Identity of a pathway for saccadic suppression

Pathways for Naturalistic Looking Behavior in Primate II. Superior Colliculus Integrates Parallel Top-down and Bottom-up Inputs

Volitional signals for gaze control are provided by multiple parallel pathways converging on the midbrain superior colliculus (SC), whose deeper layers output to the brainstem gaze circuits. In the first of two papers (Takahashi and Veale, 2023), we described the properties of gaze behavior of several species under both laboratory and natural conditions, as well as the current understanding of the brainstem and spinal cord circuits implementing gaze control in primate. In this paper, we review the parallel pathways by which sensory and task information reaches SC and how these sensory and task signals interact within SC's multilayered structure. This includes both bottom-up (world statistics) signals mediated by sensory cortex, association cortex, and subcortical structures, as well as top-down (goal and task) influences which arrive via either direct excitatory pathways from cerebral cortex, or via indirect basal ganglia relays resulting in inhibition or dis-inhibition as appropriate for alternative behaviors. Models of attention such as saliency maps serve as convenient frameworks to organize our understanding of both the separate computations of each neural pathway, as well as the interaction between the multiple parallel pathways influencing gaze. While the spatial interactions between gaze's neural pathways are relatively well understood, the temporal interactions between and within pathways will be an important area of future study, requiring both improved technical methods for measurement and improvement of our understanding of how temporal dynamics results in the observed spatiotemporal allocation of gaze.

Pathways for Naturalistic Looking Behavior in Primate I: Behavioral Characteristics and Brainstem Circuits

Organisms control their visual worlds by moving their eyes, heads, and bodies. This control of "gaze" or "looking" is key to survival and intelligence, but our investigation of the underlying neural mechanisms in natural conditions is hindered by technical limitations. Recent advances have enabled measurement of both brain and behavior in freely moving animals in complex environments, expanding on historical head-fixed laboratory investigations. We juxtapose looking behavior as traditionally measured in the laboratory against looking behavior in naturalistic conditions, finding that behavior changes when animals are free to move or when stimuli have depth or sound. We specifically focus on the brainstem circuits driving gaze shifts and gaze stabilization. The overarching goal of this review is to reconcile historical understanding of the differential neural circuits for different "classes" of gaze shift with two inconvenient truths. (1) "classes" of gaze behavior are artificial. (2) The neural circuits historically identified to control each "class" of behavior do not operate in isolation during natural behavior. Instead, multiple pathways combine adaptively and non-linearly depending on individual experience. While the neural circuits for reflexive and voluntary gaze behaviors traverse somewhat independent brainstem and spinal cord circuits, both can be modulated by feedback, meaning that most gaze behaviors are learned rather than hardcoded. Despite this flexibility, there are broadly enumerable neural pathways commonly adopted among primate gaze systems. Parallel pathways which carry simultaneous evolutionary and homeostatic drives converge in superior colliculus, a layered midbrain structure which integrates and relays these volitional signals to brainstem gaze-control circuits.

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.

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.

Input from torus longitudinalis drives binocularity and spatial summation in zebrafish optic tectum

Background

A continued effort in neuroscience aims to understand the way brain circuits consisting of diverse neuronal types generate complex behavior following sensory input. A common feature of vertebrate visual systems is that lower-order and higher-order visual areas are reciprocally connected. Feedforward projections confer visual responsiveness to higher-order visual neurons while feedback projections likely serve to modulate responses of lower-order visual neurons in a context-dependent manner. Optic tectum is the largest first-order visual brain area in zebrafish and is reciprocally connected with the torus longitudinalis (TL), a second-order visual brain area that does not receive retinal input. A functional role for feedback projections from TL to tectum has not been identified. Here we aim to understand how this feedback contributes to visual processing.

Results

In this study, we demonstrate that TL feedback projections to tectum drive binocular integration and spatial summation in a defined tectal circuit. We performed genetically targeted, cell type-specific functional imaging in tectal pyramidal neurons (PyrNs) and their two input neuron populations: retinal ganglion cells (RGCs) and neurons in TL. We find that PyrNs encode gradual changes in scene luminance using a complement of three distinct response classes that encode different light intensity ranges. Functional imaging of RGC inputs to tectum suggest that these response classes originate in the retina and RGC input specifies PyrN functional classes. In contrast, TL input serves to endow PyrNs with large, compound receptive fields that span both retinal hemifields.

Conclusions

These findings reveal a novel role for the zebrafish TL in driving binocular integration and spatial summation in tectal PyrNs. The neural circuit we describe generates a population of tectal neurons with large receptive fields tailored for detecting changes in the visual scene.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01222-x.

Mechanisms of Plasticity in Subcortical Visual Areas

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

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.

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.

Dissociable Cortical and Subcortical Mechanisms for Mediating the Influences of Visual Cues on Microsaccadic Eye Movements

Visual selection in primates is intricately linked to eye movements, which are generated by a network of cortical and subcortical neural circuits. When visual selection is performed covertly, without foveating eye movements toward the selected targets, a class of fixational eye movements, called microsaccades, is still involved. Microsaccades are small saccades that occur when maintaining precise gaze fixation on a stationary point, and they exhibit robust modulations in peripheral cueing paradigms used to investigate covert visual selection mechanisms. These modulations consist of changes in both microsaccade directions and frequencies after cue onsets. Over the past two decades, the properties and functional implications of these modulations have been heavily studied, revealing a potentially important role for microsaccades in mediating covert visual selection effects. However, the neural mechanisms underlying cueing effects on microsaccades are only beginning to be investigated. Here we review the available causal manipulation evidence for these effects' cortical and subcortical substrates. In the superior colliculus (SC), activity representing peripheral visual cues strongly influences microsaccade direction, but not frequency, modulations. In the cortical frontal eye fields (FEF), activity only compensates for early reflexive effects of cues on microsaccades. Using evidence from behavior, theoretical modeling, and preliminary lesion data from the primary visual cortex and microstimulation data from the lower brainstem, we argue that the early reflexive microsaccade effects arise subcortically, downstream of the SC. Overall, studying cueing effects on microsaccades in primates represents an important opportunity to link perception, cognition, and action through unaddressed cortical-subcortical neural interactions. These interactions are also likely relevant in other sensory and motor modalities during other active behaviors.

Active vision at the foveal scale in the primate superior colliculus

The primate superior colliculus (SC) has recently been shown to possess both a large foveal representation as well as a varied visual processing repertoire. This structure is also known to contribute to eye movement generation. Here, we describe our current understanding of how SC visual and movement-related signals interact within the realm of small eye movements associated with the foveal scale of visuomotor behavior. Within the SC's foveal representation, there is a full spectrum of visual, visual-motor, and motor-related discharge for fixational eye movements. Moreover, a substantial number of neurons only emit movement-related discharge when microsaccades are visually guided, but not when similar movements are generated toward a blank. This represents a particularly striking example of integrating vision and action at the foveal scale. Beyond that, SC visual responses themselves are strongly modulated, and in multiple ways, by the occurrence of small eye movements. Intriguingly, this impact can extend to eccentricities well beyond the fovea, causing both sensitivity enhancement and suppression in the periphery. Because of large foveal magnification of neural tissue, such long-range eccentricity effects are neurally warped into smaller differences in anatomical space, providing a structural means for linking peripheral and foveal visual modulations around fixational eye movements. Finally, even the retinal-image visual flows associated with tiny fixational eye movements are signaled fairly faithfully by peripheral SC neurons with relatively large receptive fields. These results demonstrate how studying active vision at the foveal scale represents an opportunity for understanding primate vision during natural behaviors involving ever-present foveating eye movements.NEW & NOTEWORTHY The primate superior colliculus (SC) is ideally suited for active vision at the foveal scale: it enables detailed foveal visual analysis by accurately driving small eye movements, and it also possesses a visual processing machinery that is sensitive to active eye movement behavior. Studying active vision at the foveal scale in the primate SC is informative for broader aspects of active perception, including the overt and covert processing of peripheral extra-foveal visual scene locations.

Cell type- and layer-specific convergence in core and shell neurons of the dorsal lateral geniculate nucleus

Over 40 distinct types of retinal ganglion cells (RGCs) generate parallel processing pathways in the visual system. In mice, two subdivisions of the dorsal lateral geniculate nucleus (dLGN), the core and the shell, organize distinct parallel channels to transmit visual information from the retina to the primary visual cortex (V1). To investigate how the dLGN core and shell differentially integrate visual information and other modalities, we mapped synaptic input sources to each dLGN subdivision at the cell-type level with G-deleted rabies viral vectors. The monosynaptic circuit tracing revealed that dLGN core neurons received inputs from alpha-RGCs, Layer 6 neurons of the V1, the superficial and intermediate layers of the superior colliculus (SC), the internal ventral LGN, the lower layer of the external ventral LGN (vLGNe), the intergeniculate leaf, the thalamic reticular nucleus (TRN), and the pretectal nucleus (PT). Conversely, shell neurons received inputs from alpha-RGCs and direction-selective ganglion cells of the retina, Layer 6 neurons of the V1, the superficial layer of the SC, the superficial and lower layers of the vLGNe, the TRN, the PT, and the parabigeminal nucleus. The present study provides anatomical evidence of the cell type- and layer-specific convergence in dLGN core and shell neurons. These findings suggest that dLGN core neurons integrate and process more multimodal information along with visual information than shell neurons and that LGN core and shell neurons integrate different types of information, send their own convergent information to discrete populations of the V1, and differentially contribute to visual perception and behavior.

Corollary Discharge Contributions to Perceptual Continuity Across Saccades

Our vision depends upon shifting our high-resolution fovea to objects of interest in the visual field. Each saccade displaces the image on the retina, which should produce a chaotic scene with jerks occurring several times per second. It does not. This review examines how an internal signal in the primate brain (a corollary discharge) contributes to visual continuity across saccades. The article begins with a review of evidence for a corollary discharge in the monkey and evidence from inactivation experiments that it contributes to perception. The next section examines a specific neuronal mechanism for visual continuity, based on corollary discharge that is referred to as visual remapping. Both the basic characteristics of this anticipatory remapping and the factors that control it are enumerated. The last section considers hypotheses relating remapping to the perceived visual continuity across saccades, including remapping's contribution to perceived visual stability across saccades.

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.

Parvalbumin and GABA Microcircuits in the Mouse Superior Colliculus

The mammalian superior colliculus (SC) is a sensorimotor midbrain structure responsible for orienting behaviors. Although many SC features are known, details of its intrinsic microcircuits are lacking. We used transgenic mice expressing reporter genes in parvalbumin-positive (PV⁺) and gamma aminobutyric acid-positive (GABA⁺) neurons to test the hypothesis that PV⁺ neurons co-localize GABA and form inhibitory circuits within the SC. We found more PV⁺ neurons in the superficial compared to the intermediate SC, although a larger percentage of PV⁺ neurons co-expressed GABA in the latter. Unlike PV⁺ neurons, PV⁺/GABA⁺ neurons showed predominantly rapidly inactivating spiking patterns. Optogenetic activation of PV⁺ neurons revealed direct and feedforward GABAergic inhibitory synaptic responses, as well as excitatory glutamatergic synapses. We propose that PV⁺ neurons in the SC may be specialized for a variety of circuit functions within the SC rather than forming a homogeneous, GABAergic neuronal subtype as they appear to in other regions of the brain.

Circuits for Action and Cognition: A View from the Superior Colliculus

The superior colliculus is one of the most well-studied structures in the brain, and with each new report, its proposed role in behavior seems to increase in complexity. Forty years of evidence show that the colliculus is critical for reorienting an organism toward objects of interest. In monkeys, this involves saccadic eye movements. Recent work in the monkey colliculus and in the homologous optic tectum of the bird extends our understanding of the role of the colliculus in higher mental functions, such as attention and decision making. In this review, we highlight some of these recent results, as well as those capitalizing on circuit-based methodologies using transgenic mice models, to understand the contribution of the colliculus to attention and decision making. The wealth of information we have about the colliculus, together with new tools, provides a unique opportunity to obtain a detailed accounting of the neurons, circuits, and computations that underlie complex behavior.

A neural locus for spatial-frequency specific saccadic suppression in visual-motor neurons of the primate superior colliculus

Saccades cause rapid retinal-image shifts that go perceptually unnoticed several times per second. The mechanisms for saccadic suppression have been controversial, in part because of sparse understanding of neural substrates. In this study we uncovered an unexpectedly specific neural locus for spatial frequency-specific saccadic suppression in the superior colliculus (SC). We first developed a sensitive behavioral measure of suppression in two macaque monkeys, demonstrating selectivity to low spatial frequencies similar to that observed in earlier behavioral studies. We then investigated visual responses in either purely visual SC neurons or anatomically deeper visual motor neurons, which are also involved in saccade generation commands. Surprisingly, visual motor neurons showed the strongest visual suppression, and the suppression was dependent on spatial frequency, as in behavior. Most importantly, suppression selectivity for spatial frequency in visual motor neurons was highly predictive of behavioral suppression effects in each individual animal, with our recorded population explaining up to ~74% of behavioral variance even on completely different experimental sessions. Visual SC neurons had mild suppression, which was unselective for spatial frequency and thus only explained up to ~48% of behavioral variance. In terms of spatial frequency-specific saccadic suppression, our results run contrary to predictions that may be associated with a hypothesized SC saccadic suppression mechanism, in which a motor command in the visual motor and motor neurons is first relayed to the more superficial purely visual neurons, to suppress them and to then potentially be fed back to cortex. Instead, an extraretinal modulatory signal mediating spatial-frequency-specific suppression may already be established in visual motor neurons.NEW & NOTEWORTHY Saccades, which repeatedly realign the line of sight, introduce spurious signals in retinal images that normally go unnoticed. In part, this happens because of perisaccadic suppression of visual sensitivity, which is known to depend on spatial frequency. We discovered that a specific subtype of superior colliculus (SC) neurons demonstrates spatial-frequency-dependent suppression. Curiously, it is the neurons that help mediate the saccadic command itself that exhibit such suppression, and not the purely visual ones.

A circuit for saccadic suppression in the primate brain

Saccades should cause us to see a blur as the eyes sweep across a visual scene. Specific brain mechanisms prevent this by producing suppression during saccades. Neuronal correlates of such suppression were first established in the visual superficial layers of the superior colliculus (SC) and subsequently have been observed in cortical visual areas, including the middle temporal visual area (MT). In this study, we investigated suppression in a recently identified circuit linking visual SC (SCs) to MT through the inferior pulvinar (PI). We examined responses to visual stimuli presented just before saccades to reveal a neuronal correlate of suppression driven by a copy of the saccade command, referred to as a corollary discharge. We found that visual responses were similarly suppressed in SCs, PI, and MT. Within each region, suppression of visual responses occurred with saccades into both visual hemifields, but only in the contralateral hemifield did this suppression consistently begin before the saccade (~100 ms). The consistency of the signal along the circuit led us to hypothesize that the suppression in MT was influenced by input from the SC. We tested this hypothesis in one monkey by inactivating neurons within the SC and found evidence that suppression in MT depends on corollary discharge signals from motor SC (SCi). Combining these results with recent findings in rodents, we propose a complete circuit originating with corollary discharge signals in SCi that produces suppression in visual SCs, PI, and ultimately, MT cortex.NEW & NOTEWORTHY A fundamental puzzle in visual neuroscience is that we frequently make rapid eye movements (saccades) but seldom perceive the visual blur accompanying each movement. We investigated neuronal correlates of this saccadic suppression by recording from and perturbing a recently identified circuit from brainstem to cortex. We found suppression at each stage, with evidence that it was driven by an internally generated signal. We conclude that this circuit contributes to neuronal suppression of visual signals during eye movements.

Imaging population dynamics of surround suppression in the superior colliculus

The superior colliculus (SC) plays a key role in controlling spatial attention. It is hypothesized that some forms of spatial attention, such as the detection of a single salient object arise from lateral competitive interactions between different locations within the spatial map in the SC. This hypothesis is supported by a recent in vitro study showing that a 'Mexican hat'-like pattern of synaptic connectivity is implemented in the intrinsic circuit of the superficial layer of the SC (sSC). However, the neuronal population mechanisms responsible for this pattern still remain unclear. Here, we examined how spatial response modulations, for example lateral interactions and surround suppression, are represented at the neuronal population level using in vivo two-photon calcium imaging in the mouse sSC. Observation of neuronal population responses with single-cell resolution enabled us to identify a small subset of neurons that were activated by relatively small visual stimuli (< 1° diameter), and thus allowed us to detect the exact location of the 'response center' in the sSC to a visual stimulus presented at a given location. We demonstrated that presenting two-point stimuli or one large stimulus modulated the spatial response pattern of the neuronal population, i.e. centre facilitation and surround suppression. Furthermore, we found that both GABAergic and non-GABAergic neurons showed a similar population response pattern of surround suppression. The population dynamics suggest the circuit mechanism underlying lateral inhibition and surround suppression may be supported by long-range inhibitory neurons in the sSC.

A hard-wired priority map in the superior colliculus shaped by asymmetric inhibitory circuitry

The mammalian superior colliculus (SC) is a laminar midbrain structure that translates visual signals into commands to shift the focus of attention and gaze. The SC plays an integral role in selecting targets and ultimately generating rapid eye movements to those targets. In all mammals studied to date, neurons in the SC are arranged topographically such that the location of visual stimuli and the endpoints of orienting movements form organized maps in superficial and deeper layers, respectively. The organization of these maps is thought to underlie attentional priority by assessing which regions of the visual field contain behaviorally relevant information. Using voltage imaging and patch-clamp recordings in parasagittal SC slices from the rat, we found the synaptic circuitry of the visuosensory map in the SC imposes a strong bias. Voltage imaging of responses to electrical stimulation revealed more spread in the caudal direction than the rostral direction. Pharmacological experiments demonstrated that this asymmetry arises from GABAA receptor activation rostral to the site of stimulation. Patch-clamp recordings confirmed this rostrally directed inhibitory circuit and showed that it is contained within the visuosensory layers of the SC. Stimulation of two sites showed that initial stimulation of a caudal site can take priority over subsequent stimulation of a rostral site. Taken together, our data indicate that the circuitry of the visuosensory SC is hard-wired to give higher priority to more peripheral targets, and this property is conferred by a uniquely structured, dedicated inhibitory circuit.

Spatio-temporal response properties of local field potentials in the primate superior colliculus

Local field potentials (LFPs) are becoming increasingly popular in neurophysiological studies. However, to date, most of the knowledge about LFPs has been obtained from cortical recordings. Here, we recorded single unit activity (SUA) and LFPs simultaneously from the superior colliculus (SC) of behaving rhesus monkeys. The SC is a midbrain structure that plays a central role in the visual orienting response. Previous studies have characterised the visual and visuomotor response properties of SUA in the superficial layers of the SC and the intermediate layers of the SC, respectively. We found that the signal properties of SUA were well preserved in the LFPs recorded from the SC. The SUA and LFPs had similar spatial and temporal properties, and the response properties of LFPs differed across layers, i.e. purely visual in the superficial layers of the SC but showing significant motor responses in the intermediate layers of the SC. There were also differences between SUA and LFPs. LFPs showed a significant reversal of activity following the phasic visual response, suggesting that the neighboring neurons were suppressed. The results indicate that the LFP can be used as a reliable measure of the SC activity in lieu of SUA, and open up a new way to assess sensorimotor processing within the SC.

Excitatory synaptic feedback from the motor layer to the sensory layers of the superior colliculus

Neural circuits that translate sensory information into motor commands are organized in a feedforward manner converting sensory information into motor output. The superior colliculus (SC) follows this pattern as it plays a role in converting visual information from the retina and visual cortex into motor commands for rapid eye movements (saccades). Feedback from movement to sensory regions is hypothesized to play critical roles in attention, visual image stability, and saccadic suppression, but in contrast to feedforward pathways, motor feedback to sensory regions has received much less attention. The present study used voltage imaging and patch-clamp recording in slices of rat SC to test the hypothesis of an excitatory synaptic pathway from the motor layers of the SC back to the sensory superficial layers. Voltage imaging revealed an extensive depolarization of the superficial layers evoked by electrical stimulation of the motor layers. A pharmacologically isolated excitatory synaptic potential in the superficial layers depended on stimulus strength in the motor layers in a manner consistent with orthodromic excitation. Patch-clamp recording from neurons in the sensory layers revealed excitatory synaptic potentials in response to glutamate application in the motor layers. The location, size, and morphology of responsive neurons indicated they were likely to be narrow-field vertical cells. This excitatory projection from motor to sensory layers adds an important element to the circuitry of the SC and reveals a novel feedback pathway that could play a role in enhancing sensory responses to attended targets as well as visual image stabilization.

Distinct local circuit properties of the superficial and intermediate layers of the rodent superior colliculus

The superior colliculus (SC) is critical in localizing salient visual stimuli and making decisions on the location of the next saccade. Lateral interactions across the spatial map of the SC are hypothesized to help mediate these processes. Here, we investigate lateral interactions within the SC by applying whole-cell recordings in horizontal slices of mouse SC, which maintained the local structure of the superficial (SCs) visual layer, which is hypothesized to participate in localizing salient stimuli, and the intermediate (SCi) layer, which is supposed to participate in saccade decision-making. When effects of either electrical or chemical (uncaging of free glutamate) stimuli were applied to multiple sites with various distances from the recorded cell, a pattern of center excitation-surround inhibition was found to be prominent in SCs. When the interactions of synaptic effects induced by simultaneous stimulation of two sites were tested, non-linear facilitatory or inhibitory interactions were observed. In contrast, in the SCi, stimulation induced mainly excitation, which masked underlying inhibition. The excitatory synaptic effects of stimulation applied at remote sites were summed in a near linear manner. The result suggested that SCs lateral interactions appear suitable for localizing salient stimuli, while the lateral interactions within SCi are more suitable for faithfully accumulating subthreshold signals for saccadic decision-making. Implementation of this laminar-specific organization makes the SC a unique structure for serially processing signals for saliency localization and saccadic decision-making.

Development of a neural circuit in the superior colliculus: analysis of the propagation of neuronal excitation from intermediate to superficial layers

The superior colliculus is important for orientation behaviors, in which visuomotor transformation is performed by the pathway from the superficial layer (SGS) to the intermediate layers (SGI). The opposite pathway (from the SGI to the SGS) also exists, raising the possibility of a feedback circuit, although it could be either negative (inhibitory) or positive (excitatory). In this study, we focused on the development of the feedback circuit. We used optical imaging methods that can measure neuronal population responses directly, as the orientation behaviors are determined by large population activities of superior colliculus neurons. We examined the postnatal development of the propagation pattern of neuronal excitation from the SGI to the SGS using a GABAA receptor antagonist. The optical response propagated within the SGI, but not to the SGS in infant mice that have not opened their eyes. In contrast, in young mice after eye opening, the optical response propagated initially in the SGI and then to the SGS. The GABAA receptor antagonist increased the optical response in the SGS in young mice, as well as that in the SGI in infant mice. Together, these results suggest that axons of SGI neurons terminate to the SGS during development after eye opening.

Spatiotemporal structure of visual receptive fields in macaque superior colliculus

Saccades are useful for directing the high-acuity fovea to visual targets that are of behavioral relevance. The selection of visual targets for eye movements involves the superior colliculus (SC), where many neurons respond to visual stimuli. Many of these neurons are also activated before and during saccades of specific directions and amplitudes. Although the role of the SC in controlling eye movements has been thoroughly examined, far less is known about the nature of the visual responses in this area. We have, therefore, recorded from neurons in the intermediate layers of the macaque SC, while using a sparse-noise mapping procedure to obtain a detailed characterization of the spatiotemporal structure of visual receptive fields. We find that SC responses to flashed visual stimuli start roughly 50 ms after the onset of the stimulus and last for on average ∼70 ms. About 50% of these neurons are strongly suppressed by visual stimuli flashed at certain locations flanking the excitatory center, and the spatiotemporal pattern of suppression exerts a predictable influence on the timing of saccades. This suppression may, therefore, contribute to the filtering of distractor stimuli during target selection. We also find that saccades affect the processing of visual stimuli by SC neurons in a manner that is quite similar to the saccadic suppression and postsaccadic enhancement that has been observed in the cortex and in perception. However, in contrast to what has been observed in the cortex, decreased visual sensitivity was generally associated with increased firing rates, while increased sensitivity was associated with decreased firing rates. Overall, these results suggest that the processing of visual stimuli by SC receptive fields can influence oculomotor behavior and that oculomotor signals originating in the SC can shape perisaccadic visual perception.

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

The connections of the lateral hypothalamic area juxtadorsomedial region (LHAjd) were investigated in a series of pathway-tracing experiments involving iontophoretic co-injection of the tracers Phaseolus vulgaris-leucoagglutinin (PHA-L; for outputs) and cholera toxin B subunit (CTB; for inputs). Results revealed that the LHAjd has connections with some 318 distinct gray matter regions encompassing all four subsystems-motor, sensory, cognitive, and behavioral state-included in a basic structure-function network model of the nervous system. Integration of these subsystems is necessary for the coordination and control of emotion and behavior, and in that regard the connections of the LHAjd indicate that it may have a prominent role. Furthermore, the LHAjd connections, together with the connections of other LHA differentiations studied similarly to date, indicate a distinct topographic organization that suggests each LHA differentiation has specifically differing degrees of involvement in the control of multiple behaviors. For the LHAjd, its involvement to a high degree in the control of defensive behavior, and to a lesser degree in the control of other behaviors, including ingestive and reproductive, is suggested. Moreover, the connections of the LHAjd suggest that its possible role in the control of these behaviors may be very broad in scope because they involve the somatic, neuroendocrine, and autonomic divisions of the nervous system. In addition, we suggest that connections between LHA differentiations may provide, at the level of the hypothalamus, a neuronal substrate for the coordinated control of multiple themes in the behavioral repertoire.

Cholinergic responses in GABAergic and non-GABAergic neurons in the intermediate gray layer of mouse superior colliculus

Neurons in the intermediate gray layer (SGI) of the mammalian superior colliculus (SC) receive dense cholinergic innervations from the brainstem parabrachial region. Such cholinergic inputs may influence execution of orienting behaviors. To obtain deeper insights into how the cholinergic inputs modulate the SC local circuits, we analysed the cholinergic responses in identified γ-aminobutyric acid (GABA)ergic and non-GABAergic neurons using SC slices obtained from GAD67-GFP knock-in mice. The responses of SGI neurons to cholinergic agonists were various combinations of fast inward currents mediated mainly via α4β2 and partly by α7 nicotinic receptors (nIN), slow inward currents caused by activation of M1 plus M3 muscarinic receptors (mIN), and slow outward currents caused by activation of M2 muscarinic receptors (mOUT). The most common cholinergic responses in non-GABAergic neurons was nIN + mIN + mOUT (38/68), followed by nIN + mIN (16/68), nIN + mOUT (11/68), nIN only (2/68), and no response (1/68). On the other hand, the major response pattern in GABAergic neurons was either nIN only (26/54) or nIN + mIN (21/54), followed by nIN + mOUT (4/54), mOUT only (2/54), and no response (1/54). Thus, major effects of cholinergic inputs to both SGI GABAergic and non-GABAergic neurons are excitatory, but the response patterns in these two types of SGI neurons are different. Thus, actions of the cholinergic inputs to non-GABAergic and GABAergic SGI neurons are not simple push-pull mechanisms, like excitation vs inhibition, but might cooperate to balance the level of excitation and inhibition for setting the state of the response property of the local circuit.

Circuit dynamics of the superior colliculus revealed by in vitro voltage imaging

The superior colliculus (SC) is well known for its involvement in the conversion of sensory stimuli into motor commands. This sensorimotor integration is made possible by the collective activity of multiple neuronal connections throughout the SC. Still, the majority of SC research focuses on in vivo extracellular recordings of behaving monkeys or in vitro patch-clamp recordings from lower mammals. Here, we discuss the results of an in vitro voltage-imaging technique in which population activity across the rodent SC circuitry was visualized to bridge the gap between single-cell recordings and whole-animal behavior. The high temporal and spatial resolution of the voltage-imaging technique allowed us to visualize patterns of activity following stimulation at discrete laminae. Stimulation within either the superficial or intermediate layer showed recruitment of disparate SC circuitry. These results provide insight into the circuit dynamics and neuronal populations that underlie behavior.

A circuit model for saccadic suppression in the superior colliculus

Attenuation of visual activity in the superficial layers (SLs), stratum griseum superficiale and stratum opticum, of the superior colliculus during saccades may contribute to reducing perceptual blur during saccades and also may help prevent subsequent unwanted saccades. GABAergic neurons in the intermediate, premotor, layer (SGI), stratum griseum intermedium, send an inhibitory input to SL. This pathway provided the basis for a model proposing that the SGI premotor cells that project to brainstem gaze centers and discharge before saccades also activate neighboring GABAergic neurons that suppress saccade-induced visual activity in SL. The in vitro method allowed us to test this model. We made whole-cell patch-clamp recordings in collicular slices from either rats or GAD67-GFP knock-in mice, in which GABAergic neurons could be identified by their expression of green fluorescence protein (GFP). Antidromic electrical stimulation of SGI premotor cells was produced by applying pulse currents in which their axons congregate after exiting the superior colliculus. The stimulation evoked monosynaptic EPSCs in SGI GABAergic neurons that project to SL, as would be predicted if these neurons receive excitatory input from the premotor cells. Second, IPSCs were evoked in SL neurons, some of which project to the visual thalamus. These IPSCs were polysynaptically mediated by the GABAergic neurons that were excited by the antidromically activated SGI neurons. These results support the hypothesis that collaterals of premotor neuron axons excite GABAergic neurons that inhibit SL visuosensory cells.

Signals conveyed in the pulvinar pathway from superior colliculus to cortical area MT

We previously established a functional pathway extending from the superficial layers of the superior colliculus (SC) through the inferior pulvinar (PI) to cortical area MT in the primate (Macaca mulatta). Here, we characterized the signals that this pathway conveys to cortex by recording from pulvinar neurons that we identified by microstimulation as receiving input from SC and/or projecting to MT. The basic properties of these ascending-path PI neurons resembled those of SC visual neurons. Namely, they had brisk responses to spots of light, inhibitory surrounds, and relatively large receptive fields that increased with eccentricity, as well as minimal presaccadic activity. Beyond these basic properties, there were two salient results regarding the modulatory and motion signals conveyed by this ascending pathway. First, the PI neurons appeared to convey only a subset of the modulations found in the SC: they exhibited saccadic suppression, the inhibition of activity at the time of the saccade, but did not clearly show the attentional enhancement of the visual response seen in SC. Second, directional selectivity was minimal in PI neurons belonging to the ascending path but was significantly more prominent in PI neurons receiving input from MT. This finding casts doubt on earlier assumptions that PI provides directionally selective signals to MT and instead suggests that PI derives its selectivity from MT. The identification of this pathway and its transmitted activity establishes the first functional pathway from brainstem to cortex through pulvinar and makes it possible to examine its contribution to cortical visual processing, perception, and action.

GABAergic and glutamatergic identities of developing midbrain Pitx2 neurons

Pitx2, a paired-like homeodomain transcription factor, is expressed in post-mitotic neurons within highly restricted domains of the embryonic mouse brain. Previous reports identified critical roles for PITX2 in histogenesis of the hypothalamus and midbrain, but the cellular identities of PITX2-positive neurons in these regions were not fully explored. This study characterizes Pitx2 expression with respect to midbrain transcription factor and neurotransmitter phenotypes in mid-to-late mouse gestation. In the dorsal midbrain, we identified Pitx2-positive neurons in the stratum griseum intermedium (SGI) as GABAergic and observed a requirement for PITX2 in GABAergic differentiation. We also identified two Pitx2-positive neuronal populations in the ventral midbrain, the red nucleus, and a ventromedial population, both of which contain glutamatergic precursors. Our data suggest that PITX2 is present in regionally restricted subpopulations of midbrain neurons and may have unique functions that promote GABAergic and glutamatergic differentiation.

Visual Suppression in the Superior Colliculus Around the Time of Microsaccades

Miniature eye movements jitter the retinal image unceasingly, raising the question of how perceptual continuity is achieved during visual fixation. Recent work discovered suppression of visual bursts in the superior colliculus around the time of microsaccades, tiny jerks of the eyes that support visual perception while gaze is fixed. This finding suggests that corollary discharge, supporting visual stability when rapid eye movements drastically shift the retinal image, may also exist for the smallest saccades.

Organization of GABAergic inhibition in the motor output layer of the superior colliculus

The direction and amplitude of saccadic eye movements are determined by the location of the center of gravity of burst activity over a neuronal population on the spatial map of the intermediate gray layer (SGI) of the superior colliculus (SC). GABAergic interneurons might play critical roles in shaping the activation field on the topographical map but, to understand the mechanism, basic information on the organization of inhibitory circuits is essential. In the present study, we investigated the electrophysiological and morphological properties of GABAergic neurons in SGI by whole-cell patch-clamp recordings and intracellular staining using biocytin in GAD67-GFP knock-in mice (PND17-22), in which GABAergic neurons specifically express GFP fluorescence. The most common firing properties among these GABAergic neurons (n=231) were fast spiking (58%), followed by burst spiking (29%), late spiking (8%) and, the least common, regular spiking (2%) and rapid spike inactivation (3%). Morphological analysis of axonal trajectories of intracellularly-labeled GABAergic neurons revealed three major subclasses: (i) intralaminar interneurons, which were further divided into two subclasses, local and horizontal interneurons; (ii) interlaminar interneurons; and (iii) commissural and tectofugal neurons. These results reveal distinct subsets of GABAergic neurons including neurons that mediate local and long-range inhibition in the SC, neurons that potentially modulate visual and other sensory inputs to the SC, and neurons that project to nuclei outside the SC.

Topographic organization of excitatory and inhibitory commissural connections in the superior colliculi and their functional roles in saccade generation

Our electrophysiological study showed that there are topographic connections between excitatory and inhibitory commissural neurons (CNs) in one superior colliculus (SC) and tectoreticular neurons (TRNs) in the opposite SC. To obtain morphological evidence for these topographic commissural connections between the SCs, tracers were injected into various parts of the SC, the inhibitory burst neuron (IBN) area and Forel's field H (FFH), in the cat. Retrogradely labeled CNs were classified into three types according to their somatic areas and identified as GABA-positive or -negative immunohistochemically. Caudal SC injections labeled small GABA-positive CNs (<200 μm(2)) in the deep layers of the opposite rostral SC. Rostral SC injections mainly labeled medium-sized GABA-negative CNs (200-700 μm(2)) in the upper intermediate layer of the opposite rostral SC and small GABA-positive CNs in its deeper layers. Lateral SC injections labeled small GABA-positive CNs in the opposite medial SC and mainly medium-sized GABA-negative CNs in its lateral part. Medial SC injections labeled small GABA-positive CNs in the lateral SC and medium-sized GABA-negative CNs in the medial SC. In comparison, TRNs projecting to the FFH or IBN region were large (>700 μm(2)) and medium-sized. Many of the medium-sized GABA-negative CNs were TRNs projecting to the FFH. These results indicate that mirror-symmetric excitatory pathways link medial to medial (upper field) and lateral to lateral (lower field) parts of the SCs, whereas upper and lower field representations are linked by reciprocal inhibitory pathways in the tectal commissure. These connections presumably play important roles in conjugate upward and downward vertical saccades.

Shedding new light on the role of the basal ganglia-superior colliculus pathway in eye movements

A large body of work spanning 25+ years provides compelling evidence for the involvement of the basal ganglia-superior colliculus pathway in the initiation of rapid, orienting movements of the eyes, called saccades. The role of this pathway in saccade control is similar to the role of the basal ganglia-thalamic pathway in the control of skeletal movement: a transient cessation in tonic inhibition supplied by the basal ganglia to motor structures releases movements via the direct pathway whereas a transient increase in inhibition by the basal ganglia to motor structures prevents movements via the indirect pathway. In parallel with recent advances in the study and treatment of patients with basal ganglia disease and in animal experiments in the skeletal motor system, the results of studies exploring the role of the basal ganglia-superior colliculus pathway in saccades highlight the need for a revisiting of our understanding of the role of this pathway in saccades. The discovery of many different response profiles of neurons in the substantia nigra pars reticulata of the basal ganglia and in the superior colliculus, coupled with advances in experimental and statistical techniques including sophisticated behavioral procedures and multiple neuron recording and analysis, point toward a role for the basal ganglia-superior colliculus pathway in cognitive events intervening between vision and action, such as memory, target selection and saccade choice and valuation.

Intralaminar and interlaminar activity within the rodent superior colliculus visualized with voltage imaging

The superior colliculus (SC) is a midbrain structure that plays a role in converting sensation into action. Most SC research focuses on either in vivo extracellular recordings from behaving monkeys or patch-clamp recordings from smaller mammals in vitro. However, the activity of neuronal circuits is necessary to generate behavior, and neither of these approaches measures the simultaneous activity of large populations of neurons that make up circuits. Here, we describe experiments in which we measured changes in membrane potential across the SC map using voltage imaging of the rat SC in vitro. Our results provide the first high temporal and spatial resolution images of activity within the SC. Electrical stimulation of the SC evoked a characteristic two-component optical response containing a short latency initial-spike and a longer latency after-depolarization. Single-pulse stimulation in the superficial SC evoked a pattern of intralaminar and interlaminar spread that was distinct from the spread evoked by the same stimulus applied to the intermediate SC. Intermediate layer stimulation produced a more extensive and more ventrally located activation of the superficial layers than did stimulation in the superficial SC. Together, these results indicate the recruitment of dissimilar subpopulations of circuitry depending on the layer stimulated. Field potential recordings, pharmacological manipulations, and timing analyses indicate that the patterns of activity were physiologically relevant and largely synaptically driven. Therefore, voltage imaging is a powerful technique for the study of spatiotemporal dynamics of electrical signaling across neuronal populations, providing insight into neural circuits that underlie behavior.

Microsaccadic suppression of visual bursts in the primate superior colliculus

Saccadic suppression, a behavioral phenomenon in which perceptual thresholds are elevated before, during, and after saccadic eye movements, is an important mechanism for maintaining perceptual stability. However, even during fixation, the eyes never remain still, but undergo movements including microsaccades, drift, and tremor. The neural mechanisms for mediating perceptual stability in the face of these "fixational" movements are not fully understood. Here, we investigated one component of such mechanisms: a neural correlate of microsaccadic suppression. We measured the size of short-latency, stimulus-induced visual bursts in superior colliculus neurons of adult, male rhesus macaques. We found that microsaccades caused approximately 30% suppression of the bursts. Suppression started approximately 70 ms before microsaccade onset and ended approximately 70 ms after microsaccade end, a time course similar to behavioral measures of this phenomenon in humans. We also identified a new behavioral effect of microsaccadic suppression on saccadic reaction times, even for continuously presented, suprathreshold visual stimuli. These results provide evidence that the superior colliculus is part of the mechanism for suppressing self-generated visual signals during microsaccades that might otherwise disrupt perceptual stability.

Spatial updating in monkey superior colliculus in the absence of the forebrain commissures: dissociation between superficial and intermediate layers

In previous studies, we demonstrated that the forebrain commissures are the primary pathway for remapping from one hemifield to the other. Nonetheless, remapping in lateral intraparietal cortex (LIP) across hemifield is still present in split brain monkeys. This finding indicates that a subcortical structure must contribute to remapping. The primary goal of the current study was to characterize remapping activity in the superior colliculus in intact and split brain monkeys. We recorded neurons in both the superficial and intermediate layers of the SC. We found that across-hemifield remapping was reduced in magnitude and delayed compared with within-hemifield remapping in the intermediate layers of the SC in split brain monkeys. These results mirror our previous findings in area LIP. In contrast, we found no difference in the magnitude or latency for within- compared with across-hemifield remapping in the superficial layers. At the behavioral level, we compared the performance of the monkeys on two conditions of a double-step task. When the second target remained within a single hemifield, performance remained accurate. When the second target had to be updated across hemifields, the split brain monkeys' performance was impaired. Remapping activity in the intermediate layers was correlated with the accuracy and latency of the second saccade during the across-hemifield trials. Remapping in the superficial layers was correlated with latency of the second saccade during the within- and across-hemifield trials. The differences between the layers suggest that different circuits underlie remapping in the superficial and intermediate layers of the superior colliculus.

Event-related functional MRI of cortical activity evoked by microsaccades, small visually-guided saccades, and eyeblinks in human visual cortex

We used event-related functional magnetic resonance imaging (fMRI) to determine blood oxygen-level-dependent (BOLD) signal changes following microsaccades, visually-guided saccades, and eyeblinks in retinotopically mapped visual cortical areas V1-V3 and hMT+. A deconvolution analysis revealed a similar pattern of BOLD activation following a microsaccade, 0.16 degrees voluntary saccade, and 0.16 degrees displacement of the image under conditions of fixation. In all areas, an initial increase in BOLD signal peaking at approximately 4.5 s after the event was followed by a decline and decrease below baseline. This modulation appears most pronounced for microsaccades and small voluntary saccades in V1, diminishing in strength from V1 to V3. In contrast, 0.16 degrees real motion under conditions of fixation yields the same level of BOLD signal increase in V1 through V3. BOLD signal modulates parametrically with the size of voluntary saccades (0.16 degrees , 0.38 degrees , 0.82 degrees , 1.64 degrees , and 3.28 degrees ) in V1-V3, but not in hMT+. Eyeblinks generate larger modulation that peaks by 6.5 s, and dips below baseline by 10 s post-event, and also exhibits diminishing modulation from V1 to V3. Our results are consistent with the occurrence of transient neural excitation driven by changes in input to retinal ganglion cell receptive fields that are induced by microsaccades, visually-guided saccades, or small image shifts. The pattern of results in area hMT+ exhibits no significant modulation by microsaccades, relatively small modulation by eyeblinks, and substantial responses to saccades and background jumps, suggesting that spurious image motion signal arising from microsaccades and eyeblinks is relatively diminished by hMT+.

Exploring the superior colliculus in vitro

The superior colliculus plays an important role in the translation of sensory signals that encode the location of objects in space into motor signals that encode vectors of the shifts in gaze direction called saccades. Since the late 1990s, our two laboratories have been applying whole cell patch-clamp techniques to in vitro slice preparations of rodent superior colliculus to analyze the structure and function of its circuitry at the cellular level. This review describes the results of these experiments and discusses their contributions to our understanding of the mechanisms responsible for sensorimotor integration in the superior colliculus. The experiments analyze vertical interactions between its superficial visuosensory and intermediate premotor layers and propose how they might contribute to express saccades and to saccadic suppression. They also compare and contrast the circuitry within each of these layers and propose how this circuitry might contribute to the selection of the targets for saccades and to the build-up of the premotor commands that precede saccades. Experiments also explore in vitro the roles of extrinsic inputs to the superior colliculus, including cholinergic inputs from the parabigeminal and parabrachial nuclei and GABAergic inputs from the substantia nigra pars reticulata, in modulating the activity of the collicular circuitry. The results extend and clarify our understanding of the multiple roles the superior colliculus plays in sensorimotor integration.

Nigral inhibition of GABAergic neurons in mouse superior colliculus

The current dominant concept for the control of saccadic eye movements by the basal ganglia is that release from tonic GABAergic inhibition by the substantia nigra pars reticulata (SNr) triggers burst firings of intermediate gray layer (SGI) neurons in the superior colliculus (SC) to allow saccade initiation. This hypothesis is based on the assumption that SNr cells inhibit excitatory projection neurons in the SGI. Here we show that nigrotectal fibers are connected to local GABAergic neurons in the SGI with a similar frequency to non-GABAergic neurons. This was accomplished by applying neuroanatomical tracing and slice electrophysiological experiments in GAD67-green fluorescent protein (GFP) knock-in mice, in which GABAergic neurons specifically express GFP. We also found that GABA(A), but not GABA(B), receptors subserve nigrotectal transmission. The present results revealed a novel aspect on the role of the basal ganglia in the control of saccades, e.g., the SNr not only regulates burst initiation but also modulates the spatiotemporal properties of premotor neurons via connections to local GABAergic neurons in the SC.

Spatiotemporal profiles of field potentials in mouse superior colliculus analyzed by multichannel recording

The onset and vector of orienting behaviors, such as saccades, are controlled by commands that descend from a population of neurons in deep layers of the superior colliculus (dSC). In this study, to characterize the role of the collicular local circuitry that generates the spatiotemporal pattern of command activity in the dSC neuronal population, responses evoked by single-pulse electrical stimulation in superficial layers of the superior colliculus (sSC) were analyzed by a 64-channel field potential recording system (planar electrode, 8 x 8 pattern; 150 microm interelectrode spacing) in slices obtained from 16- to 22-d-old mice. A negative field potential with short latency and short duration spatially restricted to the recording sites in sSC was evoked adjacent to the stimulation site. After bath application of 10 mum bicuculline, the same stimulus induced a large negative field response with long duration that spread from sSC to dSC. The dSC potential initially showed a positive response, presumably because of reversal of the negative potential that originated in sSC, and then a long negative response that spread horizontally as far as 1 mm. These responses disappeared after application of an NMDA receptor antagonist, 2-amino-5-phosphonovelarate, indicating that NMDA receptors have an important role in the generation of these responses. Simultaneous whole-cell patch-clamp recordings showed that the long-lasting negative field potentials corresponded to the depolarization accompanying burst spike activity of SC neurons. The present study revealed an extensive excitatory network in the dSC that may contribute to the generation of activity by a large population of neurons that discharge before a saccade.

Brain circuits for the internal monitoring of movements

Each movement we make activates our own sensory receptors, thus causing a problem for the brain: the spurious, movement-related sensations must be discriminated from the sensory inputs that really matter, those representing our environment. Here we consider circuits for solving this problem in the primate brain. Such circuits convey a copy of each motor command, known as a corollary discharge (CD), to brain regions that use sensory input. In the visual system, CD signals may help to produce a stable visual percept from the jumpy images resulting from our rapid eye movements. A candidate pathway for providing CD for vision ascends from the superior colliculus to the frontal cortex in the primate brain. This circuit conveys warning signals about impending eye movements that are used for planning subsequent movements and analyzing the visual world. Identifying this circuit has provided a model for studying CD in other primate sensory systems and may lead to a better understanding of motor and mental disorders.

Neuronal mechanisms of visual stability

Human vision is stable and continuous in spite of the incessant interruptions produced by saccadic eye movements. These rapid eye movements serve vision by directing the high resolution fovea rapidly from one part of the visual scene to another. They should detract from vision because they generate two major problems: displacement of the retinal image with each saccade and blurring of the image during the saccade. This review considers the substantial advances in understanding the neuronal mechanisms underlying this visual stability derived primarily from neuronal recording and inactivation studies in the monkey, an excellent model for systems in the human brain. For the first problem, saccadic displacement, two neuronal candidates are salient. First are the neurons in frontal and parietal cortex with shifting receptive fields that provide anticipatory activity with each saccade and are driven by a corollary discharge. These could provide the mechanism for a retinotopic hypothesis of visual stability and possibly for a transsaccadic memory hypothesis, The second neuronal mechanism is provided by neurons whose visual response is modulated by eye position (gain field neurons) or are largely independent of eye position (real position neurons), and these neurons could provide the basis for a spatiotopic hypothesis. For the second problem, saccadic suppression, visual masking and corollary discharge are well established mechanisms, and possible neuronal correlates have been identified for each.

Cholinergic responses in crossed tecto-reticular neurons of rat superior colliculus

Neurons in the intermediate gray layer (SGI) of mammalian superior colliculus (SC) receive cholinergic innervation from the brain stem parabrachial region, which seems to modulate the signal processing in the SC. To clarify its role particularly in orienting behaviors, we studied cholinergic effects on the major output neuron group of the SGI, crossed tecto-reticular neurons (cTRNs), identified by retrograde labeling from the contralateral brain stem gaze center in SC slices obtained from rats (PND 17-22) by whole cell patch-clamp techniques. Bath application of carbachol induced either 1) nicotinic inward (nIN) + muscarinic inward (mIN) (11/24) or 2) nIN + mIN + muscarinic outward (mOUT) (13/24) current responses. Transient pressure application of 1 mM acetylcholine elicited nIN in all neurons tested (n = 58). In a majority of these neurons (52/58), the nIN was completely suppressed by dihydro-beta-erythroidine, a specific antagonist for alpha4beta2 nicotinic receptor subtype. The remaining 6/58 neurons exhibited not only the slower alpha4beta2 receptor-mediated component but also a faster component that was inhibited by a specific antagonist for alpha7 nicotinic receptor, alpha-bungarotoxin. cTRNs expressing alpha7 nicotinic receptors tended to be smaller in size than those lacking alpha7 receptors. Bath application of muscarine induced two response patterns: mIN only (17/38) and mIN+ mOUT (21/38). The mIN and mOUT were mediated by M3 (plus M1) and M2 muscarinic receptors, respectively. These results suggest that a major response to cholinergic inputs to cTRNs is excitatory. This would indicate the facilitatory role of the brain stem cholinergic system in the execution of orienting behaviors including saccadic eye movements.

Quantitative and immunohistochemical analysis of neuronal types in the mouse caudal nucleus tractus solitarius: focus on GABAergic neurons

gamma-Aminobutyric acid-ergic (GABAergic) neurons are major inhibitory interneurons that are widely distributed in the central nervous system. The caudal nucleus tractus solitarius (cNTS), which plays a key role in respiratory, cardiovascular, and gastrointestinal function, contains GABAergic neurons for regulation of neuronal firing. In the present study, GABAergic neuronal organization was analyzed in relation to the location of subnuclei in the mouse cNTS. According to the differential expression of glutamate decarboxylase 67 (GAD67), vesicular glutamate transporter 2 (VGLUT2), calbindin, and tyrosine hydroxylase (TH) mRNAs, the cNTS was divided into four subnuclei: the subpostrema, dorsomedial, commissural, and medial subnuclei. The numerical density and size of soma in the four subnuclei were then quantified by an unbiased dissector analysis. Calbindin-positive cells constituted subpopulations of small non-GABAergic neurons preferentially localized in the subpostrema subnucleus. TH-positive cells constituted large neurons preferentially localized in the medial subnucleus. GABAergic neurons constituted a subpopulation of small neurons, preferentially localized in the commissural and medial subnuclei, which represented > or =50% of small cells in these subnuclei. Thus, the GABAergic small neurons were located around TH-positive large cells in the ventrolateral portion of the cNTS. This finding, in combination with results of previous studies in the rat cNTS showing that large cells originate efferents from the cNTS, suggests that GABAergic small neurons in the commissural and medial subnuclei might regulate output from the cNTS.

Regulation of burst activity through presynaptic and postsynaptic GABA(B) receptors in mouse superior colliculus

In slice preparations, electrical stimulation of the superficial gray layer (SGS) of the superior colliculus (SC) induces EPSC bursts in neurons in the intermediate gray layer (SGI) when GABA(A) receptor (GABA(A)R)-mediated inhibition is reduced. This preparation has been used as a model system to study signal processing involved in execution of short-latency orienting responses to visual stimuli such as saccadic eye movements. In the present study, we investigated the role of GABA(B) receptors (GABA(B)Rs) in modulating signal transmission in the above pathway with whole-cell patch-clamp recordings in SC slices obtained from GAD67-GFP knock-in mice. Perfusion of the slice with the GABA(B)R antagonist CGP52432 (CGP) greatly prolonged the duration of the EPSC bursts. Local application of CGP to the SGS but not to the SGI produced similar effects. Because SGS stimulation elicited bursts in GABAergic neurons in the SGS when GABA(A)Rs were blocked, these results suggest that GABA released after bursts activates GABA(B)Rs in the SGS, leading to reduced burst duration. We found both postsynaptic and presynaptic actions of GABA(B)Rs in the SGS; activation of postsynaptic GABA(B)Rs induced outward currents in narrow-field vertical cells, whereas it caused shunting inhibition in distal dendrites in wide-field vertical cells. On the other hand, activation of presynaptic GABA(B)Rs suppressed excitatory synaptic transmissions to non-GABAergic neurons in the SGS. These results indicate that synaptically released GABA can activate both presynaptic and postsynaptic GABA(B)Rs in the SGS and limit the duration of burst responses in the SC local circuit.

GABAergic pathways in the rat subcortical visual system: a comparative study in vivo and in vitro

Inhibitory pathways project from the pretectal nuclear complex to the ipsilateral superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Both pathways arise from GABAergic neurones that are located in the dorsal pretectal nuclear complex. In the present experiments, we compared the anatomy and physiology of these two pathways with the objective of determining whether they have similar functions. First, we injected retrograde axonal tracers that fluoresce at different wavelengths in the dLGN and SC of single animals to determine if individual GABAergic neurones in the pretectum project to both structures. The results showed that the dLGN and SC receive input from different cell groups. Next, morphological reconstructions of cells labelled after in-vitro whole-cell patch-clamp recordings demonstrated that the pretectal-recipient cells in the dLGN are GABAergic interneurones, whereas those in the SC are projection cells. Finally, with in-vitro whole-cell patch-clamp recordings we showed that inhibitory currents generated by both pathways are mediated by GABA(A) receptors. Taken together, these results suggest that these inhibitory projections may function to facilitate the relay of information from the dLGN to the visual cortex by suppressing the activity of dLGN interneurones, and to reduce the level of activity leaving the SC by inhibiting the projection neurones. These hypotheses will be discussed in the context of the known functions of the pretectal complex.