Experimental Brain Research

Background: Paretic-arm nonuse and trunk compensation after stroke are often interpreted as impairment or learned behavior but may also reflect task-dependent optimization. We previously showed that gravitational shoulder effort and force reserve shape these behaviors. Here, we tested whether endpoint precision is an additional constraint. Methods: Twenty-two individuals with stroke and 22 matched controls performed a seated Proximal Arm Nonuse reaching task toward small (2 cm) and large (12 cm) targets. Stroke participants were tested at normal arm weight and with the paretic arm mechanically lightened; controls were tested at normal arm weight and while holding a dumbbell corresponding to 85% of maximal voluntary shoulder force. Results: In stroke participants, trunk compensation was 15% lower for the large target than for the small target, independently of arm weight condition (F(1,13)=8.166, p=.013, pes=.39). Target size did not significantly affect trunk compensation in controls or anterior deltoid activation in either group. In stroke participants, movement time was 24% longer for the small target than for the large target (F(1,13)=8.533, p=.012, pes=.40). In controls, the small target increased movement time only when the arm was weighted (F(1,18)=14.9, p=.001, pes=.45), not at normal arm weight (F(1,18)=0.158, p=.695, pes=.01). Conclusions: Endpoint precision altered reaching after stroke partly independently of gravitational shoulder effort. Trunk compensation after stroke was therefore modulated by task demands rather than fixed by impairment, a pattern consistent with, although not a direct demonstration of, an optimal-control account. Rehabilitation should evaluate compensation by its functional role and the cost of suppressing it, not only by its deviation from typical kinematics, and should treat target size as an adjustable parameter of the task. Clinical Trial: NCT04747587. https://clinicaltrials.gov/study/NCT04747587

Standing balance relies on the continuous detection of body motion across a wide range of amplitudes and frequencies. According to Weber's law, sensory discrimination should degrade in proportion to stimulus amplitude, yet vestibular self-motion perception deviates from this prediction. Whether this deviation extends to the vestibular control of posture remains unknown. To test this hypothesis, we delivered amplitude-increasing electrical vestibular stimulation (EVS, 0-2 mA) at four frequencies (0.1, 0.2, 0.5, and 1 Hz) to 10 healthy young adults, and quantified balance discrimination thresholds (DTs) from the gain and inter-trial variability of the lateral ground reaction force, using signal detection theory. Balance control DTs increased sublinearly with EVS amplitude at all frequencies, deviating from Weber's law. Both balance-correcting response gain and variability accounted for this deviation: gain decreased more slowly than the strictly inverse relationship that would have alone produced strict adherence to Weber's law, while variability exhibited an initial increase followed by saturation at higher EVS amplitudes, a signal-dependent structure necessary to reproduce the observed DT pattern. EVS frequency modulated the shape of this deviation but affected neither overall DT nor the signal-to-noise ratio of the balance-correcting response, which instead remained similar across frequencies. This study revealed a particular property of postural control discrimination by extending the psychophysical and neural deviations from Weber's law previously reported for vestibular perception. Overall, these results provide a better understanding of the balance control discrimination process and provide potential additional assessment criteria for vestibular impairments.

Motor imagery and overt action are proposed to share common planning mechanisms while relying on distinct processes during movement performance. To investigate how repetition affects physical and imagined movements, participants physically or mentally drew paths while avoiding barriers. The barrier orientations were either repeated or different across consecutive trials, and the modality (execution or imagery) of the trials either changed or remained the same for two consecutive trials. Reaction time and movement time were analysed as measures of movement planning and task performance, respectively. Imagined movements produced longer reaction times and movement times than physical movements. Repetition of orientation reduced reaction times in both physical and imagined trials, indicating that trajectory repetition facilitated movement preparation. In contrast, previous trial type influenced reaction times only for physical movements, which were only initiated faster when preceded by another physical trial. Different effects emerged for movement time. Physical movements were faster when the same trajectory was repeated no matter the previous trial type, whereas imagined movements were unaffected by orientation and instead showed shorter durations when preceded by another imagined trial. The findings indicate that physical and imagined movements rely on more similar trajectory representations during the planning stage, but diverge during task performance. While physical movements appear to benefit from the persistence of previously generated movement trajectories, imagined trials seems more influenced by modality-specific cognitive processes. These results provide behavioural support for the Motor-Cognitive model and contribute to understanding how movement repetition influences overt and imagined actions.

Targeted stepping requires coordinating limb transport, preparatory postural adjustments, and accurate foot placement while maintaining balance under competing speed and accuracy demands. Although Fitts' law robustly describes speed-accuracy trade-offs (SATs) in upper-limb movements, its applicability to whole-body stepping remains unclear due to additional postural and visuomotor constraints. This study investigates how instructional emphasis and spatial demands modulate SAT regulation and pre-step postural dynamics during stepping. Twenty-four healthy young adults will perform a multidirectional stepping task across two directions (0 degrees and 45 degrees) and two distances (45% and 80% of leg length) under three instruction conditions: Fast, Accurate, and Fast & Accurate. Kinematic and force-plate data will quantify movement time, foot-placement error, and pre-step center-of-pressure dynamics. We hypothesize that instructional emphasis and increased postural demands will systematically alter stepping strategies and produce deviations from classical Fitts-type scaling, particularly during diagonal stepping.

Feedforward control is impaired after stroke, but whether practice improves it in the chronic stage remains unclear. In fast single-joint movements, feedforward control scales initial acceleration to movement amplitude. Feedback-error-learning theory predicts that fast practice should promote recalibration. We tested whether speed-emphasized reach training improved this scaling in the more-affected arm. Thirty-six individuals with mild-to-moderate chronic stroke completed 2,080 reaches over four sessions emphasizing Speed (n = 17) or Accuracy (n = 19). At baseline, 3 days, and 1 month after training, participants performed single-joint elbow movements of three amplitudes without online visual feedback. The slope relating initial peak acceleration to target amplitude quantified feedforward scaling. At baseline, scaling was reduced on the more-affected side for extension and flexion. Speed training increased flexion scaling at 3 days, with 14 of 17 participants improving. The group-by-session interaction showed a greater increase after Speed than Accuracy training. Scaling declined by 1 month; no training-related changes occurred for extension. Peak acceleration occurred within 100 ms. Time to peak velocity, an index of pulse width, showed no group-specific training effect. Thus, speed-emphasized training produced a short-term improvement in early flexion acceleration scaling that transferred to an untrained single-joint task and was consistent with improved feedforward control. Transfer primarily reflected acceleration pulse-height rather than pulse-width modulation.

Serial dependence, whereby current perception is systematically biased toward the most recent sensory experience, has been demonstrated across multiple sensory modalities. However, whether serial dependence occurs during active haptic perception remains unknown. Moreover, because haptic perception of object geometry depends on hand movements for sensory acquisition (i.e., active touch), it remains unclear whether serial dependence, if any, originates from perceptual processing, movement, or both. Here, we investigated whether active haptic perception exhibits attractive serial dependence using a newly developed haptic device that let participants explore virtual slopes of different heights. Participants reported perceived slope heights, and we recorded their kinematics. Perceptual judgments were systematically attracted toward the most recently experienced slope, demonstrating serial dependence during active haptic perception. By contrast, none of the kinematic parameters exhibited corresponding serial dependence. These findings show that serial dependence extends to the somatosensory perceptual domain and suggest that the observed perceptual bias was not mediated by systematic changes in hand movements.

Corticospinal-motoneuronal connections are more prominent in distal than proximal upper-limb muscles, but whether this anatomical specialization influences the capacity for activity-dependent plasticity remains unknown. Here, we tested whether Hebbian stimulation targeting corticospinal-motoneuronal connections, induces muscle-specific plasticity in proximal and distal arm muscles in healthy humans. We applied a spike-timing-dependent stimulation protocol comprising 180 paired stimuli, with transcranial magnetic stimulation timed to evoke corticospinal volleys 1-2 ms before motoneurons were antidromically activated by electrical stimulation of the brachial plexus for the biceps brachii or the ulnar nerve for the first dorsal interosseous (FDI). Corticospinal excitability was assessed by measuring motor-evoked potentials (MEPs) before and 30 min after stimulation. Hebbian stimulation increased MEP amplitudes in both muscles, but facilitation was greater in the FDI than in the biceps. Because resting motor threshold was higher for the biceps than for the FDI, in a control experiment we matched stimulation intensities between muscles. Under these conditions, Hebbian stimulation continued producing greater facilitation in the FDI than in the biceps, indicating that the difference was not attributable to stimulation intensity. Moreover, changes in MEP amplitude were associated with maximal MEP size in the FDI but not in the biceps, whereas no muscle-specific association was observed with peripheral responses (M-max), supporting a stronger relationship between corticospinal drive and Hebbian plasticity in the distal than in the proximal arm muscle. Together, these findings identify a distal-to-proximal gradient in Hebbian plasticity across arm muscles, which may contribute to optimizing targeted neuromodulation approaches for upper-limb rehabilitation.

The control of visually guided bimanual movements is thought to represent a compromise between the drive for synchrony that characterizes many bimanual movements and the competing demands that characterize the constituent unimanual movements. As a result, while visually guided bimanual movements are found to be ostensibly synchronized, they also demonstrate subtle asynchronies. Here, we conducted a fine-grained analysis of bimanual reaching movements focusing on their relative synchrony as movements unfold and the relationship with gaze behavior. Our novel analysis revealed that participants frequently shifted their gaze laterally several times during bimanual reaches and that these gaze shifts were systematically associated with changes in the speed ratio of the two limbs. More specifically, we observed a strong association between gaze shifts and the speed ratio shifting toward the limb on the attended side. This pattern of behavior was seen regardless of hand dominance and target difficulty, suggesting that it may reflect a common visuomotor strategy during bimanual reaching.

Current evidence supports resistance training for stroke recovery but autonomic dysregulation following stroke can lead to exaggerated fluctuations in blood pressure (BP) in response to exercise, thus challenging perceptions about safety and feasibility of resistance exercise. This study compared the post-exercise BP response following a single bout of leg-press exercise between people with and without stroke. Data from 11 individuals with chronic stroke (age 39-76 years, 5 female, 1-9 years post-stroke) and 19 recreationally active controls (age 18-33 years, 11 female) were included. Brachial artery BP was measured before exercise and immediately following a single set of leg-press (8 repetitions, target rating of perceived exertion 5-7/10). The difference between pre- and post-exercise BP was calculated and compared between groups using an independent t-test of equal variance. Additionally, a two-way analysis of covariance was used to examine potential cofounding effects of activity level and baseline BP on between group differences in BP response. No group differences were observed in post-exercise change in systolic (mean difference=3.61 mmHg, 95%CI:-8.36, 15.57, p=0.54); and diastolic (mean difference=5.38 mmHg, 95%CI:-6.11, 16.86, p=0.35) BP. Results were unchanged after controlling for baseline BP, and activity levels. Our work suggests that individuals with and without chronic stroke experience similar acute BP responses immediately following resistance exercise; however, work with more appropriately matched controls and greater sample sizes is needed to support these findings. Future studies may elucidate mechanisms related to the acute cardiovascular response to resistance exercise in individuals with stroke.

Hypoxia impairs cognitive function, yet its effects on navigation and intrinsic brain activity remain poorly understood. This study investigated whether a continuous exposure to hypoxia (CH) or an intermittent exposure to hypoxia (IH) differentially influence navigation performance and resting-state electroencephalography (EEG) in healthy young adults. Seventy-two participants completed sham (FiO2 = .209) and hypoxia (FiO2 = .13) sessions during either a one hour of CH or IH intervention. Navigation was assessed using the Spatial Performance Assessment for Cognitive Evaluation (SPACE), while resting-state EEG was recorded before and after the intervention in a subset of participants (n = 40). EEG spectra were parameterised into periodic (alpha and beta power and central frequency) and aperiodic (exponent and offset) components. Aligned rank transform (ART) analyses of variance examined effects of hypoxia and intervention group, while linear regression determined whether reductions in peripheral oxygen saturation (SpO2) predicted changes in navigation or EEG. Hypoxia impaired multiple components of navigation, increasing path integration distance error and egocentric pointing angle error irrespective of intervention group, while mapping accuracy was preferentially impaired following IH and perspective taking performance following CH. Hypoxia also reduced alpha and beta power, beta central frequency, and aperiodic exponent, while aperiodic offset and alpha central frequency remained stable. The reduction in aperiodic exponent was primarily evident following IH, suggesting that the temporal pattern of oxygen delivery may influence specific neurophysiological responses. Contrary to our hypothesis, SpO2 reduction did not predict individual differences in navigation performance or EEG activity. These findings demonstrate that brief, moderate hypoxic exposure can disrupt allocentric and egocentric components of spatial navigation while selectively altering periodic and aperiodic features of intrinsic brain activity.

Background: Safely walking through complex environments requires the use of visual information to plan and regulate steps, while managing competing cognitive demands. Age-related changes in visual control of gait and executive function may contribute to unsafe stepping behaviour, particularly under cognitive load. Route previewing may support visual motor planning, but its effects under dual task conditions in older adults are not well understood. Research Question: Does cognitive dual tasking influence gait and gaze behaviour during adaptive walking in older adults, and does previewing the walking route before gait initiation mitigate these effects? Methods: Thirty nine community dwelling older adults (Mage=73.4 years, SD=8.1; 14 male) completed four adaptive walking conditions, combining single and dual-task walking with and without previewing. Per condition, participants completed six trials across different 8-m long coloured walking routes. Gait characteristics were assessed using an instrumented walkway and video-analysis, while gaze behaviour was recorded using mobile eye tracking. Cognitive load was manipulated using a serial subtraction task. Gait and gaze outcomes were evaluated using linear mixed effects models. Results: Dual task performance significantly increased stepping errors, cross-steps, and double limb support, while reducing stride length (p < .001). It also significantly increased the number and duration of task irrelevant fixations (p < .001). After previewing, participants exhibited more efficient gaze behaviour, as evidenced by looking further ahead (larger forward gaze transitions) and fewer irrelevant gaze fixations (p < .002). Significant Task-by-Preview interactions revealed that previewing mitigated the negative effects of dual-tasking on stepping errors and cross steps (p < .001). Significance: Route previewing prior to gait initiation attenuates negative effects of cognitive load on adaptive gait in older adults, likely by supporting visual motor planning. These findings highlight the potential value of simple visual planning interventions for improving adaptive gait in older adults.

Axonal action potential propagation critically depends on tightly regulated intra and extracellular electrolyte concentrations and acid base homeostasis. Experimental studies have demonstrated that changes in extracellular sodium, potassium, and calcium concentrations alter axonal excitability and conduction properties. However, it remains unknown whether small variations in electrolyte concentrations within the physiological range are associated with measurable changes in nerve conduction velocity in humans. In this study, data from neurologically healthy children who underwent both nerve conduction studies and blood gas analysis were analyzed to investigate the relationship between electrolyte concentrations and median motor nerve conduction velocity. Nerve conduction velocity was significantly negatively correlated with ionized calcium concentration and positively correlated with sodium concentration, whereas no significant correlation was observed for potassium. In an a priori defined multiple linear regression model, ionized calcium, sodium, and bicarbonate remained significantly associated with nerve conduction velocity after correction for multiple testing. These findings indicate that even small variations in electrolyte concentrations within the physiological range are associated with measurable differences in peripheral nerve conduction velocity and suggest an additional contribution of acid base status to nerve conduction.

We explored the role of proprioceptive feedback from Ia afferents in intra-muscle synergies stabilizing muscle force with the help of muscle vibration, which is known to increase presynaptic inhibition of those afferents to alpha-motoneurons. Healthy young adults performed an accurate three-finger force production task by pressing with the index, middle and ring fingers of the dominant (right) hand. This was done without vibration and with vibration applied to the extrinsic finger flexors, extensors, both muscles, the palm, or biceps brachii. High-density electromyography was used to identify motor units in both finger flexors and extensors. The raster plots of motor units were converted into firing frequency functions and then subjected to principal component analysis to extract stable motor unit groups (MU-modes). The uncontrolled manifold (UCM) analysis was used to quantify the variance components affecting and not affecting force, and the synergy index in the MU-mode space. Contrary to our expectations, the synergy index did not drop under any vibration condition and showed an increase during vibration applied simultaneously to both flexor and extensor muscles. This was primarily associated with a drop in the component of variance affecting the force magnitude. The results speak against an important role of monosynaptic Ia-afferent projections to alpha-motoneurons in the formation of intra-muscle synergies. They suggest that muscle vibration can play the role of a noisy signal decreasing performance-affecting variance via stochastic resonance effects.

Eye movements have been shown to reveal the cognitive processes underlying spatial ability, but most studies investigating gaze behaviour in spatial cognition use a single, screen-based task. Therefore, it is unknown whether these gaze behaviours are preserved across naturalistic task environments, or whether their relationship with spatial performance is domain-general or task-specific. The present study tested whether established gaze behaviours are preserved and generalizable across diverse, naturalistic spatial tasks. N = 30 young adults completed three naturalistic spatial tasks varying in perceptual, cognitive, and visuomotor demands: Q-bitz, the Brick Building Task (BBT), and the Mental Rotation Test (MRT). Gaze was measured using the Pupil Labs Neon eye tracker. Within-task analyses were conducted to assess whether gaze behaviours were preserved, and cross-task analyses were conducted to assess the generalizability of preserved gaze behaviours. Within-task analyses identified source inspection and transition rate as gaze behaviours that were consistently sensitive to changing task demands. Cross-task analyses showed that source fixation duration was a domain-general predictor of performance, whereas the relationship between transition rate and performance differed by task. Exploratory sex-separated analyses also suggested a more consistent gaze-performance relationship in males. These findings show that source fixation duration may represent a common cognitive process underlying performance across naturalistic spatial tasks.

How cognitive control influences the correction of motor errors is an important question. In this study, we examined how the slow and fast processes that constitute motor adaptation are modulated by cognitive load. To address this question, we used a novel approach that differed from traditional dual-task experiments that have assessed the role of fast processes by decreasing attention to the motor adaptation task. Here, cognitive load was manipulated using a decision-making task with easy and hard difficulty levels, while simultaneously adapting to a perturbation and maintaining attention on the task. Our results showed that the hard decision-making task increased cognitive load relative to the easy task, resulting in attenuated steady-state learning. The relative contributions of the fast and slow components were assessed using a dual state-space model, which showed that the former was larger under the high-load condition, whereas the latter was larger under the easy-load condition, consistent with a competition model in which a shared error signal is partitioned between the two processes. We also showed that the effect of cognitive load on fast processes was not attributable to increased reaction times or task performance. Taken together, these results support the notion that faster explicit motor adaptation processes derive from cognitive processes involved in action selection.

Humans exhibit head movements synchronized with musical rhythms, frequently observed during instrument playing or dancing (e.g., hip hop). We conjectured that vestibular input from head movements could aid motor execution anticipatory of musical rhythms, known as sensorimotor synchronization (SMS). To examine this, we applied subthreshold galvanic vestibular stimulation (GVS) simultaneously with musical beats while healthy adults performed SMS to those beats. We designed a GVS waveform where the current rose steeply before each beat onset, which was expected to trigger intense vestibular afferent firing before the beats (as actual head movements would). Participants flexed their index finger in synchrony with the beats of syncopated auditory rhythms. In GVS trials, the SMS timing variability was significantly lower than in trials without GVS, suggesting that vestibular input stabilizes SMS to musical beats. Additionally, a control experiment using a waveform with a gradually increasing current revealed no significant difference in SMS variability between trials with and without GVS. This study provides causal evidence that the vestibular system is involved in synchronized movement to music. In addition, the results suggest that a specific parameter of the GVS waveform could influence predictive motor execution anticipatory of upcoming auditory events.

The vestibular system consists of peripheral end organs that provide the brain with temporally precise information about head position in space required for motor coordination and postural stability. Irregular vestibular afferents of the inner ear are particularly sensitive to rapid changes in acceleration, or jerk, which is necessary for making quick, corrective movements and postural adjustments. Vestibular short-latency evoked potentials (VsEPs) are waveforms that provide a measure of synchronous irregular afferent activity originating from peripheral (early peaks) and central (later peaks) vestibular pathways. These VsEPs are elicited by head jerk stimuli, commonly used preclinically. Previous studies have shown that early VsEP waves are dependent upon jerks of an adequate intensity, number, and duration. However, less is known about jerk characteristics necessary for generation of reliable VsEP signals centrally (later peaks). The present study examines the precise temporal dynamics of peripheral and central vestibular centers across a broad range of mechanical vestibular load in rats. We found differential changes in synchrony and timing driven by jerk repetition, intensity, and duration between early and late VsEP waves representing distinct anatomical components of the vestibular circuit. Our results suggest that central components are uniquely sensitive to jerk duration, intensity, and repetition, indicating a disconnect between peripheral and central neuronal recruitment, adaptive plasticity, and time-frequency dynamics. This work provides a foundation for using VsEP to assess jerk-induced physiological vestibular dysfunction and to understand how central relays differentially adapt to variations of mechanical vestibular stress compared with the precise, high-fidelity transmission observed in the periphery.

Post-stroke gait impairment is associated with altered muscle coordination. Understanding how functional muscle coordination contributes to joint control is essential for clarifying the mechanisms of gait impairment after stroke. Functional muscle network approaches quantify how muscle interactions contribute to joint kinematics. We investigated how gait-related functional muscle networks are reorganized after stroke and how this reorganization relates to motor impairment and gait dysfunction. Thirty-five participants with subacute stroke and 25 healthy controls underwent gait analysis using surface electromyography and lower-limb kinematics. We quantified functionally similar (redundant information), complementary (synergistic information), and independent (unique information) muscle interactions underlying joint kinematics. Participants with stroke exhibited greater redundant information and lower independent information than healthy controls. Stroke-specific functional muscle network modules emerged through merging modules observed in healthy controls. Stroke-specific module activation was also associated with motor impairment and gait performance. Overall, these findings identify a shift toward more redundant muscle coordination and stroke-specific reorganization of functional network modules, providing new insights into post-stroke gait dysfunction.

Detection of postoperative pain in mice is challenging, and there is a need for unbiased, automated methods. Home-cage monitoring systems are promising, but their ability to detect mild pain remains unclear. We evaluated whether home-cage activity measures can be used to detect pain-related changes in a plantar incision model of postoperative pain. First, we confirmed the presence of local hypersensitivity using von Frey filaments on 48 C57BL/6JRj mice (24 males, 24 females). Subsequently, we pair-housed 64 mice (32 males, 32 females) and allocated them to receive either a 4 mm plantar incision on the hind paw under anaesthesia, or anaesthesia alone. Baseline locomotion and voluntary wheel-running activity were recorded over five days, followed by four days of postprocedural monitoring in the home cages. Both procedure groups (incision and anaesthesia-only) showed reduced locomotion and running wheel activity relative to baseline. However, activity measures did not differ between incised and anaesthesia-only mice, and we detected no sex-related effects. Under these conditions, home cage activity measures did not reliably discriminate pain-related behavioural changes in the model, despite the model showing local mechanical hypersensitivity in the von Frey test. Together, these findings indicate that, given the mild and localised nature of the plantar incision mouse model, commonly used home-cage metrics cannot yet reliably detect subtle pain-related behavioural changes.

Background: Post-stroke spasticity is commonly assessed using the Modified Ashworth Scale (MAS), despite its limited objectivity and variable reliability. This study introduces ScoreE, a novel isokinetic dynamometry-derived metric designed to quantify elbow and wrist flexor hyper-resistance by comparing patients torque-angle profiles with a healthy reference corridor. Methods: Thirty-five post-stroke patients and twenty-one healthy controls were included. Elbow and wrist hyper-resistance was assessed using the MAS and an isokinetic dynamometer in two testing conditions: seated on the dynamometer chair (ergo chair) and seated in the patient wheelchair. Two dynamometer-derived metrics were analyzed: ScoreE and peak torque (PeakT). Feasibility, inter-rater and test-retest reliability, and associations with MAS grades were assessed. Exploratory PRE-POST changes following a 10-min local vibration session were also described. Results: ScoreE showed good to excellent inter-rater and test-retest reliability across joints and testing conditions. Inter-rater ICCs ranged from 0.81 to 0.89, and test-retest ICCs from 0.76 to 0.93. PeakT showed moderate to good reliability. In contrast, MAS agreement was lower and more variable. ScoreE showed moderate associations with MAS grades, particularly at the elbow, with weaker and less consistent associations at the wrist. MAS assessment was faster, but all assessments were well tolerated, with median pain ratings of 0/10 in all testing conditions. PRE-POST vibration changes were small and heterogeneous, preventing formal assessment of responsiveness. Conclusion: ScoreE provides a reliable, objective, and visually interpretable quantification of post-stroke elbow and wrist flexor hyper-resistance. By comparing the full torque-angle profile with normative data, this approach preserves information on both the magnitude and angular span of abnormal resistance, supporting its potential for targeted longitudinal monitoring.