Interlaminar connections of the superior colliculus in the tree shrew. II: Projections from the superficial gray to the optic layer

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

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

Unraveling circuits of visual perception and cognition through the superior colliculus

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

The Second Visual System of The Tree Shrew

This review provides a historical account of the discovery of secondary visual pathways (from retina to the superior colliculus to the dorsal thalamus and extrastriate cortex), and Vivien Casagrande's pioneering studies of this system using the tree shrew as a model. Subsequent studies of visual pathways in the tree shrew are also reviewed, beginning with a description of the organization and central projections of the tree shrew retina. The organization and connectivity of second visual system components that include the retino-recipient superior colliculus, tecto-recipient pulvinar nucleus and its projections, and the tecto-recipient dorsal lateral geniculate nucleus and its projections are detailed. Potential functions of the second visual system are discussed in the context of this work and in the context of the behavioral studies that initially inspired the secondary visual system concept.

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 diffusion tensor imaging atlas of white matter in tree shrew

Tree shrews are small mammals now commonly classified in the order of Scandentia, but have relatively closer affinity to primates than rodents. The species has a high brain-to-body mass ratio and relatively well-differentiated neocortex, and thus has been frequently used in neuroscience research, especially for studies on vision and neurological/psychiatric diseases. The available atlases on tree shrew brain provided only limited information on white matter (WM) anatomy. In this study, diffusion tensor imaging (DTI) was used to study the WM anatomy of tree shrew, with the goal to establish an image-based WM atlas. DTI and T₂-weighted anatomical images were acquired in vivo and from fixed brain samples. Deterministic tractography was used for three-dimensional reconstruction and rendering of major WM tracts. Myelin and neurofilaments staining were used to study the microstructural properties of certain WM tracts. Taking into account prior knowledge on tree shrew neuroanatomy, tractography results, and comparisons to the homologous structures in rodents and primates, an image-based WM atlas of tree shrew brain was constructed, which is available to research community upon request.

Processing of motion stimuli by cells in the optic tectum of chickens

The ability to detect motion is crucial for the survival of animals. In the avian optic tectum, motion-sensitive output neurons in the stratum griseum centrale have large dendritic fields and receive direct retinal input at their distal dendrites (bottlebrush endings). It has been hypothesized that the activation of each ending elicits a burst in the neuron. Thus, an object moving across the receptive field would lead to a fixed number of bursts, independent of movement speed. However, experimental confirmation of this hypothesis is still missing. We measured the response of tectal neurons to moving stimuli in vivo and found that in 'fast-bursting' units (~500 Hz within burst), the number of bursts was independent of stimulus speed. These results indicate that the number of bursts might indeed be related to the sequential activation of the bottlebrush endings by visual stimuli.

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.

Cortical projections to the superior colliculus in tree shrews (Tupaia belangeri)

The visuomotor functions of the superior colliculus depend not only on direct inputs from the retina, but also on inputs from neocortex. As mammals vary in the areal organization of neocortex, and in the organization of the number of visual and visuomotor areas, patterns of corticotectal projections vary. Primates in particular have a large number of visual areas projecting to the superior colliculus. As tree shrews are close relatives of primates, and they are also highly visual, we studied the distribution of cortical neurons projecting to the superior colliculus by injecting anatomical tracers into the colliculus. Since projections from visuotopically organized visual areas are expected to match the visuotopy of the superior colliculus, injections at different retinotopic locations in the superior colliculus provide information about the locations and organization of topographic areas in extrastriate cortex. Small injections in the superior colliculus labeled neurons in locations within areas 17 (V1) and 18 (V2) that are consistent with the known topography of these areas and the superior colliculus. In addition, the separate locations of clusters of labeled cells in temporal visual cortex provide evidence for five or more topographically organized areas. Injections that included deeper layers of the superior colliculus also labeled neurons in medial frontal cortex, likely in premotor cortex. Only occasional labeled neurons were observed in somatosensory or auditory cortex. Regardless of tracer injection location, we found that, unlike primates, a substantial projection to the superior colliculus from posterior parietal cortex is not a characteristic of tree shrews.

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.

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.

The macaque midbrain reticular formation sends side-specific feedback to the superior colliculus

The central mesencephalic reticular formation (cMRF) likely plays a role in gaze control, as cMRF neurons receive tectal input and provide a bilateral projection back to the superior colliculus (SC). We examined the important question of whether this feedback is excitatory or inhibitory. Biotinylated dextran amine (BDA) was injected into the cMRF of M. fascicularis monkeys to anterogradely label reticulotectal terminals and retrogradely label tectoreticular neurons. BDA labeled profiles in the ipsi- and contralateral intermediate gray layer (SGI) were examined electron microscopically. Postembedding GABA immunochemistry was used to identify putative inhibitory profiles. Nearly all (94.7%) of the ipsilateral BDA labeled terminals were GABA positive, but profiles postsynaptic to these labeled terminals were exclusively GABA negative. In addition, BDA labeled terminals were observed to contact BDA labeled dendrites, indicating the presence of a monosynaptic feedback loop connecting the cMRF and ipsilateral SC. In contrast, within the contralateral SGI, half of the BDA labeled terminals were GABA positive, while more than a third were GABA negative. All the postsynaptic profiles were GABA negative. These results indicate the cMRF provides inhibitory feedback to the ipsilateral side of the SC, but it has more complex effects on the contralateral side. The ipsilateral projection may help tune the "winner-take-all" mechanism that produces a unified saccade signal, while the contralateral projections may contribute to the coordination of activity between the two colliculi.

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.

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.

Sparse spatial sampling for the computation of motion in multiple stages

The avian retino-tecto-rotundal pathway plays a central role in motion analysis and features complex connectivity. Yet, the relation between the pathway's structural arrangement and motion computation has remained elusive. For an important type of tectal wide-field neuron, the stratum griseum centrale type I (SGC-I) neuron, we quantified its structure and found a spatially sparse but extensive sampling of the retinal projection. A computational investigation revealed that these structural properties enhance the neuron's sensitivity to change, a behaviorally important stimulus attribute, while preserving information about the stimulus location in the SGC-I population activity. Furthermore, the SGC-I neurons project with an interdigitating topography to the nucleus rotundus, where the direction of motion is computed. We showed that, for accurate direction-of-motion estimation, the interdigitating projection of tectal wide-field neurons requires a two-stage rotundal algorithm, where the second rotundal stage estimates the direction of motion from the change in the relative stimulus position represented in the first stage.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Intrinsic connectivity of human superior colliculus

The superior colliculus (SC) is believed to play an important role in sensorimotor integration and orienting behavior. It is classically divided into superficial layers predominantly containing visual neurons and deep layers containing multisensory and premotor neurons. Investigations of intrinsic connectivity within the SC in non-human species initially led to controversy regarding the existence of interlaminar connections between superficial and deep layers. It now seems more likely that such connections exist in a number of species, including non-human primates. In the latter, anatomical data concerning intrinsic SC connectivity are restricted to a limited number of intracellularly labeled neurons. No studies have been conducted to investigate the existence of intrinsic connections of human SC. In the present study, DiI (1,1'-dioctadecyl-3,3,3',3'- tetramethylindocarbocyanine perchlorate) and BDA (biotinylated dextran amine) were two tracers used in post-mortem human brains to examine intrinsic SC connections. Injections into the superficial layers revealed tangential connections within superficial layers and radial superficial-layer to deep-layer connections. Within superficial layers, horizontal connections were found over the entire rostro-caudal axis and were mostly directed laterally, i.e. toward the brachium of the inferior colliculus. Superficial-layer to deep-layer connections were more prominent in sections containing the injection site or located close to it. In these sections, an axon bundle having roughly the same diameter as the injection site crossed all deep layers, and individual axons displayed en passant or terminal boutons. The present results suggest that intrinsic connections within superficial layers and radial superficial-layers to deep-layers exist in human SC. The putative roles of these connections are discussed with regard to visual receptive field organization, as well as visuomotor and multisensory integration.

Organization of Interlaminar Interactions in the Rat Superior Colliculus

Our previous studies have shown that when slices of the rat superior colliculus (SC) are exposed to a solution containing 10 μM bicuculline and a low concentration of Mg2+ (0.1 mM), most neurons in the intermediate gray layer (stratum griseum intermediale; SGI), wide-field vertical (WFV) cells in the optic layer (stratum opticum; SO), and a minor population of neurons in the superficial gray layer (stratum griseum superficiale; SGS) exhibit spontaneous depolarization and burst firing, which are synchronous among adjacent neurons. These spontaneous and synchronous depolarizations were thought to share common mechanisms with presaccadic burst activity in SGI neurons. In the present study, we explored the site responsible for generation of synchronous depolarization of SGI neurons by performing dual whole cell recordings under different slice conditions. A pair of SGI neurons recorded in a small rectangular piece of the SGI punched out from the SC slice showed synchronous depolarization but far less frequently than those recorded in a small rectangular piece including SGS and SO. This suggests that the superficial layers are needed for triggering synchronous depolarization in the SGI. Furthermore, we recorded spontaneous depolarizations in pairs of neurons belonging to the different layers. Analysis of their synchronicity revealed that WFV cells in the SO exhibit synchronous depolarizations with both SGS and SGI neurons, and the onset of spontaneous depolarization in WFV cells precedes those of neurons in other layers. Further, when SGS and SGI neurons exhibit synchronous depolarizations, SGI neurons usually precede the SGS neurons. These observations give further evidence to the existence of interlaminar interaction between superficial and deeper layers of the SC. In addition, it is suggested that WFV cells can trigger burst activity in other layers of the SC and that there is an excitatory signal transmission from the deeper layers to the superficial layers.

Electrophysiological and morphological properties of identified crossed tecto-reticular neurons in the rat superior colliculus

Previously we classified randomly sampled neurons in the intermediate layer (SI) of the rat superior colliculus (SC) into six subclasses according to their firing responses to depolarizing current pulses and five subclasses based on their morphological properties in slice preparations. In the present study, we investigated properties of a major output cell group of the rat SC (PND 17-24), crossed tecto-reticular neurons (cTRNs), which project to the contralateral medial pontine reticular formation. The cTRNs were identified by retrograde labeling with a fluorescent tracer (n=112). We compared their properties with those of presumed interneurons (n=127). We found that a majority of cTRNs were regular spiking neurons with moderate firing frequency (73%) and were multipolar-shaped (66%). The cTRNs had larger membrane capacitance, larger soma size and lower input impedance than presumed interneurons. Electrical stimulation of the superficial gray layer induced oligosynaptic EPSPs in the cTRNs. When bicuculline was added to the extracellular solution, the EPSPs were markedly enhanced and bursting spike responses were induced. The bursting responses were suppressed by applying D-2-amino-5-phosphonovalerate. These results suggest that the cTRNs exhibit NMDA receptor-dependent bursting responses to visual inputs. These observations give insights into the neuronal mechanism of generating burst activity in cTRNs, which triggers orienting behaviors.

Laminar Specific Distribution of Lateral Excitatory Connections in the Rat Superior Colliculus

Premovement activities in neurons in the intermediate gray layer [stratum griseum intermediale (SGI)] of the mammalian superior colliculus (SC) are essential for initiation of orienting behaviors such as saccades. Our previous study demonstrated that burst activities are induced by synchronous activation of SGI neurons communicating within a local excitatory network, which depends on NMDA-receptor–dependent synaptic transmission and release from GABAA inhibition. Furthermore, dual whole cell recordings from adjacent neurons in SGI revealed that application of 10 μM bicuculline (Bic) and reduction of extracellular Mg2+ concentration (to 0.1 mM) induce spontaneous depolarization that is synchronous between neuron pairs, suggesting the recruitment of a large number of neurons communicating through intense excitatory connections. In the present study, we investigated the properties of synchronous depolarization and the fundamental structure of the lateral excitatory network that recruits a neuronal population in SC to synchronous activation, by analyzing the synchronicity of spontaneous depolarization induced in the presence of Bic plus low Mg2+. We found that 1) spontaneous depolarization exhibits bidirectional horizontal propagation among the SGI neuron pairs; 2) induction of spontaneous depolarization is not caused by activation of intrinsic voltage-dependent conductances; 3) neurons exposed to low Mg2+ alone exhibit spontaneous depolarization, although in this case the depolarization is less synchronous; and 4) neurons exposed to Bic alone exhibit synchronous depolarization, but less frequently than those exposed to both Bic and low Mg2+. Analysis of the synchronicity of spontaneous depolarization indicates that the distribution of lateral excitatory connections is markedly different among layers of SC; the SGI neurons form extensive lateral excitatory connections, whereas they are sparse or limited within subsets of neurons in the stratum griseum superficiale (SGS). Wide-field vertical neurons in the stratum opticum have features intermediate between neurons in the SGS and SGI. Such differences in the structure of lateral excitatory connections may reflect the different way signal processing is achieved in each layer of SC.

Functional connectivity between the superficial and deeper layers of the superior colliculus: an anatomical substrate for sensorimotor integration

The superior colliculus (SC) transforms both visual and nonvisual sensory signals into motor commands that control orienting behavior. Although the afferent and efferent connections of this midbrain nucleus have been well characterized, little is know about the intrinsic circuitry involved in sensorimotor integration. Transmission of visual signals from the superficial (sSC) to the deeper layers (dSC) of the SC has been implicated in both the triggering of orienting movements and the activity-dependent processes that align maps of different sensory modalities during development. However, evidence for the synaptic connectivity appropriate for these functions is lacking. In this study, we used a variety of anatomical and physiological methods to examine the functional organization of the sSC-dSC pathway in juvenile and adult ferrets. Axonal tracing in adult ferrets showed that, as in other species, sSC neurons project topographically to the dSC, providing a route for the transmission of visual signals to the multisensory output layers of the SC. We found that sSC axons terminate on dSC neurons that stain prominently for the NR1 subunit of the NMDA receptor, a subpopulation of which were identified as tectoreticulospinal projection neurons. We also show that the sSC-dSC pathway is topographically organized and mediated by monosynaptic excitatory synapses even before eye opening in young ferrets, suggesting that visual signals routed via the sSC may influence the activity of dSC neurons before the emergence of their multisensory response properties. These findings indicate that superficial- to deep-layer projections provide spatially ordered visual signals, both during development and into adulthood, directly to SC neurons that are involved in coordinating sensory inputs with motor outputs.

Local excitatory network and NMDA receptor activation generate a synchronous and bursting command from the superior colliculus

The generation of bursting spike activity in the deeper layers of the superior colliculus (SC) is a critical determinant of decision making in the initiation of orienting behaviors, such as saccades. The bursting activity exhibits a typical threshold effect that may arise from a nonlinear signal amplification process in the deeper layers of the SC. We used whole-cell patch-clamp recordings in rat SC slices to investigate the neuronal mechanism underlying the generation of such bursting activity. We found that (1) neurons in the intermediate gray layer [stratum griseum intermediale (SGI)] produce a prolonged bursting response when released from GABA(A) receptor-mediated inhibition, (2) this GABA(A) inhibition may partially arise from inhibitory interneurons within the SGI that are driven synaptically by glutamatergic excitatory inputs to the SC, (3) the bursting is not the result of the intrinsic membrane properties of individual SC neurons but is instead produced by local circuits within the SGI, (4) the bursting is mediated by activation of NMDA receptors, and (5) the bursting can be synchronous among SGI neurons. These results suggest that activation of a local excitatory network within the deeper layers of the SC and NMDA receptor-dependent synaptic transmission after release from GABA(A) inhibition are fundamental mechanisms that may explain the nonlinear signal amplification process in the deeper layers of the SC.

Release from GABA(A) receptor-mediated inhibition unmasks interlaminar connection within superior colliculus in anesthetized adult rats

The superior colliculus (SC) contains two major subdivisions, the superficial layers (sSC) and the deeper layers (dSC). sSC receives visual information from the retina and visual cortex, while dSC sends descending projections to the brainstem and spinal cord. It has not been clear whether and how sSC directly activates dSC, however recent studies in slice preparations reported that electrical stimulation of sSC induces burst firing in dSC neurons after application of bicuculline. In the present study, we tested whether sSC directly activates dSC in vivo. In isoflurane-anesthetized rats, electrical stimulation of the optic nerve (ON) induced negative field responses mainly in sSC, but not as much in dSC, under control conditions. However, after injection of bicuculline into dSC, ON stimulation induced long-lasting negative field responses in dSC. dSC neurons including the tectofugal neurons exhibited burst firing during the long-lasting negative field responses. The burst responses remained after ablation of the cerebral cortex ipsilateral to the SC recordings. These results suggest that retinal inputs induce burst responses in tectofugal neurons in dSC via subcortical pathways when the SC circuit is released from GABA(A) receptor-mediated inhibition. We propose that sSC is the main candidate for the source of the excitatory synaptic inputs to dSC among the subcortical regions.

GABAC receptors in the rat superior colliculus and pretectum participate in synaptic neurotransmission

In mammals, GABA(C) receptors seem to be specifically expressed in the retina and the subcortical visual system, with highest extraretinal expression levels in the superior colliculus (SC). Although its presence in the superficial SC has been demonstrated physiologically, a direct involvement of this receptor type in fast synaptic neurotransmission still awaits verification. We addressed the question of a possible synaptic localization of GABA(C) receptors by performing in vitro whole-cell patch-clamp recordings of inhibitory postsynaptic currents (IPSCs) in single neurons of the rat SC and the neighboring pretectal nuclear complex, where GABA(C) receptors are also expressed at significant levels. To increase the likelihood to record IPSCs we induced spontaneous activity by application of the potassium channel blocker 4-aminopyridine (4-AP) and blocked glutamate-mediated excitatory neurotransmission with kynurenic acid. All 4-AP-induced postsynaptic currents were of synaptic origin because they were completely suppressed by lidocaine or by substitution of extracellular calcium with cobalt. In 40% of the SC cells and in 60% of the pretectal neurons, IPSCs in the presence of 4-AP and kynurenic acid were only partly blocked by the selective GABA(A) receptor antagonist bicuculline. Inhibitory currents that were insensitive to bicuculline, however, could be blocked by coapplication of either the specific GABA(C) receptor antagonist 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid or picrotoxin, an unselective GABA(A) and GABA(C) receptor antagonist. We conclude that GABA(C) receptors are, at least partially, located synaptically in SC and pretectal neurons in the rat, which indicates a direct function of this receptor type for synaptic processing in both structures.

Chemoarchitecture of GABAergic neurons in the ferret superior colliculus

gamma-Aminobutyric acid (GABA)ergic neurons are thought to play a key role both in visual processing and in the complex sensory-motor transformations that take place in the mammalian superior colliculus. To understand the organization of GABAergic neurons in the ferret superior colliculus, we applied antisera to several markers of GABAergic function, including GABA, two isoforms of its synthetic enzyme glutamic acid decarboxylase (GAD-65 and GAD-67), and the GABA transporter, GAT-1. We also applied antisera to several calcium binding proteins (calbindin [CB], calretinin [CR], and parvalbumin [PV]) and neuronal nitric oxide synthase (NOS), chemical markers that colocalize with GABA in some areas of the central nervous system. The distribution of GABAergic neurons in the ferret is similar to that of other mammalian species. GABAergic neurons in the ferret superior colliculus were small, morphologically diverse, and widely distributed throughout all layers of the colliculus. As has been shown in other mammalian species, neurons expressing PV, CB, CR, and NOS were differentially distributed in layers and patches throughout the ferret colliculus. None of these markers, however, showed a distribution that mirrored that of GABAergic neurons. Furthermore, few GABAergic neurons colocalized these neurochemical markers. Only 14% of GABAergic neurons in the superficial layers and 18% of neurons in the deeper layers colocalized PV, 14% of GABAergic neurons in the superficial layers and 10% in the deeper layers colocalized CB, and only 1% of GABAergic neurons in both the superficial and deep layers colocalized nitric oxide synthase. Thus, the arrangement of GABAergic neurons in the ferret superior colliculus is broadly distributed and is distinct from other recognized organizational patterns in the superior colliculus.

Characterisation of rat superficial superior colliculus neurones: firing properties and sensitivity to GABA

Physiological, pharmacological and morphological properties of superficial superior colliculus neurones (n=93) were characterised using whole-cell patch-clamp recordings in rat brain slices. Six cell types (narrow- and wide-field vertical, horizontal, piriform, marginal and stellate) were identified based on Lucifer Yellow labelling but no cell type-specific spike pattern could be identified. Resting membrane potentials were homogeneous (mean: -67.1 +/- 0.7 mV, n=48), and spike frequencies ranged from 10 to 70 Hz (80 pA current injection). About 66% of the cells displayed regular and sustained spike production, throughout all neuronal categories. Rebound spikes and spontaneous activity were observed frequently in all cell types. Synaptically evoked action potentials appeared as single spikes (mean amplitude: 76.0 +/- 3.2 mV, n=34) followed by a fast after-hyperpolarising potential (mean amplitude: 25.4 +/- 1.4 mV, n=34) and variable late potentials (late after-depolarising and/or -hyperpolarising). Pharmacologically, a characterisation using GABA and its subtype-specific agonists indicated a strong inhibitory influence of this transmitter system on >90% of cells. The GABA(A) receptor agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (100 microM), caused a reversible hyperpolarisation (approximately 9 mV) and spike inhibition of all neurones studied. This was more pronounced for intrinsic than for synaptically evoked spikes. Assessment of the GABA(C) receptor agonist, cis-4-aminocrotonic acid (1 mM), also revealed a hyperpolarisation (approximately 3 mV) and an inhibitory action on firing, but this was not as potent and homogeneous, compared to the GABA(A) receptor agonist. Further, the GABA(B) receptor agonist, baclofen (50-100 microM), had more variable (hyperpolarising, depolarising or no change) effects on the membrane potential. It showed little modulation of current-induced action potentials but fully blocked synaptic spikes. Assessment of GABA receptor antagonist actions revealed the presence of weak tonic and strong phasic GABA(A) receptor-mediated inhibition in the superficial superior colliculus: application of the GABA(A) receptor antagonist, bicuculline (100 microM), led to a generally enhanced excitability and depolarisation (approximately 5 mV). Intrinsic firing was somewhat enhanced, but synaptic spiking was drastically potentiated and prolonged. In contrast, 1,2,5,6-tetrahydro-(pyridin-4-yl) methylphosphinic acid (TPMPA; 100 microM), the GABA(C) receptor antagonist, produced little effect on these physiological parameters. The GABA(B) receptor antagonist, CGP35348 (200 microM), caused a partial inhibition of late after-hyperpolarising potentials (approximately 30%). Uptake of GABA contributes little to endogenous inhibition in the superior colliculus slice preparation, as suggested by the action of GABA uptake inhibitors SKF89976 (50-100 microM) and nipecotic acid (200-500 microM), both had no obvious effect on physiological parameters. However, in the presence of these compounds, sub-maximal inhibitory actions of GABA were potentiated. In conclusion, different cell types in the superficial superior colliculus do not display distinct physiological properties and are subject to strong inhibitory modulation. We therefore suggest that signal processing in this brain region does not require cell type-specific encoding of information. In line with evidence provided by previous in vivo investigations, identification of visual stimuli and orientation responses appears to be realised via the network properties of the receptive fields that form topographic maps.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.

Excitatory and inhibitory circuitry in the superficial gray layer of the superior colliculus

Stratum griseum superficiale (SGS) of the superior colliculus receives a dense cholinergic input from the parabigeminal nucleus. In this study, we examined in vitro the modulatory influence of acetylcholine (ACh) on the responses of SGS neurons that project to the visual thalamus in the rat. We used whole-cell patch-clamp recording to measure the responses of these projection neurons to electrical stimulation of their afferents in the stratum opticum (SO) before and during local pressure injections of ACh. These colliculothalamic projection neurons (CTNs) were identified during the in vitro experiments by prelabeling them from the thalamus with the retrograde axonal tracer wheat germ agglutinin-apo-HRP-gold. In a group of cells that included the prelabeled neurons, EPSCs evoked by SO stimulation were significantly reduced by the application of ACh, whereas IPSC amplitudes were significantly enhanced. Similar effects were observed when the nicotinic ACh receptor agonist lobeline was used. Application of the selective GABA(B) receptor antagonist 3-[[(3,4-dichlorophenyl)-methyl]amino]propyl](diethoxymethyl)phosphinic acid blocked ACh-induced reduction in the evoked response. In contrast, the ACh-induced reduction was insensitive to application of the GABA(A) receptor antagonist bicuculline. The ACh-induced reduction was also diminished by bath application of muscimol at the low concentrations that selectively activate GABA(C) receptors. Because GABA(C) receptors may be specifically expressed by GABAergic SGS interneurons (Schmidt et al., 2001), our results support the hypothesis that ACh reduces CTN activity by nicotinic receptor-mediated excitation of local GABAergic interneurons. These interneurons in turn use GABA(B) receptors to inhibit the CTNs.

The direct visuo-motor pathway in mammalian superior colliculus; novel perspective on the interlaminar connection

The mammalian superior colliculus (SC) is a center controlling the orienting behaviors such as saccadic eye movements. The superficial layers receive visual inputs and the deeper layers send descending motor command to the brainstem and spinal cord. Existence of the interlaminar connection from the superficial to the deeper layers has been an issue of debate during the last two decades. Recent studies have proved the existence of the interlaminar connection by introducing the in vitro slice preparations. When the collicular circuit is disinhibited from gamma-amino butyric acid A (GABA(A)) receptor-mediated inhibition, the signal transmission through the interlaminar connection is enormously facilitated and neurons in the deeper layers exhibit bursting response to stimulation of the superficial layer with non-linear amplification mechanism that depends on the activation of NMDA-type glutamate receptors. In addition, the cholinergic input to the intermediate layer lowers the threshold for the bursting response and facilitates the transmission through the interlaminar connection via activation of nicotinic receptors. The signal transmission through the interlaminar connection may lead to execution of extremely short latency saccades called express saccades.

Chattering and differential signal processing in identified motion-sensitive neurons of parallel visual pathways in the chick tectum

At least three identified cell types in the stratum griseum centrale (SGC) of the chick optic tectum mediate separate pathways from the retina to different subdivisions of the thalamic nucleus rotundus. Two of these, SGC type I and type II, constitute the major direct inputs to rotundal subdivisions that process various aspects of visual information, e.g., motion and luminance changes. Here, we examined the responses of these cell types to somatic current injection and synaptic input. We used a brain slice preparation of the chick tectum and applied whole-cell patch recordings, restricted electrical stimulation of dendritic endings, and subsequent labeling with biocytin. Type I neurons responded with regular sequences of bursts ("chattering") to depolarizing current injection. Electrical stimulation of retinal afferents evoked a sharp-onset EPSP/burst response that was blocked with CNQX. The sharp-onset EPSP/burst response to synaptic stimulation persisted when the soma was hyperpolarized, thus suggesting the presence of dendritic spike generation. In contrast, the type II neurons responded to depolarizing current injection solely with an irregular sequence of individual spikes. Electrical stimulation of retinal afferents led to slow and long-lasting EPSPs that gave rise to one or several action potentials. In conclusion, the morphological distinct SGC type I and II neurons also have different response properties to retinal inputs. This difference is likely to have functional significance for the differential processing of visual information in the separate pathways from the retina to different subdivisions of the thalamic nucleus rotundus.

Disinhibition in rat superior colliculus mediated by GABAc receptors

The stratum griseum superficiale (SGS) of the superior colliculus contains a high concentration of the recently described GABA(C) receptor. In a previous study, it was postulated that activation of these receptors on inhibitory interneurons functions to disinhibit projection cells that relay visual information to the thalamus and brainstem. To test this model, we used in vitro whole-cell patch-clamp methods to measure effects of GABA and muscimol on EPSCs and IPSCs evoked in rat SGS by electrical optic layer stimulation. The neurons were filled with biocytin for later morphological characterization. As expected, bath applications of GABA and muscimol always strongly depressed evoked PSCs at concentrations of >100 and >1 micrometer, respectively. However, at lower agonist concentrations, which most likely activate GABA(C) but not GABA(A) receptors, effects were not uniform. Evoked responses were suppressed by both agonists in 48% of the neurons, whereas the remaining cells exhibited enhanced responses with increased evoked EPSCs, decreased evoked IPSCs, or both types of change. Most morphologically identified cells with suppressed responses (14 of 17 cells) had morphological characteristics of putative GABAergic interneurons, whereas almost all cells with enhanced responses (8 of 10 cells) had morphological characteristics of projection cells. Finally, all effects of GABA and muscimol at low concentrations were blocked by (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid, a specific GABA(C) receptor antagonist, but not by the specific GABA(A) receptor antagonist bicuculline. Taken together, these results indicate that in SGS, GABA(C) receptors are predominantly expressed by GABAergic neurons and that activation of these receptors leads to disinhibition of SGS projection cells.

Contribution of superficial layer neurons to premotor bursts in the superior colliculus

In vitro whole-cell patch-clamp methods were used to examine the contribution of one component of intracollicular circuitry, the superficial gray layer, to the generation of bursts of action potentials that occur in the intermediate layer and that command head and eye movements in vivo. Applying a single brief (0.5 ms) pulse of current to the superficial layer of rat collicular slices evoked prolonged bursts of excitatory postsynaptic currents (EPSCs) in the cells of the intermediate layer. The EPSCs were sufficient to elicit bursts of action potentials that lasted as long as 300 ms and resembled presaccadic command bursts. To examine the contribution of neurons within the superficial layer to the production of these bursts, we determined how superficial neurons respond to the same current pulses that evoke bursts in the intermediate layer. Recordings from 61 superficial layer cells revealed 19 neurons that produced multiple action potentials following stimulation. Nine of these 19 neurons were wide- and narrow-field vertical cells, which are known to project to the intermediate layer and could contribute to producing the EPSC bursts. The remaining cells (n = 42) did not generate trains of action potentials and 21 of these showed only subthreshold potential changes in response to the stimulus. Our results indicate that most superficial cells do not directly contribute to production of the EPSC bursts, but a small number do have the properties necessary to provide a prolonged excitatory drive to the premotor neurons.

Patterns of synchronization in the superior colliculus of anesthetized cats

Sensorimotor transformations in the mammalian superior colliculus (SC) are mediated by large sets of distributed neurons. For such distributed coding systems, stimulus superposition poses problems attributable to the merging of neural populations coding for different stimuli. Such superposition problems could be overcome by synchronization of neuronal discharges, because it allows the selection of a subset of distributed responses for further joint processing. To assess the putative role of such a temporal binding mechanism in the SC, we have applied correlation analysis to visually evoked collicular activity. We performed recordings of single-unit and multiunit activity in the SC of anesthetized and paralyzed cats with multiple electrodes. Autocorrelation analysis revealed that collicular neurons often discharged in broad (20-100 msec) bursts or with an oscillatory patterning in the alpha- and beta-frequency range. Significantly modulated cross-correlograms were observed in 50% (128 of 258) of the collicular multiunit recording pairs, and for these pairs significant correlations occurred in 44% of the stimulation epochs. For the single-unit pairs, significant interactions were observed in 14 of 48 cases studied (29%). Collicular cross-correlograms were often oscillatory, and these oscillations covered a broad frequency range of up to 100 Hz, with a predominance of oscillation frequencies in the alpha- and beta-range. In the majority of the significant correlograms (64%) the phase lag of the center peak was <5 msec. The probability of collicular synchronization increased with the overlap of the receptive fields and the proximity of the recording sites. Correlations were also observed between cells in the superficial and deep SC layers. Collicular synchronization required activation of the respective cells with a single coherent stimulus and broke down when the neurons were activated with two different stimuli. These data are consistent with the notion that collicular synchrony could define assemblies of functionally related cells.

The visuo-motor pathway in the local circuit of the rat superior colliculus

Intrinsic circuit of the superior colliculus (SC), in particular the pathway from the optic tract (OT) to neurons in the intermediate layer (SGI), was investigated by whole-cell patch-clamp recording in slice preparations obtained from 17- to 24-d-old rats. Stimulation of the OT induced monosynaptic EPSPs in neurons in the superficial gray layer (SGS) and the optic layer (SO), and disynaptic or polysynaptic EPSPs in a majority of SGI neurons. Stimulation of the SGS induced monosynaptic or oligosynaptic EPSPs in the SGI neurons. Both the monosynaptic EPSPs induced in the SGS/SO neurons by stimulation of the OT and those induced in the SGI neurons by stimulation of the SGS were mediated by AMPA- and NMDA-type glutamate receptors. Thus, we have clarified the existence of the glutamatergic excitatory pathway from the OT to the SGI neurons via SGS and SO neurons. The EPSPs in the SGI neurons induced by stimulation of the OT or SGS were remarkably enhanced by bicuculline, suggesting that the signal transmission in this pathway is under strong suppression by the GABAergic system.

Intrinsic circuitry of the superior colliculus: pharmacophysiological identification of horizontally oriented inhibitory interneurons

Much of what is known about the organization of the superior colliculus is based on the arrangement of its external connections. Consequently, there is little information regarding pathways that remain intrinsic to it, even though recent data suggest that a horizontally oriented local circuit may mediate the functional reciprocity among fixation and saccade-related neurons. Therefore, the present experiments sought physiological evidence for neurons intrinsic to the superior colliculus that might participate in a horizontally oriented local circuit. Parasagittal slices of the ferret superior colliculus were prepared for in vitro recording, and 125 intermediate/deep layer neurons were examined in response to electrical stimulation rostral or caudal to the recording site. A substantial proportion (37%) of neurons responded with a prolonged period (means = 59.3 +/- 30 ms) of poststimulus suppression of spontaneous action potential activity. Of the suppressed neurons, most (53%) were disinhibited when the excitatory amino acid receptor antagonists D-2-amino-5-phosphonovaleric acid (D-APV) and 6-nitro-7 sulphamoylbeno[f]-quinoxaline-2,3-dione (NBQX) were administered, indicating that excitatory input to inhibitory interneurons was blocked. Of the neurons that received inputs from inhibitory interneurons, all had their suppressive responses decreased or eliminated by the gamma-aminobutyric acid antagonist, bicuculline. Finally, severing the superficial layers from the slice had no effect on intermediate layer responses to intrinsic stimulation. These data provide physiological evidence for the presence of horizontally oriented inhibitory interneurons in the superior colliculus. Furthermore, these findings are consistent with the hypothesis that an intrinsic circuit, routed through interneurons, might account for the reciprocal inhibition observed among fixation and saccade-related neurons.

Role of intrinsic synaptic circuitry in collicular sensorimotor integration

The superficial gray layer of the superior colliculus contains a map that represents the visual field, whereas the underlying intermediate gray layer contains a vector map of the saccades that shift the direction of gaze. These two maps are aligned so that a particular region of the visual field is represented directly above the neurons that orient the highest acuity area of the retina toward that region. Although it has been proposed that the transmission of information from the visuosensory to the motor map plays an important role in the generation of visually guided saccades, experiments have failed to demonstrate any functional linkage between the two layers. We examined synaptic transmission between these layers in vitro by stimulating the superficial layer while using whole-cell patch-clamp methods to measure the responses of intermediate layer neurons. Stimulation of superficial layer neurons evoked excitatory postsynaptic currents in premotor cells. This synaptic input was columnar in organization, indicating that the connections between the layers link corresponding regions of the visuosensory and motor maps. Excitatory postsynaptic currents were large enough to evoke action potentials and often occurred in clusters similar in duration to the bursts of action potentials that premotor cells use to command saccades. Our results indicate the presence of functional connections between the superficial and intermediate layers and show that such connections could play a significant role in the generation of visually guided saccades.

Interlaminar connections of the superior colliculus in the tree shrew. III: The optic layer

These experiments were designed to test the idea that the optic layer in the tree shrew, Tupaia belangeri, is functionally distinct and provides a link between the visuosensory superficial and the premotor intermediate layers of the superior colliculus. First, cells in the optic layer were intracellularly labeled with biocytin in living brain slices. Compared to cells in the adjacent lower part of the superficial gray layer, which have apical dendrites that ascend toward the tectal surface, optic layer cells have dendritic fields that are restricted for the most part to the optic layer itself. The differences in dendritic-field location imply that superficial gray and optic layer cells have different patterns of input. The axons of optic layer cells terminate densely within the optic layer and, in addition, project in a horizontally restricted fashion to the overlying superficial gray and subjacent intermediate gray layers. This pattern also is different from the predominantly descending interlaminar projections of lower superficial gray layer cells. Next, cells in the intermediate gray layer were labeled in order to examine the relationships between optic layer cells and these subjacent neurons that project from the superior colliculus to oculomotor centers of the brain stem. Neurons in the upper part of the intermediate gray layer send apical dendrites into the optic layer and therefore can receive signals from the superficial gray layer either directly, from descending axons of lower superficial gray layer cells, or indirectly, through intervening optic layer cells. In contrast, lower intermediate gray layer cells have more radiate dendritic fields that are restricted to the intermediate gray layer. Thus, these lower cells must depend on descending projections from optic or upper intermediate gray layer cells for signals from the superficial gray layer. Together, these results support the idea that the optic layer is a distinct lamina that provides a link between the superficial and intermediate gray layers. They also are consistent with the traditional view that descending intracollicular projections play a role in the selection of visual targets for saccades.