bioRxiv Subject Collection: Neuroscience's Journal

Comparing Intra- and Inter-individual Correlational Brain Connectivity from Functional and Structural Neuroimaging Data
Inferring brain connectivity from inter-individual correlations has been applied to various neuroimaging modalities, such as glucose metabolic activity measured by positron emission tomography (PET) and brain structures assessed using MRI. The variability that drives these inter-individual correlations is generally attributed to individual differences, potentially influenced by factors like genetics, life experiences, and biological sex. However, it remains unclear whether long-term within-individual effects, such as aging, and state-like effects also contribute to the correlated structures, and how intra-individual correlations are compared to inter-individual correlations. In this study, we analyzed longitudinal data spanning a wide age range, examining regional brain volumes using structural MRI, and regional brain functions using both regional homogeneity (ReHo) of resting-state functional MRI and glucose metabolic activity measured with Fludeoxyglucose (18F) FDG-PET. In a first dataset from a single individual scanned over 15 years, we found that intra-individual correlations in both ReHo and regional volumes resembled resting-state functional connectivity. In a second dataset, involving multiple longitudinal points and participants for FDG-PET and MRI, we replicated these findings, showing that both intra- and inter-individual correlations were strongly associated with resting-state functional connectivity. Correlations in functional measures (i.e., ReHo or FDG-PET) showed greater similarity with resting-state connectivity than structural measures. Moreover, matrices from the same modality showed higher similarity between the two datasets, indicating modality specific contributions. These results suggest that multiple factors may contribute to both inter- and intra-individual correlational measures of connectivity. Understanding or controlling for these factors could enhance the interpretability of the inter-individual connectivity measures.

Modulation of sensory input to the spinal cord by peripheral afferent fibres. Searching for relays
A long-lasting GABA-dependent increase in the excitability of afferent fibres, and thus modulation of the sensory input to the spinal cord, may be evoked by epidural polarization. However, the direct effects of fibre polarization are short-lasting and the sustained increase in their excitability appears to be secondary to the release of GABA from nearby astrocytes. We have now investigated whether the modulation of spinal sensory input by stimulation of a peripheral nerve, previously attributed to synaptically evoked intraspinal field potentials, is evoked in a similar way. However, as neither its dependence on GABA nor its relays have been investigated, we addressed the question of whether the increase in the excitability of epidurally stimulated afferent fibres following a peripheral nerve stimulation does or does not depend on GABA and whether it might be mediated by astrocytes. The effects of conditioning stimulation of the tibial nerve were evaluated from changes in the excitability of both group I and group II muscle afferents, estimated from action potentials recorded in peripheral nerves and in field potentials recorded in the dorsal horn respectively in acute experiments on deeply anaesthetized rats. The excitability of the afferents was increased by stimulation of group II and/or cutaneous but not group I muscle afferents. The effects were significantly weakened by blocking GABA channels by gabazine, and by astrocyte toxin L-alpha-aminoadipic acid (L-AAA), indicating that the excitability of both group I and group II afferent fibres may be modulated by GABAergic astrocytes, the new role played by astrocytes.

Exploring the Abeta Plaque Microenvironment in Alzheimer's Disease Mice by Multimodal Lipid-Protein-Histology Imaging on a Benchtop Mass Spectrometer
Amyloid-beta (Abeta) plaque deposits in the brain are a hallmark of Alzheimer's disease (AD) neuropathology. Plaques consist of complex mixtures of peptides like Abeta1-42, characteristic lipids such as gangliosides, and they are targeted by reactive microglia and astrocytes. In pharmaceutical research and development it is therefore a formidable challenge to contextualize different biomolecular classes and cell types of the Abeta plaque microenvironment in a coherent experimental workflow on a single tissue section and on a benchtop imaging reader. Here, we developed such a workflow that combines lipid MALDI mass spectrometry imaging using a vacuum-stable matrix with histopathology stains and with MALDI HiPLEX-immunohistochemistry of plaques and multiple protein markers on a bench-top imaging mass spectrometer. The three data layers consisting of lipid, protein markers, and histology can be co-registered and evaluated together. Multimodal data analysis suggests extensive co-localization of Abeta plaques with the peptide precursor protein, with a defined subset of lipids and with reactive glia cells on a single brain section of APP/PS1 AD model mice. Plaque-associated lipids like ganglioside GM2 and phosphatidylinositol PI38:4 isoforms were readily identified using the tandem MS-capabilities of the mass spectrometer. Taken together, our data suggests that complex pathology involving multiple lipids, proteins and cell types can be interrogated by this spatial multiomics workflow on an easy-to-use benchtop mass spectrometer.

Assessing Behavioral and Neural Correlates of Change Detection in Spatialized Acoustic Scenes
The ability to detect changes in complex auditory scenes is crucial for human survival, yet the neural mechanisms underlying this process remain elusive. This study investigates how the presence and location of sound sources impacts active auditory change detection as well as neural correlates of passive change detection. We employed stimuli designed to minimize semantic associations while preserving naturalistic temporal envelopes and broadband spectra, presented in a spatial loudspeaker array. Behavioral change detection experiments tasked participants with detecting new sources added to spatialized and non-spatialized multi-source auditory scenes. In a passive listening experiment, participants were given a visual decoy task while neural data were collected via electroencephalography (EEG) during exposure to unattended spatialized scenes and added sources. Our behavioral experiments (N = 21 and 21) demonstrated that spatializing sounds facilitated change detection compared to non-spatialized presentation, but that performance declined with increasing number of sound sources and higher hearing thresholds at high frequencies, exclusively in spatialized conditions. Slower reaction times were also observed when changes occurred from above or behind the listener, exacerbated by a higher number of sources. EEG experiments (N = 32 and 30), using the same stimuli, showed robust change-evoked responses. However, no significant differences were detected in our analysis as a function of spatial location of the appearing source.

Structural regeneration and functional recovery of the olfactory system of zebrafish following brain injury
Olfactory dysfunction is a common outcome of brain injuries, negatively affecting quality of life. The mammalian nervous system has limited capacity for spontaneous olfactory recovery, making it challenging to study olfactory regeneration and recovery in adults. In contrast, zebrafish are an ideal model for such studies due to its extensive and lifelong regenerative abilities. In this work, we describe a model of excitotoxic injury in the olfactory bulb using quinolinic acid (QA) lesions in adult zebrafish. We observed extensive neurodegeneration in both the olfactory bulb and olfactory epithelium, including a reduction of bulbar volume, neuronal death, and impaired olfactory function. Recovery mechanisms involved tissue remodeling, cell proliferation, neurogenesis, leading to full restoration of olfactory function by 21 days. This study provides a model to further investigate the effects of excitotoxicity on olfactory dysfunction, and highlights the remarkable regenerative abilities of zebrafish, providing insights into potential therapeutic strategies for restoring olfactory function following brain injuries.

Spermidine alleviates depression via control of the stress response
Depression is a stress-associated disorder, and it represents a major global health issue. Its pathophysiology is complex and remains insufficiently understood, with current medications often showing limited efficacy and undesirable side effects. Here, we identify imbalanced polyamine levels and dysregulated autophagy as key components of the acute stress response in humans, and as hallmarks of chronic stress and depressive disorders. Moreover, conventional antidepressant pharmacotherapy increases endogenous plasma concentrations of the polyamine spermidine exclusively in patients who respond to the treatment, suggesting a link between spermidine and successful outcomes. In a clinical trial, involving drug-naive depressed individuals, three weeks of spermidine supplementation increased autophagy and alleviated symptoms of depression. Behavioral and mechanistic findings of spermidine supplementation were validated in various mouse stress and depression models. In summary, spermidine supplementation mitigates polyamine dysregulation and stimulates autophagy under pathological stress conditions, offering a novel and well-tolerated treatment approach for stress-related depressive disorders.

Cortical dynamics underlying initiation of rapid steps with contrasting postural demands
Our ability to flexibly initiate rapid visually-guided stepping movements can be measured in the form of express visuomotor responses (EVRs), which are short-latency (~100ms), goal-directed bursts of lower-limb muscle activity. Interestingly, we previously demonstrated that recruitment of anticipatory postural adjustments (APAs) interacted with the subcortically-generated EVRs in the lower limb, suggesting context-dependent top-down modulation. We investigated the associated cortical dynamics prior to and during rapid step initiation towards a salient visual target in twenty-one young, healthy individuals. We adopted two contrasting postural conditions by manipulating the stepping direction. Anterolateral steps involved low postural demands, whereas anteromedial stepping involved high postural demands. We recorded high-density EEG, surface electromyography from gluteus medius and ground-reaction forces. Independent component analysis and time-frequency statistics revealed significant, yet relatively modest differences between conditions in preparatory cortical dynamics, most evidently in primary motor areas. Following target presentation, we observed stronger theta and alpha power enhancement in the supplementary motor area, and stronger alpha and beta power decrease in primary motor, parietal and occipital clusters during APA recruitment that preceded steps under high postural demands. We found no differences in (pre)frontal areas associated with the observed EVR suppression in the high postural demand condition. Together, our findings point towards greater cortical involvement in step initiation under high postural demands as compared to more reflexive, stimulus-driven steps. This notion may be particularly relevant for populations where postural control is impaired by age or disease, as more cortical resources may need to be allocated during stepping.

Mice-side bias: Deliberative decision making in a model of rule revision reveals 'myside' confirmation bias-like cognitive processes in mice.
Confirmation or 'myside' bias--over-valuation of novel information which confirms previously internalized cognitive content (prior beliefs, rules of conduct, etc.) and corresponding under-valuation of disconfirming novel information--constitutes a serious obstacle to adaptive revision of our beliefs, especially in ambiguous or complex epistemic environments. Indeed, myside bias has become a particularly pernicious fact of societal cohesion, contributing to the propagation of fake news, to social polarization, and even to the replication crisis in experimental science. By contrast, relatively little is understood about either its neurocognitive underpinnings or its evolution, one reason for this being that the potential presence of myside bias-like tendencies in non-human animals has never been directly tested. Hence, in order to advance research in both of these directions, we designed a novel mouse model of everyday-like rule revision such that the dynamic model environment would be sufficient to call out myside bias-like behaviors providing that mice did indeed possess the particular kind of competing neurocognitive processes necessary for it to manifest. Here, we both validate that model and provide the first behavioral descriptions of myside confirmation bias-like deliberative profiles in a non-human animal. Notably, we observe that this bias does not manifest in a merely unreflective or heuristic/'system 1' manner but rather also emerges through and indeed increases deliberative behaviors, especially in contexts of low representational resolution. Several other parallels with findings from human studies of myside bias are also detailed in the discussion.

Not enough pleasure? Influence of hallucination proneness on sensory feedback processing of positive self-voice
Ample research explored changes in sensory feedback processing of the self-voice as well as the control of attention allocation in voice hearers, including both non-clinical voice hearers and voice hearers with a psychotic disorder. While the attentional bias toward negative emotional information in voice hearers with a psychotic disorder due to heightened sensitivity towards threat/danger is well established, it remains unclear how positive emotion captures or controls attention. Manipulating the certainty of sensory feedback to the self-voice, transitioning from fully neutral to entirely positive (100% neutral, 60-40% neutral-pleasure, 50-50% neutral-pleasure, 40-60% neutral-pleasure, 100% pleasure), provides an opportunity to investigate attentional control and sensory feedback processing in positive self-voice as a function of hallucination proneness (HP). Participants with different HP scores self-generated and passively listened to their own voices during EEG recordings. N100 or P200 responses to self-generated and externally-generated self-voices did not differ. Further, HP did not modulate N100/P200 responses. These null findings might result from the minimal perceptual discriminability among the five types of voices varying in pleasure content. This might have led to less variation in certainty regarding the sensory feedback to self-voice, and consequently a lack of differential engagement of attentional resources. The lack of a global N100 suppression effect prompts inquiry into the association of sense of ownership/agency and pleasure.

Spike sorting biases and information loss in a detailed cortical model
Sorting electrical signals (spikes) from extracellular recordings of large groups of connected neurons is essential to understanding brain function. Despite transformative advances in dense extracellular recordings, the activity of most cortical neurons remains undetected. Small simulations with known neuron spike times offer critical ground truth data to improve spike sorting. Yet, current simulations underestimate neuronal heterogeneity and connectivity, which can potentially make spike sorting more challenging. We simulated recordings in a detailed large-scale cortical microcircuit model to link spike sorting accuracy to neuronal heterogeneity, evaluate the performance of state-of-the-art spike sorters and examine how spike sorting impacts the retrieval of information encoded in the cortical circuit. We found that modern spike sorters accurately isolated about 15% of neurons within 50 {micro}m of the electrode shank, which contrasts with previous simulated yields but agrees with experiments. Neurons were unresolved because their spike trains were either missed (undersampling) or, when detected, incomplete or merged with other units (assignment biases). Neuron isolation quality was influenced by both anatomical and physiological factors (selection bias), improving with increased neuron firing rate, spike spatial extent, for neurons in layer 5, and excitatory neurons. We exposed the network to various stimuli to dissociate the impact of these biases on its stimulus discrimination ability. Surprisingly, undersampling did not affect discrimination capacity, but selection and assignment biases nearly reduced it by half. These findings posit realistic models as a complementary method to evaluate and improve spike sorting and, hence, brain activity representations.

A Reference Tissue Implementation of Simultaneous Multifactor Bayesian Analysis (SiMBA) of PET Time Activity Curve Data
PET analysis is conventionally performed as a two-stage process of quantification followed by analysis. We recently introduced SiMBA (Simultaneous Multifactor Bayesian Analysis), a hierarchical model that performs quantification and analysis for all brain regions of all individuals at once, and in so doing improves both the accuracy of parameter estimation as well as inferential efficiency. However until now, SiMBA has only been implemented for the two-tissue compartment model. We have now extended this general approach to also allow a non-invasive reference tissue implementation that includes both the full reference tissue model and the simplified reference tissue model. In simulated data, SiMBA improves quantitative parameter estimation accuracy, reducing error by, on average, 57% for binding potential (BPND). In considerations of statistical power, our simulation studies indicate that the efficiency of SiMBA modeling approximately corresponds to improvements that would require doubling the sample size if using conventional methods, with no increase in the false positive rate. We applied the model to PET data measured with [11C]AZ10419369, which binds selectively to the serotonin 1B receptor, in datasets collected at three different PET centres (n=139, n=44 and n=39). We show that SiMBA yields replicable inferences by comparing associations between PET parameters and age in the different datasets. Moreover, we show that time activity curve data from different centres can be combined in a single SiMBA model using covariates to control between-centre parameter differences, in order to harmonise data between centres. In summary, we present a novel approach for noninvasive quantification and analysis of PET time activity curve data which improves quantification and inferences, enables effective between-centre data harmonisation, and also yields replicable outcomes. This method has the potential to significantly expand the range of research questions which can be meaningfully tested using conventional sample sizes with PET imaging.

A neural circuit for context-dependent multimodal signaling in Drosophila
Many animals, including humans, produce multimodal displays by combining acoustic with visual or vibratory signals. However, the neural circuits that coordinate the production of multiple signals in a context-dependent manner are unknown. Multimodal behaviors could be produced by parallel circuits that independently integrate the external cues that trigger each signal. We find that multimodal signals in Drosophila are driven by a single circuit that integrates external sensory cues with internal motivational state and circuit dynamics. Drosophila males produce air-borne song and substrate-borne vibration during courtship and previous studies have identified neurons that drive courtship and singing, but the contexts and circuits that drive vibrations and coordinate multimodal signaling were not known. We show that males produce song and vibration in distinct, largely non-overlapping contexts and that brain neurons that drive song also drive vibrations with cell-type specific dynamics and via separate pre-motor pathways. This circuit also coordinates multimodal signaling with ongoing behavior, namely locomotion, to drive vibrations only when the male's vibrations can reach the female. A shared circuit facilitates the control of signal dynamics by external cues and motivational state through shared mechanisms like recurrence and mutual inhibition. A proof-of-concept circuit model shows that these motifs are sufficient to explain the behavioral dynamics. Our work shows how simple motifs can be combined in a single neural circuit to select and coordinate multiple behaviors.

Ventral tegmental area GABA neurons integrate positive and negative valence
Ventral tegmental area (VTA) dopamine (DA) neurons are classically linked to Pavlovian reward learning and reinforcement. Intermingled VTA GABA neurons are positioned to regulate dopaminergic and striatal systems, but we lack critical insight into how this population contributes to conditioned motivation in different learning contexts. Recording DA and GABA neurons across multiple conditioning paradigms, we found that GABA neurons not only actively encode appetitive and aversive cues and outcomes separately, but uniquely integrate salient events of both valences to guide reward seeking.

HCN1 channels in GABAergic amygdalar neurons underpin male-biased aggressive behaviors
Aggression behaviors typically vary between sexes, but the molecular mechanisms driving these disparities in neural coding are unclear. We found that aggression selectively activates GABAergic neurons in the posterior substantia innominata (pSI), an extend amygdala region critical for aggressive behaviors in both sexes of mice, with males exhibiting higher neuronal activity during the attack. Utilizing single-nucleus RNA sequencing, we characterized the diverse molecular landscape of pSI neurons, revealing significant differences in ion channels and hormone regulator genes that may underpin sex-specific aggression. Male GABAergic pSI neurons exhibited remarkable hyperexcitability due to increased Ih currents. Strikingly, modulating HCN1 expression not only adjusted this hyperexcitability but also influenced sexual dimorphism in aggression: silencing HCN1 in the GABAergic pSI neurons reduced male aggression, while its overexpression markedly heightened aggression in females. Furthermore, testosterone was shown to intensify aggression by upregulating HCN1 and remodeling pSI circuits. These findings provide detailed sex-specific molecular mechanisms underlying social behaviors.

Lifespan trajectory of chimpanzee brains characterized by magnetic resonance imaging histology
Chimpanzee brain maturation provides an invaluable framework for understanding the evolution of the human brain. We performed ultra-high resolution quantitative magnetic resonance imaging (qMRI) with histological validation on post mortem brains from captive and wild chimpanzees with a broad age range. We mapped developmental myelination and age-related iron accumulation across regions and layers of the neocortex. Compared to humans, chimpanzees showed more myelination and iron deposition in motor and premotor cortices, while the auditory cortex was more strongly myelinated in humans. Our model suggests that chimpanzees' cortical myelination was largely completed by the age of nine years, while iron accumulation continued throughout the lifespan. The regions with highest adult levels of myelin and iron took the longest to mature, challenging the widespread assumption that highly myelinated regions complete their development first. The reported maps and developmental curves provide a foundation for comparative neuroscience research and understanding of human brain evolution.