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

Tectal CRFR1 receptor involvement in avoidance and approach behaviors in the South African clawed frog, Xenopus laevis

Animals in the wild must balance food intake with vigilance for predators in order to survive. The optic tectum plays an important role in the integration of external (predators) and internal (energy status) cues related to predator defense and prey capture. However, the role of neuromodulators involved in tectal sensorimotor processing is poorly studied. Recently we showed that tectal CRFR1 receptor activation decreases food intake in the South African clawed frog, Xenopus laevis, suggesting that CRF may modulate food intake/predator avoidance tradeoffs. Here we use a behavioral assay modeling food intake and predator avoidance to test the role of CRFR1 receptors and energy status in this tradeoff. We tested the predictions that 1) administering the CRFR1 antagonist NBI-27914 via the optic tecta will increase food intake and feeding-related behaviors in the presence of a predator, and 2) that prior food deprivation, which lowers tectal CRF content, will increase food intake and feeding-related behaviors in the presence of a predator. Pre-treatment with NBI-27914 did not prevent predator-induced reductions in food intake. Predator exposure altered feeding-related behaviors in a predictable manner. Pretreatment with NBI-27914 reduced the response of certain behaviors to a predator but also altered behaviors irrelevant of predator presence. Although 1-wk of food deprivation altered some non-feeding behaviors related to energy conservation strategy, food intake in the presence of a predator was not altered by prior food deprivation. Collectively, our data support a role for tectal CRFR1 in modulating discrete behavioral responses during predator avoidance/foraging tradeoffs.

Ultra High Field fMRI of Human Superior Colliculi Activity during Affective Visual Processing

Research on rodents and non-human primates has established the involvement of the superior colliculus in defensive behaviours and visual threat detection. The superior colliculus has been well-studied in humans for its functional roles in saccade and visual processing, but less is known about its involvement in affect. In standard functional MRI studies of the human superior colliculus, it is challenging to discern activity in the superior colliculus from activity in surrounding nuclei such as the periaqueductal gray due to technological and methodological limitations. Employing high-field strength (7 Tesla) fMRI techniques, this study imaged the superior colliculus at high (0.75 mm isotropic) resolution, which enabled isolation of the superior colliculus from other brainstem nuclei. Superior colliculus activation during emotionally aversive image viewing blocks was greater than that during neutral image viewing blocks. These findings suggest that the superior colliculus may play a role in shaping subjective emotional experiences in addition to its visuomotor functions, bridging the gap between affective research on humans and non-human animals.

An innate brainstem self-other system involving orienting, affective responding, and polyvalent relational seeking: Some clinical implications for a "Deep Brain Reorienting" trauma psychotherapy approach

Underlying any complex relational intersubjectivity there is an inherent urge to connect, to have proximity, to engage in an experience of interpersonal contact. The hypothesis set out here is that this most basic urge to connect is dependent on circuits based in three main components: the midbrain superior colliculi (SC), the midbrain periaqueductal gray (PAG), and the mesolimbic and mesocortical dopamine systems originating in the midbrain ventral tegmental area. Firstly, there is orienting towards or away from interpersonal contact, dependent on approach and/or defensive/withdrawal areas of the SC. Secondly, there is an affective response to the contact, mediated by the PAG. Thirdly, there is an associated, affectively-loaded, seeking drive based in the mesolimbic and mesocortical dopamine systems. The neurochemical milieu of these dopaminergic systems is responsive to environmental factors, creating the possibility of multiple states of functioning with different affective valences, a polyvalent range of subjectively positive and negative experiences. The recognition of subtle tension changes in skeletal muscles when orienting to an affectively significant experience or event has clinical implications for processing of traumatic memories, including those of a relational/interpersonal nature. Sequences established at the brainstem level can underlie patterns of attachment responding that repeat over many years in different contexts. The interaction of the innate system for connection with that for alarm, through circuits based in the locus coeruleus, and that for defence, based in circuits through the PAG, can lay down deep patterns of emotional and energetic responses to relational stimuli. There may be simultaneous sequences for attachment approach and defensive aggression underlying relational styles that are so deep as to be seen as personality characteristics, for example, of borderline type. A clinical approach derived from these hypotheses, Deep Brain Reorienting, is briefly outlined as it provides a way to address the somatic residues of adverse interpersonal interactions underlying relational patterns and also the residual shock and horror of traumatic experiences. We suggest that the innate alarm system involving the SC and the locus coeruleus can generate a pre-affective shock while an affective shock can arise from excessive stimulation of the PAG. Clinically significant residues can be accessed through careful, mindful, attention to orienting-tension-affect-seeking sequences when the therapist and the client collaborate on eliciting and describing them.

Defined Cell Types in Superior Colliculus Make Distinct Contributions to Prey Capture Behavior in the Mouse

The superior colliculus (SC) plays a highly conserved role in visual processing and mediates visual orienting behaviors across species, including both overt motor orienting [1, 2] and orienting of attention [3, 4]. To determine the specific circuits within the superficial superior colliculus (sSC) that drive orienting and approach behavior toward appetitive stimuli, we explored the role of three genetically defined cell types in mediating prey capture in mice. Chemogenetic inactivation of two classically defined cell types, the wide-field (WF) and narrow-field (NF) vertical neurons, revealed that they are involved in distinct aspects of prey capture. WF neurons were required for rapid prey detection and distant approach initiation, whereas NF neurons were required for accurate orienting during pursuit as well as approach initiation and continuity. In contrast, prey capture did not require parvalbumin-expressing (PV) neurons that have previously been implicated in fear responses. The visual coding and projection targets of WF and NF cells were consistent with their roles in prey detection versus pursuit, respectively. Thus, our studies link specific neural circuit connectivity and function with stimulus detection and orienting behavior, providing insight into visuomotor and attentional mechanisms mediated by superior colliculus.

Resynthesizing behavior through phylogenetic refinement

This article proposes that biologically plausible theories of behavior can be constructed by following a method of "phylogenetic refinement," whereby they are progressively elaborated from simple to complex according to phylogenetic data on the sequence of changes that occurred over the course of evolution. It is argued that sufficient data exist to make this approach possible, and that the result can more effectively delineate the true biological categories of neurophysiological mechanisms than do approaches based on definitions of putative functions inherited from psychological traditions. As an example, the approach is used to sketch a theoretical framework of how basic feedback control of interaction with the world was elaborated during vertebrate evolution, to give rise to the functional architecture of the mammalian brain. The results provide a conceptual taxonomy of mechanisms that naturally map to neurophysiological and neuroanatomical data and that offer a context for defining putative functions that, it is argued, are better grounded in biology than are some of the traditional concepts of cognitive science.

No one knows what attention is

In this article, we challenge the usefulness of "attention" as a unitary construct and/or neural system. We point out that the concept has too many meanings to justify a single term, and that "attention" is used to refer to both the explanandum (the set of phenomena in need of explanation) and the explanans (the set of processes doing the explaining). To illustrate these points, we focus our discussion on visual selective attention. It is argued that selectivity in processing has emerged through evolution as a design feature of a complex multi-channel sensorimotor system, which generates selective phenomena of "attention" as one of many by-products. Instead of the traditional analytic approach to attention, we suggest a synthetic approach that starts with well-understood mechanisms that do not need to be dedicated to attention, and yet account for the selectivity phenomena under investigation. We conclude that what would serve scientific progress best would be to drop the term "attention" as a label for a specific functional or neural system and instead focus on behaviorally relevant selection processes and the many systems that implement them.

Dorsal raphe nucleus 5-Hydroxytryptamine 2A receptors are critical for the organisation of panic attack-like defensive behaviour and unconditioned fear-induced antinociception elicited by the chemical stimulation of superior colliculus neurons

Microinjections of N-methyl-d-aspartic acid (NMDA) in the midbrain tectum structures produce panic attack-like defensive behaviours, followed by an antinociceptive response. It has been suggested that fear-related defensive responses organised by brainstem neurons can be modulated by 5-hydroxytryptamine (5-HT). However, there is a shortage of studies showing the role of dorsal raphe nucleus (DRN) 5-HT_2A receptors in the modulation of panic-like behaviour and fear-induced antinociception organised by the superior colliculus (SC). The purpose of this study was to investigate the participation of DRN 5-HT_2A receptors in the modulation of panic attack-like behaviour and antinociception evoked by intra-SC injections of NMDA. In experiment I, the animals received microinjections of physiological saline or NMDA (6, 9 and 12 nmol) in the deep layers of the SC (dlSC). In experiment II, the most effective dose of NMDA (12 nmol) or vehicle was preceded by microinjections of vehicle or the 5-HT_2A receptor selective antagonist R-96544 at different concentrations (0.5, 5 and 10 nM) in the DRN. Both proaversive and antinociceptive effects elicited by intra-dlSC injections of NMDA were attenuated by DRN pretreatment with R-96544. In addition, a morphological analysis showed that 5-HT_2A receptors are present in GABAergic interneurons in the DRN. Taken together, these findings suggest that DRN 5-HT_2A receptors are critical for the modulation of both panic attack-like defensive behaviour organised by SC neurons and unconditioned fear-induced antinociception. A possible interaction between serotonergic inputs, GABAergic interneurons and serotonergic outputs from the DRN was also considered.

The effects of trauma on brain and body: A unifying role for the midbrain periaqueductal gray

Post-traumatic stress disorder (PTSD), a diagnosis that may follow the experience of trauma, has multiple symptomatic phenotypes. Generally, individuals with PTSD display symptoms of hyperarousal and of hyperemotionality in the presence of fearful stimuli. A subset of individuals with PTSD; however, elicit dissociative symptomatology (i.e., depersonalization, derealization) in the wake of a perceived threat. This pattern of response characterizes the dissociative subtype of the disorder, which is often associated with emotional numbing and hypoarousal. Both symptomatic phenotypes exhibit attentional threat biases, where threat stimuli are processed preferentially leading to a hypervigilant state that is thought to promote defensive behaviors during threat processing. Accordingly, PTSD and its dissociative subtype are thought to differ in their proclivity to elicit active (i.e., fight, flight) versus passive (i.e., tonic immobility, emotional shutdown) defensive responses, which are characterized by the increased and the decreased expression of the sympathetic nervous system, respectively. Moreover, active and passive defenses are accompanied by primarily endocannabinoid- and opioid-mediated analgesics, respectively. Through critical review of the literature, we apply the defense cascade model to better understand the pathological presentation of defensive responses in PTSD with a focus on the functioning of lower-level midbrain and extended brainstem systems.

Cognitive Control of Escape Behaviour

When faced with potential predators, animals instinctively decide whether there is a threat they should escape from, and also when, how, and where to take evasive action. While escape is often viewed in classical ethology as an action that is released upon presentation of specific stimuli, successful and adaptive escape behaviour relies on integrating information from sensory systems, stored knowledge, and internal states. From a neuroscience perspective, escape is an incredibly rich model that provides opportunities for investigating processes such as perceptual and value-based decision-making, or action selection, in an ethological setting. We review recent research from laboratory and field studies that explore, at the behavioural and mechanistic levels, how elements from multiple information streams are integrated to generate flexible escape behaviour.

Central distribution of oxytocin and vasopressin 1a receptors in juvenile Richardson's ground squirrels

Oxytocin and vasopressin are well-conserved peptides important to the regulation of numerous aspects of social behavior, including sociality. Research exploring the distribution of the receptors for oxytocin (Oxtr) and for vasopressin (Avpr1a) in mammals has revealed associations between receptor distribution, sociality, and species' mating systems. Given that sociality and gregariousness can be tightly linked to reproduction, these nonapeptides unsurprisingly support affiliative behaviors that are important for mating and offspring care. We localized these receptors in juvenile Richardson's ground squirrel brains to determine whether distribution patterns of Oxtr and Avpr1a that are associated with promiscuous mating systems differ in rodents that also exhibit non-reproductive affiliation. These squirrels are social, colonial, and engage in nepotistic alarm calling behavior and affiliation outside of a reproductive context. Juveniles are the most affiliative age-class and are non-reproductive; making them ideal for examining these associations. We found that juveniles had dense Oxtr binding in the dentate gyrus of the hippocampus, amygdala, lateral septum, bed nucleus of the stria terminalis and medial geniculate nucleus. Juveniles had low to modest levels of Avpr1a binding in the medial preoptic area, olfactory bulbs, nucleus accumbens, superior colliculus, and inferior colliculus. We noted Oxtr and Avpr1a binding in the social behavior neural network (SBNN), further supporting a role of these nonapeptides in modulating social behavior across taxa. Oxtr and Avpr1a binding was also present in brain regions important to auditory processing that have known projections to the SBNN. We speculate that these neural substrates may be where these nonapeptides regulate communication.

Autism Pathogenesis: The Superior Colliculus

After been exposed to the visual input, in the first year of life, the brain experiences subtle but massive changes apparently crucial for communicative/emotional and social human development. Its lack could be the explanation of the very high prevalence of autism in children with total congenital blindness. The present theory postulates that the superior colliculus is the key structure for such changes for several reasons: it dominates visual behavior during the first months of life; it is ready at birth for complex visual tasks; it has a significant influence on several hemispheric regions; it is the main brain hub that permanently integrates visual and non-visual, external and internal information (bottom-up and top-down respectively); and it owns the enigmatic ability to take non-conscious decisions about where to focus attention. It is also a sentinel that triggers the subcortical mechanisms which drive social motivation to follow faces from birth and to react automatically to emotional stimuli. Through indirect connections it also activates simultaneously several cortical structures necessary to develop social cognition and to accomplish the multiattentional task required for conscious social interaction in real life settings. Genetic or non-genetic prenatal or early postnatal factors could disrupt the SC functions resulting in autism. The timing of postnatal biological disruption matches the timing of clinical autism manifestations. Astonishing coincidences between etiologies, clinical manifestations, cognitive and pathogenic autism theories on one side and SC functions on the other are disclosed in this review. Although the visual system dependent of the SC is usually considered as accessory of the LGN canonical pathway, its imprinting gives the brain a qualitatively specific functions not supplied by any other brain structure.

The Innate Alarm System and Subliminal Threat Presentation in Posttraumatic Stress Disorder: Neuroimaging of the Midbrain and Cerebellum

BACKGROUND

The innate alarm system, a network of interconnected midbrain, other brainstem, and thalamic structures, serves to rapidly detect stimuli in the environment prior to the onset of conscious awareness. This system is sensitive to threatening stimuli and has evolved to process these stimuli subliminally for hastened responding. Despite the conscious unawareness, the presentation of subliminal threat stimuli generates increased activation of limbic structures, including the amygdala and insula, as well as emotionally evaluative structures, including the cerebellum and orbitofrontal cortex. Posttraumatic stress disorder (PTSD) is associated with an increased startle response and decreased extinction learning to conditioned threat. The role of the innate alarm system in the clinical presentation of PTSD, however, remains poorly understood.

METHODS

Here, we compare midbrain, brainstem, and cerebellar activation in persons with PTSD (n = 26) and matched controls (n = 20) during subliminal threat presentation. Subjects were presented with masked trauma-related and neutral stimuli below conscious threshold. Contrasts of subliminal brain activation for the presentation of neutral stimuli were subtracted from trauma-related brain activation. Group differences in activation, as well as correlations between clinical scores and PTSD activation, were examined. Imaging data were preprocessed utilizing the spatially unbiased infratentorial template toolbox within SPM12.

RESULTS

Analyses revealed increased midbrain activation in PTSD as compared to controls in the superior colliculus, periaqueductal gray, and midbrain reticular formation during subliminal threat as compared to neutral stimulus presentation. Controls showed increased activation in the right cerebellar lobule V during subliminal threat presentation as compared to PTSD. Finally, a negative correlation emerged between PTSD patient scores on the Multiscale Dissociation Inventory for the Depersonalization/Derealization subscale and activation in the right lobule V of the cerebellum during the presentation of subliminal threat as compared to neutral stimuli.

CONCLUSION

We interpret these findings as evidence of innate alarm system overactivation in PTSD and of the prominent role of the cerebellum in the undermodulation of emotion observed in PTSD.

The Threatful Self: Midbrain Functional Connectivity to Cortical Midline and Parietal Regions During Subliminal Trauma-Related Processing in PTSD

BACKGROUND

The innate alarm system consists of a subcortical network of interconnected midbrain, lower brainstem, and thalamic nuclei, which together mediate the detection of evolutionarily-relevant stimuli. The periaqueductal gray is a midbrain structure innervated by the innate alarm system that coordinates the expression of defensive states following threat detection. In participants with post-traumatic stress disorder, the periaqueductal gray displays overactivation during the subliminal presentation of trauma-related stimuli as well as altered resting-state functional connectivity. Aberrant functional connectivity is also reported in post-traumatic stress disorder for the default-mode network, a large-scale brain network recruited during self-referential processing and autobiographical memory. Here, research lacks investigation on the extent to which functional interactions are displayed between the midbrain and the large-scale cortical networks in post-traumatic stress disorder.

METHODS

Using a subliminal threat presentation paradigm, we investigated psycho-physiological interactions during functional neuroimaging in participants with post-traumatic stress disorder (n = 26) and healthy control subjects (n = 20). Functional connectivity of the periaqueductal gray was investigated across the whole-brain of each participant during subliminal exposure to trauma-related and neutral word stimuli.

RESULTS

As compared to controls during subliminal threat presentation, the post-traumatic stress disorder group showed significantly greater periaqueductal gray functional connectivity with regions of the default-mode network (i.e., angular gyrus, precuneus, superior frontal gyrus). Moreover, multiple regression analyses revealed that the functional connectivity between the periaqueductal gray and the regions of the default-mode network correlated positively to symptoms of avoidance and state dissociation in post-traumatic stress disorder.

CONCLUSION

Given that the periaqueductal gray engages the expression of defensive states, stronger midbrain functional coupling with the default-mode network may have clinical implications to self-referential and trauma-related processing in participants with post-traumatic stress disorder.

The orbitofrontal cortex projects to the parvafox nucleus of the ventrolateral hypothalamus and to its targets in the ventromedial periaqueductal grey matter

Although connections between the orbitofrontal cortex (OFC)-the seat of high cognitive functions-the lateral hypothalamus and the periaqueductal grey (PAG) have been recognized in the past, the precise targets of the descending fibres have not been identified. In the present study, viral tracer-transport experiments revealed neurons of the lateral (LO) and the ventrolateral (VLO) OFC (homologous to part of Area 13 in primates) to project to a circumscribed region in the ventrolateral hypothalamus, namely, the horizontally oriented, cylindrical parvalbumin- and Foxb1-expressing (parvafox) nucleus. The fine collaterals stem from coarse axons in the internal capsule and form excitatory synapses specifically with neurons of the parvafox nucleus, avoiding the rest of the hypothalamus. In its further caudal course, this contingent of LO/VLO-axons projects collaterals to the Su3- and the PV2 nuclei, which lie ventral to the aqueduct in the (PAG), where the terminals fields overlap those deriving from the parvafox nucleus itself. The targeting of the parvafox nucleus by the LO/VLO-projections, and the overlapping of their terminal fields within the PAG, suggest that the two cerebral sites interact closely. An involvement of this LO/VLO-driven circuit in the somatic manifestation of behavioural events is conceivable.

Subtypes of GABAergic cells in the inferior colliculus

The inferior colliculus occupies a central position in ascending and descending auditory pathways. A substantial proportion of its neurons are GABAergic, and these neurons contribute to intracollicular circuits as well as to extrinsic projections to numerous targets. A variety of types of evidence - morphology, physiology, molecular markers - indicate that the GABAergic cells can be divided into at least four subtypes that serve different functions. However, there has yet to emerge a unified scheme for distinguishing these subtypes. The present review discusses these criteria and, where possible, relates the different properties. In contrast to GABAergic cells in cerebral cortex, where subtypes are much more thoroughly characterized, those in the inferior colliculus contribute substantially to numerous long range extrinsic projections. At present, the best characterized subtype is a GABAergic cell with a large soma, dense perisomatic synaptic inputs and a large axon that provides rapid auditory input to the thalamus. This large GABAergic subtype projects to additional targets, and other subtypes also project to the thalamus. The eventual characterization of these subtypes can be expected to reveal multiple functions of these inhibitory cells and the many circuits to which they contribute.

Distributions of GABAergic and glutamatergic neurons in the brains of a diurnal and nocturnal rodent

Light influences the daily patterning of activity by both synchronizing internal clocks to environmental light-dark cycles and acutely modulating arousal states, a process known as masking. Masking responses are completely reversed in diurnal and nocturnal species. In nocturnal rodents, masking is mediated through a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) whose projections are similar in diurnal and nocturnal rodents. This raises the possibility that differences in responsivity to signals that these cells release might underlie chronotype differences in masking. We explored one aspect of this hypothesis by examining the distribution of excitatory and inhibitory neuronal populations in many ipRGC target areas of a diurnal species (Nile grass rat) and a nocturnal one (Norway rat). We discovered that while many of these regions were very similar in these two species, there were striking differences in the ventral lateral geniculate nucleus (vLGN; higher density of glutamate cells in Norway rats) and in the lateral habenula (LHb; GABAeric cells present in grass rats, but not Norway rats). These patterns raise the possibility that the vLGN and LHb contribute to differences in masking and/or circadian regulation of diurnal and nocturnal species.

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.

Genetically Epilepsy-Prone Rats Display Anxiety-Like Behaviors and Neuropsychiatric Comorbidities of Epilepsy

Epilepsy is associated with a variety of neuropsychiatric comorbidities, including both anxiety and depression. Despite high occurrences of depression and anxiety seen in human epilepsy populations, little is known about the etiology of these comorbidities. Experimental models of epilepsy provide a platform to disentangle the contribution of acute seizures, genetic predisposition, and underlying circuit pathologies to anxious and depressive phenotypes. Most studies to date have focused on comorbidities in acquired epilepsies; genetic models, however, allow for the assessment of affective phenotypes that occur prior to onset of recurrent seizures. Here, we tested male and female genetically epilepsy-prone rats (GEPR-3s) and Sprague-Dawley controls in a battery of tests sensitive to anxiety-like and depressive-like phenotypes. GEPR-3s showed increased anxiety-like behavior in the open field test, elevated plus maze, light-dark transition test, and looming threat test. Moreover, GEPR-3s showed impaired prepulse inhibition of the acoustic startle reflex, decreased sucrose preference index, and impaired novel object recognition memory. We also characterized defense behaviors in response to stimulation thresholds of deep and intermediate layers of the superior colliculus (DLSC), but found no difference between strains. In sum, GEPR-3s showed inherited anxiety, an effect that did not differ significantly between sexes. The anxiety phenotype in adult GEPR-3s suggests strong genetic influences that may underlie both the seizure disorder and the comorbidities seen in epilepsy.

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

The superficial layers of the superior colliculus (sSC) receive retinal input and project to thalamic regions, the dorsal lateral geniculate (dLGN) and lateral posterior (LP; or pulvinar) nuclei, that convey visual information to cortex. A critical step toward understanding the functional impact of sSC neurons on these parallel thalamo-cortical pathways is determining whether different classes of sSC neurons, which are known to respond to different features of visual stimuli, innervate overlapping or distinct thalamic targets. Here, we identified a transgenic mouse line that labels sSC neurons that project to dLGN but not LP. We utilized selective expression of fluorophores and channelrhodopsin in this and previously characterized mouse lines to demonstrate that distinct cell types give rise to sSC projections to dLGN and LP. We further show that the glutamatergic sSC cell type that projects to dLGN also provides input to the sSC cell type that projects to LP. These results clarify the cellular origin of parallel sSC-thalamo-cortical pathways and reveal an interaction between these pathways via local connections within the sSC.

NEW & NOTEWORTHY The superficial layers of the superior colliculus (sSC) project to two visual thalamic targets: the dorsal lateral geniculate (dLGN) and lateral posterior (LP) nuclei. We show that distinct excitatory sSC cell types give rise to these projections; stellate cells project to dLGN and wide-field (WF) cells project to LP. Moreover, these pathways interact via a connection within the sSC from stellate to WF cells.

Visual Function, Organization, and Development of the Mouse Superior Colliculus

The superior colliculus (SC) is the most prominent visual center in mice. Studies over the past decade have greatly advanced our understanding of the function, organization, and development of the mouse SC, which has rapidly become a popular model in vision research. These studies have described the diverse and cell-type-specific visual response properties in the mouse SC, revealed their laminar and topographic organizations, and linked the mouse SC and downstream pathways with visually guided behaviors. Here, we summarize these findings, compare them with the rich literature of SC studies in other species, and highlight important gaps and exciting future directions. Given its clear importance in mouse vision and the available modern neuroscience tools, the mouse SC holds great promise for understanding the cellular, circuit, and developmental mechanisms that underlie visual processing, sensorimotor transformation, and, ultimately, behavior.

Stimulation of the Nigrotectal Pathway at the Level of the Superior Colliculus Reduces Threat Recognition and Causes a Shift From Avoidance to Approach Behavior

Defensive behavioral responses are essential for survival in threating situations. The superior colliculus (SC) has been implicated in the generation of defensive behaviors elicited by visual, tactile and auditory stimuli. Furthermore, substantia nigra pars reticulata (SNr) neurons are known to exert a modulatory effect on midbrain tectum neural substrates. However, the functional role of this nigrotectal pathway in threating situations is still poorly understood. Using optogenetics in freely behaving mice, we activated SNr projections at the level of the SC, and assessed consequences on behavioral performance in an open field test (OFT) and the beetle mania task (BMT). The latter confronts a mouse with an erratic moving robo-beetle and allows to measure active and passive defensive responses upon frequent encounter of the threatening object. Channelrhodopsin-2 (ChR2)-mediated activation of the inhibitory nigrotectal pathway did not affect anxiety-like and exploratory behavior in the OFT, but increased the number of contacts between robo-beetle and test mouse in the BMT. Depending on the size of the arena, active avoidance responses were reduced, whereas tolerance and close following of the robo-beetle were significantly increased. We conclude from the data that the nigrotectal pathway plays holds the potential to modulate innate fear by attenuating threat recognition and causing a shift from defensive to approach behavior.

Divergent midbrain circuits orchestrate escape and freezing responses to looming stimuli in mice

Animals respond to environmental threats, e.g. looming visual stimuli, with innate defensive behaviors such as escape and freezing. The key neural circuits that participate in the generation of such dimorphic defensive behaviors remain unclear. Here we show that the dimorphic behavioral patterns triggered by looming visual stimuli are mediated by parvalbumin-positive (PV⁺) projection neurons in mouse superior colliculus (SC). Two distinct groups of SC PV⁺ neurons form divergent pathways to transmit threat-relevant visual signals to neurons in the parabigeminal nucleus (PBGN) and lateral posterior thalamic nucleus (LPTN). Activations of PV⁺ SC-PBGN and SC-LPTN pathways mimic the dimorphic defensive behaviors. The PBGN and LPTN neurons are co-activated by looming visual stimuli. Bilateral inactivation of either nucleus results in the defensive behavior dominated by the other nucleus. Together, these data suggest that the SC orchestrates dimorphic defensive behaviors through two separate tectofugal pathways that may have interactions.

In response to environmental threats, such as visual looming stimuli, mice either freeze or escape. Here the authors demonstrate that these two behaviors are mediated by separate tectofugal pathways formed by parvalbumin-positive neurons in the superior colliculus.

Cortical projections to the superior colliculus in grey squirrels (Sciurus carolinensis)

The superior colliculus is an important midbrain structure involved with integrating information from varying sensory modalities and sending motor signals to produce orienting movements towards environmental stimuli. Because of this role, the superior colliculus receives a multitude of sensory inputs from a wide variety of subcortical and cortical structures. Proportionately, the superior colliculus of grey squirrels is among the largest in size of all studied mammals, suggesting the importance of this structure in the behavioural characteristics of grey squirrels. Yet, our understanding of the connections of the superior colliculus in grey squirrels is lacking, especially with respect to possible cortical influences. In this study, we placed anatomical tracer injections within the medial aspect of the superior colliculus of five grey squirrels (Sciurus carolinensis) and analysed the areal distribution of corticotectal projecting cells in flattened cortex. V1 projections to the superior colliculus were studied in two additional animals. Our results indicate that the superior colliculus receives cortical projections from visual, higher order somatosensory, and higher order auditory regions, as well as limbic, retrosplenial and anterior cingulate cortex. Few, if any, corticotectal projections originate from primary motor, primary somatosensory or parietal cortical regions. This distribution of inputs is similar to the distribution of inputs described in other rodents such as rats and mice, yet the lack of inputs from primary somatosensory and motor cortex is features of corticotectal inputs more similar to those observed in tree shrews and primates, possibly reflecting a behavioural shift from somatosensory (vibrissae) to visual navigation.

GABAergic and non-GABAergic projections to the superior colliculus from the auditory brainstem

The superior colliculus (SC) contains an auditory space map that is shaped by projections from several subcortical auditory nuclei. Both GABAergic (inhibitory) and excitatory cells contribute to these inputs, but there are contradictory reports regarding the sources of these inputs. We used retrograde tracing techniques in guinea pigs to identify cells in the auditory brainstem that project to the SC. We combined retrograde tracing with immunohistochemistry for glutamic acid decarboxylase (GAD) to identify putative GABAergic cells that participate in this pathway. Following a tracer injection in the SC, the nucleus of the brachium of the inferior colliculus (NBIC) contained the most labeled cells, followed by the inferior colliculus (IC). Smaller populations were observed in the sagulum, paralemniscal area, periolivary nuclei and ventrolateral tegmental nucleus. Overall, only 10% of the retrogradely labeled cells were GAD immunopositive. The presumptive inhibitory cells were observed in the NBIC, IC, superior paraolivary nucleus, sagulum and paralemniscal area. We conclude that the guinea pig SC receives input from a diverse set of auditory brainstem nuclei, some of which provide GABAergic input. These diverse origins of input to the SC likely represent a variety of functions. Inputs from the NBIC and IC likely provide spatial information for guiding orienting behaviors. Inputs from subcollicular nuclei are less likely to provide spatial information; rather, they may provide a shorter route for auditory information to reach the SC, and could generate avoidance or escape responses to an external threat.

Functional Organization and Dynamic Activity in the Superior Colliculus of the Echolocating Bat, Eptesicus fuscus

Sensory-guided behaviors require the transformation of sensory information into task-specific motor commands. Prior research on sensorimotor integration has emphasized visuomotor processes in the context of simplified orienting movements in controlled laboratory tasks rather than an animal's more complete, natural behavioral repertoire. Here, we conducted a series of neural recording experiments in the midbrain superior colliculus (SC) of echolocating bats engaged in a sonar target-tracking task that invoked dynamic active sensing behaviors. We hypothesized that SC activity in freely behaving animals would reveal dynamic shifts in neural firing patterns within and across sensory, sensorimotor, and premotor layers. We recorded neural activity in the SC of freely echolocating bats (three females and one male) and replicated the general trends reported in other species with sensory responses in the dorsal divisions and premotor activity in ventral divisions of the SC. However, within this coarse functional organization, we discovered that sensory and motor neurons are comingled within layers throughout the volume of the bat SC. In addition, as the bat increased pulse rate adaptively to increase resolution of the target location with closing distance, the activity of sensory and vocal premotor neurons changed such that auditory response times decreased, and vocal premotor lead times shortened. This finding demonstrates that SC activity can be modified dynamically in concert with adaptive behaviors and suggests that an integrated functional organization within SC laminae supports rapid and local integration of sensory and motor signals for natural, adaptive behaviors.SIGNIFICANCE STATEMENT Natural sensory-guided behaviors involve the rapid integration of information from the environment to direct flexible motor actions. The vast majority of research on sensorimotor integration has used artificial stimuli and simplified behaviors, leaving open questions about nervous system function in the context of natural tasks. Our work investigated mechanisms of dynamic sensorimotor feedback control by analyzing patterns of neural activity in the midbrain superior colliculus (SC) of an echolocating bat tracking and intercepting moving prey. Recordings revealed that sensory and motor neurons comingle within laminae of the SC to support rapid sensorimotor integration. Further, we discovered that neural activity in the bat SC changes with dynamic adaptations in the animal's echolocation behavior.

Efferents of anterior cingulate areas 24a and 24b and midcingulate areas 24a' and 24b' in the mouse

The anterior cingulate cortex (ACC), constituted by areas 25, 32, 24a and 24b in rodents, plays a major role in cognition, emotion and pain. In a previous study, we described the afferents of areas 24a and 24b and those of areas 24a' and 24b' of midcingulate cortex (MCC) in mice and highlighted some density differences among cingulate inputs (Fillinger et al., Brain Struct Funct 222:1509-1532, 2017). To complete this connectome, we analyzed here the efferents of ACC and MCC by injecting anterograde tracers in areas 24a/24b of ACC and 24a'/24b' of MCC. Our results reveal a common projections pattern from both ACC and MCC, targeting the cortical mantle (intracingulate, retrosplenial and parietal associative cortex), the non-cortical basal forebrain, (dorsal striatum, septum, claustrum, basolateral amygdala), the hypothalamus (anterior, lateral, posterior), the thalamus (anterior, laterodorsal, ventral, mediodorsal, midline and intralaminar nuclei), the brainstem (periaqueductal gray, superior colliculus, pontomesencephalic reticular formation, pontine nuclei, tegmental nuclei) and the spinal cord. In addition to an overall denser ACC projection pattern compared to MCC, our analysis revealed clear differences in the density and topography of efferents between ACC and MCC, as well as between dorsal (24b/24b') and ventral (24a/24a') areas, suggesting a common functionality of these two cingulate regions supplemented by specific roles of each area. These results provide a detailed analysis of the efferents of the mouse areas 24a/24b and 24a'/24b' and achieve the description of the cingulate connectome, which bring the anatomical basis necessary to address the roles of ACC and MCC in mice.

Superior colliculus resting state networks in post-traumatic stress disorder and its dissociative subtype

OBJECTIVES

The innate alarm system (IAS) models the neurocircuitry involved in threat processing in posttraumatic stress disorder (PTSD). Here, we investigate a primary subcortical structure of the IAS model, the superior colliculus (SC), where the SC is thought to contribute to the mechanisms underlying threat-detection in PTSD. Critically, the functional connectivity between the SC and other nodes of the IAS remains unexplored.

EXPERIMENTAL DESIGN

We conducted a resting-state fMRI study to investigate the functional architecture of the IAS, focusing on connectivity of the SC in PTSD (n = 67), its dissociative subtype (n = 41), and healthy controls (n = 50) using region-of-interest seed-based analysis.

PRINCIPAL OBSERVATIONS

We observed group-specific resting state functional connectivity between the SC for both PTSD and its dissociative subtype, indicative of dedicated IAS collicular pathways in each group of patients. When comparing PTSD to its dissociative subtype, we observed increased resting state functional connectivity between the left SC and the right dorsolateral prefrontal cortex (DLPFC) in PTSD. The DLPFC is involved in modulation of emotional processes associated with active defensive responses characterising PTSD. Moreover, when comparing PTSD to its dissociative subtype, increased resting state functional connectivity was observed between the right SC and the right temporoparietal junction in the dissociative subtype. The temporoparietal junction is involved in depersonalization responses associated with passive defensive responses typical of the dissociative subtype.

CONCLUSIONS

Our findings suggest that unique resting state functional connectivity of the SC parallels the unique symptom profile and defensive responses observed in PTSD and its dissociative subtype. Hum Brain Mapp 39:563-574, 2018. © 2017 Wiley Periodicals, Inc.

Integration of Descending Command Systems for the Generation of Context-Specific Locomotor Behaviors

Over the past decade there has been a renaissance in our understanding of spinal cord circuits; new technologies are beginning to provide key insights into descending circuits which project onto spinal cord central pattern generators. By integrating work from both the locomotor and animal behavioral fields, we can now examine context-specific control of locomotion, with an emphasis on descending modulation arising from various regions of the brainstem. Here we examine approach and avoidance behaviors and the circuits that lead to the production and arrest of locomotion.

Effect of methylphenidate on visual responses in the superior colliculus in the anaesthetised rat: Role of cortical activation

The mechanism of action of psychostimulant drugs in the treatment of Attention Deficit Hyperactivity Disorder is still largely unknown, although recent evidence suggests one possibility is that the drugs affect the superior colliculus (SC). We have previously demonstrated that systemically administered d-amphetamine attenuates/abolishes visual responses to wholefield light flashes in the superficial layers of the SC in anaesthetised rats, and the present study sought to extend this work to methylphenidate (MPH). Anaesthetised rats were administered MPH at a range of doses (or saline) and subjected to monocular wholefield light flashes at two intensities, juxta-threshold and super-threshold. In contrast to d-amphetamine, systemic MPH produced an enhancement of visual activity at both intensities. Methylphenidate was also found to produce activation of the cortical EEG in anaesthetised rats. Furthermore, cortical activation induced by electrical stimulation of the pons was found to enhance visual responses in superficial layers of the SC, and when MPH was paired with pontine-induced cortical activation, the response-enhancing effects of MPH were substantially attenuated. Taken together, the results suggest that the enhancement of visual responses in the superficial layers of the SC by MPH in the anaesthetised rat is an artefact of the drug's interaction with cortical arousal.

Reticular Formation Connections Underlying Horizontal Gaze: The Central Mesencephalic Reticular Formation (cMRF) as a Conduit for the Collicular Saccade Signal

The central mesencephalic reticular formation (cMRF) occupies much of the core of the midbrain tegmentum. Physiological studies indicate that it is involved in controlling gaze changes, particularly horizontal saccades. Anatomically, it receives input from the ipsilateral superior colliculus (SC) and it has downstream projections to the brainstem, including the horizontal gaze center located in the paramedian pontine reticular formation (PPRF). Consequently, it has been hypothesized that the cMRF plays a role in the spatiotemporal transformation needed to convert spatially coded collicular saccade signals into the temporally coded signals utilized by the premotor neurons of the horizontal gaze center. In this study, we used neuroanatomical tracers to examine the patterns of connectivity of the cMRF in macaque monkeys in order to determine whether the circuit organization supports this hypothesis. Since stimulation of the cMRF produces contraversive horizontal saccades and stimulation of the horizontal gaze center produces ipsiversive saccades, this would require an excitatory cMRF projection to the contralateral PPRF. Injections of anterograde tracers into the cMRF did produce labeled terminals within the PPRF. However, the terminations were denser ipsilaterally. Since the PPRF located contralateral to the movement direction is generally considered to be silent during a horizontal saccade, we then tested the hypothesis that this ipsilateral reticuloreticular pathway might be inhibitory. The ultrastructure of ipsilateral terminals was heterogeneous, with some displaying more extensive postsynaptic densities than others. Postembedding immunohistochemistry for gamma-aminobutyric acid (GABA) indicated that only a portion (35%) of these cMRF terminals are GABAergic. Dual tracer experiments were undertaken to determine whether the SC provides input to cMRF reticuloreticular neurons projecting to the ipsilateral pons. Retrogradely labeled reticuloreticular neurons were predominantly distributed in the ipsilateral cMRF. Anterogradely labeled tectal terminals were observed in close association with a portion of these retrogradely labeled reticuloreticular neurons. Taken together, these results suggest that the SC does have connections with reticuloreticular neurons in the cMRF. However, the predominantly excitatory nature of the ipsilateral reticuloreticular projection argues against the hypothesis that this cMRF pathway is solely responsible for producing a spatiotemporal transformation of the collicular saccade signal.

Segregated fronto-cortical and midbrain connections in the mouse and their relation to approach and avoidance orienting behaviors

The orchestration of orienting behaviors requires the interaction of many cortical and subcortical areas, for example the superior colliculus (SC), as well as prefrontal areas responsible for top–down control. Orienting involves different behaviors, such as approach and avoidance. In the rat, these behaviors are at least partially mapped onto different SC subdomains, the lateral (SCl) and medial (SCm), respectively. To delineate the circuitry involved in the two types of orienting behavior in mice, we injected retrograde tracer into the intermediate and deep layers of the SCm and SCl, and thereby determined the main input structures to these subdomains. Overall the SCm receives larger numbers of afferents compared to the SCl. The prefrontal cingulate area (Cg), visual, oculomotor, and auditory areas provide strong input to the SCm, while prefrontal motor area 2 (M2), and somatosensory areas provide strong input to the SCl. The prefrontal areas Cg and M2 in turn connect to different cortical and subcortical areas, as determined by anterograde tract tracing. Even though connectivity pattern often overlap, our labeling approaches identified segregated neural circuits involving SCm, Cg, secondary visual cortices, auditory areas, and the dysgranular retrospenial cortex likely to be involved in avoidance behaviors. Conversely, SCl, M2, somatosensory cortex, and the granular retrospenial cortex comprise a network likely involved in approach/appetitive behaviors.

Anatomical characterization of subcortical descending projections to the inferior colliculus in mouse

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study was to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus (IC) of the mouse revealed that most of the subcortical descending projections originated in the brachium of the IC and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium-binding proteins calbindin, parvalbumin, or calretinin. Using two different strains of GAD-GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the IC is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the IC and amygdala plus corpus striatum as well into the IC and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium-binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the IC. J. Comp. Neurol. 525:885-900, 2017. © 2016 Wiley Periodicals, Inc.

AAV-Mediated Anterograde Transsynaptic Tagging: Mapping Corticocollicular Input-Defined Neural Pathways for Defense Behaviors

To decipher neural circuits underlying brain functions, viral tracers are widely applied to map input and output connectivity of neuronal populations. Despite the successful application of retrograde transsynaptic viruses for identifying presynaptic neurons of transduced neurons, analogous anterograde transsynaptic tools for tagging postsynaptically targeted neurons remain under development. Here, we discovered that adeno-associated viruses (AAV1 and AAV9) exhibit anterograde transsynaptic spread properties. AAV1-Cre from transduced presynaptic neurons effectively and specifically drives Cre-dependent transgene expression in selected postsynaptic neuronal targets, thus allowing axonal tracing and functional manipulations of the latter input-defined neuronal population. Its application in superior colliculus (SC) reveals that SC neuron subpopulations receiving corticocollicular projections from auditory and visual cortex specifically drive flight and freezing, two different types of defense behavior, respectively. Together with an intersectional approach, AAV-mediated anterograde transsynaptic tagging can categorize neurons by their inputs and molecular identity, and allow forward screening of distinct functional neural pathways embedded in complex brain circuits.

Secondary visual cortex is critical to the expression of surprise-induced enhancements in cue associability in rats

Considerable evidence indicates that reinforcement prediction error, the difference between the obtained and expected reinforcer values, modulates attention to potential cues for reinforcement. The surprising delivery or omission of a reinforcer enhances the associability of the stimuli that were present when the error was induced, so that they more readily enter into new associations in the future. Previous research from our laboratory identified brain circuit elements critical to the enhancement of stimulus associability by omission of an expected event and to the subsequent expression of that altered associability in more rapid learning. A key finding was that the rat posterior parietal cortex was essential during the encoding, consolidation and retrieval of associability memories that were altered by the surprising omission of an expected event in a serial prediction task. Here, we found that the function of adjacent secondary visual cortex was critical only to the expression of altered cue associability in that same task. This specialization of function is discussed in the context of broader cortical and subcortical networks for modulation of attention in associative learning, as well as recent anatomical investigations that suggest that the rodent posterior parietal cortex overlaps with and may subsume secondary visual cortex.

A quantitative comparison of the hemispheric, areal, and laminar origins of sensory and motor cortical projections to the superior colliculus of the cat

The superior colliculus (SC) is a midbrain structure central to orienting behaviors. The organization of descending projections from sensory cortices to the SC has garnered much attention; however, rarely have projections from multiple modalities been quantified and contrasted, allowing for meaningful conclusions within a single species. Here, we examine corticotectal projections from visual, auditory, somatosensory, motor, and limbic cortices via retrograde pathway tracers injected throughout the superficial and deep layers of the cat SC. As anticipated, the majority of cortical inputs to the SC originate in the visual cortex. In fact, each field implicated in visual orienting behavior makes a substantial projection. Conversely, only one area of the auditory orienting system, the auditory field of the anterior ectosylvian sulcus (fAES), and no area involved in somatosensory orienting, shows significant corticotectal inputs. Although small relative to visual inputs, the projection from the fAES is of particular interest, as it represents the only bilateral cortical input to the SC. This detailed, quantitative study allows for comparison across modalities in an animal that serves as a useful model for both auditory and visual perception. Moreover, the differences in patterns of corticotectal projections between modalities inform the ways in which orienting systems are modulated by cortical feedback. J. Comp. Neurol. 524:2623-2642, 2016. © 2016 Wiley Periodicals, Inc.

Warning! Dopaminergic Modulation of the Superior Colliculus

Recent work by Bolton et al. describes a dopaminergic input to the superior colliculus (SC) from the zona incerta, as well as the organization of D1- and D2-receptor expression in the SC. We discuss a potential role for this input in modulating SC-mediated behavior, particularly in response to aversive stimuli.

Optogenetic activation of superior colliculus neurons suppresses seizures originating in diverse brain networks

Because sites of seizure origin may be unknown or multifocal, identifying targets from which activation can suppress seizures originating in diverse networks is essential. We evaluated the ability of optogenetic activation of the deep/intermediate layers of the superior colliculus (DLSC) to fill this role. Optogenetic activation of DLSC suppressed behavioral and electrographic seizures in the pentylenetetrazole (forebrain+brainstem seizures) and Area Tempestas (forebrain/complex partial seizures) models; this effect was specific to activation of DLSC, and not neighboring structures. DLSC activation likewise attenuated seizures evoked by gamma butyrolactone (thalamocortical/absence seizures), or acoustic stimulation of genetically epilepsy prone rates (brainstem seizures). Anticonvulsant effects were seen with stimulation frequencies as low as 5 Hz. Unlike previous applications of optogenetics for the control of seizures, activation of DLSC exerted broad-spectrum anticonvulsant actions, attenuating seizures originating in diverse and distal brain networks. These data indicate that DLSC is a promising target for optogenetic control of epilepsy.

Dorsal raphe nucleus projecting retinal ganglion cells: Why Y cells?

Retinal ganglion Y (alpha) cells are found in retinas ranging from frogs to mice to primates. The highly conserved nature of the large, fast conducting retinal Y cell is a testament to its fundamental task, although precisely what this task is remained ill-defined. The recent discovery that Y-alpha retinal ganglion cells send axon collaterals to the serotonergic dorsal raphe nucleus (DRN) in addition to the lateral geniculate nucleus (LGN), medial interlaminar nucleus (MIN), pretectum and the superior colliculus (SC) has offered new insights into the important survival tasks performed by these cells with highly branched axons. We propose that in addition to its role in visual perception, the Y-alpha retinal ganglion cell provides concurrent signals via axon collaterals to the DRN, the major source of serotonergic afferents to the forebrain, to dramatically inhibit 5-HT activity during orientation or alerting/escape responses, which dis-facilitates ongoing tonic motor activity while dis-inhibiting sensory information processing throughout the visual system. The new data provide a fresh view of these evolutionarily old retinal ganglion cells.

Axon terminals from the nucleus isthmi pars parvocellularis control the ascending retinotectofugal output through direct synaptic contact with tectal ganglion cell dendrites

The optic tectum in birds and its homologue the superior colliculus in mammals both send major bilateral, nontopographic projections to the nucleus rotundus and caudal pulvinar, respectively. These projections originate from widefield tectal ganglion cells (TGCs) located in layer 13 in the avian tectum and in the lower superficial layers in the mammalian colliculus. The TGCs characteristically have monostratified arrays of brush-like dendritic terminations and respond mostly to bidimensional motion or looming features. In birds, this TGC-mediated tectofugal output is controlled by feedback signals from the nucleus isthmi pars parvocellularis (Ipc). The Ipc neurons display topographically organized axons that densely ramify in restricted columnar terminal fields overlapping various neural elements that could mediate this tectofugal control, including the retinal terminals and the TGC dendrites themselves. Whether the Ipc axons make synaptic contact with these or other tectal neural elements remains undetermined. We double labeled Ipc axons and their presumptive postsynaptic targets in the tectum of chickens (Gallus gallus) with neural tracers and performed an ultrastructural analysis. We found that the Ipc terminal boutons form glomerulus-like structures in the superficial and intermediate tectal layers, establishing asymmetric synapses with several dendritic profiles. In these glomeruli, at least two of the postsynaptic dendrites originated from TGCs. We also found synaptic contacts between retinal terminals and TGC dendrites. These findings suggest that, in birds, Ipc axons control the ascending tectal outflow of retinal signals through direct synaptic contacts with the TGCs.

Young Women do it Better: Sexual Dimorphism in Temporal Discrimination

The temporal discrimination threshold (TDT) is the shortest time interval at which two sensory stimuli presented sequentially are detected as asynchronous by the observer. TDTs are known to increase with age. Having previously observed shorter thresholds in young women than in men, in this work we sought to systematically examine the effect of sex and age on temporal discrimination. The aims of this study were to examine, in a large group of men and women aged 20–65 years, the distribution of TDTs with an analysis of the individual participant’s responses, assessing the “point of subjective equality” and the “just noticeable difference” (JND). These respectively assess sensitivity and accuracy of an individual’s response. In 175 participants (88 women) aged 20–65 years, temporal discrimination was faster in women than in men under the age of 40 years by a mean of approximately 13 ms. However, age-related decline in temporal discrimination was three times faster in women so that, in the age group of 40–65 years, the female superiority was reversed. The point of subjective equality showed a similar advantage in younger women and more marked age-related decline in women than men, as the TDT. JND values declined equally in both sexes, showing no sexual dimorphism. This observed sexual dimorphism in temporal discrimination is important for both (a) future clinical research assessing disordered mid-brain covert attention in basal-ganglia disorders, and (b) understanding the biology of this sexual dimorphism which may be genetic or hormonal.

Connections of the juxtaventromedial region of the lateral hypothalamic area in the male rat

Evolutionary conservation of the hypothalamus attests to its critical role in the control of fundamental behaviors. However, our knowledge of hypothalamic connections is incomplete, particularly for the lateral hypothalamic area (LHA). Here we present the results of neuronal pathway-tracing experiments to investigate connections of the LHA juxtaventromedial region, which is parceled into dorsal (LHAjvd) and ventral (LHAjvv) zones. Phaseolus vulgaris leucoagglutinin (PHAL, for outputs) and cholera toxin B subunit (CTB, for inputs) coinjections were targeted stereotaxically to the LHAjvd/v.

Results: LHAjvd/v connections overlapped highly but not uniformly. Major joint outputs included: Bed nuc. stria terminalis (BST), interfascicular nuc. (BSTif) and BST anteromedial area, rostral lateral septal (LSr)- and ventromedial hypothalamic (VMH) nuc., and periaqueductal gray. Prominent joint LHAjvd/v input sources included: BSTif, BST principal nuc., LSr, VMH, anterior hypothalamic-, ventral premammillary-, and medial amygdalar nuc., and hippocampal formation (HPF) field CA1. However, LHAjvd HPF retrograde labeling was markedly more abundant than from the LHAjvv; in the LSr this was reversed. Furthermore, robust LHAjvv (but not LHAjvd) targets included posterior- and basomedial amygdalar nuc., whereas the midbrain reticular nuc. received a dense input from the LHAjvd alone. Our analyses indicate the existence of about 500 LHAjvd and LHAjvv connections with about 200 distinct regions of the cerebral cortex, cerebral nuclei, and cerebrospinal trunk. Several highly LHAjvd/v-connected regions have a prominent role in reproductive behavior. These findings contrast with those from our previous pathway-tracing studies of other LHA medial and perifornical tier regions, with different connectional behavioral relations. The emerging picture is of a highly differentiated LHA with extensive and far-reaching connections that point to a role as a central coordinator of behavioral control.

Sensory Cortical Control of a Visually Induced Arrest Behavior via Corticotectal Projections

Innate defense behaviors (IDBs) evoked by threatening sensory stimuli are essential for animal survival. Although subcortical circuits are implicated in IDBs, it remains largely unclear whether sensory cortex modulates IDBs and what the underlying neural pathways are. Here, we show that optogenetic silencing of corticotectal projections from layer 5 (L5) of the mouse primary visual cortex (V1) to the superior colliculus (SC) significantly reduces an SC-dependent innate behavior (i.e., temporary suspension of locomotion upon a sudden flash of light as short as milliseconds). Surprisingly, optogenetic activation of SC-projecting neurons in V1 or their axon terminals in SC sufficiently elicits the behavior, in contrast to other major L5 corticofugal projections. Thus, via the same corticofugal projection, visual cortex not only modulates the light-induced arrest behavior, but also can directly drive the behavior. Our results suggest that sensory cortex may play a previously unrecognized role in the top-down initiation of sensory-motor behaviors.