Encoding of stimulus frequency and sensor motion in the posterior medial thalamic nucleus

Role of posterior medial thalamus in the modulation of striatal circuitry and choice behavior

The posterior medial (POm) thalamus is heavily interconnected with sensory and motor circuitry and is likely involved in behavioral modulation and sensorimotor integration. POm provides axonal projections to the dorsal striatum, a hotspot of sensorimotor processing, yet the role of POm-striatal projections has remained undetermined. Using optogenetics with mouse brain slice electrophysiology, we found that POm provides robust synaptic input to direct and indirect pathway striatal spiny projection neurons (D1- and D2-SPNs, respectively) and parvalbumin-expressing fast spiking interneurons (PVs). During the performance of a whisker-based tactile discrimination task in head-restrained mice, POm-striatal projections displayed learning-related activation correlating with anticipatory, but not reward-related, pupil dilation. Inhibition of POm-striatal axons across learning caused slower reaction times and an increase in the number of training sessions for expert performance. Our data indicate that POm-striatal inputs provide a behaviorally relevant arousal-related signal, which may prime striatal circuitry for efficient integration of subsequent choice-related inputs.

Multiple Distinct Timescales of Rapid Sensory Adaption in the Thalamocortical Circuit

Numerous studies have shown that neuronal representations in sensory pathways are far from static but are instead strongly shaped by the complex properties of the sensory inputs they receive. Adaptation dynamically shapes the neural signaling that underlies our perception of the world yet remains poorly understood. We investigated rapid adaptation across timescales from hundreds of milliseconds to seconds through simultaneous multielectrode recordings from the ventro-posteromedial nucleus of the thalamus (VPm) and layer 4 of the primary somatosensory cortex (S1) in male and female anesthetized mice in response to controlled, persistent whisker stimulation. Observations in VPm and S1 reveal a degree of adaptation that progresses through the pathway. Under these experimental conditions, signatures of two distinct timescales of rapid adaptation in the firing rates of both thalamic and cortical neuronal populations were revealed, also reflected in the synchrony of the thalamic population and in the thalamocortical synaptic efficacy that was measured in putatively monosynaptically connected thalamocortical pairs. Controlled optogenetic activation of VPm further demonstrated that the longer timescale adaptation observed in S1 is likely inherited from slow decreases in thalamic firing rate and synchrony. Despite the degraded sensory responses, adaptation induced by the controlled repetitive stimulation presented here resulted in a shift in coding strategy that favors theoretical discrimination over detection across the observed timescales of adaptation. Overall, although multiple mechanisms contribute to rapid adaptation at distinct timescales, they support a unifying framework on the role of adaptation in sensory processing.

Deciphering consciousness: The role of corticothalamocortical interactions in general anesthesia

General anesthesia is administered to millions of individuals each year, however, the precise mechanism by which it induces unconsciousness remains unclear. While some theories suggest that anesthesia shares similarities with natural sleep, targeting sleep-promoting areas and inhibiting arousal nuclei, recent research indicates a more complex process. Emerging evidence highlights the critical role of corticothalamocortical circuits, which are involved in higher cognitive functions, in controlling arousal states and modulating transitions between different conscious states during anesthesia. The administration of general anesthetics disrupts connectivity within these circuits, resulting in a reversible state of unconsciousness. This review elucidates how anesthetics impair corticothalamocortical interactions, thereby affecting the flow of information across various cortical layers and disrupting higher-order cognitive functions while preserving basic sensory processing. Additionally, the role of the prefrontal cortex in regulating arousal through both top-down and bottom-up pathways was examined. These findings highlight the intricate interplay between the cortical and subcortical networks in maintaining and restoring consciousness under anesthesia, offering potential therapeutic targets for enhancing anesthesia management.

Role of Posterior Medial Thalamus in the Modulation of Striatal Circuitry and Choice Behavior

The posterior medial (POm) thalamus is heavily interconnected with sensory and motor circuitry and is likely involved in behavioral modulation and sensorimotor integration. POm provides axonal projections to the dorsal striatum, a hotspot of sensorimotor processing, yet the role of POm-striatal projections has remained undetermined. Using optogenetics with slice electrophysiology, we found that POm provides robust synaptic input to direct and indirect pathway striatal spiny projection neurons (D1- and D2-SPNs, respectively) and parvalbumin-expressing fast spiking interneurons (PVs). During the performance of a whisker-based tactile discrimination task, POm-striatal projections displayed learning-related activation correlating with anticipatory, but not reward-related, pupil dilation. Inhibition of POm-striatal axons across learning caused slower reaction times and an increase in the number of training sessions for expert performance. Our data indicate that POm-striatal inputs provide a behaviorally relevant arousal-related signal, which may prime striatal circuitry for efficient integration of subsequent choice-related inputs.

MULTIPLE DISTINCT TIMESCALES OF RAPID SENSORY ADAPATION IN THE THALAMOCORTICAL CIRCUIT

Numerous studies have shown that neuronal representations in sensory pathways are far from static but are instead strongly shaped by the complex properties of the sensory inputs they receive. Adaptation dynamically shapes the neural signaling that underlies our perception of the world yet remains poorly understood. We investigated rapid adaptation across timescales from hundreds of milliseconds to seconds through simultaneous multi-electrode recordings from the ventro-posteromedial nucleus of the thalamus (VPm) and layer 4 of the primary somatosensory cortex (S1) in male and female anesthetized mice in response to controlled, persistent whisker stimulation. Observations in VPm and S1 reveal a degree of adaptation that progresses through the pathway. Signatures of two distinct timescales of rapid adaptation in the firing rates of both thalamic and cortical neuronal populations were revealed, also reflected in the synchrony of the thalamic population and in the thalamocortical synaptic efficacy that was measured in putatively monosynaptically connected thalamocortical pairs. Controlled optogenetic activation of VPm further demonstrated that the longer timescale adaptation observed in S1 is likely inherited from slow decreases in thalamic firing rate and synchrony. Despite the degraded sensory responses, adaptation resulted in a shift in coding strategy that favors theoretical discrimination over detection across the observed timescales of adaptation. Overall, although multiple mechanisms contribute to rapid adaptation at distinct timescales, they support a unifying framework on the role of adaptation in sensory processing.

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.

Posteromedial thalamic nucleus activity significantly contributes to perceptual discrimination

Higher-order sensory thalamic nuclei are densely connected with multiple cortical and subcortical areas, yet the role of these nuclei remains elusive. The posteromedial thalamic nucleus (POm), the higher-order thalamic nucleus in the rodent somatosensory system, is an anatomical hub broadly connected with multiple sensory and motor brain areas yet weakly responds to passive sensory stimulation and whisker movements. To understand the role of POm in sensory perception, we developed a self-initiated, two-alternative forced-choice task in freely moving mice during active sensing. Using optogenetic and chemogenetic manipulation, we show that POm plays a significant role in sensory perception and the projection from the primary somatosensory cortex to POm is critical for the contribution of POm in sensory perception during active sensing.

Effects of arousal and movement on secondary somatosensory and visual thalamus

Neocortical sensory areas have associated primary and secondary thalamic nuclei. While primary nuclei transmit sensory information to cortex, secondary nuclei remain poorly understood. We recorded juxtasomally from secondary somatosensory (POm) and visual (LP) nuclei of awake mice while tracking whisking and pupil size. POm activity correlated with whisking, but not precise whisker kinematics. This coarse movement modulation persisted after facial paralysis and thus was not due to sensory reafference. This phenomenon also continued during optogenetic silencing of somatosensory and motor cortex and after lesion of superior colliculus, ruling out a motor efference copy mechanism. Whisking and pupil dilation were strongly correlated, possibly reflecting arousal. Indeed LP, which is not part of the whisker system, tracked whisking equally well, further indicating that POm activity does not encode whisker movement per se. The semblance of movement-related activity is likely instead a global effect of arousal on both nuclei. We conclude that secondary thalamus monitors behavioral state, rather than movement, and may exist to alter cortical activity accordingly.

Higher-Order Thalamic Encoding of Somatosensory Patterns and Bilateral Events

The function of the higher-order sensory thalamus remains unclear. Here, the posterior medial (POm) nucleus of the thalamus was examined by in vivo extracellular recordings in anesthetized rats across a variety of contralateral, ipsilateral, and bilateral whisker sensory patterns. We found that POm was highly sensitive to multiwhisker stimuli involving diverse spatiotemporal interactions. Accurate increases in POm activity were produced during the overlapping time between spatial signals reflecting changes in the spatiotemporal structure of sensory patterns. In addition, our results showed for first time that POm was also able to respond to tactile stimulation of ipsilateral whiskers. This finding challenges the notion that the somatosensory thalamus only computes unilateral stimuli. We found that POm also integrates signals from both whisker pads and described how this integration is generated. Our results showed that ipsilateral activity reached one POm indirectly from the other POm and demonstrated a transmission of sensory activity between both nuclei through a functional POm-POm loop formed by thalamocortical, interhemispheric, and corticothalamic projections. The implication of different cortical areas was investigated revealing that S1 plays a central role in this POm-POm loop. Accordingly, the subcortical and cortical inputs allow POm but not the ventral posteromedial thalamic nucleus (VPM) to have sensory information from both sides of the body. This finding is in agreement with the higher-order nature of POm and can be considered to functionally differentiate and classify these thalamic nuclei. A possible functional role of these higher-order thalamic patterns of integrated activity in brain function is discussed.

An increase in dendritic plateau potentials is associated with experience-dependent cortical map reorganization

Here we describe a mechanism for cortical map plasticity. Classically, representational map changes are thought to be driven by changes within cortico-cortical circuits, e.g., Hebbian plasticity of synaptic circuits that lost vs. maintained an excitatory drive from the first-order thalamus, possibly steered by neuromodulatory forces from deep brain regions. Our work provides evidence for an additional gating mechanism, provided by plateau potentials, which are driven by higher-order thalamic feedback. Higher-order thalamic neurons are characterized by broad receptive fields, and the plateau potentials that they evoke strongly facilitate long-term potentiation and elicit spikes. We show that these features combined constitute a powerful driving force for the fusion or expansion of sensory representations within cortical maps.

The organization of sensory maps in the cerebral cortex depends on experience, which drives homeostatic and long-term synaptic plasticity of cortico-cortical circuits. In the mouse primary somatosensory cortex (S1) afferents from the higher-order, posterior medial thalamic nucleus (POm) gate synaptic plasticity in layer (L) 2/3 pyramidal neurons via disinhibition and the production of dendritic plateau potentials. Here we address whether these thalamocortically mediated responses play a role in whisker map plasticity in S1. We find that trimming all but two whiskers causes a partial fusion of the representations of the two spared whiskers, concomitantly with an increase in the occurrence of POm-driven N-methyl-D-aspartate receptor-dependent plateau potentials. Blocking the plateau potentials restores the archetypical organization of the sensory map. Our results reveal a mechanism for experience-dependent cortical map plasticity in which higher-order thalamocortically mediated plateau potentials facilitate the fusion of normally segregated cortical representations.

Circuit mechanisms for cortical plasticity and learning

The cerebral cortex integrates sensory information with emotional states and internal representations to produce coherent percepts, form associations, and execute voluntary actions. For the cortex to optimize perception, its neuronal network needs to dynamically retrieve and encode new information. Over the last few decades, research has started to provide insight into how the cortex serves these functions. Building on classical Hebbian plasticity models, the latest hypotheses hold that throughout experience and learning, streams of feedforward, feedback, and modulatory information operate in selective and coordinated manners to alter the strength of synapses and ultimately change the response properties of cortical neurons. Here, we describe cortical plasticity mechanisms that involve the concerted action of feedforward and long-range feedback input onto pyramidal neurons as well as the implication of local disinhibitory circuit motifs in this process.

Inferring thalamocortical monosynaptic connectivity in vivo

As the tools to simultaneously record electrophysiological signals from large numbers of neurons within and across brain regions become increasingly available, this opens up for the first time the possibility of establishing the details of causal relationships between monosynaptically connected neurons and the patterns of neural activation that underlie perception and behavior. Although recorded activity across synaptically connected neurons has served as the cornerstone for much of what we know about synaptic transmission and plasticity, this has largely been relegated to ex vivo preparations that enable precise targeting under relatively well-controlled conditions. Analogous studies in vivo, where image-guided targeting is often not yet possible, rely on indirect, data-driven measures, and as a result such studies have been sparse and the dependence upon important experimental parameters has not been well studied. Here, using in vivo extracellular single-unit recordings in the topographically aligned rodent thalamocortical pathway, we sought to establish a general experimental and computational framework for inferring synaptic connectivity. Specifically, attacking this problem within a statistical signal detection framework utilizing experimentally recorded data in the ventral-posterior medial (VPm) region of the thalamus and the homologous region in layer 4 of primary somatosensory cortex (S1) revealed a trade-off between network activity levels needed for the data-driven inference and synchronization of nearby neurons within the population that results in masking of synaptic relationships. Here, we provide a framework for establishing connectivity in multisite, multielectrode recordings based on statistical inference, setting the stage for large-scale assessment of synaptic connectivity within and across brain structures.NEW & NOTEWORTHY Despite the fact that all brain function relies on the long-range transfer of information across different regions, the tools enabling us to measure connectivity across brain structures are lacking. Here, we provide a statistical framework for identifying and assessing potential monosynaptic connectivity across neuronal circuits from population spiking activity that generalizes to large-scale recording technologies that will help us to better understand the signaling within networks that underlies perception and behavior.

The effect of GABAergic neurotransmission on the seizure-related activity of the laterodorsal thalamic nuclei and the somatosensory cortex in a genetic model of absence epilepsy

The present study aimed to investigate the alterations of the GABAergic system in the laterodorsal nucleus (LDN) of the thalamus and the somatosensory cortex (SC) in an experimental model of absence seizure. The effects of pharmacological manipulation of both GABAA and GABAB receptor subunits in the LDN on the generation of spike-wave discharges (SWD) were evaluated. The experiments were carried out in four groups of both WAG/Rij and Wistar rats with 2 and 6 months of age. The expressions of various GABA receptor subunits were studied in the LDN and SC. Furthermore, recordings of unit activity from the LDN and electrocorticography were simultaneously monitored before, during, and after the application of GABAA and GABAB antagonists in the LDN. The generation of SWD in the older WAG/Rij rats was associated with significant alterations in the expression of GABAARα1, GABAARβ3, and GABABR2 subunits in the LDN as well as GABAARα1, GABAARβ3, GABAARγ2, and GABABR2 subunits in the SC. Furthermore, the occurrence of SWD was associated with a significant reduction of gene expression of GABAARα1 and increase of GABAARβ3 in the LDN as well as reduction of GABAARα1, GABAARβ3, GABAARγ2, and GABABR2 in the SC. The microionthophoretic application of the GABAA antagonist bicuculline resulted in a significant increase in the population firing rate of LDN neurons as well as the mean number and duration of SWD. The application of the GABAB antagonist CGP35348 significantly increased the population firing rate of LDN neurons but decreased the mean number of SWD. Our data indicate the regulatory effect of the GABAergic system of the LDN and SC in absence seizures.

The zona incerta system: Involvement in attention and movement

The zona incerta (ZI) is a large structure made of four neurochemically defined regions (at least, in rodents). It is globally involved in complex connections with telencephalic and brainstem centers. In this work, we focus on some of the anatomical links this structure develops with the cerebral cortex and the tectum. We also point to its integration within a larger basal ganglia network. The functions of this region are still mysterious, even if recent works suggest its participation in behavioral expression. Studies about the functional organization of the vibrissal system have provided the first integrated model, illustrating the ZI's role in sensory-motor programing. In addition, ZI connections with the superior colliculus and the cerebral cortex as well as recent behavioral studies point to this region playing a role in cognitive processes related to attention toward salient stimuli.

General Anesthesia Decouples Cortical Pyramidal Neurons

The mystery of general anesthesia is that it specifically suppresses consciousness by disrupting feedback signaling in the brain, even when feedforward signaling and basic neuronal function are left relatively unchanged. The mechanism for such selectiveness is unknown. Here we show that three different anesthetics have the same disruptive influence on signaling along apical dendrites in cortical layer 5 pyramidal neurons in mice. We found that optogenetic depolarization of the distal apical dendrites caused robust spiking at the cell body under awake conditions that was blocked by anesthesia. Moreover, we found that blocking metabotropic glutamate and cholinergic receptors had the same effect on apical dendrite decoupling as anesthesia or inactivation of the higher-order thalamus. If feedback signaling occurs predominantly through apical dendrites, the cellular mechanism we found would explain not only how anesthesia selectively blocks this signaling but also why conscious perception depends on both cortico-cortical and thalamo-cortical connectivity.

Anatomically and functionally distinct thalamocortical inputs to primary and secondary mouse whisker somatosensory cortices

Subdivisions of mouse whisker somatosensory thalamus project to cortex in a region-specific and layer-specific manner. However, a clear anatomical dissection of these pathways and their functional properties during whisker sensation is lacking. Here, we use anterograde trans-synaptic viral vectors to identify three specific thalamic subpopulations based on their connectivity with brainstem. The principal trigeminal nucleus innervates ventral posterior medial thalamus, which conveys whisker-selective tactile information to layer 4 primary somatosensory cortex that is highly sensitive to self-initiated movements. The spinal trigeminal nucleus innervates a rostral part of the posterior medial (POm) thalamus, signaling whisker-selective sensory information, as well as decision-related information during a goal-directed behavior, to layer 4 secondary somatosensory cortex. A caudal part of the POm, which apparently does not receive brainstem input, innervates layer 1 and 5A, responding with little whisker selectivity, but showing decision-related modulation. Our results suggest the existence of complementary segregated information streams to somatosensory cortices.

The (Un)Conscious Mouse as a Model for Human Brain Functions: Key Principles of Anesthesia and Their Impact on Translational Neuroimaging

In recent years, technical and procedural advances have brought functional magnetic resonance imaging (fMRI) to the field of murine neuroscience. Due to its unique capacity to measure functional activity non-invasively, across the entire brain, fMRI allows for the direct comparison of large-scale murine and human brain functions. This opens an avenue for bidirectional translational strategies to address fundamental questions ranging from neurological disorders to the nature of consciousness. The key challenges of murine fMRI are: (1) to generate and maintain functional brain states that approximate those of calm and relaxed human volunteers, while (2) preserving neurovascular coupling and physiological baseline conditions. Low-dose anesthetic protocols are commonly applied in murine functional brain studies to prevent stress and facilitate a calm and relaxed condition among animals. Yet, current mono-anesthesia has been shown to impair neural transmission and hemodynamic integrity. By linking the current state of murine electrophysiology, Ca2+ imaging and fMRI of anesthetic effects to findings from human studies, this systematic review proposes general principles to design, apply and monitor anesthetic protocols in a more sophisticated way. The further development of balanced multimodal anesthesia, combining two or more drugs with complementary modes of action helps to shape and maintain specific brain states and relevant aspects of murine physiology. Functional connectivity and its dynamic repertoire as assessed by fMRI can be used to make inferences about cortical states and provide additional information about whole-brain functional dynamics. Based on this, a simple and comprehensive functional neurosignature pattern can be determined for use in defining brain states and anesthetic depth in rest and in response to stimuli. Such a signature can be evaluated and shared between labs to indicate the brain state of a mouse during experiments, an important step toward translating findings across species.

Recruitment of GABAergic Interneurons in the Barrel Cortex during Active Tactile Behavior

Neural computation involves diverse types of GABAergic inhibitory interneurons that are integrated with excitatory (E) neurons into precisely structured circuits. To understand how each neuron type shapes sensory representations, we measured firing patterns of defined types of neurons in the barrel cortex while mice performed an active, whisker-dependent object localization task. Touch excited fast-spiking (FS) interneurons at short latency, followed by activation of E neurons and somatostatin-expressing (SST) interneurons. Touch only weakly modulated vasoactive intestinal polypeptide-expressing (VIP) interneurons. Voluntary whisker movement activated FS neurons in the ventral posteromedial nucleus (VPM) target layers, a subset of SST neurons and a majority of VIP neurons. Together, FS neurons track thalamic input, mediating feedforward inhibition. SST neurons monitor local excitation, providing feedback inhibition. VIP neurons are activated by non-sensory inputs, disinhibiting E and FS neurons. Our data reveal rules of recruitment for interneuron types during behavior, providing foundations for understanding computation in cortical microcircuits.

Chronic pain after blast-induced traumatic brain injury in awake rats

Explosive blast-induced traumatic brain injury (blast-TBI) in military personnel is a leading cause of injury and persistent neurological abnormalities, including chronic pain. We previously demonstrated that chronic pain after spinal cord injury results from central sensitization in the posterior thalamus (PO). The presence of persistent headaches and back pain in veterans with blast-TBI suggests a similar involvement of thalamic sensitization. Here, we tested the hypothesis that pain after blast-TBI is associated with abnormal increases in activity of neurons in PO thalamus. We developed a novel model with two unique features: (1) blast-TBI was performed in awake, un-anesthetized rats, to simulate the human experience and to eliminate confounds of anesthesia and surgery inherent in other models; (2) only the cranium, rather than the entire body, was exposed to a collimated blast wave, with the blast wave striking the posterior cranium in the region of the occipital crest and foramen magnum. Three weeks after blast-TBI, rats developed persistent, ongoing spontaneous pain. Contrary to our hypothesis, we found no significant differences in the activity of PO neurons, or of neurons in the spinal trigeminal nucleus. There were also no significant changes in gliosis in either of these structures. This novel model will allow future studies on the pathophysiology of chronic pain after blast-TBI.

High-order thalamic inputs to primary somatosensory cortex are stronger and longer lasting than cortical inputs

Layer (L) 2/3 pyramidal neurons in the primary somatosensory cortex (S1) are sparsely active, spontaneously and during sensory stimulation. Long-range inputs from higher areas may gate L2/3 activity. We investigated their in vivo impact by expressing channelrhodopsin in three main sources of feedback to rat S1: primary motor cortex, secondary somatosensory cortex, and secondary somatosensory thalamic nucleus (the posterior medial nucleus, POm). Inputs from cortical areas were relatively weak. POm, however, more robustly depolarized L2/3 cells and, when paired with peripheral stimulation, evoked action potentials. POm triggered not only a stronger fast-onset depolarization but also a delayed all-or-none persistent depolarization, lasting up to 1 s and exhibiting alpha/beta-range oscillations. Inactivating POm somata abolished persistent but not initial depolarization, indicating a recurrent circuit mechanism. We conclude that secondary thalamus can enhance L2/3 responsiveness over long periods. Such timescales could provide a potential modality-specific substrate for attention, working memory, and plasticity.

Neural coding: A single neuron's perspective

What any sensory neuron knows about the world is one of the cardinal questions in Neuroscience. Information from the sensory periphery travels across synaptically coupled neurons as each neuron encodes information by varying the rate and timing of its action potentials (spikes). Spatiotemporally correlated changes in this spiking regimen across neuronal populations are the neural basis of sensory representations. In the somatosensory cortex, however, spiking of individual (or pairs of) cortical neurons is only minimally informative about the world. Recent studies showed that one solution neurons implement to counteract this information loss is adapting their rate of information transfer to the ongoing synaptic activity by changing the membrane potential at which spike is generated. Here we first introduce the principles of information flow from the sensory periphery to the primary sensory cortex in a model sensory (whisker) system, and subsequently discuss how the adaptive spike threshold gates the intracellular information transfer from the somatic post-synaptic potential to action potentials, controlling the information content of communication across somatosensory cortical neurons.

Posterior Thalamic Nucleus Modulation of Tactile Stimuli Processing in Rat Motor and Primary Somatosensory Cortices

Rodents move rhythmically their facial whiskers and compute differences between signals predicted and those resulting from the movement to infer information about objects near their head. These computations are carried out by a large network of forebrain structures that includes the thalamus and the primary somatosensory (S1BF) and motor (M1wk) cortices. Spatially and temporally precise mechanorreceptive whisker information reaches the S1BF cortex via the ventroposterior medial thalamic nucleus (VPM). Other whisker-related information may reach both M1wk and S1BF via the axons from the posterior thalamic nucleus (Po). However, Po axons may convey, in addition to direct sensory signals, the dynamic output of computations between whisker signals and descending motor commands. It has been proposed that this input may be relevant for adjusting cortical responses to predicted vs. unpredicted whisker signals, but the effects of Po input on M1wk and S1BF function have not been directly tested or compared in vivo. Here, using electrophysiology, optogenetics and pharmacological tools, we compared in adult rats M1wk and S1BF in vivo responses in the whisker areas of the motor and primary somatosensory cortices to passive multi-whisker deflection, their dependence on Po activity, and their changes after a brief intense activation of Po axons. We report that the latencies of the first component of tactile-evoked local field potentials in M1wk and S1BF are similar. The evoked potentials decrease markedly in M1wk, but not in S1BF, by injection in Po of the GABA_A agonist muscimol. A brief high-frequency electrical stimulation of Po decreases the responsivity of M1wk and S1BF cells to subsequent whisker stimulation. This effect is prevented by the local application of omega-agatoxin, suggesting that it may in part depend on GABA release by fast-spiking parvalbumin (PV)-expressing cortical interneurons. Local optogenetic activation of Po synapses in different cortical layers also diminishes M1wk and S1BF responses. This effect is most pronounced in the superficial layers of both areas, known to be the main source and target of their reciprocal cortico-cortical connections.

Sensory Processing in the Dorsolateral Striatum: The Contribution of Thalamostriatal Pathways

The dorsal striatum has two functionally-defined subdivisions: a dorsomedial striatum (DMS) region involved in mediating goal-directed behaviors that require conscious effort, and a dorsolateral striatum (DLS) region involved in the execution of habitual behaviors in a familiar sensory context. Consistent with its presumed role in forming stimulus-response (S-R) associations, neurons in DLS receive massive inputs from sensorimotor cortex and are responsive to both active and passive sensory stimulation. While several studies have established that corticostriatal inputs contribute to the stimulus-induced responses observed in the DLS, there is growing awareness that the thalamus has a significant role in conveying sensory-related information to DLS and other parts of the striatum. The thalamostriatal projections to DLS originate mainly from the caudal intralaminar region, which contains the parafascicular (Pf) nucleus, and from higher-order thalamic nuclei such as the medial part of the posterior (POm) nucleus. Based on recent findings, we hypothesize that the thalamostriatal projections from these two regions exert opposing influences on the expression of behavioral habits. This article reviews the subcortical circuits that regulate the transmission of sensory information through these thalamostriatal projection systems, and describes the evidence that indicates these circuits could be manipulated to ameliorate the symptoms of Parkinson's disease (PD) and related neurological disorders.

Control of Somatosensory Cortical Processing by Thalamic Posterior Medial Nucleus: A New Role of Thalamus in Cortical Function

Current knowledge of thalamocortical interaction comes mainly from studying lemniscal thalamic systems. Less is known about paralemniscal thalamic nuclei function. In the vibrissae system, the posterior medial nucleus (POm) is the corresponding paralemniscal nucleus. POm neurons project to L1 and L5A of the primary somatosensory cortex (S1) in the rat brain. It is known that L1 modifies sensory-evoked responses through control of intracortical excitability suggesting that L1 exerts an influence on whisker responses. Therefore, thalamocortical pathways targeting L1 could modulate cortical firing. Here, using a combination of electrophysiology and pharmacology in vivo, we have sought to determine how POm influences cortical processing. In our experiments, single unit recordings performed in urethane-anesthetized rats showed that POm imposes precise control on the magnitude and duration of supra- and infragranular barrel cortex whisker responses. Our findings demonstrated that L1 inputs from POm imposed a time and intensity dependent regulation on cortical sensory processing. Moreover, we found that blocking L1 GABAergic inhibition or blocking P/Q-type Ca2+ channels in L1 prevents POm adjustment of whisker responses in the barrel cortex. Additionally, we found that POm was also controlling the sensory processing in S2 and this regulation was modulated by corticofugal activity from L5 in S1. Taken together, our data demonstrate the determinant role exerted by the POm in the adjustment of somatosensory cortical processing and in the regulation of cortical processing between S1 and S2. We propose that this adjustment could be a thalamocortical gain regulation mechanism also present in the processing of information between cortical areas.

Whisking-Related Changes in Neuronal Firing and Membrane Potential Dynamics in the Somatosensory Thalamus of Awake Mice

The thalamus transmits sensory information to the neocortex and receives neocortical, subcortical, and neuromodulatory inputs. Despite its obvious importance, surprisingly little is known about thalamic function in awake animals. Here, using intracellular and extracellular recordings in awake head-restrained mice, we investigate membrane potential dynamics and action potential firing in the two major thalamic nuclei related to whisker sensation, the ventral posterior medial nucleus (VPM) and the posterior medial group (Pom), which receive distinct inputs from brainstem and neocortex. We find heterogeneous state-dependent dynamics in both nuclei, with an overall increase in action potential firing during active states. Whisking increased putative lemniscal and corticothalamic excitatory inputs onto VPM and Pom neurons, respectively. A subpopulation of VPM cells fired spikes phase-locked to the whisking cycle during free whisking, and these cells may therefore signal whisker position. Our results suggest differential processing of whisking comparing thalamic nuclei at both sub- and supra-threshold levels.

Vibrissa Self-Motion and Touch Are Reliably Encoded along the Same Somatosensory Pathway from Brainstem through Thalamus

Active sensing involves the fusion of internally generated motor events with external sensation. For rodents, active somatosensation includes scanning the immediate environment with the mystacial vibrissae. In doing so, the vibrissae may touch an object at any angle in the whisk cycle. The representation of touch and vibrissa self-motion may in principle be encoded along separate pathways, or share a single pathway, from the periphery to cortex. Past studies established that the spike rates in neurons along the lemniscal pathway from receptors to cortex, which includes the principal trigeminal and ventral-posterior-medial thalamic nuclei, are substantially modulated by touch. In contrast, spike rates along the paralemniscal pathway, which includes the rostral spinal trigeminal interpolaris, posteromedial thalamic, and ventral zona incerta nuclei, are only weakly modulated by touch. Here we find that neurons along the lemniscal pathway robustly encode rhythmic whisking on a cycle-by-cycle basis, while encoding along the paralemniscal pathway is relatively poor. Thus, the representations of both touch and self-motion share one pathway. In fact, some individual neurons carry both signals, so that upstream neurons with a supralinear gain function could, in principle, demodulate these signals to recover the known decoding of touch as a function of vibrissa position in the whisk cycle.

Thalamic relay or cortico-thalamic processing? Old question, new answers

What are the functions implemented by neurons in the sensory nuclei of the thalamus? It seems that this question has accompanied cortical and thalamic studies since their onset some 6 decades ago. Over the years, the simplistic, traditional view of thalamic neurons as mere relays of sensory information has given way to more sophisticated views, of which several alternative hypotheses have been proposed. This commentary briefly reviews the 2 current major hypotheses and shows how a new, pioneering experiment, published in Cerebral Cortex by Groh, Acsady and colleagues, discriminates between them. The commentary further elaborates on the thalamo-cortical processing suggested by the new findings, the general sensory-motor scheme to which these findings may be relevant, and the possible roles such thalamo-cortical processing may have in sensory-motor control.

Modulation of artificial whisking related signals in barrel cortex

Rats use rhythmic whisker movements, called active whisking, to sense the environment, which include whisker protractions followed by retractions at various frequencies. Using a proxy of active whisking in anesthetized rats, called artificial whisking, which is induced by electrically stimulating the facial motor nerve, we characterized the neural responses evoked in the barrel cortex by whisking in air (without contact) and on a surface (with contact). Neural responses were compared between distinct network states consisting of cortical deactivation (synchronized slow oscillations) and activation (desynchronized state) produced by neuromodulation (cholinergic or noradrenergic stimulation in neocortex or thalamus). Here we show that population responses in the barrel cortex consist of a robust signal driven by the onset of the whisker protraction followed by a whisking retraction signal that emerges during low frequency whisking on a surface. The whisking movement onset signal is suppressed by increasing whisking frequency, is controlled by cortical synaptic inhibition, is suppressed during cortical activation states, is little affected by whisking on a surface, and is ubiquitous in ventroposterior medial (VPM) thalamus, barrel cortex, and superior colliculus. The whisking retraction signal codes the duration of the preceding whisker protraction, is present in thalamocortical networks but not in superior colliculus, and is robust during cortical activation; a state associated with natural exploratory whisking. The expression of different whisking signals in forebrain and midbrain may define the sensory processing abilities of those sensorimotor circuits. Whisking related signals in the barrel cortex are controlled by network states that are set by neuromodulators.

Tactile object localization by anticipatory whisker motion

Rodents use rhythmic protractions of their whiskers to locate objects in space. The amplitude of these protractions is reduced when whiskers contact objects, leading to a tendency of whiskers to only lightly touch the environment. While the impact of this process on the sensory input has been studied, little is known about how sensory input causes this change in the motor pattern. Here, using high-speed imaging of whisking in mice, we simultaneously measured whisker contacts and the resulting whisking motion. We found that mice precisely target their whisker protractions to the distance at which they expect objects. This modulation does not depend on the current sensory input and remains stable for at least one whisking cycle when there is no object contact or when the object position is changed. As a result, the timing and other information carried by whisker contacts encodes how well each protraction was matched to the object, functioning as an error signal. Whisker contacts can thus encode a mismatch between expected object locations and the actual environment.

Specific activation of the paralemniscal pathway during nociception

Two main neuronal pathways connect facial whiskers to the somatosensory cortex in rodents: (i) the lemniscal pathway, which originates in the brainstem principal trigeminal nucleus and is relayed in the ventroposterior thalamic nucleus and (ii) the paralemniscal pathway, originating in the spinal trigeminal nucleus and relayed in the posterior thalamic nucleus. While lemniscal neurons are readily activated by whisker contacts, the contribution of paralemniscal neurons to perception is less clear. Here, we functionally investigated these pathways by manipulating input from the whisker pad in freely moving mice. We report that while lemniscal neurons readily respond to neonatal infraorbital nerve sectioning or whisker contacts in vivo, paralemniscal neurons do not detectably respond to these environmental changes. However, the paralemniscal pathway is specifically activated upon noxious stimulation of the whisker pad. These findings reveal a nociceptive function for paralemniscal neurons in vivo that may critically inform context-specific behaviour during environmental exploration.

Rhythmic whisking area (RW) in rat primary motor cortex: an internal monitor of movement-related signals?

Vibrissae-related sensorimotor cortex controls whisking movements indirectly via modulation of lower-level sensorimotor loops and a brainstem central pattern generator (CPG). Two different whisker representations in primary motor cortex (vM1) affect whisker movements in different ways. Prolonged microstimulation in RF, a larger anterior subregion of vM1, gives rise to complex face movements and whisker retraction while the same stimulation evokes large-amplitude rhythmic whisker movement in a small caudo-medial area (RW). To characterize the motor cortex representation of explorative whisking movements, here we recorded RW units in head-fixed rats trained to contact a moving object with one whisker. RW single units were found to encode two aspects of whisker movement independently, albeit on slow time scales (hundreds of milliseconds). The first is whisker position. The second consists of speed (absolute velocity), intensity (instantaneous power), and frequency (spectral centroid). The coding for the latter three parameters was tightly correlated and realized by a continuum of RW responses-ranging from a preference of movement to a preference of rest. Information theory analysis indicated that RW spikes carry most information about position and frequency, while intensity and speed are less well represented. Further, investigating multiple and single RW units, we found a lack of phase locking, movement anticipation, and contact-related tactile responses. These findings suggest that RW neither programs detailed whisker trajectories nor initiates them. Nor does it play a role in processing object touch. Its relationship to whisking is thus indirect and may be related to movement monitoring, perhaps using feedback from the CPG.

Thalamostriatal projections from the medial posterior and parafascicular nuclei have distinct topographic and physiologic properties

The dorsolateral striatum (DLS) is critical for executing sensorimotor behaviors that depend on stimulus-response (S-R) associations. In rats, the DLS receives it densest inputs from primary somatosensory (SI) cortex, but it also receives substantial input from the thalamus. Much of rat DLS is devoted to processing whisker-related information, and thalamic projections to these whisker-responsive DLS regions originate from the parafascicular (Pf) and medial posterior (POm) nuclei. To determine which thalamic nucleus is better suited for mediating S-R associations in the DLS, we compared their input-output connections and neuronal responses to repetitive whisker stimulation. Tracing experiments demonstrate that POm projects specifically to the DLS, but the Pf innervates both dorsolateral and dorsomedial parts of the striatum. The Pf nucleus is innervated by whisker-sensitive sites in the superior colliculus, and these sites also send dense projections to the zona incerta, a thalamic region that sends inhibitory projections to the POm. These data suggest that projections from POm to the DLS are suppressed by incertal inputs when the superior colliculus is activated by unexpected sensory stimuli. Simultaneous recordings with two electrodes indicate that POm neurons are more responsive and habituate significantly less than Pf neurons during repetitive whisker stimulation. Response latencies are also shorter in POm than in Pf, which is consistent with the fact that Pf receives its whisker information via synaptic relays in the superior colliculus. These findings indicate that, compared with the Pf nucleus, POm transmits somatosensory information to the DLS with a higher degree of sensory fidelity.

A disinhibitory circuit mediates motor integration in the somatosensory cortex

The influence of motor activity on sensory processing is crucial for perception and motor execution. However, the underlying circuits are not known. To unravel the circuit by which activity in the primary vibrissal motor cortex (vM1) modulates sensory processing in the primary somatosensory barrel cortex (S1), we used optogenetics to examine the long-range inputs from vM1 to the various neuronal elements in S1. We found that S1-projecting vM1 pyramidal neurons strongly recruited vasointestinal peptide (VIP)-expressing GABAergic interneurons, a subset of serotonin receptor-expressing interneurons. These VIP interneurons preferentially inhibited somatostatin-expressing interneurons, neurons that target the distal dendrites of pyramidal cells. Consistent with this vM1-mediated disinhibitory circuit, the activity of VIP interneurons in vivo increased and that of somatostatin interneurons decreased during whisking. These changes in firing rates during whisking depended on vM1 activity. Our results suggest previously unknown circuitry by which inputs from motor cortex influence sensory processing in sensory cortex.

Simultaneous top-down modulation of the primary somatosensory cortex and thalamic nuclei during active tactile discrimination

The rat somatosensory system contains multiple thalamocortical loops (TCLs) that altogether process, in fundamentally different ways, tactile stimuli delivered passively or actively sampled. To elucidate potential top-down mechanisms that govern TCL processing in awake, behaving animals, we simultaneously recorded neuronal ensemble activity across multiple cortical and thalamic areas while rats performed an active aperture discrimination task. Single neurons located in the primary somatosensory cortex (S1), the ventroposterior medial, and the posterior medial thalamic nuclei of the trigeminal somatosensory pathways exhibited prominent anticipatory firing modulations before the whiskers touching the aperture edges. This cortical and thalamic anticipatory firing could not be explained by whisker movements or whisker stimulation, because neither trigeminal ganglion sensory-evoked responses nor EMG activity were detected during the same period. Both thalamic and S1 anticipatory activity were predictive of the animal's discrimination accuracy. Inactivation of the primary motor cortex (M1) with muscimol affected anticipatory patterns in S1 and the thalamus, and impaired the ability to predict the animal's performance accuracy based on thalamocortical anticipatory activity. These findings suggest that neural processing in TCLs is launched in anticipation of whisker contact with objects, depends on top-down effects generated in part by M1 activity, and cannot be explained by the classical feedforward model of the rat trigeminal system.

Motor cortex stimulation activates the incertothalamic pathway in an animal model of spinal cord injury

We have shown previously that electrical stimulation of the motor cortex reduces spontaneous painlike behaviors in animals with spinal cord injury (SCI). Because SCI pain behaviors are associated with abnormal inhibition in the inhibitory nucleus zona incerta (ZI) and because inactivation of the ZI blocks motor cortex stimulation (MCS) effects, we hypothesized that the antinociceptive effects of MCS are due to enhanced inhibitory inputs from ZI to the posterior thalamus (Po)-an area heavily implicated in nociceptive processing. To test this hypothesis, we used a rodent model of SCI pain and performed in vivo extracellular electrophysiological recordings in single well-isolated neurons in anesthetized rats. We recorded spontaneous activity in ZI and Po from 48 rats before, during, and after MCS (50 μA, 50 Hz; 300-ms pulses). We found that MCS enhanced spontaneous activity in 35% of ZI neurons and suppressed spontaneous activity in 58% of Po neurons. The majority of MCS-enhanced ZI neurons (81%) were located in the ventrorateral subdivision of ZI-the area containing Po-projecting ZI neurons. In addition, we found that inactivation of ZI using muscimol (GABAA receptor agonist) blocked the effects of MCS in 73% of Po neurons. Although we cannot eliminate the possibility that muscimol spread to areas adjacent to ZI, these findings support our hypothesis and suggest that MCS produces antinociception by activating the incertothalamic pathway.

PERSPECTIVE

This article describes a novel brain circuit that can be manipulated, in rats, to produce antinociception. These results have the potential to significantly impact the standard of care currently in place for the treatment of patients with intractable pain.

Thalamic POm projections to the dorsolateral striatum of rats: potential pathway for mediating stimulus-response associations for sensorimotor habits

The dorsolateral part of the striatum (DLS) represents the initial stage for processing sensorimotor information in the basal ganglia. Although the DLS receives much of its input from the primary somatosensory (SI) cortex, peripheral somesthetic stimulation activates the DLS at latencies that are shorter than the response latencies recorded in the SI cortex. To identify the subcortical regions that transmit somesthetic information directly to the DLS, we deposited small quantities of retrograde tracers at DLS sites that displayed consistent time-locked responses to controlled whisker stimulation. The neurons that were retrogradely labeled by these injections were located mainly in the sensorimotor cortex and, to a lesser degree, in the amygdala and thalamus. Quantitative analysis of neuronal labeling in the thalamus indicated that the strongest thalamic input to the whisker-sensitive part of the DLS originates from the medial posterior nucleus (POm), a somesthetic-related region that receives inputs from the spinal trigeminal nucleus. Anterograde tracer injections in POm confirmed that this thalamic region projects to the DLS neuropil. In subsequent experiments, simultaneous recordings from POm and the DLS during whisker stimulation showed that POm consistently responds before the DLS. These results suggest that POm could transmit somesthetic information to the DLS, and this modality-specific thalamostriatal pathway may cooperate with the thalamostriatal projections that originate from the intralaminar nuclei.

Neuronal basis for object location in the vibrissa scanning sensorimotor system

An essential issue in perception is how the location of an object is estimated from tactile signals in the context of self-generated changes in sensor configuration. Here, we review the pathways and dynamics of neuronal signals that encode touch in the rodent vibrissa sensorimotor system. Rodents rhythmically scan an array of long, facial hairs across a region of interest. Behavioral evidence shows that these animals maintain knowledge of the azimuthal position of their vibrissae. Electrophysiological measurements have identified a reafferent signal of the azimuth that is coded in normalized coordinates, broadcast throughout primary sensory cortex and provides strong modulation of signals of vibrissa contact. Efferent signals in motor cortex report the range of the scan. Collectively, these signals allow the rodent to form a percept of object location.

Interconnected cortical networks between primary somatosensory cortex septal columns and posterior parietal cortex in rat

Visual and somesthetic cues are used for spatial processing in the posterior parietal cortex (PPC) of the mammalian brain. In rats, somatic information collected by the mystacial whiskers is critically involved in constructing a neural representation of the external space. Here, we delineated the topography of the cortical pathway from the primary somatosensory cortex (SI) that may deliver vibrissal cues to PPC for spatial processing. For anterograde tracing, we made small injections of biotinylated dextran amine (BDA) into SI barrel cortex. The injections in the regions directly above the septal compartments produced dense terminals in PPC, whereas injections above the center of the barrels resulted in sparse terminals. For retrograde tracing, we made large injections of cholera toxin subunit B (CtB) in PPC. Retrogradely labeled neurons within SI barrel cortex formed multiple, parallel strips. In layer IV, these strips of labeled neurons were confined within the septal rows, extending from barrel arc position 0 to 5. In the extragranular layers, labeled neurons were clustered primarily within the vertical extensions of the septal rows and extended to the edges of neighboring barrel columns. Based on these findings, in which SI projections to PPC arise mainly from the septal columns, we hypothesize that septal columns may form interconnected cortical networks that engage in spatial information processing contingent on somestheic cues.

Three-dimensional axon morphologies of individual layer 5 neurons indicate cell type-specific intracortical pathways for whisker motion and touch

The cortical output layer 5 contains two excitatory cell types, slender- and thick-tufted neurons. In rat vibrissal cortex, slender-tufted neurons carry motion and phase information during active whisking, but remain inactive after passive whisker touch. In contrast, thick-tufted neurons reliably increase spiking preferably after passive touch. By reconstructing the 3D patterns of intracortical axon projections from individual slender- and thick-tufted neurons, filled in vivo with biocytin, we were able to identify cell type-specific intracortical circuits that may encode whisker motion and touch. Individual slender-tufted neurons showed elaborate and dense innervation of supragranular layers of large portions of the vibrissal area (total length, 86.8 ± 5.5 mm). During active whisking, these long-range projections may modulate and phase-lock the membrane potential of dendrites in layers 2 and 3 to the whisking cycle. Thick-tufted neurons with soma locations intermingling with those of slender-tufted ones display less dense intracortical axon projections (total length, 31.6 ± 14.3 mm) that are primarily confined to infragranular layers. Based on anatomical reconstructions and previous measurements of spiking, we put forward the hypothesis that thick-tufted neurons in rat vibrissal cortex receive input of whisker motion from slender-tufted neurons onto their apical tuft dendrites and input of whisker touch from thalamic neurons onto their basal dendrites. During tactile-driven behavior, such as object location, near-coincident input from these two pathways may result in increased spiking activity of thick-tufted neurons and thus enhanced signaling to their subcortical targets.

Whisking in air: encoding of kinematics by VPM neurons in awake rats

Rodent whisking behavior generates two types of neural signals: one produced by whisker contact with objects; the other by movements in air. While kinematic signals generated by contact reliably activate neurons at all levels of the trigeminal neuraxis, the extent to which the kinematics of whisking in air are reliably encoded at each level remains unclear. Previously, we showed that the responses of trigeminal ganglion (TG) neurons in awake, head-fixed rats are correlated with whisking kinematic parameters, but that individual neurons may differ substantially in the reliability of their kinematic encoding. Here, we extend that analysis to neurons in the ventral posterior medial (VPM) nucleus. Three possible coding strategies were examined: (1) firing rate across an entire movement; (2) the probability of individual spikes as a function of the instantaneous movement trajectory; and (3) the coherence between spikes and whisking. While VPM neurons were clearly responsive to variations in whisker kinematics during whisking in air, the encoding of whisker kinematics by VPM neurons was less consistent than that of TG neurons. Furthermore, we found that, in VPM as in TG, movement direction is an important determinant of unit responsiveness during whisking in air.

Abnormal anterior pretectal nucleus activity contributes to central pain syndrome

Central pain syndrome (CPS) is a debilitating condition that affects a large number of patients with a primary lesion or dysfunction in the CNS, most commonly due to spinal cord injury, stroke, and multiple sclerosis lesions. The pathophysiological processes underlying the development and maintenance of CPS are poorly understood. We have recently shown, in an animal model of CPS, that neurons in the posterior thalamic nucleus (PO) have increased spontaneous and evoked activity. We also demonstrated that these changes are due to suppressed inhibitory inputs from the zona incerta (ZI). The anterior pretectal nucleus (APT) is a diencephalic nucleus that projects on both the PO and ZI, suggesting that it might be involved in the pathophysiology of CPS. Here we test the hypothesis that CPS is associated with abnormal APT activity by recording single units from APT in anesthetized rats with CPS resulting from spinal cord lesions. The firing rate of APT neurons was increased in spinal-lesioned animals, compared with sham-operated controls. This increase was due to a selective increase in firing of tonic neurons that project to and inhibit ZI and an increase in bursts in fast bursting and slow rhythmic neurons. We also show that, in normal animals, suppressing APT results in increased PO spontaneous activity and evoked responses in a subpopulation of PO neurons. Taken together, these findings suggest that APT regulates ZI inputs to PO and that enhanced APT activity during CPS contributes to the abnormally high activity of PO neurons in CPS.

Spiking in primary somatosensory cortex during natural whisking in awake head-restrained rats is cell-type specific

Sensation involves active movement of sensory organs, but it remains unknown how position or movement of sensory organs is encoded in cortex. In the rat whisker system, each whisker is represented by an individual cortical (barrel) column. Here, we quantified in awake, head-fixed rats the impact of natural whisker movements on action potential frequencies of single (identified) neurons located in different layers of somatosensory (barrel) cortex. In all layers, we found only weak correlations between spiking and whisker position or velocity. Conversely, whisking significantly increased spiking rate in a subset of neurons located preferentially in layer 5A. This finding suggests that whisker movement could be encoded by population responses of neurons within all layers and by single slender-tufted pyramids in layer 5A.

Zona incerta: a role in central pain

Central pain syndrome (CPS) is a debilitating condition that affects a large number of patients with a primary lesion or dysfunction in the CNS. Despite its discovery over a century ago, the pathophysiological processes underlying the development and maintenance of CPS are poorly understood. We recently demonstrated that activity in the posterior thalamus (PO) is tightly regulated by inhibitory inputs from zona incerta (ZI). Here we test the hypothesis that CPS is associated with abnormal inhibitory regulation of PO by ZI. We recorded single units from ZI and PO in animals with CPS resulting from spinal cord lesions. Consistent with our hypothesis, the spontaneous firing rate and somatosensory evoked responses of ZI neurons were lower in lesioned animals compared with sham-operated controls. In PO, neurons recorded from lesioned rats exhibited significantly higher spontaneous firing rates and greater responses to noxious and innocuous stimuli applied to the hindpaw and to the face. These changes were not associated with increased afferent drive from the spinal trigeminal nucleus or changes in the ventroposterior thalamus. Thus CPS can result from suppressed inputs from the inhibitory nucleus zona incerta to the posterior thalamus.

Differential response patterns in the si barrel and septal compartments during mechanical whisker stimulation

A growing body of evidence suggests that the barrel and septal regions in layer IV of rat primary somatosensory (SI) cortex may represent separate processing channels. To assess this view, pairs of barrel and septal neurons were recorded simultaneously in the anesthetized rat while a 4x4 array of 16 whiskers was mechanically stimulated at 4, 8, 12, and 16 Hz. Compared with barrel neurons, regular-spiking septal neurons displayed greater increases in response latencies as the frequency of whisker stimulation increased. Cross-correlation analysis indicated that the incidence and strength of neuronal coordination varied with the relative spatial configuration (within vs. across rows) and compartmental location (barrel vs. septa) of the recorded neurons. Barrel and septal neurons were strongly coordinated if both neurons were in close proximity and resided in the same row. Some barrel neurons were weakly coordinated, but only if they resided in the same row. By contrast, the strength of coordination among pairs of septal neurons did not vary with their spatial proximity or their spatial configuration within the arcs and rows of the barrel field. These differential responses provide further support for the view that the barrel and septal regions represent the cortical gateway for processing streams that encode specific aspects of the sensorimotor information associated with whisking behavior.

Neural coding and contextual influences in the whisker system

A fundamental problem in neuroscience, to which Prof. Segundo has made seminal contributions, is to understand how action potentials represent events in the external world. The aim of this paper is to review the issue of neural coding in the context of the rodent whiskers, an increasingly popular model system. Key issues we consider are: the role of spike timing; mechanisms of spike timing; decoding and context-dependence. Significant insight has come from the development of rigorous, information theoretic frameworks for tackling these questions, in conjunction with suitably designed experiments. We review both the theory and experimental studies. In contrast to the classical view that neurons are noisy and unreliable, it is becoming clear that many neurons in the subcortical whisker pathway are remarkably reliable and, by virtue of spike timing with millisecond-precision, have high bandwidth for conveying sensory information. In this way, even small (approximately 200 neuron) subcortical modules are able to support the sensory processing underlying sophisticated whisker-dependent behaviours. Future work on neural coding in cortex will need to consider new findings that responses are highly dependent on context, including behavioural and internal states.

Whisking in air: encoding of kinematics by trigeminal ganglion neurons in awake rats

Active sensing requires the brain to distinguish signals produced by external inputs from those generated by the animal's own movements. Because the rodent whisker musculature lacks proprioceptors, we asked whether trigeminal ganglion neurons encode the kinematics of the rat's own whisker movements in air. By examining the role of kinematics, we have extended previous findings showing that many neurons that respond during such movements do not do so consistently. Nevertheless, the majority ( approximately 70%) of trigeminal ganglion neurons display significant correlations between firing rate and a kinematic parameter, and a subset, approximately 30%, represent kinematics with high reliability. Preferential firing to movement direction was observed but was strongly modulated by movement amplitude and speed. However, in contrast to the precise time-locking that occurs in response to active whisker contacts, whisker movements in air generate temporally dispersed responses that are not time-locked to the onset of either protractions or retractions.

Phase-to-rate transformations encode touch in cortical neurons of a scanning sensorimotor system

Sensory perception involves the dual challenge of encoding external stimuli and managing the influence of changes in body position that alter the sensory field. To examine mechanisms used to integrate sensory signals elicited by both external stimuli and motor activity, we recorded from rats trained to rhythmically sweep their vibrissa in search of a target. We found a select population of neurons in primary somatosensory cortex that are transiently excited by the confluence of touch by a single vibrissa and the phase of vibrissa motion in the whisk cycle; different units have different preferred phases. This conditional response enables the rodent to estimate object position in a coordinate frame that is normalized to the trajectory of the motor output, as defined by phase in the whisk cycle, rather than angle of the vibrissa relative to the face. The underlying computation is consistent with gating by an inhibitory shunt.