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

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.

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.

Transient enhancement of inhibition following visual cortical lesions in the mouse superior colliculus

Numerous studies have investigated the effects of lesions of the primary visual cortex (V1) on visual responses in neurons of the superficial layer of the superior colliculus (sSC), which receives visual information from both the retina and V1. However, little is known about the changes in the local circuit dynamics of the sSC after receiving V1 lesions. Here, we show that surround inhibition of sSC neurons is transiently enhanced following V1 lesions in mice and that this enhancement may be attributed to alterations in the balance between excitatory and inhibitory inputs to sSC neurons. Extracellular recordings in vivo revealed that sSC neuronal responses to large visual stimuli were transiently reduced at about 1 week after visual cortical lesions compared with normal mice and that this reduction was partially recovered at about 1 month after the lesions. By using whole-cell patch-clamp recordings from sSC neurons in slice preparations obtained from mice that had received visual cortical lesions at 1 week prior to the recordings, we found cell type-dependent changes in the balance between excitation and inhibition. In non-GABAergic cells, inhibition predominated over excitation, whereas the excitation-inhibition balance did not change in GABAergic neurons. These results suggest that enhanced inhibition may be partially responsible for the reduced responses to large visual stimuli in some sSC neurons. Thus, we propose that the enhanced surround inhibition shortly after visual cortical lesions may prevent hyperexcitability in the sSC local circuit, contributing to reconstructing the finely tuned receptive field organization of sSC neurons after the visual cortical lesions.

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.

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.

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.

Competitive integration of visual and preparatory signals in the superior colliculus during saccadic programming

Efficient behavior requires that internally specified motor plans be integrated with incoming sensory information. Motor preparation and visual signals converge in the intermediate and deep layers of the superior colliculus (SC) to influence saccade planning and execution; however, the mechanism by which these sometimes conflicting signals are combined remains unclear. We studied this issue by presenting visual distractors as monkeys prepared saccades toward an upcoming target whose timing and location were fully predictable. Monkeys made more distractor-directed errors when the spatial location of visual distractors more closely coincided with the saccadic goal. Concomitant pretarget activity of SC visuomotor neurons, whose response fields were centered on the saccadic goal, was similarly increased by the presentation of nearby distractors and inhibited by the presentation of distant distractors. Finally, subthreshold microstimulation of the SC shifted the pattern of distractor-directed errors away from the saccadic goal toward that specified by the site of stimulation. Together, our results suggest that the likelihood of saccade generation is influenced by the spatial register of internal motor preparation signals and external sensory signals across the topographically organized SC map.

Properties of cholinergic responses in neurons in the intermediate grey layer of rat superior colliculus

The intermediate grey layer (SGI) of superior colliculus (SC) receives cholinergic innervation from brainstem parabrachial region. To clarify the action of cholinergic inputs to local circuits in the SGI, we investigated the effect of cholinergic agonists and antagonists on a large number of randomly sampled neurons in Wistar rat SGI (n=246) using whole-cell patch clamp technique in slices of the rat SC. Responses of the recorded cells (n=98) to bath application of carbachol were classified into five patterns: (i) nicotinic inward only (n=14); (ii) nicotinic inward+muscarinic inward (n=26); (iii) nicotinic inward+muscarinic inward+muscarinic outward (n=39); (iv) nicotinic inward+muscarinic outward (n=13) and (v) muscarinic outward only (n=4). Among these, a majority of morphologically identified projection neurons exhibited either response pattern (ii) (9/28) or (iii) (15/28), which suggested that the primary action of cholinergic inputs on the SGI output is excitatory. Nicotinic receptor subtypes involved in the nicotinic current were examined by testing the effects of antagonists on the currents induced by bath application of 1,1-dimethyl-4-phenyl-piperazinium or transient pressure application of acetylcholine (ACh). Muscarinic receptor subtypes involved in the muscarinic inward and outward currents were investigated by examining the effects of antagonists on muscarine-induced currents. The results showed that nicotinic inward currents are mediated mainly by alpha4beta2 and partly by alpha7 nicotinic receptors and that muscarinic inward and outward currents are mediated by M3 (plus M1) and M2 muscarinic receptors, respectively.

Cortico–basal ganglia circuit mechanism for a decision threshold in reaction time tasks

Growing evidence from primate neurophysiology and modeling indicates that in reaction time tasks, a perceptual choice is made when the firing rate of a selective cortical neural population reaches a threshold. This raises two questions: what is the neural substrate of the threshold and how can it be adaptively tuned according to behavioral demands? Using a biophysically based network model of spiking neurons, we show that local dynamics in the superior colliculus gives rise to an all-or-none burst response that signals threshold crossing in upstream cortical neurons. Furthermore, the threshold level depends only weakly on the efficacy of the cortico-collicular pathway. In contrast, the threshold and the rate of reward harvest are sensitive to, and hence can be optimally tuned by, the strength of cortico-striatal synapses, which are known to be modifiable by dopamine-dependent plasticity. Our model provides a framework to describe the main computational steps in a reaction time task and suggests that separate brain pathways are critical to the detection and adjustment of a decision threshold.

Recruitment of local excitatory circuits in the superior colliculus following deafferentation and the regeneration of retinocollicular inputs

The local synaptic connectivity in the superficial gray layer of the superior colliculus (SC) was assessed following retinal ganglion cell axonal regeneration through a peripheral nerve graft into the SC of Lister Hooded rats, using in vitro brain slice techniques. Repair was effected between the ipsilateral eye and SC, following bilateral lesion of optic nerves and ablation of ipsilateral occipital cortex. Deafferentation surgery alone resulted in a complete loss of synaptic potentials of extrinsic origin, once both retinal and cortical inputs were removed. Stimulation of graft insertion sites elicited synaptic responses comprising monosynaptic and network-mediated depolarising events. This activity, together with similar spontaneous bursts of depolarising events and action potential firing, was generated by the activation of non-N-methyl-D-aspartate glutamate receptors. This behaviour may reflect the development of a local recurrent synaptic connectivity following the repair surgery, as both evoked and spontaneous responses developed into large long-lasting bursts of excitatory activity when inhibition mediated by GABA receptors was blocked. These results suggest that the ultrastructural changes in the superficial layers of the SC resulting from deafferentation are reflected functionally at the synaptic level in the target structure even after repair. Such changes are likely to compromise the ability of the target structure to function normally during information processing. Therefore, although axons regenerating along peripheral nerve grafts can make functional synaptic connections, their efficacy in activating the target structure will probably be compromised by local changes in synaptic connectivity.

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.