Distribution, morphology, and synaptic targets of corticothalamic terminals in the cat lateral posterior-pulvinar complex that originate from the posteromedial lateral suprasylvian cortex

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.

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.

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.

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.

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.

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.

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.

Distribution and Morphology of Cortical Terminals in the Cat Thalamus from the Anterior Ectosylvian Sulcus

Two main types of cortical terminals have been identified in the cat thalamus. Large (type II) have been proposed to drive the response properties of thalamic cells while smaller (type I) are believed to modulate those properties. Among the cat's visual cortical areas, the anterior ectosylvian visual area (AEV) is considered as one of the highest areas in the hierarchical organization of the visual system. Whereas the connections from the AEV to the thalamus have been recognized, their nature (type I or II) is presently not known. In this study, we assessed and compared the relative contribution of type I and type II inputs to thalamic nuclei originating from the AEV. The anterograde tracer BDA was injected in the AEV of five animals. Results show that (1) both type I and II terminals from AEV are present in the Lateral Posterior- Pulvinar complex, the lateral median suprageniculate complex and the medial and dorsal geniculate nuclei (2) type I terminals significantly outnumber the type II terminals in almost all nuclei studied. Our results indicate that neurons in the AEV are more likely to modulate response properties in the thalamus rather than to determine basic organization of receptive fields of thalamic cells.

Asymmetric Direct Reciprocal Connections Between Primary Visual and Somatosensory Cortices of the Mouse

Several studies show direct connections between primary sensory cortices involved in multisensory integration. The purpose of this study is to understand the microcircuitry of the reciprocal connections between visual and somatosensory cortices. The laminar distribution of retrogradely labeled cell bodies in V1 and in the somatosensory cortex both in (S1BF) and outside (S1) the barrel field was studied to provide layer indices in order to determine whether the connections are of feedforward, feedback or lateral type. Single axons were reconstructed and the size of their swellings was stereologically sampled. The negative layer indices in S1 and S1BF and the layer index near zero in V1 indicate that the connection from S1BF to V1 is of feedback type while the opposite is of lateral type. The greater incidence of larger axonal swellings in the projection from V1 to S1BF strongly suggests that S1BF receives a stronger driver input from V1 and that S1BF inputs to V1 have a predominant modulatory influence.

The mouse pulvinar nucleus: Organization of the tectorecipient zones

Comparative studies have greatly contributed to our understanding of the organization and function of visual pathways of the brain, including that of humans. This comparative approach is a particularly useful tactic for studying the pulvinar nucleus, an enigmatic structure which comprises the largest territory of the human thalamus. This review focuses on the regions of the mouse pulvinar that receive input from the superior colliculus, and highlights similarities of the tectorecipient pulvinar identified across species. Open questions are discussed, as well as the potential contributions of the mouse model for endeavors to elucidate the function of the pulvinar nucleus.

Corticothalamic axon morphologies and network architecture

Recent commentaries on the role of the thalamus consider a wide sphere of influence beyond sensory-motor transformation, to include task-relevant cognitive processes. In this short review, I reconsider known anatomic features of corticothalamic connectivity, primarily for macaque monkey, and discuss these as part of an intricate network architecture consistent with multiple connectional recombinations and a diversity of functional tasks. Drawing mainly on results from single axon analysis for the two broad classes of corticothalamic (CT) connections, I review the strikingly complementary spatial parameters of their extrinsic CT arbors in relation to intrinsic cortical collaterals. That is, CT neurons in layer 5 (class II) have spatially compact (low divergent) thalamic fields, but highly spatially divergent cortical collaterals. In contrast, CT neurons in layer 6 (class I) have highly divergent thalamic fields, but delimited, low divergent cortical collaterals. CT convergence in the thalamus is technically more difficult to analyze, but one can infer a low convergence of terminations from layer 5, in contrast with CT terminations from layer 6, which are highly convergent. Reciprocating thalamocortical (TC) axons have multiple clustered and divergent arbors. What to conclude from these relationships requires further investigation of activity patterns and networks under different conditions. Specific parameters are suggestive of selective recruitment of distributed postsynaptic networks and ordered activity sequences; but are these separable systems, operating cooperatively or in parallel (L.5 low divergent/low convergent vs. L. 6 high divergent/high convergent)?

Synaptic organization of striate cortex projections in the tree shrew: A comparison of the claustrum and dorsal thalamus

The tree shrew (Tupaia belangeri) striate cortex is reciprocally connected with the dorsal lateral geniculate nucleus (dLGN), the ventral pulvinar nucleus (Pv), and the claustrum. In the Pv or the dLGN, striate cortex projections are thought to either strongly "drive", or more subtly "modulate" activity patterns respectively. To provide clues to the function of the claustrum, we compare the synaptic arrangements of striate cortex projections to the dLGN, Pv, and claustrum, using anterograde tracing and electron microscopy. Tissue was additionally stained with antibodies against γ-aminobutyric acid (GABA) to identify GABAergic interneurons and non-GABAergic projection cells. The striate cortex terminals were largest in the Pv (0.94 ± 0.08 μm² ), intermediate in the claustrum (0.34 ± 0.02 μm² ), and smallest in the dLGN (0.24 ± 0.01 μm² ). Contacts on interneurons were most common in the Pv (39%), intermediate in the claustrum (15%), and least common in the dLGN (12%). In the claustrum, non-GABAergic terminals (0.34 ± 0.01 μm² ) and striate cortex terminals were not significantly different in size. The largest terminals in the claustrum were GABAergic (0.51 ± 0.02 μm² ), and these terminals contacted dendrites and somata that were significantly larger (1.90 ± 0.30 μm² ) than those contacted by cortex or non-GABAergic terminals (0.28 ± 0.02 μm² and 0.25 ± 0.02 μm² , respectively). Our results indicate that the synaptic organization of the claustrum does not correspond to a driver/modulator framework. Instead, the circuitry of the claustrum suggests an integration of convergent cortical inputs, gated by GABAergic circuits. J. Comp. Neurol. 525:1403-1420, 2017. © 2016 Wiley Periodicals, Inc.

Thalamic Circuit Diversity: Modulation of the Driver/Modulator Framework

The idea that dorsal thalamic inputs can be divided into “drivers”, which provide the primary excitatory drive for the relay of information to cortex, and “modulators”, which alter the gain of signal transmission, has provided a valuable organizing principle for the study of thalamic function. This view further promoted the identification of “first order” and “higher order” thalamic nuclei, based on the origin of their driving inputs. Since the introduction of this influential terminology, a number of studies have revealed the existence of a wide variety of thalamic organizational schemes. For example, some thalamic nuclei are not innervated by typical driver inputs, but instead receive input from terminals which exhibit features distinct from those of either classic drivers or modulators. In addition, many thalamic nuclei contain unique combinations of convergent first order, higher order, and/or other “driver-like” inputs that do not conform with the driver/modulator framework. The assortment of synaptic arrangements identified in the thalamus are reviewed and discussed from the perspective that this organizational diversity can dramatically increase the computational capabilities of the thalamus, reflecting its essential roles in sensory, motor, and sensory-motor circuits.

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.

Analysis of excitatory synapses in the guinea pig inferior colliculus: a study using electron microscopy and GABA immunocytochemistry

The inferior colliculus (IC) integrates ascending auditory input from the lower brainstem and descending input from the auditory cortex. Understanding how IC cells integrate these inputs requires identification of their synaptic arrangements. We describe excitatory synapses in the dorsal cortex, central nucleus, and lateral cortex of the IC (ICd, ICc and IClc) in guinea pigs. We used electron microscopy (EM) and post-embedding anti-GABA immunogold histochemistry on aldehyde-fixed tissue from pigmented adult guinea pigs. Excitatory synapses were identified by round vesicles, asymmetric synaptic junctions, and gamma-aminobutyric acid-immunonegative (GABA-negative) presynaptic boutons. Excitatory synapses constitute ∼60% of the synapses in each IC subdivision. Three types can be distinguished by presynaptic profile area and number of mitochondrial profiles. Large excitatory (LE) boutons are more than 2 μm(2) in area and usually contain five or more mitochondrial profiles. Small excitatory (SE) boutons are usually less than 0.7 μm(2) in area and usually contain 0 or 1 mitochondria. Medium excitatory (ME) boutons are intermediate in size and usually contain 2 to 4 mitochondria. LE boutons are mostly confined to the ICc, while the other two types are present throughout the IC. Dendritic spines are the most common target of excitatory boutons in the IC dorsal cortex, whereas dendritic shafts are the most common target in other IC subdivisions. Finally, each bouton type terminates on both gamma-aminobutyric acid-immunopositive (GABA+) and GABA-negative (i.e., glutamatergic) targets, with terminations on GABA-negative profiles being much more frequent. The ultrastructural differences between the three types of boutons presumably reflect different origins and may indicate differences in postsynaptic effect. Despite such differences in origins, each of the bouton types contact both GABAergic and non-GABAergic IC cells, and could be expected to activate both excitatory and inhibitory IC circuits.

Triadic synaptic interactions of large corticothalamic terminals in non-lemniscal thalamic nuclei of the cat auditory system

Large corticothalamic (CT) terminals, presumed to originate from cortical layer 5 pyramidal cells, are distributed predominantly in non-specific thalamic nuclei in mammals. In the auditory system, little is known about whether these CT projections participate in the synaptic aggregation referred to as the triad. We studied synaptic interactions of these terminals with neuronal elements in one of the auditory non-lemniscal thalamic nuclei, the dorsal nucleus of the medial geniculate complex (MGC), in cats. After injections of an anterograde tracer in the primary auditory cortex, areas containing labeled large terminals were examined using an electron microscope. It was revealed that a fraction of large CT terminals participated in complicated synaptic arrangements: labeled terminals making synaptic contacts with vesicle-free dendrites, probably of thalamic principal neurons, and/or vesicle-filled neuronal profiles, probably of presynaptic dendrites (PSDs) of interneurons. In reconstructions or even in single sections, we found that these synaptic connections participated in triadic arrangements. Thus, PSDs postsynaptic to the labeled CT terminals were in turn presynaptic to the vesicle-free dendrites.

Synaptic organization of connections between the temporal cortex and pulvinar nucleus of the tree shrew

We examined the synaptic organization of reciprocal connections between the temporal cortex and the dorsal (Pd) and central (Pc) subdivisions of the tree shrew pulvinar nucleus, regions innervated by the medial and lateral superior colliculus, respectively. Both Pd and Pc subdivisions project topographically to 2 separate regions of the temporal cortex; small injections of anterograde tracers placed in either Pd or Pc labeled 2 foci of terminals in the temporal cortex. Pulvinocortical pathways innervated layers I-IV, with beaded axons oriented perpendicular to the cortical surface, where they synapsed with spines that did not contain gamma amino butyric acid (GABA), likely located on the apical dendrites of pyramidal cells. Projections from the temporal cortex to the Pd and Pc originate from layer VI cells, and form small terminals that contact small caliber non-GABAergic dendrites. These results suggest that cortical terminals are located distal to tectopulvinar terminals on the dendritic arbors of Pd and Pc projection cells, which subsequently contact pyramidal cells in the temporal cortex. This circuitry could provide a mechanism for the pulvinar nucleus to activate subcortical visuomotor circuits and modulate the activity of other visual cortical areas. The potential relation to primate tecto-pulvino-cortical pathways is discussed.

Excitatory actions of substance P in the rat lateral posterior nucleus

The lateral posterior nucleus (LP) receives inputs from both neocortex and superior colliculus (SC), and is involved with integration and processing of higher-level visual information. Relay neurons in LP contain tachykinin receptors and are innervated by substance P (SP)-containing SC neurons and by layer V neurons of the visual cortex. In this study, we investigated the actions of SP on LP relay neurons using whole-cell recording techniques. SP produced a graded depolarizing response in LP neurons along the rostro-caudal extent of the lateral subdivision of LP nuclei (LPl), with a significantly larger response in rostral LPl neurons compared with caudal LPl neurons. In rostral LPl, SP (5-2000 nm) depolarized nearly all relay neurons tested (> 98%) in a concentration-dependent manner. Voltage-clamp experiments revealed that SP produced an inward current associated with a decreased conductance. The inward current was mediated primarily by neurokinin receptor (NK)(1) tachykinin receptors, although significantly smaller inward currents were produced by specific NK2 and NK3 receptor agonists. The selective NK(1) receptor antagonist RP67580 attenuated the SP-mediated response by 71.5% and was significantly larger than the attenuation of the SP response obtained by NK2 and NK3 receptor antagonists, GR159897 and SB222200, respectively. The SP-mediated response showed voltage characteristics consistent with a K+ conductance, and was attenuated by Cs+, a K+ channel blocker. Our data suggest that SP may modulate visual information that is being processed and integrated in the LPl with inputs from collicular sources.

Axon morphologies and convergence patterns of projections from different sensory-specific cortices of the anterior ectosylvian sulcus onto multisensory neurons in the cat superior colliculus

Corticofugal projections to the thalamus reveal 2 axonal morphologies, each associated with specific physiological attributes. These determine the functional characteristics of thalamic neurons. It is not clear, however, whether such features characterize the corticofugal projections that mediate multisensory integration in superior colliculus (SC) neurons. The cortico-collicular projections from cat anterior ectosylvian sulcus (AES) are derived from its visual, auditory, and somatosensory representations and are critical for multisensory integration. Following tracer injections into each subdivision, 2 types of cortico-collicular axons were observed. Most were categorized as type I and consisted of small-caliber axons traversing long distances without branching, bearing mainly small boutons. The less frequent type II had thicker axons, more complex branching patterns, larger boutons, and more complex terminal boutons. Following combinatorial injections of 2 different fluorescent tracers into defined AES subdivisions, fibers from each were seen converging onto individual SC neurons and indicate that such convergence, like that in the corticothalamic system, is mediated by 2 distinct morphological types of axon terminals. Nevertheless, and despite the conservation of axonal morphologies across different subcortical systems, it is not yet clear if the concomitant physiological attributes described in the thalamus are directly applicable to multisensory integration.

Synaptic organization of thalamocortical axon collaterals in the perigeniculate nucleus and dorsal lateral geniculate nucleus

We examined the synaptic targets of large non-gamma-aminobutyric acid (GABA)-ergic profiles that contain round vesicles and dark mitochondria (RLD profiles) in the perigeniculate nucleus (PGN) and the dorsal lateral geniculate nucleus (dLGN). RLD profiles can provisionally be identified as the collaterals of thalamocortical axons, because their ultrastrucure is distinct from all other previously described dLGN inputs. We also found that RLD profiles are larger than cholinergic terminals and contain the type 2 vesicular glutamate transporter. RLD profiles are distributed throughout the PGN and are concentrated within the interlaminar zones (IZs) of the dLGN, regions distinguished by dense binding of Wisteria floribunda agglutinin (WFA). To determine the synaptic targets of thalamocortical axon collaterals, we examined RLD profiles in the PGN and dLGN in tissue stained for GABA. For the PGN, we found that all RLD profiles make synaptic contacts with GABAergic PGN somata, dendrites, and spines. In the dLGN, RLD profiles primarily synapse with GABAergic dendrites that contain vesicles (F2 profiles) and non-GABAergic dendrites in glomerular arrangements that include triads. Occasional synapses on GABAergic somata and proximal dendrites were also observed in the dLGN. These results suggest that correlated dLGN activity may be enhanced via direct synaptic contacts between thalamocortical cells, whereas noncorrelated activity (such as that occurring during binocular rivalry) could be suppressed via thalamocortical collateral input to PGN cells and dLGN interneurons.

Evidence for nonreciprocal organization of the mouse auditory thalamocortical-corticothalamic projection systems

We tested the hypothesis that information is routed from one area of the auditory cortex (AC) to another via the dorsal division of the medial geniculate body (MGBd) by analyzing the degree of reciprocal connectivity between the auditory thalamus and cortex. Biotinylated dextran amine injected into the primary AC (AI) or anterior auditory field (AAF) of mice produced large, "driver-type" terminals primarily in the MGBd, with essentially no such terminals in the ventral MGB (MGBv). In contrast, small, "modulator-type" terminals were found primarily in the MGBv, and this coincided with areas of retrogradely labeled thalamocortical cell bodies. After MGBv injections, anterograde label was observed in layers 4 and 6 of the AI and AAF, which coincided with retrogradely labeled layer 6 cell bodies. After MGBd injections, thalamocortical terminals were seen in layers 1, 4, and 6 of the secondary AC and dorsoposterior AC, which coincided with labeled layer 6 cell bodies. Notably, after MGBd injection, a substantial number of layer 5 cells were labeled in all AC areas, whereas very few were seen after MGBv injection. Further, the degree of anterograde label in layer 4 of cortical columns containing labeled layer 6 cell bodies was greater than in columns containing labeled layer 5 cell bodies. These data suggest that auditory layer 5 corticothalamic projections are targeted to the MGBd in a nonreciprocal fashion and that the MGBd may route this information to the nonprimary AC.

Complex motion selectivity in PMLS cortex following early lesions of primary visual cortex in the cat

In the cat, the analysis of visual motion cues has generally been attributed to the posteromedial lateral suprasylvian cortex (PMLS) (Toyama et al., 1985; Rauschecker et al., 1987; Rauschecker, 1988; Kim et al., 1997). The responses of neurons in this area are not critically dependent on inputs from the primary visual cortex (VC), as lesions of VC leave neuronal response properties in PMLS relatively unchanged (Spear & Baumann, 1979; Spear, 1988; Guido et al., 1990b). However, previous studies have used a limited range of visual stimuli. In this study, we assessed whether neurons in PMLS cortex remained direction-selective to complex motion stimuli following a lesion of VC, particularly to complex random dot kinematograms (RDKs). Unilateral aspiration of VC was performed on post-natal days 7–9. Single unit extracellular recordings were performed one year later in the ipsilateral PMLS cortex. As in previous studies, a reduction in the percentage of direction selective neurons was observed with drifting sinewave gratings. We report a previously unobserved phenomenon with sinewave gratings, in which there is a greater modulation of firing rate at the temporal frequency of the stimulus in animals with a lesion of VC, suggesting an increased segregation of ON and OFF sub-regions. A significant portion of neurons in PMLS cortex were direction selective to simple (16/18) and complex (11/16) RDKs. However, the strength of direction selectivity to both stimuli was reduced as compared to normals. The data suggest that complex motion processing is still present, albeit reduced, in PMLS cortex despite the removal of VC input. The complex RDK motion selectivity is consistent with both geniculo-cortical and extra-geniculate thalamo-cortical pathways in residual direction encoding.

Two streams of attention-dependent beta activity in the striate recipient zone of cat's lateral posterior-pulvinar complex

Local field potentials from different visual cortical areas and subdivisions of the cat's lateral posterior-pulvinar complex of the thalamus (LP-P) were recorded during a behavioral task based on delayed spatial discrimination of visual or auditory stimuli. During visual but not auditory attentive tasks, we observed an increase of beta activity (12-25 Hz) as calculated from signals recorded from the caudal part of the lateral zone of the LP-P (LPl-c) as well as from cortical areas 17 and 18 and the complex located at the middle suprasylvian sulcus (MSS). This beta activity appeared only in the trials that ended with a successful response, proving its relationship to the mechanism of visual attention. In contrast, no enhanced beta activity was observed in the rostral part of the lateral zone of the LP-P and in the pulvinar proper. Two subregions of LPl-c (ventromedial and dorsolateral) were distinguished by visually related, attentional beta activity of low (12-18 Hz) and high (18-25 Hz) frequencies, respectively. At the same time, area 17 exhibited attentional activation in the whole beta range, and an increase of power in low-frequency beta was observed in the medial bank of MSS, whereas cortical area 18 and the lateral bank of the MSS were activated in the high beta range. Phase-correlation analysis revealed that two distinct corticothalamic systems were synchronized by the beta activity of different frequencies. One comprised of cortical area 17, ventromedial region of LPl-c, and medial MSS, the second involved area 18 and the dorsolateral LPl-c. Our observations suggest that LPl-c belongs to the wide corticothalamic attentional system, which is functionally segregated by distinct streams of beta activity.

Overlapping visual response latency distributions in visual cortices and LP-pulvinar complex of the cat

The visual system of the cat is considered to be organized in both a serial and parallel manner. Studies of visual onset latencies generally suggest that parallel processing occurs throughout the dorsal stream. These studies are at odds with the proposed hierarchies of visual areas based on termination patterns of cortico-cortical projections. In previous studies, a variety of stimuli have been used to compute latencies, and this is problematic as latencies are known to depend on stimulus parameters. This could explain the discrepancy between latency and neuroanatomical based studies. Therefore, the first aim of the present study was to determine whether latencies increased along the hierarchy of visual areas when the same stimuli are used. In addition, the effect of stimulus complexity was assessed. Visual onset latencies were calculated for area 17, PMLS, AMLS, and AEV neurons. Latencies were also computed from neurons in the lateral posterior (LP)-pulvinar complex given the importance of this extrageniculate complex in cortical intercommunication. Latency distributions from all regions overlapped substantially, and no significant difference was present, regardless of the type of stimulus used. The onset latencies in the LP-pulvinar complex were comparable to those seen in cortical areas. The data suggest that the initial processing of information in the visual system is parallel, despite the presence of a neuroanatomical hierarchy. Simultaneous response onsets among cortical areas and the LP-pulvinar suggest that the latter is more than a simple relay station for information headed to cortex. The data are consistent with proposals of the LP-pulvinar as a center for the integration and distribution of information from/to multiple cortical areas.