Patterned brain stimulation is commonly employed to modulate brain circuits and treat neuropsychiatric disorders. Although widely used in clinical settings, there remains limited understanding of how stimulation alters neural oscillations and how these effects relate to baseline evoked network responses. To address this question, we applied 15 minutes of 10Hz focal electrical stimulation delivered as 5-s trains separated by 10-s rest intervals, a pattern resembling 'excitatory' transcranial magnetic stimulation, to 14 patients with epilepsy undergoing intracranial electroencephalography in premotor, parietal, and temporal cortical sites. We quantified spectral features of cortico-cortical evoked potentials (CCEPs) before and after stimulation. We hypothesized that the temporal and spectral components of the CCEP predicted the location and degree of stimulation-induced modulation. Across patients, low frequency power (alpha, beta) showed the broadest change, while the magnitude of change was stronger in high frequencies (beta, gamma). Next, we demonstrated that regions with stronger pre-stimulation evoked spectral responses were more likely to exhibit larger post-stimulation spectral changes. These findings were specific to frequency in a temporal window. Post-stimulation power changes were driven by interaction between direction of change in pre-stimulation power and temporal window. Finally, regions exhibiting early increases and late decreases in pre-stimulation power exhibited power changes after stimulation. These patterns were consistent across premotor, parietal, and temporal sites, but the magnitude and latency varied by stimulation location. Together, these findings demonstrate that time-frequency pre-stimulation features predict post-stimulation modulation of human neural oscillations and highlight temporally specific, state-dependent properties of stimulation responses in humans, thereby improving our understanding of stimulation-associated changes in human brain networks and helping inform more effective brain stimulation techniques.

Songbirds, in particular zebra finches (Taeniopygia guttata), provide a powerful model for investigating the neural mechanisms of learned vocal behavior. Researchers typically rely on the acoustic structure of birdsong to quantify vocal behavior. As a more direct measure of motor control, we present VIBE: Vocal acoustic Inversion to Biomechanical Estimates, an open-source pipeline that recovers the biomechanical control parameters of song production from the pitch and amplitude of the acoustic waveform. Biomechanical models of the songbird syrinx describe vocal production with two continuously varying parameters: α and β, representing subsyringeal air sac pressure and syringeal muscle tension, respectively. Recovering these parameters from song acoustics provides a motor-based coordinate system against which neural activity or other dependent variables can be directly compared. Because α and β are the coupled control parameters of a nonlinear oscillator, their joint recovery is non-trivial. VIBE addresses this through iterative optimization of the governing normal-form equations. We validate VIBE against recorded air sac pressure across 44 songs from twelve birds, showing that the recovered α corresponds to empirically measured air sac pressure. Pairing VIBE with Neuropixels recordings from RA in five birds, we find that RA activity is well predicted by the recovered parameters, and that α and β add predictive power beyond the acoustic features of song. By recovering biomechanical control parameters from the acoustic signal, VIBE makes the biomechanical coordinate system of song production accessible to the broader songbird research community.

Dynamic interactions amongst competing motor programs shape behavioural output. Here, we use bath application of biogenic amines octopamine (OA) and tyramine (TA) to explore operating principles of competition amongst central pattern generating (CPG) networks controlling locomotion in 3[rd] instar Drosophila larvae. OA and TA share the same synthesis pathway and are contained in overlapping sets of neurons. Bath application of OA promoted fictive forwards locomotion, suppressed backwards locomotion, and induced bouts of fictive head sweeps during wash periods that were proportional to the promotion of forward waves. In contrast, TA, the conceptualised inhibitory antagonist to OA, promoted collisions and overlap of motor programmes, activity-dependent inhibition, and bouts of silence during wash periods. Co-application of OA and TA revealed bimodal state-dependent modulation with forward bias promotion exclusively if the pre-application network was not already forward dominant. Overall, our work provides insights into how adrenergic-like systems can modulate dynamics of motor competition among interacting CPGs within locomotor networks.

Central sensitization can produce secondary mechanical hyperalgesia; however, secondary thermal hyperalgesia remains controversial. This study investigated whether topical capsaicin alters temporal summation to tonic heat-pain within regions of secondary mechanical hyperalgesia. Twenty-six healthy adults (19 women, 7 men; 22.5 ± 3.5 yr) completed a participant- and investigator-masked, placebo-controlled, crossover protocol (≥1-wk washout) with topical capsaicin (0.075% Zostrix) or placebo (Lubriderm) applied to C5-C6 dermatomes. Temporal summation to tonic heat pain was assessed at 46.5°C for 120 s and quantified as the area under the pain-time curve (AUC). Mechanical summation used weighted pinprick (256 mN, 16 stimuli, 1 Hz) and was quantified using Sumscore. Outcomes were measured at baseline, 10-, 20-, and 30-min postintervention. Secondary mechanical hyperalgesia was induced postcapsaicin, with a significant main effect of intervention [F(1,24) = 4.77, P = 0.039]; the main effect of time and the time × intervention interaction were not significant, and no individual time point comparison survived multiplicity adjustment. Temporal summation to tonic heat pain within secondary hyperalgesic regions showed no significant effects of time, intervention, or time × intervention (P > 0.05). Within-subject analysis revealed 18/26 participants (69%; 95% CI: 0.48 to 0.86) with higher peak thermal AUC postcapsaicin versus placebo (two-sided P = 0.076), but exhibiting substantial individual heterogeneity. Topical capsaicin-induced central sensitization produced significant secondary mechanical hyperalgesia but no statistically significant group-level increase in temporal summation of heat pain. This modality-specific pattern using the current protocol does not exclude a moderate thermal effect. The substantial interindividual variability observed in this exploratory within-subject analysis was not statistically significant and is hypothesis generating.NEW & NOTEWORTHY This is, to our knowledge, the first double-blind, placebo-controlled, crossover study to quantify temporal summation of heat pain within regions of secondary hyperalgesia using an extended tonic thermal stimulus. Capsaicin-induced central sensitization produced significant secondary mechanical hyperalgesia without a statistically significant group-level increase in tonic heat summation. The findings describe a modality-specific pattern under this protocol and should not be interpreted as proof that thermal sensitization is absent.

The brain contributes to maintaining balance both while standing and during step initiation, where cortical preparatory activity and sensory modulation may be tuned to the task goal. Standing requires maintaining postural equilibrium, whereas stepping involves propelling the body toward a target while preserving stability. These demands change with the intended step direction, suggesting that cortical processing may be flexibly adjusted according to directional goals. Foot-sole cutaneous receptors are relevant to postural control, as they signal load distribution that supports upright posture and step initiation. This project aimed to investigate how sensory modulation, and preparatory cortical processes differ across stepping tasks with differing directional goals. Quiet standing, straight stepping, and diagonal stepping tasks were performed by 31 healthy participants, while EEG was recorded. Standing trials were conducted only with electrical stimulation, whereas stepping trials were performed both with and without stimulation (2 Hz at 125% perceptual threshold) delivered to the left (stance) foot sole to elicit somatosensory evoked potentials (SEPs). Trials without stimulation allowed examination of cortical preparatory activity. eLORETA source localization was applied across all conditions to identify cortical generators. SEP amplitudes were larger during both stepping tasks compared to standing, and diagonal stepping SEPs were larger compared to straight stepping. In non-stimulation trials, frontal, central, and parietal regions showed greater preparatory activity during diagonal compared with straight stepping, particularly in early preparation stages. These findings suggest that somatosensory processing and preparatory cortical activity are flexibly tuned to the directional goal of the task, consistent with optimal feedback control principles.

Motor systems must flexibly select between competing outputs while preserving stability of rhythmic outputs. In Drosophila larvae, the isolated central nervous system is capable of maintaining rhythmicity by generating multiple different fictive motor programs. The biogenic amines octopamine and tyramine are known to regulate larval locomotion, however, how the tdc[2+] octopaminergic/tyraminergic system regulates motor program competition is not well understood. Here, we combine dual-colour calcium imaging and optogenetic manipulation to explore how tdc[2+] neurons track, permit, and bias fictive motor activity in 3[rd] instar Drosophila larvae. We find that tdc[2+] activity in the larval ventral nerve cord is strongly coupled to motor neuron activity across multiple fictive behaviours, indicating that the system is recruited broadly across the motor repertoire. Optogenetic depolarisation of tdc[2+]neurons increases motor root bursting and induces a robust fictive forward bias, whereas optogenetic hyperpolarisation suppresses or abolishes fictive rhythms and generates a short-lasting, post-inhibitory rebound in fictive activity. Spatially-restricted stimulation reveals that posterior abdominal activation is especially effective at promoting fictive forward activity. Separating VNC-residing from brain-residing tdc[2+] populations further shows that activation of descending brain-residing tdc[2+] projections is sufficient to recapitulate this forward bias. Finally, tdc[2+] activation induces short-lived post-stimulation changes in motor programme probability, including transient elevation of competing fictive backward instantaneous frequency. Together, these findings suggest that tdc[2+] neurons act as a permissive and biasing modulatory layer within larval motor circuits, linking adrenergic-like signalling to motor programme competition.

The glial glutamate transporters play a crucial role in clearing glutamate from the synaptic space, thereby regulating its concentration and maintaining synaptic homeostasis. As the main glutamate transporter in astrocytes, GLT-1 ensures efficient neurotransmitter clearance, contributing to the excitatory-inhibitory balance in neural circuits. Given its essential role in synaptic function, modulation of GLT-1 has been explored as a potential approach to altering synaptic transmission and plasticity. The objective of this study was to examine hippocampal synaptic plasticity in juvenile rats following the administration of Ceftriaxone (CTX), a compound widely reported to influence GLT-1 function and glutamate homeostasis. Our study primarily focused on functional electrophysiological outcomes rather than direct molecular measurements. A 0.5-μL intracerebroventricular (ICV) microinjection of 0.5 mM Ceftriaxone was administered to male Wistar rats. Field excitatory postsynaptic potentials (fEPSP) were recorded from the CA1 region following Schaffer collateral stimulation. The study found that 29 days of CTX treatment significantly weakened long-term potentiation (LTP) in CA3-CA1 synapses. A significant decrease in the paired-pulse index was observed following ceftriaxone injection at interpulse intervals (IPIs) of 20 and 80 ms. Compared with the control group, the Ceftriaxone-treated group showed a significantly higher maximum current intensity required to elicit the maximal synaptic response. This study suggests that, beyond its antimicrobial properties, Ceftriaxone can significantly influence electrophysiological properties in the hippocampus. Such findings highlight the significant impact of ceftriaxone on synaptic function and suggest its potential to modulate neural network activity.NEW & NOTEWORTHY Long-term intracerebroventricular ceftriaxone, a GLT-1 activator, was tested in vivo in juvenile rats without a disease model. Our results reveal three reliable effects in CA3-CA1 synapses: attenuated LTP, reduced paired-pulse indices, and a shift in the input-output curves (higher current needed for maximal response). Together, these data reveal that sustained astrocytic glutamate clearance dampens excitatory gain under strong input, extending prior slice or short-term studies to a chronic in vivo context.

Glabrous skin on the human palms and soles is specialised for mechanical interaction and is characterised by papillary ridges, an absence of hair follicles, and a thick stratum corneum. While gross morphological differences, such as the increased thickness of the plantar stratum corneum, are well-documented, the fine-scale structural features associated with individual ridges and whether they help explain differences in tactile perception remain poorly understood. Here, we employed optical coherence tomography, automated image segmentation, and 3D reconstruction to generate high-resolution models of the stratum corneum and viable epidermis across six sites on the hand and foot for 27 participants. Our morphological analysis revealed that while the stratum corneum was thicker on the foot than the hand, the thickness of the viable epidermis remained remarkably consistent across all sites. Furthermore, papillary ridge width increased in a distal-to-proximal fashion and was larger on the foot. We also found that internal ridge architecture was considerably more pronounced than the topography observable on the skin surface. Correlating these structural parameters with psychophysical measures, specifically absolute force and two-point discrimination thresholds, demonstrated distinct drivers for tactile sensitivity. Force thresholds were primarily governed by a combination of skin thickness and innervation density, and additionally on the foot by papillary ridge width and internal ridge depth. In contrast, spatial acuity was best predicted by innervation and papillary ridge width. These findings precisely quantify glabrous skin morphology and offer new insights into the process of mechanotransduction by highlighting how tissue architecture might contribute to human tactile perception.

Lifting a paper cup without crushing it requires precise regulation of finger force. In humans, such control is thought to rely primarily on the contralateral corticospinal tract (CST), but the cortical mechanisms underlying graded force modulation remain unclear. To address this, we used online repetitive transcranial magnetic stimulation (rTMS) as a transient interference during an isometric index-finger force-tracking task. Eleven healthy adults increased and decreased index-finger isometric force at low (0-20% maximal voluntary contraction, MVC) and high (0-60% MVC) levels, while 2-s trains of 10 Hz rTMS were delivered over contralateral or ipsilateral primary motor cortex (M1; cM1, iM1) or a midline control site (Pz). Accuracy, consistency, and force smoothness were quantified using RMSE, SDAE, and normalized YankRMS within the 2-s stimulation windows, and outcomes were normalized to Pz. Factorial mixed-effects analysis showed a significant main effect of stimulation-site (Condition) across all three measures (all p < 0.0022). Holm-adjusted comparisons showed greater disruption during cM1 than iM1 stimulation and no-TMS, whereas iM1 did not differ from no-TMS. For YankRMS, the Condition × Intensity interaction was significant (p=0.0031), with a larger cM1-iM1 difference at the higher force level (60% MVC) than at the lower force level (20% MVC; interaction contrast, p=0.0033). No other task-dependent interactions were significant. These findings provide time-locked causal evidence that ongoing cM1 activity is essential for dynamic index-finger force regulation, with the contralateral-ipsilateral asymmetry becoming more pronounced at higher force demands.

Descending corticospinal and afferent pathways underlying spinally evoked motor potential both contribute to motor output, yet how their interaction at the spinal and peripheral levels is organized spatially within a muscle remains unclear. This study investigated the spatiotemporal characteristics of descending modulation of spinally evoked motor potentials by combining subthreshold transcranial magnetic stimulation (TMS) with transcutaneous spinal cord stimulation (tSCS) delivered at the L1-L2 interspinous space in nine participants and at the T12-L1 interspinous space in one participant. In Experiment 1, spinally evoked motor potentials were recorded from multiple lower-limb muscles at various interstimulus intervals (ISIs) defined relative to central conduction time (CCT). Subthreshold TMS facilitated spinally evoked motor potentials from CCT onward across all recorded muscles, with additional bilateral facilitation observed at longer ISIs. In Experiment 2, high-density surface electromyography (HDsEMG) revealed distinct intramuscular activation patterns in the tibialis anterior. The center of gravity (CoG) of TMS-induced motor evoked potentials was located more proximally than that of spinally evoked motor potentials. Notably, the CoG of facilitation maps was shifted further proximally than that of both single-stimulus responses. These findings suggest that descending and afferent inputs preferentially recruit partially distinct motoneuron pools within the same muscle. The proximal bias of facilitation indicates recruitment of additional motoneurons rather than uniform amplification of existing activity. Together, these results demonstrate that the interaction between descending and afferent inputs is both timing-dependent and spatially non-uniform, providing new insight into sensorimotor integration in the human lower limb.

The availability of a pre-existing cognitive-motor schema accelerates the learning of novel motor information. The encoding of a novel schema-compatible, compared to -incompatible, motor sequence was recently shown to be supported by the left primary motor cortex (M1). However, causal evidence for the role of M1 in schema-mediated motor learning is currently lacking. In the current study, we aimed to address this knowledge gap by transiently disrupting M1 using inhibitory continuous theta burst stimulation (cTBS). Forty-eight young healthy participants learned a bimanual motor sequence task (cognitive-motor schema). Twenty-four hours later, they learned a novel sequence whose ordinal schematic structure was compatible with that learned on the previous day. To provide causal evidence for a role of M1 on such schema-mediated motor learning, we applied either cTBS or sham stimulation to the left M1 immediately prior to encoding the schema-compatible novel sequence. Electromyography results showed no evidence for an effect of left M1 cTBS on corticospinal excitability as measured with motor-evoked potentials. Similarly, behavioral results indicated no significant effect of cTBS on subsequent schema-mediated motor sequence learning. Altogether, the present data do not provide evidence for a causal role of the left M1 in schema-mediated motor sequence learning.

Dendritic spines are the principal postsynaptic sites of excitatory transmission. For over a century, their shape has been sorted into discrete categories such as filopodia, thin, long-thin, stubby, mushroom, and branched, largely by Golgi-Cox impregnation and, more recently, confocal microscopy. Yet both approaches share a fundamental limitation. Here, using the human H01 petavoxel reconstruction imaged by electron microscopy, we show that spine shape classification depends strongly on viewing angle. A total of 445 spines on layer 4 basal dendrites of five pyramidal neurons were classified from an arbitrary initial viewpoint (angle 1) and then reclassified after rotation in Neuroglancer (angle 2). Only 20.9% kept their category, so chance-corrected agreement was negligible (Cohen's kappa = 0.027), and a McNemar-Bowker test showed that the reclassifications were directional rather than random. Comparing morphometric measurements against three-dimensional ground-truth shapes further showed that spine length is systematically distorted by rotation across spine types, whereas caliber parameters (neck diameter, head diameter, and head area) generally remain stable except in mushroom spines, which are highly sensitive to viewing angles in both length and caliber. These observations demonstrate that the rigid Golgi-Cox and confocal taxonomies conflate true spine morphology with the arbitrary angle of view, cautioning against treating spine categories from single-viewpoint imaging as reliable descriptors of shape. Our results therefore support, as best practice, the use of continuous and coarse morphometric descriptors. More broadly, they mark the limits of optical resolution for dendritic spine analysis and signal a shift toward three-dimensional, electron microscopy-based reconstruction as the reference for studying spine morphology.NEW & NOTEWORTHY Discrete classification of dendritic spines underpins research on synaptic plasticity, development, and disease yet rests on two-dimensional images whose viewing angle is uncontrolled. By arbitrary rotation of dendrites, four of five spines changed category with viewpoint alone, exposing bias in Golgi-Cox and confocal classifications. Comparing morphometric measurements against ground-truth shapes also showed that spines are distorted by rotation. Our results support, as best practice, replacing the rigid six-category scheme with continuous and coarse morphometric descriptors.

Interlimb asymmetries in motor control are well documented in simplified tasks, but their expression in coordinated reach-to-grasp actions under perturbation remains unclear. Here, we examined whether differences between the dominant (DH) and non-dominant hand (NDH) emerge during online adjustments of reach-to-grasp movements. Participants (n = 12; 5 female, 7 male) performed reach-to-grasp actions in a virtual environment under control, visual perturbation, and mechanical perturbation conditions. Kinematic and electromyographic (EMG) measures were analyzed, and baseline-corrected values were used to isolate perturbation-specific effects. Under control conditions, differences between hands were minimal and limited to peak transport velocity and proximal muscle activity. During perturbations, analyses performed prior to baseline correction revealed a significant Hand × Perturbation interaction for index-finger curvature, indicating hand-dependent distal reorganization during mechanical perturbation. Crucially, applying baseline correction to isolate perturbation-driven effects eliminated most global kinematic asymmetries, while revealing greater transport deceleration and triceps brachii activity in the NDH and greater index-finger curvature and extensor indicis activity in the DH during mechanical perturbation. In contrast, visual perturbations elicited largely symmetric responses across both limbs. These results indicate that interlimb differences in reach-to-grasp are not expressed as stable, fixed traits, but emerge selectively under mechanical perturbation. The observed pattern suggests that lateralized coordination strategies depend on the specific demands of the task, with the DH showing greater distal reorganization and the NDH exhibiting stronger proximal stabilization-related responses.