors in superficial layers and D2 receptors in deep layers. However, it remains unknown if A13 dopamine neurons control behaviours through their effect on afferents within the superior colliculus. We propose that A13 dopamine neurons may play a critical role in processing information in the superior colliculus, modifying behavioural responses to visual cues, and propose some testable hypotheses regarding dopamine's effect on visual perception.

Incerto-thalamic modulation of fear via GABA and dopamine

Fear generalization and deficits in extinction learning are debilitating dimensions of Post-Traumatic Stress Disorder (PTSD). Most understanding of the neurobiology underlying these dimensions comes from studies of cortical and limbic brain regions. While thalamic and subthalamic regions have been implicated in modulating fear, the potential for incerto-thalamic pathways to suppress fear generalization and rescue deficits in extinction recall remains unexplored. We first used patch-clamp electrophysiology to examine functional connections between the subthalamic zona incerta and thalamic reuniens (RE). Optogenetic stimulation of GABAergic ZI → RE cell terminals in vitro induced inhibitory post-synaptic currents (IPSCs) in the RE. We then combined high-intensity discriminative auditory fear conditioning with cell-type-specific and projection-specific optogenetics in mice to assess functional roles of GABAergic ZI → RE cell projections in modulating fear generalization and extinction recall. In addition, we used a similar approach to test the possibility of fear generalization and extinction recall being modulated by a smaller subset of GABAergic ZI → RE cells, the A13 dopaminergic cell population. Optogenetic stimulation of GABAergic ZI → RE cell terminals attenuated fear generalization and enhanced extinction recall. In contrast, optogenetic stimulation of dopaminergic ZI → RE cell terminals had no effect on fear generalization but enhanced extinction recall in a dopamine receptor D1-dependent manner. Our findings shed new light on the neuroanatomy and neurochemistry of ZI-located cells that contribute to adaptive fear by increasing the precision and extinction of learned associations. In so doing, these data reveal novel neuroanatomical substrates that could be therapeutically targeted for treatment of PTSD.

The tectonigral pathway regulates appetitive locomotion in predatory hunting in mice

Appetitive locomotion is essential for animals to approach rewards, such as food and prey. The neuronal circuitry controlling appetitive locomotion is unclear. In a goal-directed behavior-predatory hunting, we show an excitatory brain circuit from the superior colliculus (SC) to the substantia nigra pars compacta (SNc) to enhance appetitive locomotion in mice. This tectonigral pathway transmits locomotion-speed signals to dopamine neurons and triggers dopamine release in the dorsal striatum. Synaptic inactivation of this pathway impairs appetitive locomotion but not defensive locomotion. Conversely, activation of this pathway increases the speed and frequency of approach during predatory hunting, an effect that depends on the activities of SNc dopamine neurons. Together, these data reveal that the SC regulates locomotion-speed signals to SNc dopamine neurons to enhance appetitive locomotion in mice.

Organization of the inputs and outputs of the mouse superior colliculus

The superior colliculus (SC) receives diverse and robust cortical inputs to drive a range of cognitive and sensorimotor behaviors. However, it remains unclear how descending cortical input arising from higher-order associative areas coordinate with SC sensorimotor networks to influence its outputs. Here, we construct a comprehensive map of all cortico-tectal projections and identify four collicular zones with differential cortical inputs: medial (SC.m), centromedial (SC.cm), centrolateral (SC.cl) and lateral (SC.l). Further, we delineate the distinctive brain-wide input/output organization of each collicular zone, assemble multiple parallel cortico-tecto-thalamic subnetworks, and identify the somatotopic map in the SC that displays distinguishable spatial properties from the somatotopic maps in the neocortex and basal ganglia. Finally, we characterize interactions between those cortico-tecto-thalamic and cortico-basal ganglia-thalamic subnetworks. This study provides a structural basis for understanding how SC is involved in integrating different sensory modalities, translating sensory information to motor command, and coordinating different actions in goal-directed behaviors.

Objective and comprehensive re-evaluation of anxiety-like behaviors in mice using the Behavior Atlas

Various animal models of anxiety have been developed to evaluate anxiety and anxiolytic drugs. However, non-uniform measuring paradigms, variability in apparatus use and individual differences in animals confound study results. In this study, when all animals were included in the data analysis, we found no significant differences between control and stressed mice using standard behavioral paradigms for assessing anxiety (elevated plus maze and open field test). To provide a better assessment of anxiety, we therefore used a machine learning approach to analyze the behavioral patterns of each animal, and selected typical subjects in each group for use as a training set according to classical anxiety parameters. Spontaneous behaviors in these animals were captured by multi-view cameras and decomposed into sub-second modules using Behavior Atlas, and six behavioral features providing statistically significant difference between stressed and control mice were identified. Combined with low-dimensional embedding and clustering, new features were used to discriminate stressed mice from controls, in both the training set and all objects. Our results show Behavior Atlas is a powerful approach for identifying new potential biomarkers in an unbiased fashion. Our approach can complement classical measuring paradigms to objectively and comprehensively evaluate anxiety-like behaviors.

Inhibitory neurons in the superior colliculus mediate selection of spatially-directed movements

Decision making is a cognitive process that mediates behaviors critical for survival. Choosing spatial targets is an experimentally-tractable form of decision making that depends on the midbrain superior colliculus (SC). While physiological and computational studies have uncovered the functional topographic organization of the SC, the role of specific SC cell types in spatial choice is unknown. Here, we leveraged behavior, optogenetics, neural recordings and modeling to directly examine the contribution of GABAergic SC neurons to the selection of opposing spatial targets. Although GABAergic SC neurons comprise a heterogeneous population with local and long-range projections, our results demonstrate that GABAergic SC neurons do not locally suppress premotor output, suggesting that functional long-range inhibition instead plays a dominant role in spatial choice. An attractor model requiring only intrinsic SC circuitry was sufficient to account for our experimental observations. Overall, our study elucidates the role of GABAergic SC neurons in spatial choice.

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.

Specific populations of basal ganglia output neurons target distinct brain stem areas while collateralizing throughout the diencephalon

Basal ganglia play a central role in regulating behavior, but the organization of their outputs to other brain areas is incompletely understood. We investigate the largest output nucleus, the substantia nigra pars reticulata (SNr), and delineate the organization and physiology of its projection populations in mice. Using genetically targeted viral tracing and whole-brain anatomical analysis, we identify over 40 SNr targets that encompass a roughly 50-fold range of axonal densities. Retrograde tracing from the volumetrically largest targets indicates that the SNr contains segregated subpopulations that differentially project to functionally distinct brain stem regions. These subpopulations are electrophysiologically specialized and topographically organized and collateralize to common diencephalon targets, including the motor and intralaminar thalamus as well as the pedunculopontine nucleus and the midbrain reticular formation. These findings establish that SNr signaling is organized as dense, parallel outputs to specific brain stem targets concurrent with extensive collateral branches that encompass the majority of SNr axonal boutons.

Superior Colliculus Controls the Activity of the Rostromedial Tegmental Nuclei in an Asymmetrical Manner

Dopaminergic (DA) neurons of the midbrain are involved in controlling orienting and approach of animals toward relevant external stimuli. The firing of DA neurons is regulated by many brain structures; however, the sensory input is provided predominantly by the ipsilateral superior colliculus (SC). It is suggested that SC also innervates the contralateral rostromedial tegmental nucleus (RMTg)-the main inhibitory input to DA neurons. Therefore, this study aimed to describe the physiology and anatomy of the SC-RMTg pathway. To investigate the anatomic connections within the circuit of interest, anterograde, retrograde, and transsynaptic tract-tracing studies were performed on male Sprague Dawley rats. We have observed that RMTg is monosynaptically innervated predominantly by the lateral parts of the intermediate layer of the contralateral SC. To study the physiology of this neuronal pathway, we conducted in vivo electrophysiological experiments combined with optogenetics; the activity of RMTg neurons was recorded using silicon probes, while either contralateral or ipsilateral SC was optogenetically stimulated. Obtained results revealed that activation of the contralateral SC excites the majority of RMTg neurons, while stimulation of the ipsilateral SC evokes similar proportions of excitatory or inhibitory responses. Consequently, single-unit recordings showed that the activation of RMTg neurons innervated by the contralateral SC, or stimulation of contralateral SC-originating axon terminals within the RMTg, inhibits midbrain DA neurons. Together, the anatomy and physiology of the discovered brain circuit suggest its involvement in the orienting and motivation-driven locomotion of animals based on the direction of external sensory stimuli.SIGNIFICANCE STATEMENT Dopaminergic neurons are the target of predominantly ipsilateral, excitatory innervation originating from the superior colliculus. However, we demonstrate in our study that SC inhibits the activity of dopaminergic neurons on the contralateral side of the brain via the rostromedial tegmental nucleus. In this way, sensory information received by the animal from one hemifield could induce opposite effects on both sides of the dopaminergic system. It was shown that the side to which an animal directs its behavior is a manifestation of asymmetry in dopamine release between left and right striatum. Animals tend to move oppositely to the hemisphere with higher striatal dopamine concentration. This explains how the above-described circuit might guide the behavior of animals according to the direction of incoming sensory stimuli.

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.

The zona incerta system: Involvement in attention and movement

The zona incerta (ZI) is a large structure made of four neurochemically defined regions (at least, in rodents). It is globally involved in complex connections with telencephalic and brainstem centers. In this work, we focus on some of the anatomical links this structure develops with the cerebral cortex and the tectum. We also point to its integration within a larger basal ganglia network. The functions of this region are still mysterious, even if recent works suggest its participation in behavioral expression. Studies about the functional organization of the vibrissal system have provided the first integrated model, illustrating the ZI's role in sensory-motor programing. In addition, ZI connections with the superior colliculus and the cerebral cortex as well as recent behavioral studies point to this region playing a role in cognitive processes related to attention toward salient stimuli.

Inhibition of the Deep and Intermediate Layers of the Superior Colliculus Disrupts Sensorimotor Gating in Monkeys

The deep and intermediate layers of the superior colliculus (DLSC) respond to visual, auditory, and tactile inputs and act as a multimodal sensory association area. In turn, activity in the DLSC can drive orienting and avoidance responses-such as saccades and head and body movements-across species, including in rats, cats, and non-human primates. As shown in rodents, DLSC also plays a role in regulating pre-pulse inhibition (PPI) of the acoustic startle response (ASR), a form of sensorimotor gating. DLSC lesions attenuate PPI and electrical stimulation of DLSC inhibits the startle response. While the circuitry mediating PPI is well-characterized in rodents, less is known about PPI regulation in primates. Two recent studies from our labs reported a species difference in the effects of pharmacological inhibition of the basolateral amygdala and substantia nigra pars reticulata (SNpr) on PPI between rats and macaques: in rats, inhibition of these structures decreased PPI, while in macaques, it increased PPI. Given that the SNpr sends direct inhibitory projections to DLSC, we next sought to determine if this species difference was similarly evident at the level of DLSC. Here, we transiently inactivated DLSC in four rhesus macaques by focal microinfusion of the GABAA receptor agonist muscimol. Similar to findings reported in rodents, we observed that bilateral inhibition of the DLSC in macaques significantly disrupted PPI. The impairment was specific to the PPI as the ASR itself was not affected. These results indicate that our previously reported species divergence at the level of the SNpr is not due to downstream differences at the level of the DLSC. Species differences at the level of the SNpr and basolateral amygdala emphasize the importance of studying the underlying circuitry in non-human primates, as impairment in PPI has been reported in several disorders in humans, including schizophrenia, autism, and PTSD.

A specialized reciprocal connectivity suggests a link between the mechanisms by which the superior colliculus and parabigeminal nucleus produce defensive behaviors in rodents

The parabigeminal nucleus (PBG) is the mammalian homologue to the isthmic complex of other vertebrates. Optogenetic stimulation of the PBG induces freezing and escape in mice, a result thought to be caused by a PBG projection to the central nucleus of the amygdala. However, the isthmic complex, including the PBG, has been classically considered satellite nuclei of the Superior Colliculus (SC), which upon stimulation of its medial part also triggers fear and avoidance reactions. As the PBG-SC connectivity is not well characterized, we investigated whether the topology of the PBG projection to the SC could be related to the behavioral consequences of PBG stimulation. To that end, we performed immunohistochemistry, in situ hybridization and neural tracer injections in the SC and PBG in a diurnal rodent, the Octodon degus. We found that all PBG neurons expressed both glutamatergic and cholinergic markers and were distributed in clearly defined anterior (aPBG) and posterior (pPBG) subdivisions. The pPBG is connected reciprocally and topographically to the ipsilateral SC, whereas the aPBG receives afferent axons from the ipsilateral SC and projected exclusively to the contralateral SC. This contralateral projection forms a dense field of terminals that is restricted to the medial SC, in correspondence with the SC representation of the aerial binocular field which, we also found, in O. degus prompted escape reactions upon looming stimulation. Therefore, this specialized topography allows binocular interactions in the SC region controlling responses to aerial predators, suggesting a link between the mechanisms by which the SC and PBG produce defensive behaviors.

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.

Dynamic Contextual Modulation in Superior Colliculus of Awake Mouse

The responses of neurons in the visual pathway depend on the context in which a stimulus is presented. Responses to predictable stimuli are usually suppressed, highlighting responses to unexpected stimuli that might be important for behavior. Here, we established how context modulates the response of neurons in the superior colliculus (SC), a region important in orienting toward or away from visual stimuli. We made extracellular recordings from single units in the superficial layers of SC in awake mice. We found strong suppression of visual response by spatial context (surround suppression) and temporal context (adaptation). Neurons showing stronger surround suppression also showed stronger adaptation effects. In neurons where it was present, surround suppression was dynamic and was reduced by adaptation. Adaptation's effects further revealed two components to surround suppression: one component that was weakly tuned for orientation and adaptable, and another component that was more strongly tuned but less adaptable. The selectivity of the tuned component was flexible, such that suppression was stronger when the stimulus over the surround matched that over the receptive field. Our results therefore reveal strong interactions between spatial and temporal context in regulating the flow of signals through mouse SC, and suggest the presence of a subpopulation of neurons that might signal novelty in either space or time.

Call-specific patterns of neural activation in auditory processing of Richardson's ground squirrel alarm calls

INTRODUCTION

Richardson's ground squirrels use alarm calls to warn conspecifics about potential predatory threats. Chirp calls typically indicate high levels of threat from airborne predators, while whistle calls are associated with lower levels of threat from terrestrial predators. These types of calls primarily elicit escape behaviors and increased vigilance in receivers, respectively. While much is known about the neural mechanisms involved in the production of vocalizations, less is known about the mechanisms important for the perception of alarm calls by receivers, and whether changes in perceived risk are associated with unique patterns of neuronal activation. Thus, to determine whether alarm calls associated with different levels of predation risk result in differential neuronal activation, we used immunohistochemistry to identify and quantify c-Fos immunopositive cells in brain regions important in stress, fear, danger, and reward, following alarm call reception.

METHODS

We exposed 29 female Richardson's ground squirrels (10 control, 10 whistle receivers, and 9 chirp receivers) to playbacks of whistles, chirps, or a no-vocalization control. We then assessed neuronal activation via c-Fos immunohistochemistry in 12 brain regions.

RESULTS

Ground squirrels receiving high-threat "chirp" vocalizations had reduced neuronal activation in the medial amygdala and superior colliculus compared with controls. It is likely that changes in activity in these brain regions serve to alter the balance between approach and avoidance in turn promoting escape behaviors.

CONCLUSIONS

Thus, we conclude that in Richardson's ground squirrels, these brain regions are important for the perception of risk resulting from receiving alarm calls and allow for appropriate behavioral responses by receivers.

Perineuronal nets and subtypes of GABAergic cells differentiate auditory and multisensory nuclei in the intercollicular area of the midbrain

The intercollicular region, which lies between the inferior and superior colliculi in the midbrain, contains neurons that respond to auditory, visual, and somatosensory stimuli. Golgi studies have been used to parse this region into three distinct nuclei: the intercollicular tegmentum (ICt), the rostral pole of the inferior colliculus (ICrp), and the nucleus of the brachium of the IC (NBIC). Few reports have focused on these nuclei, especially the ICt and the ICrp, possibly due to lack of a marker that distinguishes these areas and is compatible with modern methods. Here, we found that staining for GABAergic cells and perineuronal nets differentiates these intercollicular nuclei in guinea pigs. Further, we found that the proportions of four subtypes of GABAergic cells differentiate intercollicular nuclei from each other and from adjacent inferior collicular subdivisions. Our results support earlier studies that suggest distinct morphology and functions for intercollicular nuclei, and provide staining methods that differentiate intercollicular nuclei and are compatible with most modern techniques. We hope that this will help future studies to further characterize the intercollicular region.

The influence of subcortical shortcuts on disordered sensory and cognitive processing

The very earliest stages of sensory processing have the potential to alter how we perceive and respond to our environment. These initial processing circuits can incorporate subcortical regions, such as the thalamus and brainstem nuclei, which mediate complex interactions with the brain's cortical processing hierarchy. These subcortical pathways, many of which we share with other animals, are not merely vestigial but appear to function as 'shortcuts' that ensure processing efficiency and preservation of vital life-preserving functions, such as harm avoidance, adaptive social interactions and efficient decision-making. Here, we propose that functional interactions between these higher-order and lower-order brain areas contribute to atypical sensory and cognitive processing that characterizes numerous neuropsychiatric disorders.