Pulvinar Modulates Contrast Responses in the Visual Cortex as a Function of Cortical Hierarchy.

The pulvinar is the largest extrageniculate visual nucleus in mammals. Given its extensive reciprocal connectivity with the visual cortex, it allows the cortico-thalamocortical transfer of visual information. Nonetheless, knowledge of the nature of the pulvinar inputs to the cortex remains elusive. We investigated the impact of silencing the pulvinar on the contrast response function of neurons in 2 distinct hierarchical cortical areas in the cat (areas 17 and 21a). Pulvinar inactivation altered the response gain in both areas, but with larger changes observed in area 21a. A theoretical model was proposed, simulating the pulvinar contribution to cortical contrast responses by modifying the excitation-inhibition balanced state of neurons across the cortical hierarchy. Our experimental and theoretical data showed that the pulvinar exerts a greater modulatory influence on neuronal activity in area 21a than in the primary visual cortex, indicating that the pulvinar impact on cortical visual neurons varies along the cortical hierarchy.

Stronger responses to darks along the ventral pathway of the cat visual cortex.

Light increments (brights) and decrements (darks) are differently processed throughout the early visual system. It is well known that a bias towards faster and stronger responses to darks is present in the retina, lateral geniculate nucleus and primary visual cortex. In humans, psychophysical and neurophysiological data indicate that darks are better detected than brights, suggesting that the dark bias found in early visual areas is transmitted across the cortical hierarchy. Here, we tested this assumption by investigating the spatiotemporal features of responses to brights and darks in area 21a, a gateway area of the cat ventral stream, using reverse correlation analysis of a sparse noise stimulus. The receptive field of most 21a neurons exhibited larger dark subfields. Additionally, the amplitude of the responses to darks was considerably greater than those evoked by brights. In the temporal domain, no differences were found between the response peak latency. Thus, the present study supports the notion that bright/dark asymmetries are transmitted throughout the cortical hierarchy and further, that the luminance processing varies as a function of the position in the cortical hierarchy, dark preference being strongly enhanced (in the spatial domain and response amplitude) along the ventral pathway.

Spatiotemporal Profile of Voltage-Sensitive Dye Responses in the Visual Cortex of Tree Shrews Evoked by Electric Microstimulation of the Dorsal Lateral Geniculate and Pulvinar Nuclei.

The primary visual cortex (V1) receives its main thalamic drive from the dorsal lateral geniculate nucleus (dLGN) through synaptic contacts terminating primarily in cortical layer IV. In contrast, the projections from the pulvinar nucleus to the cortex are less clearly defined. The pulvinar projects predominantly to layer I in V1, and layer IV in extrastriate areas. These projection patterns suggest that the pulvinar nucleus most strongly influences (drives) activity in cortical areas beyond V1. Should this hypothesis be true, one would expect the spatiotemporal responses evoked by pulvinar activation to be different in V1 and extrastriate areas, reflecting the different connectivity patterns. We investigated this issue by analyzing the spatiotemporal dynamics of cortical visual areas' activity following thalamic electrical microstimulation in tree shrews, using optical imaging and voltage-sensitive dyes. As expected, electrical stimulation of the dLGN induced fast and local responses in V1, as well as in extrastriate and contralateral cortical areas. In contrast, electrical stimulation of the pulvinar induced fast and local responses in extrastriate areas, followed by weak and diffuse activation in V1 and contralateral cortical areas. This study highlights spatiotemporal cortical activation characteristics induced by stimulation of first (dLGN) and high-order (pulvinar) thalamic nuclei. SIGNIFICANCE STATEMENT: The pulvinar nucleus represents the main extrageniculate thalamic visual structure in higher-order mammals, but its exact role remains enigmatic. The pulvinar receive prominent inputs from virtually all visual cortical areas. Cortico-thalamo-cortical pathways through the pulvinar nuclei may then provide a complementary route for corticocortical information flow. One step toward the understanding of the role of transthalamic corticocortical pathways is to determine the nature of the signals transmitted between the cortex and the thalamus. By performing, for the first time, high spatiotemporal mesoscopic imaging on tree shrews (the primate's closest relative) through the combination of voltage-sensitive dye recordings and brain stimulation, we revealed clear evidence of distinct thalamocortical functional connectivity pattern originating from the geniculate nucleus and the pulvinar nuclei.

Scotopic vision in the monkey is modulated by the G protein-coupled receptor 55.

The endogenous cannabinoid system plays important roles in the retina of mice and monkeys via their classic CB1 and CB2 receptors. We have previously reported that the G protein-coupled receptor 55 (GPR55), a putative cannabinoid receptor, is exclusively expressed in rod photoreceptors in the monkey retina, suggesting its possible role in scotopic vision. To test this hypothesis, we recorded full-field electroretinograms (ERGs) after the intravitreal injection of the GPR55 agonist lysophosphatidylglucoside (LPG) or the selective GPR55 antagonist CID16020046 (CID), under light- and dark-adapted conditions. Thirteen vervet monkeys (Chlorocebus sabaeus) were used in this study: four controls (injected with the vehicle dimethyl sulfoxide, DMSO), four injected with LPG and five with CID. We analyzed amplitudes and latencies of the a-wave (photoreceptor responses) and the b-wave (rod and cone system responses) of the ERG. Our results showed that after injection of LPG, the amplitude of the scotopic b-wave was significantly higher, whereas after the injection of CID, it was significantly decreased, compared to the vehicle (DMSO). On the other hand, the a-wave amplitude, and the a-wave and b-wave latencies, of the scotopic ERG responses were not significantly affected by the injection of either compound. Furthermore, the photopic ERG waveforms were not affected by either drug. These results support the hypothesis that GPR55 plays an instrumental role in mediating scotopic vision.

Expression and Function of the Endocannabinoid System in the Retina and the Visual Brain.

Endocannabinoids are important retrograde modulators of synaptic transmission throughout the nervous system. Cannabinoid receptors are seven transmembrane G-protein coupled receptors favoring Gi/o protein. They are known to play an important role in various processes, including metabolic regulation, craving, pain, anxiety, and immune function. In the last decade, there has been a growing interest for endocannabinoids in the retina and their role in visual processing. The purpose of this review is to characterize the expression and physiological functions of the endocannabinoid system in the visual system, from the retina to the primary visual cortex, with a main interest regarding the retina, which is the best-described area in this system so far. It will show that the endocannabinoid system is widely present in the retina, mostly in the through pathway where it can modulate neurotransmitter release and ion channel activity, although some evidence also indicates possible mechanisms via amacrine, horizontal, and Müller cells. The presence of multiple endocannabinoid ligands, synthesizing and catabolizing enzymes, and receptors highlights various pharmacological targets for novel therapeutic application to retinal diseases.

A Comparative Analysis of the Endocannabinoid System in the Retina of Mice, Tree Shrews, and Monkeys.

The endocannabinoid (eCB) system is widely expressed in various parts of the central nervous system, including the retina. The localization of the key eCB receptors, particularly CB1R and CB2R, has been recently reported in rodent and primate retinas with striking interspecies differences. Little is known about the distribution of the enzymes involved in the synthesis and degradation of these eCBs. We therefore examined the expression and localization of the main components of the eCB system in the retina of mice, tree shrews, and monkeys. We found that CB1R and FAAH distributions are well-preserved among these species. However, expression of NAPE-PLD is circumscribed to the photoreceptor layer only in monkeys. In contrast, CB2R expression is variable across these species; in mice, CB2R is found in retinal neurons but not in glial cells; in tree shrews, CB2R is expressed in Müller cell processes of the outer retina and in retinal neurons of the inner retina; in monkeys, CB2R is restricted to Müller cells. Finally, the expression patterns of MAGL and DAGLα are differently expressed across species. Overall, these results provide evidence that the eCB system is differently expressed in the retina of these mammals and suggest a distinctive role of eCBs in visual processing.

Cannabinoid Receptors CB1 and CB2 Modulate the Electroretinographic Waves in Vervet Monkeys.

The expression patterns of the cannabinoid receptor type 1 (CB1R) and the cannabinoid receptor type 2 (CB2R) are well documented in rodents and primates. In vervet monkeys, CB1R is present in the retinal neurons (photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells) and CB2R is exclusively found in the retinal glia (Müller cells). However, the role of these cannabinoid receptors in normal primate retinal function remains elusive. Using full-field electroretinography in adult vervet monkeys, we recorded changes in neural activity following the blockade of CB1R and CB2R by the intravitreal administration of their antagonists (AM251 and AM630, resp.) in photopic and scotopic conditions. Our results show that AM251 increases the photopic a-wave amplitude at high flash intensities, whereas AM630 increases the amplitude of both the photopic a- and b-waves. In scotopic conditions, both blockers increased the b-wave amplitude but did not change the a-wave amplitude. These findings suggest an important role of CB1R and CB2R in primate retinal function.

Spatiotemporal profiles of receptive fields of neurons in the lateral posterior nucleus of the cat LP-pulvinar complex.

The pulvinar is the largest extrageniculate thalamic visual nucleus in mammals. It establishes reciprocal connections with virtually all visual cortexes and likely plays a role in transthalamic cortico-cortical communication. In cats, the lateral posterior nucleus (LP) of the LP-pulvinar complex can be subdivided in two subregions, the lateral (LPl) and medial (LPm) parts, which receive a predominant input from the striate cortex and the superior colliculus, respectively. Here, we revisit the receptive field structure of LPl and LPm cells in anesthetized cats by determining their first-order spatiotemporal profiles through reverse correlation analysis following sparse noise stimulation. Our data reveal the existence of previously unidentified receptive field profiles in the LP nucleus both in space and time domains. While some cells responded to only one stimulus polarity, the majority of neurons had receptive fields comprised of bright and dark responsive subfields. For these neurons, dark subfields' size was larger than that of bright subfields. A variety of receptive field spatial organization types were identified, ranging from totally overlapped to segregated bright and dark subfields. In the time domain, a large spectrum of activity overlap was found, from cells with temporally coinciding subfield activity to neurons with distinct, time-dissociated subfield peak activity windows. We also found LP neurons with space-time inseparable receptive fields and neurons with multiple activity periods. Finally, a substantial degree of homology was found between LPl and LPm first-order receptive field spatiotemporal profiles, suggesting a high integration of cortical and subcortical inputs within the LP-pulvinar complex.

Impaired functional organization in the visual cortex of muscarinic receptor knock-out mice.

Acetylcholine modulates maturation and neuronal activity through muscarinic and nicotinic receptors in the primary visual cortex. However, the specific contribution of different muscarinic receptor subtypes in these neuromodulatory mechanisms is not fully understood. The present study evaluates in vivo the functional organization and the properties of the visual cortex of different groups of muscarinic receptor knock-out (KO) mice. Optical imaging of intrinsic signals coupled to continuous and episodic visual stimulation paradigms was used. Retinotopic maps along elevation and azimuth were preserved among the different groups of mice. However, compared to their wild-type counterparts, the apparent visual field along elevation was larger in M2/M4-KO mice but smaller in M1-KO. There was a reduction in the estimated relative receptive field size of V1 neurons in M1/M3-KO and M1-KO mice. Spatial frequency and contrast selectivity of V1 neuronal populations were affected only in M1/M3-KO and M1-KO mice. Finally, the neuronal connectivity was altered by the absence of M2/M4 muscarinic receptors. All these effects suggest the distinct roles of different subtypes of muscarinic receptors in the intrinsic organization of V1 and a strong involvement of the muscarinic transmission in the detectability of visual stimuli.

The pulvinar as a hub of visual processing and cortical integration.

The pulvinar nucleus of the thalamus is a crucial component of the visual system and plays significant roles in sensory processing and cognitive integration. The pulvinar's extensive connectivity with cortical regions allows for bidirectional communication, contributing to the integration of sensory information across the visual hierarchy. Recent findings underscore the pulvinar's involvement in attentional modulation, feature binding, and predictive coding. In this review, we highlight recent advances in clarifying the pulvinar's circuitry and function. We discuss the contributions of the pulvinar to signal modulation across the global cortical network and place these findings within theoretical frameworks of cortical processing, particularly the global neuronal workspace (GNW) theory and predictive coding.

Dynamic Ultrasound Localization Microscopy Without ECG-Gating.

OBJECTIVE: Dynamic Ultrasound Localization Microscopy (DULM) has first been developed for non-invasive Pulsatility measurements in the rodent brain. DULM relies on the localization and tracking of microbubbles (MBs) injected into the bloodstream, to obtain highly resolved velocity and density cine-loops. Previous DULM techniques required ECG-gating, limiting its application to specific datasets, and increasing acquisition time. The objective of this study is to eliminate the need for ECG-gating in DULM experiments by introducing a motion-matching method for time registration. METHODS: We developed a motion-matching algorithm based on tissue Doppler that leverages the cyclic tissue motion within the brain. Tissue Doppler was estimated for each group of frames in the acquisitions, at multiple locations identified as local maxima in the skin above the skull. Subsequently, each group of frames was time-registered to a reference group by delaying it based on the maximum correlation value between their respective tissue Doppler signals. This synchronization ensured that each group of frames aligned with the brain tissue motion of the reference group, and consequently, with its cardiac cycle. As a result, velocities of MBs could be averaged to retrieve flow velocity variations over time. RESULTS: Initially validated in ECG-gated acquisitions in a rat model (n = 1), the proposed method was successfully applied in a mice model in 2D (n = 3) and in a feline model in 3D (n = 1). Performing time-registration with the proposed motion-matching method or by using ECG-gating leads to similar results. For the first time, dynamic velocity and density cine-loops were extracted without the need for any information on the animal ECG, and complex dynamic markers such as the Pulsatility index were estimated. CONCLUSION: Results suggest that DULM can be performed without external gating, enabling the use of DULM on any ULM dataset where enough MBs are detectable. Time registration by motion-matching represents a significant advancement in DULM techniques, making DULM more accessible by simplifying its experimental complexity.

Quantitative pulsatility measurements using 3D dynamic ultrasound localization microscopy.

A rise in blood flow velocity variations (i.e. pulsatility) in the brain, caused by the stiffening of upstream arteries, is associated with cognitive impairment and neurodegenerative diseases. The study of this phenomenon requires brain-wide pulsatility measurements, with large penetration depth and high spatiotemporal resolution. The development of dynamic ultrasound localization microscopy (DULM), based on ULM, has enabled pulsatility measurements in the rodent brain in 2D. However, 2D imaging accesses only one slice of the brain and measures only 2D-projected and hence biased velocities . Herein, we present 3D DULM: using a single ultrasound scanner at high frame rate (1000-2000 Hz), this method can produce dynamic maps of microbubbles flowing in the bloodstream and extract quantitative pulsatility measurements in the cat brain with craniotomy and in the mouse brain through the skull, showing a wide range of flow hemodynamics in both large and small vessels. We highlighted a decrease in pulsatility along the vascular tree in the cat brain, which could be mapped with ultrasound down to a few tens of micrometers for the first time. We also performed an intra-animal validation of the method by showing consistent measurements between the two sides of the Willis circle in the mouse brain. Our study provides the first step towards a new biomarker that would allow the detection of dynamic abnormalities in microvessels in the brain, which could be linked to early signs of neurodegenerative diseases.

The dorsal lateral geniculate nucleus and the pulvinar as essential partners for visual cortical functions.

In most neuroscience textbooks, the thalamus is presented as a structure that relays sensory signals from visual, auditory, somatosensory, and gustatory receptors to the cerebral cortex. But the function of the thalamic nuclei goes beyond the simple transfer of information. This is especially true for the second-order nuclei, but also applies to first-order nuclei. First order thalamic nuclei receive information from the periphery, like the dorsal lateral geniculate nucleus (dLGN), which receives a direct input from the retina. In contrast, second order thalamic nuclei, like the pulvinar, receive minor or no input from the periphery, with the bulk of their input derived from cortical areas. The dLGN refines the information received from the retina by temporal decorrelation, thereby transmitting the most "relevant" signals to the visual cortex. The pulvinar is closely linked to virtually all visual cortical areas, and there is growing evidence that it is necessary for normal cortical processing and for aspects of visual cognition. In this article, we will discuss what we know and do not know about these structures and propose some thoughts based on the knowledge gained during the course of our careers. We hope that these thoughts will arouse curiosity about the visual thalamus and its important role, especially for the next generation of neuroscientists.

Cortical recurrence supports resilience to sensory variance in the primary visual cortex.

Our daily endeavors occur in a complex visual environment, whose intrinsic variability challenges the way we integrate information to make decisions. By processing myriads of parallel sensory inputs, our brain is theoretically able to compute the variance of its environment, a cue known to guide our behavior. Yet, the neurobiological and computational basis of such variance computations are still poorly understood. Here, we quantify the dynamics of sensory variance modulations of cat primary visual cortex neurons. We report two archetypal neuronal responses, one of which is resilient to changes in variance and co-encodes the sensory feature and its variance, improving the population encoding of orientation. The existence of these variance-specific responses can be accounted for by a model of intracortical recurrent connectivity. We thus propose that local recurrent circuits process uncertainty as a generic computation, advancing our understanding of how the brain handles naturalistic inputs.

Distributions of vesicular glutamate transporters 1 and 2 in the visual thalamus and associated areas of the cat.

Glutamate is packaged in vesicles via two main vesicular transporter (VGLUT) proteins, VGLUT1 and VGLUT2, which regulate its storage and release from synapses of excitatory neurons. Studies in rodents, primates, ferrets, and tree shrews suggest that these transporters may identify distinct subsets of excitatory projections in visual structures, particularly in thalamocortical pathways where they tend to correlate with modulatory and driver projections, respectively. Despite being a well-studied model of thalamocortical connectivity, little is known about their expression pattern in the cat visual system. To expand current knowledge on their distribution and how they correlated with known driver and modulator projecting sites, we examined the protein expression patterns of VGLUT1 and VGLUT2 in the visual thalamus of the cat (lateral geniculate nucleus and the pulvinar complex). We also studied their expression pattern in relevant visual structures projecting to or receiving significant thalamic projections, such as the primary visual cortex and the superior colliculus. Our results indicate that both VGLUTs are consistently present throughout the cat visual system and show laminar or nuclei specificity in their distribution, which suggests, as in other species, that VGLUT1 and VGLUT2 represent distinct populations of glutamatergic projections.

Reliable, Fast and Stable Contrast Response Function Estimation.

A study was conducted to determine stable cortical contrast response functions (CRFs) accurately and repeatedly in the shortest possible experimentation time. The method consisted of searching for experimental temporal aspects (number and duration of trials and number and distribution of contrasts used) with a model based on inhomogeneous Poisson spike trains to varying contrast levels. The set of values providing both short experimental duration and maximizing fit of the CRFs were saved, and then tested on cats' visual cortical neurons. Our analysis revealed that 4 sets of parameters with less or equal to 6 experimental visual contrasts satisfied our premise of obtaining good CRFs' performance in a short recording period, in which the number of trials seems to be the experimental condition that stabilizes the fit.

Corticothalamic Projections Gate Alpha Rhythms in the Pulvinar.

Two types of corticothalamic (CT) terminals reach the pulvinar nucleus of the thalamus, and their distribution varies according to the hierarchical level of the cortical area they originate from. While type 2 terminals are more abundant at lower hierarchical levels, terminals from higher cortical areas mostly exhibit type 1 axons. Such terminals also evoke different excitatory postsynaptic potential dynamic profiles, presenting facilitation for type 1 and depression for type 2. As the pulvinar is involved in the oscillatory regulation between intercortical areas, fundamental questions about the role of these different terminal types in the neuronal communication throughout the cortical hierarchy are yielded. Our theoretical results support that the co-action of the two types of terminals produces different oscillatory rhythms in pulvinar neurons. More precisely, terminal types 1 and 2 produce alpha-band oscillations at a specific range of connectivity weights. Such oscillatory activity is generated by an unstable transition of the balanced state network's properties that it is found between the quiescent state and the stable asynchronous spike response state. While CT projections from areas 17 and 21a are arranged in the model as the empirical proportion of terminal types 1 and 2, the actions of these two cortical connections are antagonistic. As area 17 generates low-band oscillatory activity, cortical area 21a shifts pulvinar responses to stable asynchronous spiking activity and vice versa when area 17 produces an asynchronous state. To further investigate such oscillatory effects through corticothalamo-cortical projections, the transthalamic pathway, we created a cortical feedforward network of two cortical areas, 17 and 21a, with CT connections to a pulvinar-like network with two cortico-recipient compartments. With this model, the transthalamic pathway propagates alpha waves from the pulvinar to area 21a. This oscillatory transfer ceases when reciprocal connections from area 21a reach the pulvinar, closing the CT loop. Taken together, results of our model suggest that the pulvinar shows a bi-stable spiking activity, oscillatory or regular asynchronous spiking, whose responses are gated by the different activation of cortico-pulvinar projections from lower to higher-order areas such as areas 17 and 21a.

Laminar distribution of cortical projection neurons to the pulvinar: A comparative study in cats and mice.

The cortical processing of visual information is thought to follow a hierarchical framework. This framework of connections between visual areas is based on the laminar patterns of direct feedforward and feedback cortico-cortical projections. However, this view ignores the cortico-thalamo-cortical projections to the pulvinar nucleus in the thalamus, which provides an alternative transthalamic information transfer between cortical areas. It was proposed that corticothalamic (CT) pathways follow a similar hierarchical pattern as cortico-cortical connections. Two main types of CT projections have been recognized: drivers and modulators. Drivers originate mainly in Layer 5 whereas modulators are from Layer 6. Little is known about the laminar distribution of these projections to the pulvinar across visual cortical areas. Here, we analyzed the distribution of CT neurons projecting to the lateral posterior (LP) thalamus in two species: cats and mice. Injections of the retrograde tracer fragment B of cholera toxin (CTb) were performed in the LP. The morphology and cortical laminar distribution of CTb-labeled neurons was assessed. In cats, neurons were mostly found in Layer 6 except in Area 17, where they were mostly in Layer 5. In contrast, CT neurons in mice were mostly located in Layer 6 across all areas. Thus, our results demonstrate that CT projections in mice do not follow the same organization as cats suggesting that the transthalamic pathways play distinct roles in these species.

Hierarchical Organization of Corticothalamic Projections to the Pulvinar.

Signals from lower cortical visual areas travel to higher-order areas for further processing through cortico-cortical projections, organized in a hierarchical manner. These signals can also be transferred between cortical areas via alternative cortical transthalamic routes involving higher-order thalamic nuclei like the pulvinar. It is unknown whether the organization of transthalamic pathways may reflect the cortical hierarchy. Two axon terminal types have been identified in corticothalamic (CT) pathways: the types I (modulators) and II (drivers) characterized by thin axons with small terminals and by thick axons and large terminals, respectively. In cats, projections from V1 to the pulvinar complex comprise mainly type II terminals, whereas those from extrastriate areas include a combination of both terminals suggesting that the nature of CT terminals varies with the hierarchical order of visual areas. To test this hypothesis, distribution of CT terminals from area 21a was charted and compared with 3 other visual areas located at different hierarchical levels. Results demonstrate that the proportion of modulatory CT inputs increases along the hierarchical level of cortical areas. This organization of transthalamic pathways reflecting cortical hierarchy provides new and fundamental insights for the establishment of more accurate models of cortical signal processing along transthalamic cortical pathways.