Neuronal activity during development: permissive or instructive?

Dendritic growth and synaptic organization from activity-independent cues and local activity-dependent plasticity

Dendritic branching and synaptic organization shape single-neuron and network computations. How they emerge simultaneously during brain development as neurons become integrated into functional networks is still not mechanistically understood. Here, we propose a mechanistic model in which dendrite growth and the organization of synapses arise from the interaction of activity-independent cues from potential synaptic partners and local activity-dependent synaptic plasticity. Consistent with experiments, three phases of dendritic growth - overshoot, pruning, and stabilization - emerge naturally in the model. The model generates stellate-like dendritic morphologies that capture several morphological features of biological neurons under normal and perturbed learning rules, reflecting biological variability. Model-generated dendrites have approximately optimal wiring length consistent with experimental measurements. In addition to establishing dendritic morphologies, activity-dependent plasticity rules organize synapses into spatial clusters according to the correlated activity they experience. We demonstrate that a trade-off between activity-dependent and -independent factors influences dendritic growth and synaptic location throughout development, suggesting that early developmental variability can affect mature morphology and synaptic function. Therefore, a single mechanistic model can capture dendritic growth and account for the synaptic organization of correlated inputs during development. Our work suggests concrete mechanistic components underlying the emergence of dendritic morphologies and synaptic formation and removal in function and dysfunction, and provides experimentally testable predictions for the role of individual components.

Neurotrophin NT-4/5 Promotes Structural Changes in Neurons of the Developing Visual Cortex

Current hypotheses on the mechanisms underlying the development and plasticity of the ocular dominance system through competitive interactions between pathways serving the two eyes strongly suggest the involvement of neurotrophins and their high affinity receptors. In the cat, infusion of the tyrosine kinase B ligand (trkB), neurotrophin-4/5 (NT-4/5), abolishes ocular dominance plasticity that follows monocular deprivation (Gillespie et al., 2000), while tyrosine kinase A and C ligands (trkA and trkC) do not have this effect. One interpretation of this finding is that NT-4/5 causes overgrowth and sprouting of thalamocortical and/or corticocortical terminals, leading to promiscuous neuronal connections which override the experience-dependent fine tuning of connections based on correlated activity. The present study tested whether neurons in cortical regions infused with NT-4/5 showed anatomical changes compatible with this hypothesis. Cats at the peak of the critical period received chronic infusion NT-4/5 into visual cortical areas 17/18 via an osmotic minipump. Visual cortical neurons were labeled in fixed slices using the DiOlistics methods (Gan et al., 2000) and analyzed in confocal microscopy. Infusion of NT-4/5 induced a significant increase of spine-like processes on primary dendrites and a distinctive sprouting of protuberances from neuronal somata in all layers. The increase of neuronal membrane was paralleled by an increase in density of the presynaptic marker synaptophysin in infused areas, suggesting an increase in the numbers of synapses. A contingent of these newly formed synapses may feed into inhibitory circuits, as suggested by an increase of GAD-65 immunostaining in NT-4/5 affected areas. These anatomical changes are consistent with the physiological changes in such animals, suggesting that excess trkB neurotrophin can stimulate the formation of promiscuous connections during the critical period.

Activity-induced MeCP2 phosphorylation regulates retinogeniculate synapse refinement

Mutations in MECP2 give rise to Rett syndrome (RTT), an X-linked neurodevelopmental disorder that results in broad cognitive impairments in females. While the exact etiology of RTT symptoms remains unknown, one possible explanation for its clinical presentation is that loss of MECP2 causes miswiring of neural circuits due to defects in the brain's capacity to respond to changes in neuronal activity and sensory experience. Here, we show that MeCP2 is phosphorylated at four residues in the mouse brain (S86, S274, T308, and S421) in response to neuronal activity, and we generate a quadruple knock-in (QKI) mouse line in which all four activity-dependent sites are mutated to alanines to prevent phosphorylation. QKI mice do not display overt RTT phenotypes or detectable gene expression changes in two brain regions. However, electrophysiological recordings from the retinogeniculate synapse of QKI mice reveal that while synapse elimination is initially normal at P14, it is significantly compromised at P20. Notably, this phenotype is distinct from the synapse refinement defect previously reported for Mecp2 null mice, where synapses initially refine but then regress after the third postnatal week. We thus propose a model in which activity-induced phosphorylation of MeCP2 is critical for the proper timing of retinogeniculate synapse maturation specifically during the early postnatal period.

Experience-dependent structural plasticity of pyramidal neurons in the developing sensory cortices

Sensory experience regulates the structural and functional wiring of neuronal circuits, during development and throughout adulthood. Here, we review current knowledge of how experience affects structural plasticity of pyramidal neurons in the sensory cortices. We discuss the pros and cons of existing labeling approaches, as well as what structural parameters are most plastic. We further discuss how recent advances in sparse labeling of specific neuronal subtypes, as well as development of techniques that allow fast, high resolution imaging in large fields, would enable future studies to address currently unanswered questions in the field of structural plasticity.

Activity-Induced MeCP2 Phosphorylation Regulates Retinogeniculate Synapse Refinement

Mutations in MECP2 give rise to Rett syndrome (RTT), an X-linked neurodevelopmental disorder that results in broad cognitive impairments in females. While the exact etiology of RTT symptoms remains unknown, one possible explanation for its clinical presentation is that loss of MeCP2 causes miswiring of neural circuits due to defects in the brain's capacity to respond to changes in neuronal activity and sensory experience. Here we show that MeCP2 is phosphorylated at four residues in the brain (S86, S274, T308, and S421) in response to neuronal activity, and we generate a quadruple knock-in (QKI) mouse line in which all four activity-dependent sites are mutated to alanines to prevent phosphorylation. QKI mice do not display overt RTT phenotypes or detectable gene expression changes in two brain regions. However, electrophysiological recordings from the retinogeniculate synapse of QKI mice reveal that while synapse elimination is initially normal at P14, it is significantly compromised at P20. Notably, this phenotype is distinct from that previously reported for Mecp2 null mice, where synapses initially refine but then regress after the third postnatal week. We thus propose a model in which activity-induced phosphorylation of MeCP2 is critical for the proper timing of retinogeniculate synapse maturation specifically during the early postnatal period.

SIGNIFICANCE STATEMENT

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder that predominantly affects girls. RTT is caused by loss of function mutations in a single gene MeCP2. Girls with RTT develop normally during their first year of life, but then experience neurological abnormalities including breathing and movement difficulties, loss of speech, and seizures. This study investigates the function of the MeCP2 protein in the brain, and how MeCP2 activity is modulated by sensory experience in early life. Evidence is presented that sensory experience affects MeCP2 function, and that this is required for synaptic pruning in the brain. These findings provide insight into MeCP2 function, and clues as to what goes awry in the brain when the function of MeCP2 is disrupted.

Endocannabinoid-dependent formation of columnar axonal projection in the mouse cerebral cortex

Columnar structure is one of the most fundamental morphological features of the cerebral cortex and is thought to be the basis of information processing in higher animals. Yet, how such a topographically precise structure is formed is largely unknown. Formation of columnar projection of layer 4 (L4) axons is preceded by thalamocortical formation, in which type 1 cannabinoid receptors (CB1R) play an important role in shaping barrel-specific targeted projection by operating spike timing-dependent plasticity during development (Itami et al., J. Neurosci. 36, 7039-7054 [2016]; Kimura & Itami, J. Neurosci. 39, 3784-3791 [2019]). Right after the formation of thalamocortical projections, CB1Rs start to function at L4 axon terminals (Itami & Kimura, J. Neurosci. 32, 15000-15011 [2012]), which coincides with the timing of columnar shaping of L4 axons. Here, we show that the endocannabinoid 2-arachidonoylglycerol (2-AG) plays a crucial role in columnar shaping. We found that L4 axon projections were less organized until P12 and then became columnar after CB1Rs became functional. By contrast, the columnar organization of L4 axons was collapsed in mice genetically lacking diacylglycerol lipase α, the major enzyme for 2-AG synthesis. Intraperitoneally administered CB1R agonists shortened axon length, whereas knockout of CB1R in L4 neurons impaired columnar projection of their axons. Our results suggest that endocannabinoid signaling is crucial for shaping columnar axonal projection in the cerebral cortex.

A discrete neuronal population coordinates brain-wide developmental activity

In vertebrates, stimulus-independent activity accompanies neural circuit maturation throughout the developing brain1,2. The recent discovery of similar activity in the developing Drosophila central nervous system suggests that developmental activity is fundamental to the assembly of complex brains3. How such activity is coordinated across disparate brain regions to influence synaptic development at the level of defined cell types is not well understood. Here we show that neurons expressing the cation channel transient receptor potential gamma (Trpγ) relay and pattern developmental activity throughout the Drosophila brain. In trpγ mutants, activity is attenuated globally, and both patterns of activity and synapse structure are altered in a cell-type-specific manner. Less than 2% of the neurons in the brain express Trpγ. These neurons arborize throughout the brain, and silencing or activating them leads to loss or gain of brain-wide activity. Together, these results indicate that this small population of neurons coordinates brain-wide developmental activity. We propose that stereotyped patterns of developmental activity are driven by a discrete, genetically specified network to instruct neural circuit assembly at the level of individual cells and synapses. This work establishes the fly brain as an experimentally tractable system for studying how activity contributes to synapse and circuit formation.

Oxytocin Regulates Synaptic Transmission in the Sensory Cortices in a Developmentally Dynamic Manner

The development and stabilization of neuronal circuits are critical to proper brain function. Synapses are the building blocks of neural circuits. Here we examine the effects of the neuropeptide oxytocin on synaptic transmission in L2/3 pyramidal neurons of the barrel field of the primary somatosensory cortex (S1BF). We find that perfusion of oxytocin onto acute brain slices significantly increases the frequency of miniature excitatory postsynaptic currents (mEPSC) of S1BF L2/3 pyramidal neurons at P10 and P14, but reduces it at the later ages of P22 and P28; the transition occurs at around P18. Since oxytocin expression is itself regulated by sensory experience, we also examine whether the effects of oxytocin on excitatory synaptic transmission correlate with that of sensory experience. We find that, indeed, the effects of sensory experience and oxytocin on excitatory synaptic transmission of L2/3 pyramidal neurons both peak at around P14 and plateau around P18, suggesting that they regulate a specific form of synaptic plasticity in L2/3 pyramidal neurons, with a sensitive/critical period ending around P18. Consistently, oxytocin receptor (Oxtr) expression in glutamatergic neurons of the upper layers of the cerebral cortex peaks around P14. By P28, however, Oxtr expression becomes more prominent in GABAergic neurons, especially somatostatin (SST) neurons. At P28, oxytocin perfusion increases inhibitory synaptic transmission and reduces excitatory synaptic transmission, effects that result in a net reduction of neuronal excitation, in contrast to increased excitation at P14. Using oxytocin knockout mice and Oxtr conditional knockout mice, we show that loss-of-function of oxytocin affects baseline excitatory synaptic transmission, while Oxtr is required for oxytocin-induced changes in excitatory synaptic transmission, at both P14 and P28. Together, these results demonstrate that oxytocin has complex and dynamic functions in regulating synaptic transmission in cortical L2/3 pyramidal neurons. These findings add to existing knowledge of the function of oxytocin in regulating neural circuit development and plasticity.

Light stimulation during postnatal development is not determinant for glutamatergic neurotransmission from the retinohypothalamic tract to the suprachiasmatic nucleus in rats

The hypothalamic suprachiasmatic nucleus (SCN) is the leading circadian pacemaker in mammals, which synchronizes with environmental light through the retinohypothalamic tract (RHT). Although the SCN regulates circadian rhythms before birth, postnatal synaptic changes are needed for the RHT-SCN pathway to achieve total functional development. However, it is unknown whether visual experience affects developmental maturation. Here, we studied the effects of constant darkness (DD) rearing on the physiology (at pre- and postsynaptic levels) of glutamatergic neurotransmission between RHT and SCN during postnatal development in rats. Upon recording spontaneous and evoked excitatory postsynaptic currents (EPSCs) by electrical stimulation of RHT fibers, we found that DD animals at early postnatal ages (P3-19) exhibited different frequencies of spontaneous EPSCs and lower synaptic performance (short-term depression, release sites, and recruitment of RHT fibers) when compared with their normal light/dark (LD) counterparts. At the oldest age evaluated (P30-35), there was a synaptic response strengthening (probability of release, vesicular re-filling rate, and reduced synaptic depression) in DD rats, which functionally equaled (or surmounted) that of LD animals. Control experiments evaluating EPSCs in ventral SCN neurons of LD rats during day and night revealed no significant differences in spontaneous or evoked EPSCs by high-frequency trains in the RHT at any postnatal age. Our results suggest that DD conditions induce a compensatory mechanism in the glutamatergic signaling of the circadian system to increase the chances of synchronization to light at adult ages, and that the synaptic properties of RHT terminals during postnatal development are not critically influenced by environmental light.

Building a circuit through correlated spontaneous neuronal activity in the developing vertebrate and invertebrate visual systems

During the development of the vertebrate nervous systems, genetic programs assemble an immature circuit that is subsequently refined by neuronal activity evoked by external stimuli. However, prior to sensory experience, the intrinsic property of the developing nervous system also triggers correlated network-level neuronal activity, with retinal waves in the developing vertebrate retina being the best documented example. Spontaneous activity has also been found in the visual system of Drosophila Here, we compare the spontaneous activity of the developing visual system between mammalian and Drosophila and suggest that Drosophila is an emerging model for mechanistic and functional studies of correlated spontaneous activity.

Efferent feedback controls bilateral auditory spontaneous activity

In the developing auditory system, spontaneous activity generated in the cochleae propagates into the central nervous system to promote circuit formation. The effects of peripheral firing patterns on spontaneous activity in the central auditory system are not well understood. Here, we describe wide-spread bilateral coupling of spontaneous activity that coincides with the period of transient efferent modulation of inner hair cells from the brainstem medial olivocochlear system. Knocking out α9/α10 nicotinic acetylcholine receptors, a requisite part of the efferent pathway, profoundly reduces bilateral correlations. Pharmacological and chemogenetic experiments confirm that the efferent system is necessary for normal bilateral coupling. Moreover, auditory sensitivity at hearing onset is reduced in the absence of pre-hearing efferent modulation. Together, these results demonstrate how afferent and efferent pathways collectively shape spontaneous activity patterns and reveal the important role of efferents in coordinating bilateral spontaneous activity and the emergence of functional responses during the prehearing period.

Qualitative review on N-methyl-D-aspartate receptor expression in rat spinal cord during the postnatal development: Implications for central sensitization and pain

The N‐methyl‐D‐aspartate receptor (NMDAR) is an important mediator of central sensitization and nociception in the rat spinal dorsal horn. The NMDAR subunits and splice variants determine the properties of the receptor. Understanding the expression of NMDAR subunits in spinal cord during the neonatal development is important as it may have consequences for the process of central sensitization and nociception in later life. In this review, a systematic literature search was conducted using three databases: Medline, Embase, and PubMed. A quality assessment was performed on predetermined entities of bias. Thirteen articles were identified to be relevant. The results show that NMDAR subunits and splice variants are dynamically expressed during postnatal development in the spinal dorsal horn. During the first 2 weeks, the expression of less excitable GluN2A subunit and more sensitive GluN2B subunit increases while the expression of high excitable GluN2C subunit decreases. During the 2nd week of postnatal development GluN1 subunits with exon 21 spliced in but exon 22 spliced out are predominantly expressed, increasing phosphorylation, and transport to the membrane. The data suggest that in rats, the nociceptive system is most susceptible to central sensitization processes during the first two postnatal weeks. This may have important consequences for nociception and pain responses in later life. From this, we conclude that targeted therapy directed toward specific NMDAR subunits is a promising candidate for mechanism‐based treatment of pain in neonates.

Visual experience is not necessary for the development of face-selectivity in the lateral fusiform gyrus

The fusiform face area responds selectively to faces and is causally involved in face perception. How does face-selectivity in the fusiform arise in development, and why does it develop so systematically in the same location across individuals? Preferential cortical responses to faces develop early in infancy, yet evidence is conflicting on the central question of whether visual experience with faces is necessary. Here, we revisit this question by scanning congenitally blind individuals with fMRI while they haptically explored 3D-printed faces and other stimuli. We found robust face-selective responses in the lateral fusiform gyrus of individual blind participants during haptic exploration of stimuli, indicating that neither visual experience with faces nor fovea-biased inputs is necessary for face-selectivity to arise in the lateral fusiform gyrus. Our results instead suggest a role for long-range connectivity in specifying the location of face-selectivity in the human brain.

Imaging the Emergence of Behavior

In this issue of Cell, Wan et al. (2019) track comprehensively the development of individual neurons, along with their activity, during zebrafish spinal cord development. They find that mostly motor neurons are the founders of initially small neuronal-activity ensembles, coalescing into larger populations establishing the first motor patterns.

A neural mechanism of nuclear receptor expression and regionalization

Spatially restricted expression of genes by global circulating inducers (hormones, secreted proteins, growth factors, neuromodulators, etc.) was a prerequisite for the evolution of animals. Far from a random occurrence, it is a systematically occurring, certain event, implying that specific information is invested for it to happen. In this minireview, we show for the first time that the expression and regionalization takes place at the level of receptors via a neural mechanism and make an attempt to reconstruct the causal chain from neural signaling to expression of nuclear receptors.

Self-Generated Whisker Movements Drive State-Dependent Sensory Input to Developing Barrel Cortex

Cortical development is an activity-dependent process [1-3]. Regarding the role of activity in the developing somatosensory cortex, one persistent debate concerns the importance of sensory feedback from self-generated movements. Specifically, recent studies claim that cortical activity is generated intrinsically, independent of movement [3, 4]. However, other studies claim that behavioral state moderates the relationship between movement and cortical activity [5-7]. Thus, perhaps inattention to behavioral state leads to failures to detect movement-driven activity [8]. Here, we resolve this issue by associating local field activity (i.e., spindle bursts) and unit activity in the barrel cortex of 5-day-old rats with whisker movements during wake and myoclonic twitches of the whiskers during active (REM) sleep. Barrel activity increased significantly within 500 ms of whisker movements, especially after twitches. Also, higher-amplitude movements were more likely to trigger barrel activity; when we controlled for movement amplitude, barrel activity was again greater after a twitch than a wake movement. We then inverted the analysis to assess the likelihood that increases in barrel activity were preceded within 500 ms by whisker movements: at least 55% of barrel activity was attributable to sensory feedback from whisker movements. Finally, when periods with and without movement were compared, 70%-75% of barrel activity was movement related. These results confirm the importance of sensory feedback from movements in driving activity in sensorimotor cortex and underscore the necessity of monitoring sleep-wake states to ensure accurate assessments of the contributions of the sensory periphery to activity in developing somatosensory cortex.

Multiple Morphological Factors Underlie Experience-Dependent Cross-Modal Plasticity in the Developing Sensory Cortices

Sensory experience regulates the structural and functional wiring of sensory cortices. In previous work, we showed that whisker deprivation (WD) from birth not only reduced excitatory synaptic transmission of layer (L) 2/3 pyramidal neurons of the correspondent barrel cortex in mice, but also cross-modally reduced synaptic transmission of L2/3 pyramidal neurons in other sensory cortices. Here, we used in utero electroporation, in combination with optical clearing, to examine the main morphological components regulating neural circuit wiring, namely presynaptic bouton density, spine density, as well as dendrite and axon arbor lengths. We found that WD from P0 to P14 reduced presynaptic bouton density in both L4 and L2/3 inputs to L2/3 pyramidal neurons, as well as spine density across the dendritic tree of L2/3 pyramidal neurons, in the barrel field of the primary somatosensory cortex. The cross-modal effects in the primary auditory cortex were manifested mostly as reduced dendrite and axon arbor size, as well as reduced bouton density of L2/3 inputs. Increasing sensory experience by rearing mice in an enriched environment rescued the effects of WD. Together, these results demonstrate that multiple morphological factors contribute to experience-dependent structural plasticity during early wiring of the sensory cortices.

An early phase of instructive plasticity before the typical onset of sensory experience

While early experience with moving stimuli is necessary for the development of direction selectivity in visual cortex of carnivores, it is unclear whether experience exerts a permissive or instructive influence. To test if the specific parameters of the experienced stimuli could instructively sculpt the emergent responses, visually naive ferrets were exposed to several hours of experience with unusual spatiotemporal patterns. In the most immature ferrets, cortical neurons developed selectivity to these patterns, indicating an instructive influence. In animals that were 1–10 days more mature, exposure to the same patterns led to a developmentally-typical increase in direction selectivity. We conclude that visual development progresses via an early phase of instructive plasticity, when the specific patterns of neural activity shape the specific parameters of the emerging response properties, followed by a late phase of permissive maturation, when sensory-driven activity merely serves to enhance the response properties already seeded in cortical circuits.

Brain circuits exhibit different amounts of plasticity over different developmental stages. Here authors show that there is a transition of the influence of spatiotemporal patterns, from instructive to permissive, around the time of the onset of visual experience.

Increased Anxiety-Related Behavior, Impaired Cognitive Function and Cellular Alterations in the Brain of Cend1-deficient Mice

Cend1 is a neuronal-lineage specific modulator involved in coordination of cell cycle exit and differentiation of neuronal precursors. We have previously shown that Cend1^-/- mice show altered cerebellar layering caused by increased proliferation of granule cell precursors, delayed radial granule cell migration and compromised Purkinje cell differentiation, leading to ataxic gait and deficits in motor coordination. To further characterize the effects of Cend1 genetic ablation we determined herein a range of behaviors, including anxiety and exploratory behavior in the elevated plus maze (EPM), associative learning in fear conditioning, and spatial learning and memory in the Morris water maze (MWM). We observed significant deficits in all tests, suggesting structural and/or functional alterations in brain regions such as the cortex, amygdala and the hippocampus. In agreement with these findings, immunohistochemistry revealed reduced numbers of γ amino butyric acid (GABA) GABAergic interneurons, but not of glutamatergic projection neurons, in the adult cerebral cortex. Reduced GABAergic interneurons were also observed in the amygdala, most notably in the basolateral nucleus. The paucity in GABAergic interneurons in adult Cend1^-/- mice correlated with increased proliferation and apoptosis as well as reduced migration of neuronal progenitors from the embryonic medial ganglionic eminence (MGE), the origin of these cells. Further we noted reduced GABAergic neurons and aberrant neurogenesis in the adult dentate gyrus (DG) of the hippocampus, which has been previously shown to confer spatial learning and memory deficits. Our data highlight the necessity of Cend1 expression in the formation of a structurally and functionally normal brain phenotype.

Emergence of mature cortical activity in wakefulness and sleep in healthy preterm and full-term infants

Study Objectives

Cortical activity patterns develop rapidly over the equivalent of the last trimester of gestation, in parallel with the establishment of sleep architecture. However, the emergence of mature cortical activity in wakefulness compared with sleep states in healthy preterm infants is poorly understood.

Methods

To investigate whether the cortical activity has a different developmental profile in each sleep-wake state, we recorded 11-channels electroencephalography (EEG), electrooculography (EOG), and respiratory movement for 1 hr from 115 infants 34 to 43 weeks-corrected age, with 0.5-17 days of postnatal age. We characterized the trajectory of δ, θ, and α-β oscillations in wakefulness, rapid eye movement (REM) sleep, and non-REM sleep by calculating the power spectrum of the EEG, averaged across artifact-free epochs.

Results

δ-Oscillations in wakefulness and REM sleep decrease with corrected age, particularly in the temporal region, but not in non-REM sleep. θ-Oscillations increase with corrected age in sleep, especially non-REM sleep, but not in wakefulness. On the other hand, α-β oscillations decrease predominantly with postnatal age, independently of sleep-wake state, particularly in the occipital region.

Conclusions

The developmental trajectory of δ and θ rhythms is state-dependent and results in changed cortical activity patterns between states with corrected age, which suggests that these frequency bands may have particular functional roles in each state. Interestingly, postnatal age is associated with a decrease in α-β oscillations overlying primary visual cortex in every sleep-wake state, suggesting that postnatal experience (including the first visual input through open eyes during periods of wakefulness) is associated with resting-state visual cortical activity changes.

Dopaminergic responses in the core part of the nucleus accumbens to subcutaneous MK801 administration are increased following postnatal transient blockade of the prefrontal cortex

Schizophrenia is a complex and devastating neuropsychiatric disease thought to result from impaired connectivity between several integrative regions, stemming from developmental failures. In particular, the left prefrontal cortex of schizophrenia patients seems to be targeted by such early developmental disturbances. Data obtained over the last three decades support the hypothesis of a dopaminergic dysfunction in schizophrenia. Striatal dopaminergic dysregulation in schizophrenia may result from a dysconnection between the prefrontal cortex and the striatum (dorsal and ventral) involving glutamatergic N-methyl-d-aspartate (NMDA) receptors. In the context of animal modeling of the pathophysiology of schizophrenia, the present study was designed to investigate the effects of MK 801 (dizocilpine) on locomotor activity and dopaminergic responses in the left core part of the nucleus accumbens (ventral striatum) in adult rats following neonatal tetrodotoxin inactivation of the left prefrontal cortex (infralimbic/prelimbic region) at postnatal day 8. Dopaminergic variations were recorded in the nucleus accumbens by means of in vivo voltammetry in freely moving adult animals. Following MK 801 administration, and in comparison to control (PBS) animals, animals microinjected with tetrodotoxin display locomotor hyperactivity and increased extracellular dopamine levels in the core part of the nucleus accumbens. These findings suggest neonatal functional inactivation of the prefrontal cortex may lead to a dysregulation of dopamine release in the core part of the nucleus accumbens involving NMDA receptors. The results obtained may provide new insight into the involvement of NMDA receptors in the pathophysiology of schizophrenia and suggest that future studies should look carefully at the core of the nucleus accumbens.

Genetic and activity-dependent mechanisms underlying interneuron diversity

The proper construction of neural circuits requires the generation of diverse cell types, their distribution to defined regions, and their specific and appropriate wiring. A major objective in neurobiology has been to understand the molecular determinants that link neural birth to terminal specification and functional connectivity, a task that is especially daunting in the case of cortical interneurons. Considerable evidence supports the idea that an interplay of intrinsic and environmental signalling is crucial to the sequential steps of interneuron specification, including migration, selection of a settling position, morphogenesis and synaptogenesis. However, when and how these influences merge to support the appropriate terminal differentiation of different classes of interneurons remains uncertain. In this Review, we discuss a wealth of recent findings that have advanced our understanding of the developmental mechanisms that contribute to the diversification of interneurons and suggest areas of particular promise for further investigation.

Propagation and synchronization of reverberatory bursts in developing cultured networks

Developing networks of neural systems can exhibit spontaneous, synchronous activities called neural bursts, which can be important in the organization of functional neural circuits. Before the network matures, the activity level of a burst can reverberate in repeated rise-and-falls in periods of hundreds of milliseconds following an initial wave-like propagation of spiking activity, while the burst itself lasts for seconds. To investigate the spatiotemporal structure of the reverberatory bursts, we culture dissociated, rat cortical neurons on a high-density multi-electrode array to record the dynamics of neural activity over the growth and maturation of the network. We find the synchrony of the spiking significantly reduced following the initial wave and the activities become broadly distributed spatially. The synchrony recovers as the system reverberates until the end of the burst. Using a propagation model we infer the spreading speed of the spiking activity, which increases as the culture ages. We perform computer simulations of the system using a physiological model of spiking networks in two spatial dimensions and find the parameters that reproduce the observed resynchronization of spiking in the bursts. An analysis of the simulated dynamics suggests that the depletion of synaptic resources causes the resynchronization. The spatial propagation dynamics of the simulations match well with observations over the course of a burst and point to an interplay of the synaptic efficacy and the noisy neural self-activation in producing the morphology of the bursts.

Oxytocin modulation of neural circuits for social behavior

Oxytocin is a hypothalamic neuropeptide that has gained attention for the effects on social behavior. Recent findings shed new light on the mechanisms of oxytocin in synaptic plasticity and adaptively modifying neural circuits for social interactions such as conspecific recognition, pair bonding, and maternal care. Here, we review several of these newer studies on oxytocin in the context of previous findings, with an emphasis on social behavior and circuit plasticity in various brain regions shown to be enriched for oxytocin receptors. We provide a framework that highlights current circuit-level mechanisms underlying the widespread action of oxytocin. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 169-189, 2017.

Spatiotemporal Features of Retinal Waves Instruct the Wiring of the Visual Circuitry

Coordinated spontaneous activity is present in different sensory systems during early stages of development. This activity is thought to play a critical role in the development of sensory representations before the maturation of sensory experience. In the visual system, the mechanisms by which spatiotemporal properties of retinal spontaneous activity, called retinal waves, drive developmental events has been well studied. Recent advancements in pharmacological, genetic, and optogenetic manipulations have provided further understanding of the contribution of specific spatiotemporal properties of retinal waves to eye-specific segregation and retinotopic refinement of retinofugal projections. Here we review some of the recent progress in understanding the role of retinal waves in the early stages of visual system development, prior to the maturation of vision.

Neonatal Prefrontal Inactivation Results in Reversed Dopaminergic Responses in the Shell Subregion of the Nucleus Accumbens to NMDA Antagonists

Striatal dopaminergic dysregulation in schizophrenia could result from a prefronto-striatal dysconnectivity, of neurodevelopmental origin, involving N-methyl-d-aspartate (NMDA) receptors. The dorsomedian shell part of the nucleus accumbens is a striatal subregion of particular interest inasmuch as it has been described as the common target region for antipsychotics. Moreover, NMDA receptors located on the dopaminergic endings have been reported in the shell. The present study examines in adult rats the effects of early functional inactivation of the left prefrontal cortex on behavioral and dopaminergic responses in the dorsomedian shell part of the nucleus accumbens following administration of two noncompetitive NMDA receptor antagonists, ketamine, and dizocilpine (MK-801). The results showed that postnatal blockade of the prefrontal cortex led to increased locomotor activity as well as increased extracellular dopamine levels in the dorsomedian shell following administration of both noncompetitive NMDA receptor antagonists, and, more markedly, after treatment with the more specific one, MK-801, whereas decreased dopaminergic levels were observed in respective controls. These data suggest a link between NMDA receptor dysfunctioning and dopamine dysregulation at the level of the dorsomedian shell part of the nucleus accumbens. They may help to understand the pathophysiology of schizophrenia in a neurodevelopmental perspective.

Experience-dependent emergence of beta and gamma band oscillations in the primary visual cortex during the critical period

Neural oscillatory activities have been shown to play important roles in neural information processing and the shaping of circuit connections during development. However, it remains unknown whether and how specific neural oscillations emerge during a postnatal critical period (CP), in which neuronal connections are most substantially modified by neural activity and experience. By recording local field potentials (LFPs) and single unit activity in developing primary visual cortex (V1) of head-fixed awake mice, we here demonstrate an emergence of characteristic oscillatory activities during the CP. From the pre-CP to CP, the peak frequency of spontaneous fast oscillatory activities shifts from the beta band (15–35 Hz) to the gamma band (40–70 Hz), accompanied by a decrease of cross-frequency coupling (CFC) and broadband spike-field coherence (SFC). Moreover, visual stimulation induced a large increase of beta-band activity but a reduction of gamma-band activity specifically from the CP onwards. Dark rearing of animals from the birth delayed this emergence of oscillatory activities during the CP, suggesting its dependence on early visual experience. These findings suggest that the characteristic neuronal oscillatory activities emerged specifically during the CP may represent as neural activity trait markers for the experience-dependent maturation of developing visual cortical circuits.

Defects During Mecp2 Null Embryonic Cortex Development Precede the Onset of Overt Neurological Symptoms

MeCP2 is associated with several neurological disorders; of which, Rett syndrome undoubtedly represents the most frequent. Its molecular roles, however, are still unclear, and data from animal models often describe adult, symptomatic stages, while MeCP2 functions during embryonic development remain elusive. We describe the pattern and timing of Mecp2 expression in the embryonic neocortex highlighting its low but consistent expression in virtually all cells and show the unexpected occurrence of transcriptional defects in the Mecp2 null samples at a stage largely preceding the onset of overt symptoms. Through the deregulated expression of ionic channels and glutamatergic receptors, the lack of Mecp2 during early neuronal maturation leads to the reduction in the neuronal responsiveness to stimuli. We suggest that such features concur to morphological alterations that begin affecting Mecp2 null neurons around the perinatal age and become evident later in adulthood. We indicate MeCP2 as a key modulator of the transcriptional mechanisms regulating cerebral cortex development. Neurological phenotypes of MECP2 patients could thus be the cumulative result of different adverse events that are already present at stages when no obvious signs of the pathology are evident and are worsened by later impairments affecting the central nervous system during maturation and maintenance of its functionality.

Spatial pattern of spontaneous retinal waves instructs retinotopic map refinement more than activity frequency

Spontaneous activity during early development is necessary for the formation of precise neural connections, but it remains uncertain whether activity plays an instructive or permissive role in brain wiring. In the visual system, retinal ganglion cell (RGC) projections to the brain form two prominent sensory maps, one reflecting eye of origin and the other retinotopic location. Recent studies provide compelling evidence supporting an instructive role for spontaneous retinal activity in the development of eye-specific projections, but evidence for a similarly instructive role in the development of retinotopy is more equivocal. Here, we report on experiments in which we knocked down the expression of β2-containing nicotinic acetylcholine receptors (β2-nAChRs) specifically in the retina through a Cre-loxP recombination strategy. Overall levels of spontaneous retinal activity in retina-specific β2-nAChR mutant mice (Rx-β2cKO), examined in vitro and in vivo, were reduced to a degree comparable to that observed in whole animal β2-nAChR mouse mutants (β2KO). However, many residual spontaneous waves in Rx-β2cKO mice displayed local propagating features with strong correlations between nearby but not distant RGCs typical of waves observed in wild-type (WT) but not β2KO mice. We further observed that eye-specific segregation was disrupted in Rx-β2cKO mice, but retinotopy was spared in a competition-dependent manner. These results suggest that propagating patterns of spontaneous retinal waves are essential for normal development of the retinotopic map, even while overall activity levels are significantly reduced, and support an instructive role for spontaneous retinal activity in both eye-specific segregation and retinotopic refinement.

Development of multisensory integration from the perspective of the individual neuron

The ability to use cues from multiple senses in concert is a fundamental aspect of brain function. It maximizes the brain’s use of the information available to it at any given moment and enhances the physiological salience of external events. Because each sense conveys a unique perspective of the external world, synthesizing information across senses affords computational benefits that cannot otherwise be achieved. Multisensory integration not only has substantial survival value but can also create unique experiences that emerge when signals from different sensory channels are bound together. However, neurons in a newborn’s brain are not capable of multisensory integration, and studies in the midbrain have shown that the development of this process is not predetermined. Rather, its emergence and maturation critically depend on cross-modal experiences that alter the underlying neural circuit in such a way that optimizes multisensory integrative capabilities for the environment in which the animal will function.

Modeling development in retinal afferents: retinotopy, segregation, and ephrinA/EphA mutants

During neural development, neurons extend axons to target areas of the brain. Through processes of growth, branching and retraction these axons establish stereotypic patterns of connectivity. In the visual system, these patterns include retinotopic organization and the segregation of individual axons onto different subsets of target neurons based on the eye of origin (ocular dominance) or receptive field type (ON or OFF). Characteristic disruptions to these patterns occur when neural activity or guidance molecule expression is perturbed. In this paper we present a model that explains how these developmental patterns might emerge as a result of the coordinated growth and retraction of individual axons and synapses responding to position-specific markers, trophic factors and spontaneous neural activity. This model derives from one presented earlier (Godfrey et al., 2009) but which is here extended to account for a wider range of phenomena than previously described. These include ocular dominance and ON-OFF segregation and the results of altered ephrinA and EphA guidance molecule expression. The model takes into account molecular guidance factors, realistic patterns of spontaneous retinal wave activity, trophic molecules, homeostatic mechanisms, axon branching and retraction rules and intra-axonal signaling mechanisms that contribute to the survival of nearby synapses on an axon. We show that, collectively, these mechanisms can account for a wider range of phenomena than previous models of retino-tectal development.

Refinement but not maintenance of visual receptive fields is independent of visual experience

Visual deprivation is reported to prevent or delay the development of mature receptive field (RF) properties in primary visual cortex (V1) in several species. In contrast, visual deprivation neither prevents nor delays refinement of RF size in the superior colliculus (SC) of Syrian hamsters, although vision is required for RF maintenance in the SC. Here, we report that, contrary to expectation, visual cortical RF refinement occurs normally in dark-reared animals. As in the SC, a brief period of visual experience is required to maintain V1 RF refinement in adulthood. Whereas in the SC, 3 days of visual experience within a sensitive period (P37-40) was sufficient to protect RFs from deprivation-induced enlargement in adulthood, 7 days (P33-40) were required for RF size maintenance in V1. Thus, spontaneous activity is sufficient for RF refinement at these 2 levels of the visual pathway, and visual input is necessary only to prevent deprivation-induced RF enlargement in adulthood. These studies show that sensory experience during a late juvenile sensitive period protects the visual pathway against sensory deprivation in adulthood, and suggest that more importance may have been placed on the role of early visual experience in visual RF development than is warranted.

Role of adenylate cyclase 1 in retinofugal map development

The development of topographic maps of the sensory periphery is sensitive to the disruption of adenylate cyclase 1 (AC1) signaling. AC1 catalyzes the production of cAMP in a Ca2+/calmodulin-dependent manner, and AC1 mutant mice (AC1−/−) have disordered visual and somatotopic maps. However, the broad expression of AC1 in the brain and the promiscuous nature of cAMP signaling have frustrated attempts to determine the underlying mechanism of AC1-dependent map development. In the mammalian visual system, the initial coarse targeting of retinal ganglion cell (RGC) projections to the superior colliculus (SC) and lateral geniculate nucleus (LGN) is guided by molecular cues, and the subsequent refinement of these crude projections occurs via an activity-dependent process that depends on spontaneous retinal waves. Here, we show that AC1−/− mice have normal retinal waves but disrupted map refinement. We demonstrate that AC1 is required for the emergence of dense and focused termination zones and elimination of inaccurately targeted collaterals at the level of individual retinofugal arbors. Conditional deletion of AC1 in the retina recapitulates map defects, indicating that the locus of map disruptions in the SC and dorsal LGN of AC1−/− mice is presynaptic. Finally, map defects in mice without AC1 and disrupted retinal waves (AC1−/−;β2−/− double KO mice) are no worse than those in mice lacking only β2−/−, but loss of AC1 occludes map recovery in β2−/− mice during the second postnatal week. These results suggest that AC1 in RGC axons mediates the development of retinotopy and eye-specific segregation in the SC and dorsal LGN.

What can we get from 'barrels': the rodent barrel cortex as a model for studying the establishment of neural circuits

Sensory inputs triggered by external stimuli are projected into discrete arrays of neuronal modules in the primary sensory cortex. This whisker-to-barrel pathway has gained in popularity as a model system for studying the development of cortical circuits and sensory processing because its clear patterns facilitate the identification of genetically modified mice with whisker map deficits and make possible coordinated in vitro and in vivo electrophysiological studies. Numerous whisker map determinants have been identified in the past two decades. In this review, we summarize what have we learned from the detailed studies conducted in various mutant mice with cortical whisker map deficits. We will specifically focus on the anatomical and functional establishment of the somatosensory thalamocortical circuits.

The emergence of somatotopic maps of the body in S1 in rats: the correspondence between functional and anatomical organization

Most of what we know about cortical map development and plasticity comes from studies in mice and rats, and for the somatosensory cortex, almost exclusively from the whisker-dominated posteromedial barrel fields. Whiskers are the main effector organs of mice and rats, and their representation in cortex and subcortical pathways is a highly derived feature of murine rodents. This specialized anatomical organization may therefore not be representative of somatosensory cortex in general, especially for species that utilize other body parts as their main effector organs, like the hands of primates. For these reasons, we examined the emergence of whole body maps in developing rats using electrophysiological recording techniques. In P5, P10, P15, P20 and adult rats, multiple recordings were made in the medial portion of S1 in each animal. Subsequently, these functional maps were related to anatomical parcellations of S1 based on a variety of histological stains. We found that at early postnatal ages (P5) medial S1 was composed almost exclusively of the representation of the vibrissae. At P10, other body part representations including the hindlimb and forelimb were present, although these were not topographically organized. By P15, a clear topographic organization began to emerge coincident with a reduction in receptive field size. By P20, body maps were adult-like. This study is the first to describe how topography of the body develops in S1 in any mammal. It indicates that anatomical parcellations and functional maps are initially incongruent but become tightly coupled by P15. Finally, because anatomical and functional specificity of developing barrel cortex appears much earlier in postnatal life than the rest of the body, the entire primary somatosensory cortex should be considered when studying general topographic map formation in development.

GABA expression and regulation by sensory experience in the developing visual system

The developing retinotectal system of the Xenopus laevis tadpole is a model of choice for studying visual experience-dependent circuit maturation in the intact animal. The neurotransmitter gamma-aminobutyric acid (GABA) has been shown to play a critical role in the formation of sensory circuits in this preparation, however a comprehensive neuroanatomical study of GABAergic cell distribution in the developing tadpole has not been conducted. We report a detailed description of the spatial expression of GABA immunoreactivity in the Xenopus laevis tadpole brain at two key developmental stages: stage 40/42 around the onset of retinotectal innervation and stage 47 when the retinotectal circuit supports visually-guided behavior. During this period, GABAergic neurons within specific brain structures appeared to redistribute from clusters of neuronal somata to a sparser, more uniform distribution. Furthermore, we found that GABA levels were regulated by recent sensory experience. Both ELISA measurements of GABA concentration and quantitative analysis of GABA immunoreactivity in tissue sections from the optic tectum show that GABA increased in response to a 4 hr period of enhanced visual stimulation in stage 47 tadpoles. These observations reveal a remarkable degree of adaptability of GABAergic neurons in the developing brain, consistent with their key contributions to circuit development and function.

An instructive role for patterned spontaneous retinal activity in mouse visual map development

Complex neural circuits in the mammalian brain develop through a combination of genetic instruction and activity-dependent refinement. The relative role of these factors and the form of neuronal activity responsible for circuit development is a matter of significant debate. In the mammalian visual system, retinal ganglion cell projections to the brain are mapped with respect to retinotopic location and eye of origin. We manipulated the pattern of spontaneous retinal waves present during development without changing overall activity levels through the transgenic expression of β2-nicotinic acetylcholine receptors in retinal ganglion cells of mice. We used this manipulation to demonstrate that spontaneous retinal activity is not just permissive, but instructive in the emergence of eye-specific segregation and retinotopic refinement in the mouse visual system. This suggests that specific patterns of spontaneous activity throughout the developing brain are essential in the emergence of specific and distinct patterns of neuronal connectivity.

Visual experience-dependent maturation of correlated neuronal activity patterns in a developing visual system

The functional properties of neural circuits become increasingly robust over development. This allows circuits to optimize their output in response to a variety of input. However, it is not clear whether this optimization is a function of hardwired circuit elements, or whether it requires neural experience to develop. We performed rapid in vivo imaging of calcium signals from bulk-labeled neurons in the Xenopus laevis optic tectum to resolve the rapid spatiotemporal response properties of populations of developing tectal neurons in response to visual stimuli. We found that during a critical time in tectal development, network activity becomes increasingly robust, more correlated, and more synchronous. These developmental changes require normal visual input during development and are disrupted by NMDAR blockade. Our data show that visual activity and NMDAR activation are critical for the maturation of tectal network dynamics during visual system development.

Beyond dreams: do sleep-related movements contribute to brain development?

Conventional wisdom has long held that the twitches of sleeping infants and adults are by-products of a dreaming brain. With the discovery of active (or REM) sleep in the 1950s and the recognition soon thereafter that active sleep is characterized by inhibition of motor outflow, researchers elaborated on conventional wisdom and concluded that sleep-related twitches are epiphenomena that result from incomplete blockade of dream-related cortical activity. This view persists despite the fact that twitching is unaffected in infants and adults when the cortex is disconnected from the brainstem. In 1966, Roffwarg and colleagues introduced the ontogenetic hypothesis, which addressed the preponderance of active sleep in early infancy. This hypothesis posited that the brainstem mechanisms that produce active sleep provide direct ascending stimulation to the forebrain and descending stimulation to the musculature, thereby promoting brain and neuromuscular development. However, this hypothesis and the subsequent work that tested it did not directly address the developmental significance of twitching or sensory feedback as a contributor to activity-dependent development. Here I review recent findings that have inspired an elaboration of the ontogenetic hypothesis. Specifically, in addition to direct brainstem activation of cortex during active sleep, sensory feedback arising from limb twitches produces discrete and substantial activation of somatosensory cortex and, beyond that, of hippocampus. Delineating how twitching during active sleep contributes to the establishment, refinement, and maintenance of neural circuits may aid our understanding of the early developmental events that make sensorimotor integration possible. In addition, twitches may prove to be sensitive and powerful tools for assessing somatosensory function in humans across the lifespan as well as functional recovery in individuals with injuries or conditions that affect sensorimotor function.

An abrupt developmental shift in callosal modulation of sleep-related spindle bursts coincides with the emergence of excitatory-inhibitory balance and a reduction of somatosensory cortical plasticity

Transecting the corpus callosum of postnatal day (P)1-6 rats disinhibits the production of spindle bursts (SBs) within primary somatosensory cortex (S1), most notably during periods of sleep-related myoclonic twitching. Here we investigated developmental changes in this callosally mediated disinhibition and its association with cortical plasticity. Recordings in P2-15 subjects revealed that callosotomy-induced disinhibition is a transient feature of early development that disappears abruptly after P6. This abrupt switch was accompanied by sharp decreases in myoclonic twitching and equally sharp increases in spontaneous SBs and in the number of GABAergic and glutamatergic presynaptic terminals in S1. Expression of the K+Cl- cotransporter 2 (KCC2) also increased across these ages. To determine whether these developmental changes are associated with alterations in cortical plasticity, pups were callosotomized at P1, P6, or P8, and tested over the subsequent week. Regardless of age, callosotomy immediately disrupted SBs evoked by forepaw stimulation. Over the next week, the P1 and P6 callosotomy groups exhibited full recovery of function; in contrast, the P8 group did not exhibit recovery of function, thus indicating an abrupt decrease in cortical plasticity between P6 and P8. Together, our data demonstrate that callosotomy-induced disinhibition is a transient phenomenon whose disappearance coincides with the onset of increased intrinsic connectivity, establishment of excitatory-inhibitory balance, and diminished plasticity in S1. Accordingly, our findings indicate that callosotomy-induced disinhibition of twitch-related SBs is a bioassay of somatosensory cortical plasticity and, in addition, support the hypothesis that myoclonic twitches, like retinal waves, actively contribute to cortical development and plasticity.

The applicability of spike time dependent plasticity to development

Spike time dependent plasticity (STDP) has been observed in both developing and adult animals. Theoretical studies suggest that it implicitly leads to both competition and homeostasis in addition to correlation-based plasticity, making it a good candidate to explain developmental refinement and plasticity in a number of systems. However, it has only been observed to play a clear role in development in a small number of cases. Because the fast time scales necessary to elicit STDP, it would likely be inefficient in governing synaptic modifications in the absence of fast correlations in neural activity. In contrast, later stages of development often depend on sensory inputs that can drive activity on much faster time scales, suggesting a role in STDP in many sensory systems after opening of the eyes and ear canals. Correlations on fast time scales can be also be present earlier in developing microcircuits, such as those produced by specific transient "teacher" circuits in the cerebral cortex. By reviewing examples of each case, we suggest that STDP is not a universal rule, but rather might be masked or phased in, depending on the information available to instruct refinement in different developing circuits. Thus, this review describes selected cases where STDP has been studied in developmental contexts, and uses these examples to suggest a more general framework for understanding where it could be playing a role in development.

Chromatic and luminance contrast sensitivity in fullterm and preterm infants

In order to investigate the contributions of visual experience vs. preprogrammed mechanisms on visual development, the current study compared contrast sensitivity in preterm vs. fullterm infants. If development is tied to time since conception, preterm infants should match the developmental trajectories of fullterm infants when plotted in postterm age. By contrast, if development is influenced by visual experience, preterm and fullterm infants should match when plotted in postnatal age. Luminance (light/dark) and chromatic (red/green) contrast sensitivities (CS) were measured in 25 preterm (born, on average, 6.6 weeks early) and 77 fullterm infants, between 1 and 6 months postterm. In the first few months, luminance CS was found to be predicted by postterm age, suggesting that preprogrammed development is sufficient to account for luminance CS. By contrast, chromatic CS exceeded that predicted by postterm age, which suggests that time since birth confers a benefit on chromatic CS. The preterms' 6.6 weeks of additional time since birth is roughly equivalent to 3.7 weeks of development in chromatic CS. In sum, these results suggest that chromatic CS is more influenced by early postnatal visual experience than luminance CS, which may have implications for development of parvocellular and magnocellular pathways.

Effects of gestational length, gender, postnatal age, and birth order on visual contrast sensitivity in infants

To investigate effects of visual experience versus preprogrammed mechanisms on visual development, we used multiple regression analysis to determine the extent to which a variety of variables (that differ in the extent to which they are tied to visual experience) predict luminance and chromatic (red/green) contrast sensitivity (CS), which are mediated by the magnocellular (M) and parvocellular (P) subcortical pathways, respectively. Our variables included gestational length (GL), birth weight (BW), gender, postnatal age (PNA), and birth order (BO). Two-month-olds (n = 60) and 6-month-olds (n = 122) were tested. Results revealed that (1) at 2 months, infants with longer GL have higher luminance CS; (2) at both ages, CS significantly increases over a approximately 21-day range of PNA, but this effect is stronger in 2- than 6-month-olds and stronger for chromatic than luminance CS; (3) at 2 months, boys have higher luminance CS than girls; and (4) at 2 months, firstborn infants have higher CS, while at 6 months, non-firstborn infants have higher CS. The results for PNA/GL are consistent with the possibility that P pathway development is more influenced by variables tied to visual experience (PNA), while M pathway development is more influenced by variables unrelated to visual experience (GL). Other variables, including prenatal environment, are also discussed.

Interareal coordination of columnar architectures during visual cortical development

The formation of cortical columns is often conceptualized as a local process in which synaptic microcircuits confined to the volume of the emerging column are established and selectively refined. Many neurons, however, while wiring up locally are simultaneously building macroscopic circuits spanning widely distributed brain regions, such as different cortical areas or the two brain hemispheres. Thus, it is conceivable that interareal interactions shape the local column layout. Here we show that the columnar architectures of different areas of the cat visual cortex in fact develop in a coordinated manner, not adequately described as a local process. This is revealed by comparing the layouts of orientation columns (i) in left/right pairs of brain hemispheres and (ii) in areas V1 and V2 of individual brain hemispheres. Whereas the size of columns varied strongly within all areas considered, columns in different areas were typically closely matched in size if they were mutually connected. During development, we find that such mutually connected columns progressively become better matched in size as the late phase of the critical period unfolds. Our results suggest that one function of critical-period plasticity is to progressively coordinate the functional architectures of different cortical areas--even across hemispheres.