bioRxiv Subject Collection: Neuroscience's Journal

Developmental axon diameter growth of central nervous system axons does not depend on ensheathment or myelination by oligodendrocytes
Myelination facilitates the rapid conduction of action potentials along axons. In the central nervous system (CNS), myelinated axons vary over 100-fold in diameter, with conduction speed scaling linearly with increasing diameter. Axon diameter and myelination are closely interlinked, with axon diameter exerting a strong influence on myelination. Conversely, myelinating Schwann cells in the peripheral nervous system can both positively and negatively affect axon diameter. However, whether axon diameter is regulated by CNS oligodendrocytes is less clear. Here, we investigated CNS axon diameter growth in the absence of myelin using mouse (Mbpshi/shi and Myrf conditional knockout) and zebrafish (olig2 morpholino) models. We find that neither the ensheathment of axons, nor the formation of compact myelin are required for CNS axons to achieve appropriate and diverse diameters. This indicates that developmental CNS axon diameter growth is independent of myelination, and shows that myelinating cells of CNS and PNS differentially influence axonal morphology.

A critical initialization for biological neural networks
Artificial neural networks learn faster if they are initialized well. Good initializations can generate high-dimensional macroscopic dynamics with long timescales. It is not known if biological neural networks have similar properties. Here we show that the eigenvalue spectrum and dynamical properties of large-scale neural recordings in mice (two-photon and electrophysiology) are similar to those produced by linear dynamics governed by a random symmetric matrix that is critically normalized. An exception was hippocampal area CA1: population activity in this area resembled an efficient, uncorrelated neural code, which may be optimized for information storage capacity. Global emergent activity modes persisted in simulations with sparse, clustered or spatial connectivity. We hypothesize that the spontaneous neural activity reflects a critical initialization of whole-brain neural circuits that is optimized for learning time-dependent tasks.

Brain dynamics during architectural experience: prefrontal and hippocampal regions track aesthetics and spatial complexity
Architectural experience involves processing the spatial layout of an environment and our emotional reaction to it. However, these two processes are largely studied separately. Here we used functional magnetic resonance imaging (fMRI) and first-person movies of journeys through buildings and cities to determine the contribution of different brain regions to spatial and aesthetic aspects of the built environment. During scanning, participants watched 48 movies that show first-person-view travel through different spaces; immediately after each video, they either judged the spatial layout complexity or valence of the environment. After scanning, participants also reported the memorability of the spaces encountered. Activity in brain regions previously linked to valence processing (e.g. ventromedial prefrontal cortex) were modulated by aesthetic qualities of the stimuli (i.e. increased for pleasant spaces compared to unpleasant spaces) and the task (more active when judging valence), whereas activity in brain regions linked with spatial processing (e.g. parahippocampal regions) increased in complex layouts compared to simple layouts. The hippocampus and parahippocampal cortex were associated with the memorability of spaces and were modulated by both aesthetic and spatial qualities. We also tested for curvature, fascination, coherence and hominess - qualities linked to aesthetic judgement in architecture. We replicated findings activating right lingual gyrus for fascination, left inferior occipital gyrus for coherence, and left cuneus for hominess, and found inverse curvature (increasing rectilinearity) activated spatial, valence and visual processing regions. Overall, these findings provide important insights into how different brain regions respond whilst experiencing new buildings and city spaces, which is needed to advance the field of neuroarchitecture.

Large extracellular vesicles containing mitochondria (EVMs) derived from Alzheimer disease cells harbor pathologic functional and molecular profiles and spread mitochondrial dysfunctions
In addition to small extracellular vesicles known as exosomes, cells release large extracellular vesicles containing mitochondria (EVMs). However, the molecular and functional characteristics of EVMs, as well as the impact of EVM secretion on the spreading of mitochondrial dysfunction between cells, remain unknown in the context of Alzheimer Disease (AD). Here, we provide an ultrastructural, biochemical, and functional characterization of EVMs isolated from cells expressing the amyloid precursor protein (APP) with the familial Swedish mutation (APPswe). We identified differential proteomic and lipidomic signatures in APPswe-derived EVMs compared to control EVMs and revealed a specific proteomic profile in EVMs isolated from conditioned media of fibroblasts from AD patients at the prodromal stage of the disease. Our findings show that APP-C terminal fragments (APP-CTFs) pathogenic accumulation in cells potentiates EVM secretion through the budding of the plasma membrane. We lastly demonstrated that APP-CTFs loaded EVMs are active carriers of dysfunctional mitochondria that transfer mitochondrial pathology to naive control recipient cells.

Natural body-selective motion processing in humans: Evidence from event-related potentials
As social species, perception of body motion is crucial to daily interactions, yet the underlying neural processes and their dynamics are not fully understood. While well-established EEG evidence has identified temporal markers of biological body motion, and recent fMRI research points to naturalistic body networks, the temporal markers of higher-level, naturalistic body motion processes remain unknown. The present study bridges this gap, showing that at the stage of P2, the cortex may already distinguish between the global motion of naturalistic bodies compared to objects. Exploratory analyses point to a similar body-specific modulation across stimulus species, namely human and monkey. The findings of the present study expand our understanding of body motion processes, in line with emerging, integrative models of multi-faceted body processes.

High-field fMRI at 7 Tesla reveals topographic responses tuned to number in the developing human brain
In the adult brain, hemodynamic responses to visually presented numerosities reveal receptive field-like tuning curves in topographically organized maps across association cortices. It is currently unknown whether such tuned topographic responses to numerosities can also be detected in the developing brain. Here we conducted a 7 Tesla fMRI experiment in which we presented a large set of visual dot displays to 11-12-year-old children and adults. We found that developing hemodynamic responses indeed revealed logarithmic Gaussian tuning to quantitative information. Tuning models explained comparable amounts of variance in children and adults. In most subjects, six bilateral cortical maps consistently exhibited topographic responses to numerosities. The present study goes beyond previous work by uncovering a population code for quantity detection in individual developing human brains. Our work lays a foundation for a model-based neuroimaging approach to individual cognitive differences in the context of developmental dyscalculia and mathematical giftedness.

Association between theta-band resting-state functional connectivity and declarative memory abilities in children
Declarative memory formation critically relies on the synchronization of brain oscillations in the theta (4-8 Hz) frequency band within specific brain networks. The development of this capacity is closely linked to the functional organization of these networks already at rest. However, the relationship between theta-band resting-state functional connectivity and declarative memory abilities remains unexplored in children. Here, using magnetoencephalography, we examined the association between declarative memory performance and pre-learning resting-state functional connectivity across frequency bands in 32 school-aged children. Declarative memory was assessed as the percentage of correct retrieval of 50 new associations between non-objects and magical functions, while resting-state functional connectivity was measured through power envelope correlation of the theta, alpha, low and high beta frequency bands. We found that stronger theta-band resting-state functional connectivity within occipito-temporo-frontal networks correlated with better declarative memory retrieval, while no correlation was observed in the alpha and beta frequency bands. These findings suggest that the functional brain architecture at rest, specifically involving theta-band oscillations, supports declarative memory in children. This mechanism may facilitate the subsequent rapid transformation of sensory input into visuo-semantic representations, highlighting the critical role of theta-band connectivity in early cognitive development.

Monocarboxylate transporter 2 is required for the maintenance of myelin and axonal integrity by oligodendrocytes
Neurodegenerative pathologies including multiple sclerosis (MS) are consistently associated with energy deficit in the central nervous system (CNS). This might directly impact myelinating oligodendrocytes as these are particularly vulnerable to metabolic insults. Importantly, oligodendroglial dysfunction and myelin alterations occur in most, if not all neurodegenerative diseases, and are associated with axonal pathology/loss. Thus, elucidating metabolic mechanisms required for oligodendroglial myelin maintenance and axonal support might be crucial to identify therapeutic targets to achieve neuroprotection. While monocarboxylates are important energy fuels for the CNS, their role in myelinating oligodendrocyte function remains unclear. Here we show that, just like neurons, myelinating oligodendrocytes express high affinity monocarboxylate transporter 2 (MCT2) both in mice and humans, which is downregulated in progressive MS. While deletion of MCT2 in mouse oligodendrocytes did not affect the survival of these cells, it resulted in downregulation of lipid synthesis-associated enzymes and failure of myelin maintenance. Moreover, axonal upregulation of lactate dehydrogenase A concomitant with axonal damage was observed but could be alleviated by ketogenic diet. We conclude that oligodendroglial MCT2 is required for myelin maintenance and axonal support, which becomes altered in progressive MS, but may be compensated for by specific metabolic therapies.

Epigenome-wide analysis reveals novel DNA methylation signatures significantly associated with the infant pupillary light reflex, a candidate intermediate phenotype for autism.
Autism is a highly heterogeneous neurodevelopmental condition, currently diagnosed based on behavioural characteristics. Candidate early intermediate phenotypes, such as the Pupillary Light Reflex (PLR), a reflexive constriction of the pupil in response to increased optical luminance, may provide insights into etiological factors and potential biomarkers, such as DNA methylation (DNAm), involved in the emergence of autism. We conducted epigenome-wide DNAm association analyses of 9-, 14-, 24-month PLR onset latency and constriction amplitude in a sample of 51 infants enriched for autism family history, using buccal DNA collected at 9-months. Our epigenome-wide analysis (EWAS) identified four stringently significant differentially methylated probes (p < 2.4 x 10-7) associated with cross-section PLR latency measurements at 14- and 24-months, and with 14- to 24-month PLR latency developmental change. Differentially methylated probes associated with PLR amplitude were identified, but at a less stringent discovery threshold (p < 5 x 10-5). Our region analyses identified several significant differentially methylated regions associated with both PLR latency and amplitude Downstream exploratory pathway analysis identified enrichment for multiple developmental biological processes, as well as several susceptibility genes to autism and related neurodevelopmental conditions including NR4A2, HNRNPU and NAV2. Our findings provide novel insight into the role of DNAm in PLR development and illuminate biological mechanisms underpinning altered PLR in infancy in emerging autism.

Neural mechanisms of object prioritization in vision
Selective attention is widely thought to be sensitive to visual objects. This is commonly demonstrated in cueing studies, which show that when attention is deployed to a known target location that happens to fall on a visual object, responses to targets that unexpectedly appear at other locations on that object are faster and more accurate, as if the object in its entirety has been visually prioritized. However, this notion has recently been challenged by results suggesting that putative object-based effects may reflect the influence of hemifield anisotropies in attentional deployment, or of unacknowledged influences of perceptual complexity and visual clutter. Studies employing measures of behaviour provide limited opportunity to address these challenges. Here, we used EEG to directly measure the influence of task-irrelevant objects on the deployment of visual attention. We had participants complete a simple visual cueing task involving identification of a target that appeared at either a cued location or elsewhere. Throughout each experimental trial, displays contained task-irrelevant rectangle stimuli that could be oriented horizontally or vertically. We derived two cue-elicited indices of attentional deployment (lateralized alpha oscillations and the ADAN component of the event-related potential) and found that these were sensitive to the otherwise irrelevant orientation of the rectangles. Our results demonstrate that the allocation of visual attention is influenced by objects boundaries, supporting models of object-based attentional prioritization.

Social dysfunction relates to altered default mode network functional integrity across neuropsychiatric disorders: A replication and generalization study
Background: Social dysfunction is an early manifestation of neuropsychiatric disorders that may relate to altered Default Mode Network (DMN) integrity. This study aimed to replicate previous findings linking social dysfunction with diminished resting-state DMN functional connectivity and altered task-based DMN functional activation in response to emotional faces across schizophrenia (SZ), Alzheimer's disease (AD), and healthy controls (HC), and to extend these findings to major depressive disorder (MDD). Methods: Resting-state fMRI and task-based fMRI data on implicit facial emotional processing were acquired in an overlapping cohort (resting-state fMRI: N=167; SZ=32, MDD=44, AD=29, HC=62. Task-based fMRI: N=152; SZ=30, MDD=42, AD=26, HC=54). Additionally, mega-analyses (N=317 for resting-state fMRI; N=291 for task-based fMRI) of the current and a prior independent sample were conducted. Social dysfunction was indexed with the Social Functioning Scale (SFS) and the De Jong-Gierveld Loneliness (LON) scale. Results: The association between higher mean SFS+LON social dysfunction scores and diminished DMN connectivity within the dorsomedial prefrontal cortex across SZ/AD/HC participants was replicated, and extended to MDD patients. Similar observations within the dorsomedial and rostromedial prefrontal cortex were found in the mega-analysis. Associations between social dysfunction and DMN activation in response to sad and happy faces were not replicated or found in the mega-analysis. Conclusions: Diminished dorsomedial prefrontal cortex DMN connectivity emerged as a transdiagnostic neurobiological marker for social dysfunction, suggesting a potential treatment target for precision medicine approaches. DMN functional responses to emotional faces may not be a sensitive biomarker for social dysfunction.

Collateral connectomes of Esr1-positive hypothalamic neurons modulate defensive behavior plasticity
The ventromedial hypothalamus (VMH) projects to the periaqueductal gray (PAG) and anterior hypothalamic nucleus (AHN), mediating freezing and escape behaviors, respectively. We investigated VMH collateral (VMH-coll) neurons, which innervate both PAG and AHN, to elucidate their role in postsynaptic processing and defensive behavior plasticity. Using all-optical voltage imaging of 22,151 postsynaptic neurons ex vivo, we found that VMH-coll neurons engage inhibitory mechanisms at both synaptic ends and can induce synaptic circuit plasticity. In vivo optogenetic activation of the VMH-coll somas induced escape behaviors. We identified an Esr1-expressing VMH-coll subpopulation with postsynaptic connectome resembling that of wild-type collaterals on the PAG side. Activation of Esr1+VMH-coll neurons evoked freezing and unexpected flattening behavior, previously not linked to the VMH. Neuropeptides such as PACAP and dynorphin modulated both Esr1+VMH-coll connectomes. In vivo kappa-opioid receptor antagonism impaired Esr1+VMH-coll-mediated defensive behaviors. These findings unveiled the central role of VMH-coll pathways in innate defensive behavior plasticity.

Astroglial deficiency for oligophrenin-1 contributes to intellectual disability
Neurodevelopmental disorders, including X-linked intellectual disability, autism spectrum disorder or schizophrenia, can result from the mutation of oligophrenin-1 (Ophn1), encoding a Rho-GTPase-activating protein. Ophn1 regulates synaptic development and function, in part via cytoskeleton reorganization, and is expressed in both neurons and astrocytes. Despite the crucial role of astrocytes in synapse function, altered in neurodevelopmental disorders, and their Ophn1 expression, the specific impact of astroglial Ophn1 deficiency on synaptic transmission and behavior remains unknown. Here, we show that Ophn1 deficiency postnatally in hippocampal astrocytes impairs synaptic transmission, short-term plasticity and spatial working memory in adults. This involves an adenosine A1 receptor-dependent presynaptic mechanism associated with astroglial morphological rearrangements resulting in increased astroglial synapse coverage. The structural, functional and behavioral alterations induced by astroglial Ophn1 deficiency are rescued in adults by pharmacological inhibition of the RhoA/ROCK pathway. Our findings uncover an important role for astroglial Ophn1 deficiency in synaptic and behavioral dysfunctions, pointing to a novel cellular therapeutic target for neurodevelopmental disorders.

Decoding Gene Networks Controlling Hypothalamic and Prethalamic Neuron Development.
Neuronal subtypes derived from the embryonic hypothalamus and prethalamus regulate many essential physiological processes, yet the gene regulatory networks controlling their development remain poorly understood. Using single-cell RNA- and ATAC-sequencing, we analyzed mouse hypothalamic and prethalamic development from embryonic day 11 to postnatal day 8, profiling 660,000 cells in total. This identified key transcriptional and chromatin dynamics driving regionalization, neurogenesis, and differentiation. This identified multiple distinct neural progenitor populations, as well as gene regulatory networks that control their spatial and temporal identities, and their terminal differentiation into major neuronal subtypes. Integrating these results with large-scale genome-wide association study data, we identified a central role for transcription factors controlling supramammillary hypothalamic development in a broad range of metabolic and cognitive traits. Recurring cross-repressive regulatory relationships were observed between transcription factors that induced prethalamic and tuberal hypothalamic identity on the one hand and mammillary and supramammillary hypothalamic identity on the other. In postnatal animals, Dlx1/2 was found to severely disrupt GABAergic neuron specification in both the hypothalamus and prethalamus, resulting in a loss of inhibition of thalamic neurons, hypersensitivity to cold, and behavioral hyperactivity. By identifying core gene regulatory networks controlling the specification and differentiation of major hypothalamic and prethalamic neuronal cell types, this study provides a roadmap for future efforts aimed at preventing and treating a broad range of homeostatic and cognitive disorders.

Attentional enhancement and suppression of stimulus-synchronized BOLD oscillations
Visual cortical neurons synchronize their firing rates to periodic visual stimuli. EEG is commonly used to study directed attention by frequency-tagging brain responses to multiple stimuli oscillating at different frequencies, but is limited by its coarse spatial resolution. Here we leverage frequency-tagging fMRI (ft-fMRI) to study the influence of directed attention on the fine-grained spatiotemporal dynamics of competing stimulus-driven visual cortical oscillations. Our analysis reveals that distinct populations of visual cortical neurons exhibit in-phase (enhancing) or antiphase (suppressive) synchronization with the oscillating stimuli. Directed attention homogeneously increases the amplitude of anti-phase BOLD oscillations across the visual hierarchy, consistent with a distributed suppressive field. In contrast, attentional modulation of in phase BOLD oscillations increases hierarchically from V1 to hV4. The strength of anti-phase, but not in-phase, modulation predicted psychophysical correlates of attentional performance. Our results strongly corroborate the biased competition model of attention and unveil a novel BOLD correlate of attentional suppression.

Humans can walk economically on uneven terrain without deliberative optimization with a Simple Feedback Control Policy
Humans proactively anticipate and adjust for uneven terrain as they traverse it. They modulate their forward speed and momentum ahead of upcoming steps, perhaps to optimize energy expenditure as they prefer to do for level ground. But actual optimization is a complex, deliberative process and seems not to describe how human act in real time during continuous walking. Here we show that human uneven walking strategies are predicted well by an control scheme that requires no deliberation yet is nearly optimal. The experimentally measured speed trajectory for a single upward step (a kernel) predicts the strategy for a complex terrain sequence, by repeatedly delaying and weighting (convolving) it for each step height. Conversely, the same strategy can be deconvolved to predict for six other terrains about as well as deliberative energy minimization Composability is predicted by a simple model of walking, which requires work to redirect the body center of mass velocity between pendulum-like steps. These dynamics obey linear superposition, so that a simple kernel, learnable through experience, is sufficient to compose other strategies with lookahead information about upcoming step heights. Walking dynamics may allow humans to act quickly and nearly optimally without thinking slowly.

Homeostatic regulation of a motor circuit through temperature sensing rather than activity sensing
Homeostasis is a driving principle in physiology. To achieve homeostatic control of neural activity, neurons monitor their activity levels and then initiate corrective adjustments in excitability when activity strays from a set point. However, fluctuations in the brain microenvironment, such as temperature, pH, and other ions represent some of the most common perturbations to neural function in animals. Therefore, it is unclear if activity sensing is a universal strategy for different types of perturbations or if stability may arise by sensing specific environmental cues. Here we show the respiratory network of amphibians mounts a fast homeostatic response to restore motor function following inactivity caused by cooling over the physiological range. This response was not initiated by inactivity, but rather, by temperature. Compensation involved cold-activation of the noradrenergic system via mechanisms that involve inhibition of the Na+/K+ ATPase, causing {beta}-adrenoceptor signaling that enhanced network excitability. Thus, acute cooling initiates a modulatory response that opposes inactivity and enhances network excitability. As the nervous system of all animals is subjected to changes in the microenvironment, some circuits may have selected regulatory systems tuned to environmental variables in place of, or in addition to, activity-dependent control mechanisms.

Whole-body coarticulation reflects expertise in sport climbing
Taking sport climbing as a testbed, we explored coarticulation in naturalistic motor-behavior at the level of whole-body kinematics. Participants were instructed to execute a series of climbing routes, each composed of two initial foot-moves equal in all routes, and two subsequent hand-moves differing across routes in a set of eight possible configurations. The goal was assessing whether climbers modulate the execution of a given move depending on which moves come next in the plan. Coarticulation was assessed by training a set of classifiers and estimating how well the whole-body (or single-joint) kinematics during a given stage of the climbing execution could predict its future unfolding. Results showed that most participants engage in coarticulation, with temporal and bodily patterns that depend on expertise. Non-climbers tend to prepare the next-to-come move right before its onset and only after the end of the previous move. Rather, expert-climbers (and to a smaller extent, beginner-climbers) show early coarticulation during the execution of the previous move and engage in adjustments that involve the coordination of a larger number of joints across the body. These results demonstrate coarticulation effects in whole-body naturalistic motor behavior and as a function of expertise. Furthermore, the enhanced coarticulation found in expert-climbers provides hints for experts engaging in more refined mental processes converting abstract instructions (e.g., move the right hand to a given location) into motor simulations involving whole-body coordination. Overall, these results contribute to advancing our current knowledge of the rich interplay between cognition and motor control.

Perceived risk of forward versus backward balance disturbances while walking in young and older adults
Falls, which often result from trips or slips, pose a major health concern, particularly among older adults. Experiencing falls or near-falls from balance disturbances can shape subsequent gait-related decisions, as individuals may avoid situations they perceive as risky. Here, we explore whether perceptions of risk are sensitive to the direction of previously experienced balance disturbances - forward or backward - and whether these perceptions change with age. Twenty young and twenty older adults walked on a split-belt treadmill while performing a two-alternative forced-choice task, where they indicated their preference between a forward-falling trip and a backward-falling slip. We varied the perturbation magnitudes using an adaptive staircase algorithm to obtain multiple trip-slip equivalence points. Using a mixed-effects linear model, we estimated the slope of the trip-slip relationship, which quantified the direction and strength of the sensitivity of perceived risk to perturbation type. To assess reliability, we repeated the procedure on a second day. Additionally, we investigated two potential reasons underlying any observed sensitivity - 1) emotional responses measured by state anxiety, and 2) physical responses measured by peak center of mass velocity. We found that both young and older adults perceived slips to be riskier than trips, with no group difference in sensitivity. The relative sensitivity to slips versus trips was moderately reliable across two days of testing, though most participants were less sensitive to perturbation direction on the second day. Neither state anxiety responses nor peak center of mass (CoM) velocity explained the directional sensitivity, though CoM velocity was higher during slips than trips for both age groups. These results suggest that the characteristics of experienced balance disturbances may influence behavioral decisions. Objectively measuring individual differences in the perceived risk of balance disturbances and the ability to recover from these disturbances, can potentially improve fall risk assessment and inform personalized training interventions.

Layer 1 NDNF interneurons form distinct subpopulations with opposite activation patterns during sleep in freely behaving mice
Non-rapid eye movement (NREM) sleep facilitates memory consolidation by transferring information from the hippocampus to the neocortex. Recent evidence suggests that this transfer occurs primarily when hippocampal sharp-wave ripples (SWRs) and thalamocortical spindles are synchronized. In this study, we asked what role cortical layer 1 NDNF-expressing (L1 NDNF) interneurons play during NREM sleep in gating information transfer during SWR-spindle synchronization. Using simultaneous cell-specific calcium imaging and local field potential recordings in freely moving mice, we discovered that L1 NDNF neurons form cell assemblies tuned to specific sleep stages, exhibiting differential responses to spindle synchronization. L1 NDNF neurons mediate slow inhibition through GABAB receptors. Systemic application of a GABAB receptor antagonist increased pyramidal neuron excitability during NREM sleep, enhanced inhibitory responses during SWRs, and disrupted SWR-spindle coupling. Overall, these findings suggest an important contribution of L1 NDNF neuron-mediated slow inhibition to the synchronization of sleep oscillations with potential implications for memory consolidation processes.

Quantifying Differences in Neural Population Activity With Shape Metrics
Quantifying differences across species and individuals is fundamental to many fields of biology. However, it remains challenging to draw detailed functional comparisons between large populations of interacting neurons. Here, we introduce a general framework for comparing neural population activity in terms of shape distances. This approach defines similarity in terms of explicit geometric transformations, which can be flexibly specified to obtain different measures of population-level neural similarity. Moreover, differences between systems are defined by a distance that is symmetric and satisfies the triangle inequality, enabling downstream analyses such as clustering and nearest-neighbor regression. We demonstrate this approach on datasets spanning multiple behavioral tasks (navigation, passive viewing of images, and decision making) and species (mice and non-human primates), highlighting its potential to measure functional variability across subjects and brain regions, as well as its ability to relate neural geometry to animal behavior.