Thalamocortical function in developing sensory circuits

Development of the basic architecture of neocortical circuitry in the human fetus as revealed by the coupling spatiotemporal pattern of synaptogenesis along with microstructure and macroscale in vivo MR imaging

In humans, a quantifiable number of cortical synapses appears early in fetal life. In this paper, we present a bridge across different scales of resolution and the distribution of synapses across the transient cytoarchitectonic compartments: marginal zone (MZ), cortical plate (CP), subplate (SP), and in vivo MR images. The tissue of somatosensory cortex (7-26 postconceptional weeks (PCW)) was prepared for electron microscopy, and classified synapses with a determined subpial depth were used for creating histograms matched to the histological sections immunoreacted for synaptic markers and aligned to in vivo MR images (1.5 T) of corresponding fetal ages (maternal indication). Two time periods and laminar patterns of synaptogenesis were identified: an early and midfetal two-compartmental distribution (MZ and SP) and a late fetal three-compartmental distribution (CP synaptogenesis). During both periods, a voluminous, synapse-rich SP was visualized on the in vivo MR. Another novel finding concerns the phase of secondary expansion of the SP (13 PCW), where a quantifiable number of synapses appears in the upper SP. This lamina shows a T2 intermediate signal intensity below the low signal CP. In conclusion, the early fetal appearance of synapses shows early differentiation of putative genetic mechanisms underlying the synthesis, transport and assembly of synaptic proteins. "Pioneering" synapses are likely to play a morphogenetic role in constructing of fundamental circuitry architecture due to interaction between neurons. They underlie spontaneous, evoked, and resting state activity prior to ex utero experience. Synapses can also mediate genetic and environmental triggers, adversely altering the development of cortical circuitry and leading to neurodevelopmental disorders.

More Than a Small Brain: The Importance of Studying Neural Function during Development

The nervous system contains complex circuits comprising thousands of cell types and trillions of connections. Here, we discuss how the field of "developmental systems neuroscience" combines the molecular and genetic perspectives of developmental neuroscience with the (typically adult-focused) functional perspective of systems neuroscience. This combination of approaches is critical to understanding how a handful of cells eventually produce the wide range of behaviors necessary for survival. Functional circuit development typically lags behind neural connectivity, leading to intermediate stages of neural activity that are either not seen in adults or, if present, are considered pathophysiological. Developmental systems neuroscience examines these intermediate stages of neural activity, mapping out the critical phases and inflection points of neural circuit function to understand how neural activity and behavior emerge across development. Beyond understanding typical development, this approach provides invaluable insight into the pathophysiology of neurodevelopmental disorders by identifying when and how functional development diverges between health and disease. We argue that developmental systems neuroscience will identify important periods of neural development, reveal novel therapeutic windows for treatment, and set the stage to answer fundamental questions about the brain in health and disease.

Revisiting the functional monitoring of brain development in premature neonates. A new direction in clinical care and research

The first 1000 days of life are of paramount importance for neonatal development. Premature newborns are exposed early to the external environment, modifying the fetal exposome and leading to overexposure in some sensory domains and deprivation in others. The resulting neurodevelopmental effects may persist throughout the individual's lifetime. Several neonatal neuromonitoring techniques can be used to investigate neural mechanisms in early postnatal development. EEG is the most widely used, as it is easy to perform, even at the patient's bedside. It is not expensive and provides information with a high temporal resolution and relatively good spatial resolution when performed in high-density mode. Functional near-infrared spectroscopy (fNIRS), a technique for monitoring vascular network dynamics, can also be used at the patient's bedside. It is not expensive and has a good spatial resolution at the cortical surface. These two techniques can be combined for simultaneous monitoring of the neuronal and vascular networks in premature newborns, providing insight into neurodevelopment before term. However, the extent to which more general conclusions about fetal development can be drawn from findings for premature neonates remains unclear due to considerable differences in environmental and medical situations. Fetal MEG (fMEG, as an alternative to EEG for preterm infants) and fMRI (as an alternative to fNIRS for preterm infants) can also be used to investigate fetal neurodevelopment on a trimester-specific basis. These techniques should be used for validation purposes as they are the only tools available for evaluating neuronal dysfunction in the fetus at the time of the gene-environment interactions influencing transient neuronal progenitor populations in brain structures. But what do these techniques tell us about early neurodevelopment? We address this question here, from two points of view. We first discuss spontaneous neural activity and its electromagnetic and hemodynamic correlates. We then explore the effects of stimulating the immature developing brain with information from exogenous sources, reviewing the available evidence concerning the characteristics of electromagnetic and hemodynamic responses. Once the characteristics of the correlates of neural dynamics have been determined, it will be essential to evaluate their possible modulation in the context of disease and in at-risk populations. Evidence can be collected with various neuroimaging techniques targeting both spontaneous and exogenously driven neural activity. A multimodal approach combining the neuromonitoring of different functional compartments (neuronal and vascular) is required to improve our understanding of the normal functioning and dysfunction of the brain and to identify neurobiomarkers for predicting the neurodevelopmental outcome of premature neonate and fetus. Such an approach would provide a framework for exploring early neurodevelopment, paving the way for the development of tools for earlier diagnosis in these vulnerable populations, thereby facilitating preventive, rescue and reparative neurotherapeutic interventions.

Network state transitions during cortical development

Mammalian cortical networks are active before synaptogenesis begins in earnest, before neuronal migration is complete, and well before an animal opens its eyes and begins to actively explore its surroundings. This early activity undergoes several transformations during development. The most important of these is a transition from episodic synchronous network events, which are necessary for patterning the neocortex into functionally related modules, to desynchronized activity that is computationally more powerful and efficient. Network desynchronization is perhaps the most dramatic and abrupt developmental event in an otherwise slow and gradual process of brain maturation. In this Review, we summarize what is known about the phenomenology of developmental synchronous activity in the rodent neocortex and speculate on the mechanisms that drive its eventual desynchronization. We argue that desynchronization of network activity is a fundamental step through which the cortex transitions from passive, bottom-up detection of sensory stimuli to active sensory processing with top-down modulation.

Development of reciprocal connections between the dorsal lateral geniculate nucleus and the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) serves as an important node between the thalamus and neocortex, regulating thalamocortical rhythms and sensory processing in a state dependent manner. Disruptions in TRN circuitry also figures prominently in several neurodevelopmental disorders including epilepsy, autism, and attentional defects. An understanding of how and when connections between TRN and 1st order thalamic nuclei, such as the dorsal lateral geniculate nucleus (dLGN), develop is lacking. We used the mouse visual thalamus as a model system to study the organization, pattern of innervation and functional responses between TRN and the dLGN. Genetically modified mouse lines were used to visualize and target the feedforward and feedback components of these intra-thalamic circuits and to understand how peripheral input from the retina impacts their development.Retrograde tracing of thalamocortical (TC) afferents through TRN revealed that the modality-specific organization seen in the adult, is present at perinatal ages and seems impervious to the loss of peripheral input. To examine the formation and functional maturation of intrathalamic circuits between the visual sector of TRN and dLGN, we examined when projections from each nuclei arrive, and used an acute thalamic slice preparation along with optogenetic stimulation to assess the maturation of functional synaptic responses. Although thalamocortical projections passed through TRN at birth, feedforward axon collaterals determined by vGluT2 labeling, emerged during the second postnatal week, increasing in density through the third week. Optogenetic stimulation of TC axon collaterals in TRN showed infrequent, weak excitatory responses near the end of week 1. During weeks 2-4, responses became more prevalent, grew larger in amplitude and exhibited synaptic depression during repetitive stimulation. Feedback projections from visual TRN to dLGN began to innervate dLGN as early as postnatal day 2 with weak inhibitory responses emerging during week 1. During week 2-4, inhibitory responses continued to grow larger, showing synaptic depression during repetitive stimulation. During this time TRN inhibition started to suppress TC spiking, having its greatest impact by week 4-6. Using a mutant mouse that lacks retinofugal projections revealed that the absence of retinal input led to an acceleration of TRN innervation of dLGN but had little impact on the development of feedforward projections from dLGN to TRN. Together, these experiments reveal how and when intrathalamic connections emerge during early postnatal ages and provide foundational knowledge to understand the development of thalamocortical network dynamics as well as neurodevelopmental diseases that involve TRN circuitry.

Coincident development and synchronization of sleep-dependent delta in the cortex and medulla

In early development, active sleep is the predominant sleep state before it is supplanted by quiet sleep. In rats, the developmental increase in quiet sleep is accompanied by the sudden emergence of the cortical delta rhythm (0.5-4 Hz) around postnatal day 12 (P12). We sought to explain the emergence of the cortical delta by assessing developmental changes in the activity of the parafacial zone (PZ), a medullary structure thought to regulate quiet sleep in adults. We recorded from the PZ in P10 and P12 rats and predicted an age-related increase in neural activity during increasing periods of delta-rich cortical activity. Instead, during quiet sleep, we discovered sleep-dependent rhythmic spiking activity-with intervening periods of total silence-phase locked to a local delta rhythm. Moreover, PZ and cortical delta were coherent at P12 but not at P10. PZ delta was also phase locked to respiration, suggesting sleep-dependent modulation of PZ activity by respiratory pacemakers in the ventral medulla. Disconnecting the main olfactory bulbs from the cortex did not diminish cortical delta, indicating that the influence of respiration on delta at this age is not mediated indirectly through nasal breathing. Finally, we observed an increase in parvalbumin-expressing terminals in the PZ across these ages, supporting a role for local GABAergic inhibition in the PZ's rhythmicity. The unexpected discovery of delta-rhythmic neural activity in the medulla-when cortical delta is also emerging-provides a new perspective on the brainstem's role in regulating sleep and promoting long-range functional connectivity in early development.

Restoring thalamocortical circuit dysfunction by correcting HCN channelopathy in Shank3 mutant mice

Thalamocortical (TC) circuits are essential for sensory information processing. Clinical and preclinical studies of autism spectrum disorders (ASDs) have highlighted abnormal thalamic development and TC circuit dysfunction. However, mechanistic understanding of how TC dysfunction contributes to behavioral abnormalities in ASDs is limited. Here, our study on a Shank3 mouse model of ASD reveals TC neuron hyperexcitability with excessive burst firing and a temporal mismatch relationship with slow cortical rhythms during sleep. These TC electrophysiological alterations and the consequent sensory hypersensitivity and sleep fragmentation in Shank3 mutant mice are causally linked to HCN2 channelopathy. Restoring HCN2 function early in postnatal development via a viral approach or lamotrigine (LTG) ameliorates sensory and sleep problems. A retrospective case series also supports beneficial effects of LTG treatment on sensory behavior in ASD patients. Our study identifies a clinically relevant circuit mechanism and proposes a targeted molecular intervention for ASD-related behavioral impairments.

DELTA-RHYTHMIC ACTIVITY IN THE MEDULLA DEVELOPS COINCIDENT WITH CORTICAL DELTA IN SLEEPING INFANT RATS

In early development, active sleep is the predominant sleep state before it is supplanted by quiet sleep. In rats, the developmental increase in quiet sleep is accompanied by the sudden emergence of the cortical delta rhythm (0.5-4 Hz) around postnatal day 12 (P12). We sought to explain the emergence of cortical delta by assessing developmental changes in the activity of the parafacial zone (PZ), a medullary structure thought to regulate quiet sleep in adults. We recorded from PZ in P10 and P12 rats and predicted an age-related increase in neural activity during increasing periods of delta-rich cortical activity. Instead, during quiet sleep we discovered sleep-dependent rhythmic spiking activity-with intervening periods of total silence-phase-locked to a local delta rhythm. Moreover, PZ and cortical delta were coherent at P12, but not at P10. PZ delta was also phase-locked to respiration, suggesting sleep-dependent modulation of PZ activity by respiratory pacemakers in the ventral medulla. Disconnecting the main olfactory bulbs from the cortex did not diminish cortical delta, indicating that the influence of respiration on delta at this age is not mediated indirectly through nasal breathing. Finally, we observed an increase in parvalbumin-expressing terminals in PZ across these ages, supporting a role for GABAergic inhibition in PZ's rhythmicity. The discovery of delta-rhythmic neural activity in the medulla-when cortical delta is also emerging-opens a new path to understanding the brainstem's role in regulating sleep and synchronizing rhythmic activity throughout the brain.

Levodopa improved different motor symptoms in patients with Parkinson's disease by reducing the functional connectivity of specific thalamic subregions

BACKGROUND

The thalamus is an important relay station for the motor circuit of human. Levodopa can reverse the clinical manifestations by modulating the function of motor circuits, but its detailed mechanisms are still not fully understood. We aimed to explore (1) the mechanism by which levodopa modulates the functional connectivity (FC) in the subregions of the thalamus; (2) the relationship between the changed FC and the improvement of motor symptoms in Parkinson's disease (PD) patients.

METHODS

Resting-state functional MRI was used to scan 36 PD patients and 37 healthy controls. The FC between the subregions in the thalamus and the whole brain was measured and compared under different medication states of PD patients. The correlation between the improvement of motor symptoms and changes in FC in the thalamus subregions was examined.

RESULTS

The PD on state exhibited decreased FC between the right pre-motor thalamus and the right postcentral gyrus, as well as the right lateral pre-frontal thalamus and the right postcentral gyrus. These decreases were positively correlated with the improvement of resting tremor. The PD on state also exhibited decreased FC between the left lateral pre-frontal thalamus and right paracentral lobule, which was positively correlated with the improvement of bradykinesia.

CONCLUSIONS

This study demonstrates that levodopa treats PD by decreasing the FC between the thalamus subregions and pre/post-central cortex. Our results provide a basis for further exploration of the functional activity of thalamic subregions and offer new insights into the precision treatment in PD patients.

Continuity and change in neural plasticity through embryonic morphogenesis, fetal activity-dependent synaptogenesis, and infant memory consolidation

There is an apparent continuity in human neural development that can be traced to venerable themes of vertebrate morphogenesis that have shaped the evolution of the reptilian telencephalon (including both primitive three-layered cortex and basal ganglia) and then the subsequent evolution of the mammalian six-layered neocortex. In this theoretical analysis, we propose that an evolutionary-developmental analysis of these general morphogenetic themes can help to explain the embryonic development of the dual divisions of the limbic system that control the dorsal and ventral networks of the human neocortex. These include the archicortical (dorsal limbic) Papez circuits regulated by the hippocampus that organize spatial, contextual memory, as well as the paleocortical (ventral limbic) circuits that organize object memory. We review evidence that these dorsal and ventral limbic divisions are controlled by the differential actions of brainstem lemnothalamic and midbrain collothalamic arousal control systems, respectively, thereby traversing the vertebrate subcortical neuraxis. These dual control systems are first seen shaping the phyletic morphogenesis of the archicortical and paleocortical foundations of the forebrain in embryogenesis. They then provide dual modes of activity-dependent synaptic organization in the active (lemnothalamic) and quiet (collothalamic) stages of fetal sleep. Finally, these regulatory systems mature to form the major systems of memory consolidation of postnatal development, including the rapid eye movement (lemnothalamic) consolidation of implicit memory and social attachment in the first year, and then-in a subsequent stage-the non-REM (collothalamic) consolidation of explicit memory that is integral to the autonomy and individuation of the second year of life.

Neural decoding reveals specialized kinematic tuning after an abrupt cortical transition

The primary motor cortex (M1) exhibits a protracted period of development, including the development of a sensory representation long before motor outflow emerges. In rats, this representation is present by postnatal day (P) 8, when M1 activity is "discontinuous." Here, we ask how the representation changes upon the transition to "continuous" activity at P12. We use neural decoding to predict forelimb movements from M1 activity and show that a linear decoder effectively predicts limb movements at P8 but not at P12; instead, a nonlinear decoder better predicts limb movements at P12. The altered decoder performance reflects increased complexity and uniqueness of kinematic information in M1. We next show that M1's representation at P12 is more susceptible to "lesioning" of inputs and "transplanting" of M1's encoding scheme from one pup to another. Thus, the emergence of continuous M1 activity signals the developmental onset of more complex, informationally sparse, and individualized sensory representations.

cFOS as a biomarker of activity maturation in the hippocampal formation

We explored the potential for cFOS expression as a marker of functional development of "resting-state" waking activity in the extended network of the hippocampus and entorhinal cortex. We examined sleeping and awake mice at (P)ostnatal days 5, 9, 13, and 17 as well as in adulthood. We find that cFOS expression is state-dependent even at 5 days old, with reliable staining occurring only in the awake mice. Even during waking, cFOS expression was rare and weak at P5. The septal nuclei, entorhinal cortex layer (L)2, and anterodorsal thalamus were exceptional in that they had robust cFOS expression at P5 that was similar to or greater than in adulthood. Significant P5 expression was also observed in the dentate gyrus, entorhinal cortex L6, postsubiculum L4-6, ventral subiculum, supramammillary nucleus, and posterior hypothalamic nucleus. The expression in these regions grew stronger with age, and the expression in new regions was added progressively at P9 and P13 by which point the overall expression pattern in many regions was qualitatively similar to the adult. Six regions-CA1, dorsal subiculum, postsubiculum L2-3, reuniens nucleus, and perirhinal and postrhinal cortices-were very late developing, mostly achieving adult levels only after P17. Our findings support a number of developmental principles. First, early spontaneous activity patterns induced by muscle twitches during sleep do not induce robust cFOS expression in the extended hippocampal network. Second, the development of cFOS expression follows the progressive activation along the trisynaptic circuit, rather than birth date or cellular maturation. Third, we reveal components of the egocentric head-direction and theta-rhythm circuits as the earliest cFOS active circuits in the forebrain. Our results suggest that cFOS staining may provide a reliable and sensitive biomarker for hippocampal formation activity development, particularly in regard to the attainment of a normal waking state and synchronizing rhythms such as theta and gamma.

Maturational networks of human fetal brain activity reveal emerging connectivity patterns prior to ex-utero exposure

A key feature of the fetal period is the rapid emergence of organised patterns of spontaneous brain activity. However, characterising this process in utero using functional MRI is inherently challenging and requires analytical methods which can capture the constituent developmental transformations. Here, we introduce a novel analytical framework, termed "maturational networks" (matnets), that achieves this by modelling functional networks as an emerging property of the developing brain. Compared to standard network analysis methods that assume consistent patterns of connectivity across development, our method incorporates age-related changes in connectivity directly into network estimation. We test its performance in a large neonatal sample, finding that the matnets approach characterises adult-like features of functional network architecture with a greater specificity than a standard group-ICA approach; for example, our approach is able to identify a nearly complete default mode network. In the in-utero brain, matnets enables us to reveal the richness of emerging functional connections and the hierarchy of their maturational relationships with remarkable anatomical specificity. We show that the associative areas play a central role within prenatal functional architecture, therefore indicating that functional connections of high-level associative areas start emerging prior to exposure to the extra-utero environment.

Synaptic and circuit mechanisms prevent detrimentally precise correlation in the developing mammalian visual system

The developing visual thalamus and cortex extract positional information encoded in the correlated activity of retinal ganglion cells by synaptic plasticity, allowing for the refinement of connectivity. Here, we use a biophysical model of the visual thalamus during the initial visual circuit refinement period to explore the role of synaptic and circuit properties in the regulation of such neural correlations. We find that the NMDA receptor dominance, combined with weak recurrent excitation and inhibition characteristic of this age, prevents the emergence of spike-correlations between thalamocortical neurons on the millisecond timescale. Such precise correlations, which would emerge due to the broad, unrefined connections from the retina to the thalamus, reduce the spatial information contained by thalamic spikes, and therefore we term them 'parasitic' correlations. Our results suggest that developing synapses and circuits evolved mechanisms to compensate for such detrimental parasitic correlations arising from the unrefined and immature circuit.

Cortical Representation of Movement Across the Developmental Transition to Continuous Neural Activity

Primary motor cortex (M1) exhibits a protracted period of development that includes the establishment of a somatosensory map long before motor outflow emerges. In rats, the sensory representation is established by postnatal day (P) 8 when cortical activity is still "discontinuous." Here, we ask how the representation survives the sudden transition to noisy "continuous" activity at P12. Using neural decoding to predict forelimb movements based solely on M1 activity, we show that a linear decoder is sufficient to predict limb movements at P8, but not at P12; in contrast, a nonlinear decoder effectively predicts limb movements at P12. The change in decoder performance at P12 reflects an increase in both the complexity and uniqueness of kinematic information available in M1. We next show that the representation at P12 is more susceptible to the deleterious effects of "lesioning" inputs and to "transplanting" M1's encoding scheme from one pup to another. We conclude that the emergence of continuous cortical activity signals the developmental onset in M1 of more complex, informationally sparse, and individualized sensory representations.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Preterm neonates distinguish rhythm violation through a hierarchy of cortical processing

Rhythm is a fundamental component of the auditory world, present even during the prenatal life. While there is evidence that some auditory capacities are already present before birth, whether and how the premature neural networks process auditory rhythm is yet not known. We investigated the neural response of premature neonates at 30-34 weeks gestational age to violations from rhythmic regularities in an auditory sequence using high-resolution electroencephalography and event-related potentials. Unpredicted rhythm violations elicited a fronto-central mismatch response, indicating that the premature neonates detected the rhythmic regularities. Next, we examined the cortical effective connectivity underlying the elicited mismatch response using dynamic causal modeling. We examined the connectivity between cortical sources using a set of 16 generative models that embedded alternate hypotheses about the role of the frontal cortex as well as backward fronto-temporal connection. Our results demonstrated that the processing of rhythm violations was not limited to the primary auditory areas, and as in the case of adults, encompassed a hierarchy of temporo-frontal cortical structures. The result also emphasized the importance of top-down (backward) projections from the frontal cortex in explaining the mismatch response. Our findings demonstrate a sophisticated cortical structure underlying predictive rhythm processing at the onset of the thalamocortical and cortico-cortical circuits, two months before term.

Sleep, plasticity, and sensory neurodevelopment

A defining feature of early infancy is the immense neural plasticity that enables animals to develop a brain that is functionally integrated with a growing body. Early infancy is also defined as a period dominated by sleep. Here, we describe three conceptual frameworks that vary in terms of whether and how they incorporate sleep as a factor in the activity-dependent development of sensory and sensorimotor systems. The most widely accepted framework is exemplified by the visual system where retinal waves seemingly occur independent of sleep-wake states. An alternative framework is exemplified by the sensorimotor system where sensory feedback from sleep-specific movements activates the brain. We prefer a third framework that encompasses the first two but also captures the diverse ways in which sleep modulates activity-dependent development throughout the nervous system. Appreciation of the third framework will spur progress toward a more comprehensive and cohesive understanding of both typical and atypical neurodevelopment.

The Effects of Prenatal Exposure to Neighborhood Crime on Neonatal Functional Connectivity

BACKGROUND

Maternal exposure to adversity during pregnancy has been found to affect infant brain development; however, the specific effect of prenatal crime exposure on neonatal brain connectivity remains unclear. Based on existing research, we hypothesized that living in a high-crime neighborhood during pregnancy would affect neonatal frontolimbic connectivity over and above other individual- and neighborhood-level adversity and that these associations would be mediated by maternal psychosocial stress.

METHODS

Participants included 399 pregnant women, recruited as part of the eLABE (Early Life Adversity, Biological Embedding, and Risk for Developmental Precursors of Mental Disorders) study. In the neonatal period, 319 healthy, nonsedated infants were scanned using resting-state functional magnetic resonance imaging (repetition time = 800 ms; echo time = 37 ms; voxel size = 2.0 × 2.0 × 2.0 mm3; multiband = 8) on a Prisma 3T scanner and had at least 10 minutes of high-quality data. Crime data at the block group level were obtained from Applied Geographic Solution. Linear regressions and mediation models tested associations between crime, frontolimbic connectivity, and psychosocial stress.

RESULTS

Living in a neighborhood with high property crime during pregnancy was related to weaker neonatal functional connectivity between the thalamus-anterior default mode network (aDMN) (β = -0.15, 95% CI = -0.25 to -0.04, p = .008). Similarly, high neighborhood violent crime was related to weaker functional connectivity between the thalamus-aDMN (β = -0.16, 95% CI = -0.29 to -0.04, p = .01) and amygdala-hippocampus (β = -0.16, 95% CI = -0.29 to -0.03, p = .02), controlling for other types of adversity. Psychosocial stress partially mediated relationships between the thalamus-aDMN and both violent and property crime.

CONCLUSIONS

These findings suggest that prenatal exposure to crime is associated with weaker neonatal limbic and frontal functional brain connections, providing another reason for targeted public policy interventions to reduce crime.

Not so spontaneous: Multi-dimensional representations of behaviors and context in sensory areas

Sensory areas are spontaneously active in the absence of sensory stimuli. This spontaneous activity has long been studied; however, its functional role remains largely unknown. Recent advances in technology, allowing large-scale neural recordings in the awake and behaving animal, have transformed our understanding of spontaneous activity. Studies using these recordings have discovered high-dimensional spontaneous activity patterns, correlation between spontaneous activity and behavior, and dissimilarity between spontaneous and sensory-driven activity patterns. These findings are supported by evidence from developing animals, where a transition toward these characteristics is observed as the circuit matures, as well as by evidence from mature animals across species. These newly revealed characteristics call for the formulation of a new role for spontaneous activity in neural sensory computation.

Epigenetic and Transcriptional Regulation of Spontaneous and Sensory Activity Dependent Programs During Neuronal Circuit Development

Spontaneous activity generated before the onset of sensory transduction has a key role in wiring developing sensory circuits. From axonal targeting, to synapse formation and elimination, to the balanced integration of neurons into developing circuits, this type of activity is implicated in a variety of cellular processes. However, little is known about its molecular mechanisms of action, especially at the level of genome regulation. Conversely, sensory experience-dependent activity implements well-characterized transcriptional and epigenetic chromatin programs that underlie heterogeneous but specific genomic responses that shape both postnatal circuit development and neuroplasticity in the adult. In this review, we focus on our knowledge of the developmental processes regulated by spontaneous activity and the underlying transcriptional mechanisms. We also review novel findings on how chromatin regulates the specificity and developmental induction of the experience-dependent program, and speculate their relevance for our understanding of how spontaneous activity may act at the genomic level to instruct circuit assembly and prepare developing neurons for sensory-dependent connectivity refinement and processing.

Sleep as a window on the sensorimotor foundations of the developing hippocampus

The hippocampal formation plays established roles in learning, memory, and related cognitive functions. Recent findings also suggest that the hippocampus integrates sensory feedback from self-generated movements to modulate ongoing motor responses in a changing environment. Such findings support the view of Bland and Oddie (Behavioural Brain Research, 2001, 127, 119-136) that the hippocampus is a site of sensorimotor integration. In further support of this view, we review neurophysiological evidence in developing rats that hippocampal function is built on a sensorimotor foundation and that this foundation is especially evident early in development. Moreover, at those ages when the hippocampus is first establishing functional connectivity with distant sensory and motor structures, that connectivity is preferentially expressed during periods of active (or REM) sleep. These findings reinforce the notion that sleep, as the predominant state of early infancy, provides a critical context for sensorimotor development, including development of the hippocampus and its associated network.

Neurophysiology of the Developing Cerebral Cortex: What We Have Learned and What We Need to Know

This review article aims to give a brief summary on the novel technologies, the challenges, our current understanding, and the open questions in the field of the neurophysiology of the developing cerebral cortex in rodents. In the past, in vitro electrophysiological and calcium imaging studies on single neurons provided important insights into the function of cellular and subcellular mechanism during early postnatal development. In the past decade, neuronal activity in large cortical networks was recorded in pre- and neonatal rodents in vivo by the use of novel high-density multi-electrode arrays and genetically encoded calcium indicators. These studies demonstrated a surprisingly rich repertoire of spontaneous cortical and subcortical activity patterns, which are currently not completely understood in their functional roles in early development and their impact on cortical maturation. Technological progress in targeted genetic manipulations, optogenetics, and chemogenetics now allow the experimental manipulation of specific neuronal cell types to elucidate the function of early (transient) cortical circuits and their role in the generation of spontaneous and sensory evoked cortical activity patterns. Large-scale interactions between different cortical areas and subcortical regions, characterization of developmental shifts from synchronized to desynchronized activity patterns, identification of transient circuits and hub neurons, role of electrical activity in the control of glial cell differentiation and function are future key tasks to gain further insights into the neurophysiology of the developing cerebral cortex.

A new role for visual experience in top-down cortical development

In this issue of Neuron, Ibrahim et al. (2021) examine the rules by which top-down connections are made on visual cortical layer 1 interneurons, discovering activity-dependent cooperative interactions with visual input that are specific to neurogliaform cells and anterior cingulate cortex.

Sensory Coding of Limb Kinematics in Motor Cortex across a Key Developmental Transition

Primary motor cortex (M1) undergoes protracted development in mammals, functioning initially as a sensory structure. Throughout the first postnatal week in rats, M1 is strongly activated by self-generated forelimb movements-especially by the twitches that occur during active sleep. Here, we quantify the kinematic features of forelimb movements to reveal receptive-field properties of individual units within the forelimb region of M1. At postnatal day 8 (P8), nearly all units were strongly modulated by movement amplitude, especially during active sleep. By P12, only a minority of units continued to exhibit amplitude tuning, regardless of behavioral state. At both ages, movement direction also modulated M1 activity, though to a lesser extent. Finally, at P12, M1 population-level activity became more sparse and decorrelated, along with a substantial alteration in the statistical distribution of M1 responses to limb movements. These findings reveal a transition toward a more complex and informationally rich representation of movement long before M1 develops its motor functionality.SIGNIFICANCE STATEMENT Primary motor cortex (M1) plays a fundamental role in the generation of voluntary movements and motor learning in adults. In early development, however, M1 functions as a prototypical sensory structure. Here, we demonstrate in infant rats that M1 codes for the kinematics of self-generated limb movements long before M1 develops its capacity to drive movements themselves. Moreover, we identify a key transition during the second postnatal week in which M1 activity becomes more informationally complex. Together, these findings further delineate the complex developmental path by which M1 develops its sensory functions in support of its later-emerging motor capacities.

Spontaneous activity in developing thalamic and cortical sensory networks

Developing sensory circuits exhibit different patterns of spontaneous activity, patterns that are related to the construction and refinement of functional networks. During the development of different sensory modalities, spontaneous activity originates in the immature peripheral sensory structures and in the higher-order central structures, such as the thalamus and cortex. Certainly, the perinatal thalamus exhibits spontaneous calcium waves, a pattern of activity that is fundamental for the formation of sensory maps and for circuit plasticity. Here, we review our current understanding of the maturation of early (including embryonic) patterns of spontaneous activity and their influence on the assembly of thalamic and cortical sensory networks. Overall, the data currently available suggest similarities between the developmental trajectory of brain activity in experimental models and humans, which in the future may help to improve the early diagnosis of developmental disorders.

Somatostatin interneurons restrict cell recruitment to retinally driven spontaneous activity in the developing cortex

During early development, before the eyes open, synaptic refinement of sensory networks depends on activity generated by developing neurons themselves. In the mouse visual system, retinal cells spontaneously depolarize and recruit downstream neurons to bursts of activity, where the number of recruited cells determines the resolution of synaptic retinotopic refinement. Here we show that during the second post-natal week in mouse visual cortex, somatostatin (SST)-expressing interneurons control the recruitment of cells to retinally driven spontaneous activity. Suppressing SST interneurons increases cell participation and allows events to spread farther along the cortex. During the same developmental period, a second type of high-participation, retina-independent event occurs. During these events, cells receive such large excitatory charge that inhibition is overwhelmed and large parts of the cortex participate in each burst. These results reveal a role of SST interneurons in restricting retinally driven activity in the visual cortex, which may contribute to the refinement of retinotopy.

Twitches emerge postnatally during quiet sleep in human infants and are synchronized with sleep spindles

In humans and other mammals, the stillness of sleep is punctuated by bursts of rapid eye movements (REMs) and myoclonic twitches of the limbs.¹ Like the spontaneous activity that arises from the sensory periphery in other modalities (e.g., retinal waves),² sensory feedback arising from twitches is well suited to drive activity-dependent development of the sensorimotor system.³ It is partly because of the behavioral activation of REM sleep that this state is also called active sleep (AS), in contrast with the behavioral quiescence that gives quiet sleep (QS)-the second major stage of sleep-its name. In human infants, for which AS occupies 8 h of each day,⁴ twitching helps to identify the state;^5-8 nonetheless, we know little about the structure and functions of twitching across development. Recently, in sleeping infants,⁹ we documented a shift in the temporal expression of twitching beginning around 3 months of age that suggested a qualitative change in how twitches are produced. Here, we combine behavioral analysis with high-density electroencephalography (EEG) to demonstrate that this shift reflects the emergence of limb twitches during QS. Twitches during QS are not only unaccompanied by REMs, but they also occur synchronously with sleep spindles, a hallmark of QS. As QS-related twitching increases with age, sleep spindle rate also increases along the sensorimotor strip. The emerging synchrony between subcortically generated twitches and cortical oscillations suggests the development of functional connectivity among distant sensorimotor structures, with potential implications for detecting and explaining atypical developmental trajectories.

Input-Independent Homeostasis of Developing Thalamocortical Activity

The isocortex of all mammals studied to date shows a progressive increase in the amount and continuity of background activity during early development. In humans the transition from a discontinuous (mostly silent, intermittently bursting) cortex to one that is continuously active is complete soon after birth and is a critical prognostic indicator. In the visual cortex of rodents this switch from discontinuous to continuous background activity occurs during the 2 d before eye-opening, driven by activity changes in relay thalamus. The factors that regulate the timing of continuity development, which enables mature visual processing, are unknown. Here, we test the role of the retina, the primary input, in the development of continuous spontaneous activity in the visual cortex of mice using depth electrode recordings from enucleated mice in vivo. Bilateral enucleation at postnatal day (P)6, one week before the onset of continuous activity, acutely silences cortex, yet firing rates and early oscillations return to normal within 2 d and show a normal developmental trajectory through P12. Enucleated animals showed differences in silent period duration and continuity on P13 that resolved on P16, and an increase in low frequency power that did not. Our results show that the timing of cortical activity development is not determined by the major driving input to the system. Rather, even during a period of rapid increase in firing rates and continuity, neural activity in the visual cortex is under homeostatic control that is largely robust to the loss of the primary input.

Polynomial, piecewise-Linear, Step (PLS): A Simple, Scalable, and Efficient Framework for Modeling Neurons

Biological neurons can be modeled with different levels of biophysical/biochemical details. The accuracy with which a model reflects the actual physiological processes and ultimately the information function of a neuron, can range from very detailed to a schematic phenomenological representation. This range exists due to the common problem: one needs to find an optimal trade-off between the level of details needed to capture the necessary information processing in a neuron and the computational load needed to compute 1 s of model time. An increase in modeled network size or model-time, for which the solution should be obtained, makes this trade-off pivotal in model development. Numerical simulations become incredibly challenging when an extensive network with a detailed representation of each neuron needs to be modeled over a long time interval to study slow evolving processes, e.g., development of the thalamocortical circuits. Here we suggest a simple, powerful and flexible approach in which we approximate the right-hand sides of differential equations by combinations of functions from three families: Polynomial, piecewise-Linear, Step (PLS). To obtain a single coherent framework, we provide four core principles in which PLS functions should be combined. We show the rationale behind each of the core principles. Two examples illustrate how to build a conductance-based or phenomenological model using the PLS-framework. We use the first example as a benchmark on three different computational platforms: CPU, GPU, and mobile system-on-chip devices. We show that the PLS-framework speeds up computations without increasing the memory footprint and maintains high model fidelity comparable to the fully-computed model or with lookup-table approximation. We are convinced that the full range of neuron models: from biophysical to phenomenological and even to abstract models, may benefit from using the PLS-framework.

The Use of Deep Learning and VR Technology in Film and Television Production From the Perspective of Audience Psychology

As the development of artificial intelligence (AI) technology, the deep-learning (DL)-based Virtual Reality (VR) technology, and DL technology are applied in human-computer interaction (HCI), and their impacts on modern film and TV works production and audience psychology are analyzed. In film and TV production, audiences have a higher demand for the verisimilitude and immersion of the works, especially in film production. Based on this, a 2D image recognition system for human body motions and a 3D recognition system for human body motions based on the convolutional neural network (CNN) algorithm of DL are proposed, and an analysis framework is established. The proposed systems are simulated on practical and professional datasets, respectively. The results show that the algorithm's computing performance in 2D image recognition is 7-9 times higher than that of the Open Pose method. It runs at 44.3 ms in 3D motion recognition, significantly lower than the Open Pose method's 794.5 and 138.7 ms. Although the detection accuracy has dropped by 2.4%, it is more efficient and convenient without limitations of scenarios in practical applications. The AI-based VR and DL enriches and expands the role and application of computer graphics in film and TV production using HCI technology theoretically and practically.

Back to basics: the neuronal substrates and mechanisms that underlie the electroencephalogram in premature neonates

Electroencephalography is the only clinically available technique that can address the premature neonate normal and pathological functional development week after week. The changes in the electroencephalogram (EEG) result from gradual structural and functional modifications that arise during the last trimester of pregnancy. Here, we review the structural changes over time that underlie the establishment of functional immature neural networks, the impact of certain anatomical specificities (fontanelles, connectivity, etc.) on the EEG, limitations in EEG interpretation, and the utility of high-resolution EEG (HR-EEG) in premature newborns (a promising technique with a high degree of spatiotemporal resolution). In particular, we classify EEG features according to whether they are manifestations of endogenous generators (i.e. theta activities that coalesce with a slow wave or delta brushes) or come from a broader network. Furthermore, we review publications on EEG in premature animals because the data provide a better understanding of what is happening in premature newborns. We then discuss the results and limitations of functional connectivity analyses in premature newborns. Lastly, we report on the magnetoelectroencephalographic studies of brain activity in the fetus. A better understanding of complex interactions at various structural and functional levels during normal neurodevelopment (as assessed using electroencephalography as a benchmark method) might lead to better clinical care and monitoring for premature neonates.

The intimate relationship between coalescent generators in very premature human newborn brains: Quantifying the coupling of nested endogenous oscillations

Temporal theta slow-wave activity (TTA-SW) in premature infants is a specific neurobiomarker of the early neurodevelopment of perisylvian networks observed as early as 24 weeks of gestational age (wGA). It is present at the turning point between non-sensory driven spontaneous networks and cortical network functioning. Despite its clinical importance, the underlying mechanisms responsible for this spontaneous nested activity and its functional role have not yet been determined. The coupling between neural oscillations at different timescales is a key feature of ongoing neural activity, the characteristics of which are determined by the network structure and dynamics. The underlying mechanisms of cross-frequency coupling (CFC) are associated with several putative functions in adults. In order to show that this generic mechanism is already in place early in the course of development, we analyzed electroencephalography recordings from sleeping preterm newborns (24-27 wGA). Employing cross-frequency phase-amplitude coupling analyses, we found that TTAs were orchestrated by the SWs defined by a precise temporal relationship. Notably, TTAs were synchronized to the SW trough, and were suppressed during the SW peak. Spontaneous endogenous TTA-SWs constitute one of the very early signatures of the developing temporal neural networks with key functions, such as language and communication. The presence of a fine-tuned relationship between the slow activity and the TTA in premature neonates emphasizes the complexity and relative maturity of the intimate mechanisms that shape the CFC, the disruption of which can have severe neurodevelopmental consequences.

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

GABAergic interneurons excite neonatal hippocampus in vivo

GABAergic interneurons are proposed to be critical for early activity and synapse formation by directly exciting, rather than inhibiting, neurons in developing hippocampus and neocortex. However, the role of GABAergic neurons in the generation of neonatal network activity has not been tested in vivo, and recent studies have challenged the excitatory nature of early GABA. By locally manipulating interneuron activity in unanesthetized neonatal mice, we show that GABAergic neurons are excitatory in CA1 hippocampus at postnatal day 3 (P3) and are responsible for most of the spontaneous firing of pyramidal cells at that age. Hippocampal interneurons become inhibitory by P7, whereas visual cortex interneurons are already inhibitory by P3 and remain so throughout development. These regional and age-specific differences are the result of a change in chloride reversal potential, because direct activation of light-gated anion channels in glutamatergic neurons drives CA1 firing at P3, but silences it at P7 in CA1, and at all ages in visual cortex. This study in the intact brain reveals that GABAergic interneuron excitation is essential for network activity in neonatal hippocampus and confirms that visual cortical interneurons are inhibitory throughout early postnatal development.

Progressive alignment of inhibitory and excitatory delay may drive a rapid developmental switch in cortical network dynamics

Nervous system maturation occurs on multiple levels-synaptic, circuit, and network-at divergent timescales. For example, many synaptic properties mature gradually, whereas emergent network dynamics can change abruptly. Here we combine experimental and theoretical approaches to investigate a sudden transition in spontaneous and sensory evoked thalamocortical activity necessary for the development of vision. Inspired by in vivo measurements of timescales and amplitudes of synaptic currents, we extend the Wilson and Cowan model to take into account the relative onset timing and amplitudes of inhibitory and excitatory neural population responses. We study this system as these parameters are varied within amplitudes and timescales consistent with developmental observations to identify the bifurcations of the dynamics that might explain the network behaviors in vivo. Our findings indicate that the inhibitory timing is a critical determinant of thalamocortical activity maturation; a gradual decay of the ratio of inhibitory to excitatory onset time drives the system through a bifurcation that leads to a sudden switch of the network spontaneous activity from high-amplitude oscillations to a nonoscillatory active state. This switch also drives a change from a threshold bursting to linear response to transient stimuli, also consistent with in vivo observation. Thus we show that inhibitory timing is likely critical to the development of network dynamics and may underlie rapid changes in activity without similarly rapid changes in the underlying synaptic and cellular parameters.NEW & NOTEWORTHY Relying on a generalization of the Wilson-Cowan model, which allows a solid analytic foundation for the understanding of the link between maturation of inhibition and network dynamics, we propose a potential explanation for the role of developing excitatory/inhibitory synaptic delays in mediating a sudden switch in thalamocortical visual activity preceding vision onset.

Impact of prematurity on neurodevelopment

The consequences of prematurity on brain functional development are numerous and diverse, and impact all brain functions at different levels. Prematurity occurs between 22 and 36 weeks of gestation. This period is marked by extreme dynamics in the physiologic maturation, structural, and functional processes. These different processes appear sequentially or simultaneously. They are dependent on genetic and/or environmental factors. Disturbance of these processes or of the fine-tuning between them, when caring for premature children, is likely to induce disturbances in the structural and functional development of the immature neural networks. These will appear as impairments in learning skills progress and are likely to have a lasting impact on the development of children born prematurely. The level of severity depends on the initial alteration, whether structural or functional. In this chapter, after having briefly reviewed the neurodevelopmental, structural, and functional processes, we describe, in a nonexhaustive manner, the impact of prematurity on the different brain, motor, sensory, and cognitive functions.

THE DEVELOPING BRAIN REVEALED DURING SLEEP

Given the prevalence of sleep in early development, any satisfactory account of infant brain activity must consider what happens during sleep. Only recently, however, has it become possible to record sleep-related brain activity in newborn rodents. Using such methods in rat pups, it is now clear that sleep, more so than wake, provides a critical context for the processing of sensory input and the expression of functional connectivity throughout the sensorimotor system. In addition, sleep uniquely reveals functional activity in the developing primary motor cortex, which establishes a somatosensory map long before its role in motor control emerges. These findings will inform our understanding of the developmental processes that contribute to the nascent sense of embodiment in human infants.

Behavioral states modulate sensory processing in early development

Purpose of Review

Sleep-wake states modulate cortical activity in adults. In infants, however, such modulation is less clear; indeed, early cortical activity comprises bursts of neural activity driven predominantly by peripheral sensory input. Consequently, in many studies of sensory development in rodents, sensory processing has been carefully investigated, but the modulatory role of behavioral state has typically been ignored.

Recent Findings

In the developing visual and somatosensory systems, it is now known that sleep and wake states modulate sensory processing. Further, in both systems, the nature of this modulation shifts rapidly during the second postnatal week, with subcortical nuclei changing how they gate sensory inputs.

Summary

The interactions among sleep and wake movements, sensory processing, and development are dynamic and complex. Now that established methods exist to record neural activity in unanesthetized infant animals, we can provide a more comprehensive understanding of how infant sleep-wake states interact with sensory-driven responses to promote developmental plasticity.

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Developmental 'awakening' of primary motor cortex to the sensory consequences of movement

Before primary motor cortex (M1) develops its motor functions, it functions like a somatosensory area. Here, by recording from neurons in the forelimb representation of M1 in postnatal day (P) 8–12 rats, we demonstrate a rapid shift in its sensory responses. At P8-10, M1 neurons respond overwhelmingly to feedback from sleep-related twitches of the forelimb, but the same neurons do not respond to wake-related movements. By P12, M1 neurons suddenly respond to wake movements, a transition that results from opening the sensory gate in the external cuneate nucleus. Also at P12, fewer M1 neurons respond to individual twitches, but the full complement of twitch-related feedback observed at P8 is unmasked through local disinhibition. Finally, through P12, M1 sensory responses originate in the deep thalamorecipient layers, not primary somatosensory cortex. These findings demonstrate that M1 initially establishes a sensory framework upon which its later-emerging role in motor control is built.

The Superior Function of the Subplate in Early Neocortical Development

During early development the structure and function of the cerebral cortex is critically organized by subplate neurons (SPNs), a mostly transient population of glutamatergic and GABAergic neurons located below the cortical plate. At the molecular and morphological level SPNs represent a rather diverse population of cells expressing a variety of genetic markers and revealing different axonal-dendritic morphologies. Electrophysiologically SPNs are characterized by their rather mature intrinsic membrane properties and firing patterns. They are connected via electrical and chemical synapses to local and remote neurons, e.g., thalamic relay neurons forming the first thalamocortical input to the cerebral cortex. Therefore SPNs are robustly activated at pre- and perinatal stages by the sensory periphery. Although SPNs play pivotal roles in early neocortical activity, development and plasticity, they mostly disappear by programmed cell death during further maturation. On the one hand, SPNs may be selectively vulnerable to hypoxia-ischemia contributing to brain damage, on the other hand there is some evidence that enhanced survival rates or alterations in SPN distribution may contribute to the etiology of neurological or psychiatric disorders. This review aims to give a comprehensive and up-to-date overview on the many functions of SPNs during early physiological and pathophysiological development of the cerebral cortex.

Thalamus Controls Development and Expression of Arousal States in Visual Cortex

Two major checkpoints of development in cerebral cortex are the acquisition of continuous spontaneous activity and the modulation of this activity by behavioral state. Despite the critical importance of these functions, the circuit mechanisms of their development remain unknown. Here we use the rodent visual system as a model to test the hypothesis that the locus of circuit change responsible for the developmental acquisition of continuity and state dependence measured in sensory cortex is relay thalamus, rather than the local cortical circuitry or the interconnectivity of the two structures. We conducted simultaneous recordings in the dorsal lateral geniculate nucleus (dLGN) and primary visual cortex (VC) of awake, head-fixed male and female rats using linear multielectrode arrays throughout early development. We find that activity in dLGN becomes continuous and positively correlated with movement (a measure of state dependence) on P13, the same day as VC, and that these properties are not dependent on VC activity. By contrast, silencing dLGN after P13 causes activity in VC to become discontinuous and movement to suppress, rather than augment, cortical firing, effectively reversing development. Thalamic bursting, a core characteristic of non-aroused states, emerged later, on P16, suggesting these processes are developmentally independent. Together our results indicate that cellular or circuit changes in relay thalamus are critical drivers for the maturation of background activity, which occurs around term in humans.SIGNIFICANCE STATEMENT The developing brain acquires two crucial features, continuous spontaneous activity and its modulation by arousal state, around term in humans and before the onset of sensory experience in rodents. This developmental transition in cortical activity, as measured by electroencephalogram (EEG), is an important milestone for normal brain development and indicates a good prognosis for babies born preterm and/or suffering brain damage such as hypoxic-ischemic encephalopathy. By using the awake rodent visual system as a model, we identify changes occurring at the level of relay thalamus, the major input to cortex, as the critical driver of EEG maturation. These results could help understand the circuit basis of human EEG development to improve diagnosis and treatment of infants in vulnerable situations.