Experimental Brain Research

Most stroke patients do not use their paretic limb whereas they are able to. The Constraint-Induced Movement Therapy (CIMT) is effective to reverse this non-use behaviour in some patients but is inapplicable or unsuccessful on others. Here, we investigate how much non-use could come from shoulder weakness instead of the behavioural conditioning treated by the CIMT. We asked 26 healthy participants to reach a target while holding a dumbbell. We found that 18/26 participants exhibit proximal arm non-use when loaded and that non-use reduces shoulder torque of final posture. We either found that non-use improves accuracy in a high gravity field. Following optimal control policy, we explain how the non-use could be an adaptative solution when the shoulder is weak. Our results show the need to include muscular strength into cost function used to model human movement. The framework presented here suggests that psychological non-use could be treated effectively with CIMT, while physiological non-use, resulting from shoulder weakness, might respond better to anti-gravity muscles strengthening.

Sensorimotor conflict is theorised to contribute to the maintenance of some pathological pain conditions, such as Complex Regional Pain Syndrome (CRPS). We therefore tested whether sensorimotor adaptation is impaired in people with CRPS by characterising their adaption to lateral prismatic shifts in vision. People with unilateral upper limb CRPS Type I (n = 17), and pain-free individuals (n = 18; matched for age, sex, and handedness) completed prism adaptation with their affected/non-dominant and non-affected/dominant arm, in a counterbalanced order. We examined 1) the rate at which participants compensated for the optical shift during prism exposure (i.e. strategic recalibration), 2) endpoint errors made directly after prism adaptation (sensorimotor realignment) and their retention, and 3) kinematic markers associated with feedforward motor control and sensorimotor realignment. We found no evidence that strategic recalibration was different between people with CRPS and controls, including no evidence for differences in a kinematic marker associated with trial-by-trial changes in movement plans. Participants made significant endpoint errors in the prism adaptation after-effect phase, which are indicative of sensorimotor realignment. Overall, the magnitude of this realignment was not found to differ between people with CRPS and pain-free controls. However, people with CRPS made greater endpoint errors when using their affected hand than their non-affected hand, whereas no such difference was seen in controls. Taken together, these findings suggest that strategic control and sensorimotor realignment were not impaired for either arm in people with CRPS. In contrast, they provide some evidence that there is a greater propensity for sensorimotor realignment in CRPS, consistent with more flexible representations of the body and peripersonal space. Our study challenges the theory that sensorimotor conflict might underlie pathological pain that is maintained in the absence of tissue pathology.

Issuing a goal-directed action requires specifying the goal of the action as well as planning the hand trajectory to obtain it. Accumulating results suggest that planning a straight point-to-point trajectory is more efficient and likely to involve simpler optimization process compared to the planning of trajectories with more complex shapes (e.g., curved trajectories). We sought to find evidence for the qualitative difference between the two planning modes through the investigation of reaction times (RT) in a pointing task performed with the wrist. In experiment 1, 18 subjects performed delayed straight and curved via-point reaching movements to arrays of 2 or 8 targets. Both trajectory type and number of possible targets affected RT. In experiment 2 (N=14), we demonstrate a switching cost between the issuing of the two types of trajectories, irrespective of changes in target position. Unexpectedly, trajectory type did not affect RT in experiment 2, likely due to the lack of target pre-cuing in experiment 2. Our results suggest that the planning of curved and straight trajectories differ in their memory load during pre-planning and requires a time-consuming update of the motor commands when switching between straight and curved plans.

Current models of mental body representations (MBRs) indicate that tactile inputs feed several of them for different functions, implying that altering tactile inputs may affect MBRs differently. Here we tested this hypothesis by leveraging Repetitive Somatosensory Stimulation (RSS), known to improve tactile perception by modulating primary somatosensory cortex (SI) activity, and measured its effects over the body image, the body model and the superficial schema in a randomized sham-controlled, double-blind cross-over study. Results show that RSS affected the body image, participants perceiving their finger size as being smaller after RSS. While previous work showed increase of finger size perception after tactile anesthesia (Gandevia & Phegan 1999), these findings reveal that tactile inputs can diametrically modulate the body image. In contrast, RSS did not alter the body model or superficial schema. In addition, we report a novel mislocalization pattern, with a bias towards the middle finger in the distal phalanges that reverses towards the thumb in the proximal phalanx, enriching the known distortions of the superficial schema. Overall, these findings provide novel insights into the functional organization of MBRs and their relationships with somatosensory information. Reducing the perceived body size through RSS could be useful in helping treat body image disturbance.

When people observe conflicting visual and proprioceptive cues about their static hand position, visuo-proprioceptive recalibration results. Recalibration also occurs during gradual or abrupt visuomotor adaptation, in response to both the cue conflict and sensory prediction errors experienced as the hand reaches to a target. Here we asked whether creating a cue conflict gradually vs. abruptly, or providing error feedback, affects recalibration in a static hand. We examined participants responses to a 70 mm visuo-proprioceptive conflict, imposed by shifting the visual cue forward from the proprioceptive cue (static left hand). Participants pointed with their unseen right hand to indicate perceived bimodal and unimodal cue positions. Conflict was introduced gradually (groups 1 and 2) or abruptly (groups 3 and 4), with performance feedback present (groups 2 and 4) or absent (groups 1 and 3). For abrupt groups, most behavioral change occurred immediately after the conflict began. No-feedback groups (1 and 3) showed comparable magnitudes of overall recalibration, indicating that abrupt and gradual conflicts result in similar recalibration but with different timings. Motor adaptation was evident in the indicator hand with performance feedback (groups 2 and 4). However, performance on a static ruler task suggests proprioceptive recalibration also occurred despite the presence of feedback. Control groups confirmed accurate performance on the pointing task despite the visual cue shift. These findings highlight the distinct timing of recalibration mechanisms for gradual versus abrupt cue conflicts and potential smaller contribution of error mechanisms for a static conflict. New and NoteworthyThe brain may handle spatial conflicts between visual and proprioceptive cues differently for a dynamic hand undergoing visuomotor adaptation than for a static hand. In a static hand, abrupt conflict triggered immediate recalibration without further adjustment, and feedback had little impact on recalibration. This suggests varying roles of multisensory and error mechanisms across motor contexts, underscoring the importance of examining a variety of motor contexts to understand and predict behavior.

Many individuals who undergo limb amputation experience persistent phantom limb pain (PLP). The underlying mechanism of PLP is unknown, but the phenomenon has been associated with reorganization in sensorimotor cortex following amputation. The traditional view is that cortical reorganization degrades the missing limbs representation. However, recent work suggests that an amputated limbs cortical representation remains intact and that reorganization reflects a retargeting of efferent projections to residual muscles proximal to the amputation site. Evidence of retargeting has only been shown in individuals with upper limb amputations, and the relationship of retargeting to PLP is controversial. This study assessed retargeting and its relationship to PLP in 10 individuals with lower limb amputations. We recorded electromyographic (EMG) activity in a residual thigh muscle (vastus lateralis, VL) in patients with above-knee amputations during cyclical movements of the foot. VL activity on the amputated side was compared to that recorded on patients intact side while they moved their phantom and intact feet, respectively. VL activity in the patient group was also compared to VL activity from a sample of 9 control participants with no amputation. We show that phantom foot movement is associated with greater VL activity in the amputated leg than that seen in the intact leg as well as that exhibited by controls. The magnitude of residual VL activity was also positively related to ratings of PLP. These results provide the first support for retargeting in lower limb amputees and suggest that retargeting is related to the experience of phantom pain. New and NoteworthyPrevious work has only examined retargeting in upper limb amputees. This study provides evidence for retargeting in lower limb amputees and suggests that retargeting is related to phantom limb pain.

BackgroundTraumatic brachial plexus injury (TBPI) is a potentially debilitating event, that usually affects young men following car or motorbike accidents. TBPI interferes with hand sensorimotor function, is associated with chronic pain, and causes cortical reorganization. Interactions between the somatosensory and motor cortices are of fundamental importance for motor control. The hands and face stand out as regions of high functionality with a privileged interaction existing between them, as reflected by the proximity and extension of their representations. Face-hand sensorimotor interactions have been demonstrated in healthy subjects. ObjectiveThe aim of this study was to investigate changes in the sensorimotor interaction in the hand and between the face and the hand in TBPI patients in order to better understand the plasticity of face-hand sensorimotor circuits following TBPI. MethodThe experimental design consisted of activating the representation of a hand muscle using transcranial magnetic stimulation (TMS) preceded by an electrical stimulation (ES) applied to the hand or face, which allows the investigation of the cortical reorganization resulting from TBPI. In the paradigm called afferent inhibition (AI), the motor evoked potential (MEP) in a target muscle is significantly reduced by a previous peripheral ES. AI can be evoked in short-latency (SAI) or long-latency (LAI) interstimulus intervals. Nine TBPI patients participated: five had partial sensorimotor function in their hands and were evaluated on the injured side (TBPI-I group) and four had complete loss of sensorimotor function in their hands and were evaluated on the uninjured side (TBPI-UI group). A control group (CG) included 18 healthy adults. A detailed clinical evaluation complemented the analysis. ResultsThe results showed preserved hand sensorimotor integration for TBPI patients at SAI intervals, but not at LAI intervals. For the face-to-hand sensorimotor integration, the results showed no inhibition at SAI intervals for the TBPI patients. For LAI intervals, a facilitation effect was observed for the TBPI patients, an effect we termed long afferent facilitation or LAF. LAF positively correlated with results in the Central Sensitization Inventory and in the Disabilities Arm, Shoulder, and Hand questionnaire. ConclusionThese results point to the existence of an inhibitory regulation system between the representations of the face and the hand that seems to be suppressed in TBPI and correlates with pain. Moreover, brain changes arising from TBPI are not restricted to the hemisphere contralateral to the injured limb, but extend to both hemispheres.

Motor skill learning involves multiple mechanisms, including use-dependent learning (UDL), reinforcement learning (RL), and error-based learning (EBL). These operate over different time scales and neural pathways, contributing uniquely to skill acquisition, consolidation, and transfer. Here we asked how these mechanisms support the acquisition of a spatially complex maze navigation task in five groups of healthy young adults. Groups received one of five types of feedback during training of their unseen dominant hand: UDL (no feedback), RL (binary success/failure feedback with a static threshold), RLA (binary feedback with an adaptive threshold), RLB (binary feedback with adaptive threshold and brief flash of cursor feedback), or EBL (continuous real-time cursor feedback). Skill, transfer, and proprioceptive acuity were assessed pre- and post- training using a speed-accuracy function (SAF) for each hand and a two-alternative forced-choice shape discrimination task. Results showed that UDL and RL groups exhibited no improvement post-training, while EBL, RLA, and RLB groups demonstrated accuracy improvements. EBL and RLB participants experienced a significant reduction in movement variability, with EBL showing a greater decrease compared to UDL. The left-hand SAF revealed improvements in accuracy across all groups except UDL. All groups showed reduced variability in the left hand, suggesting intermanual transfer, with EBL transferring more variability improvements than UDL. No significant proprioceptive changes were observed in any group. These findings provide new insights into motor skill learning, emphasizing that even minimal feedback can facilitate complex skill acquisition and transfer and has significant implications for studies where error-based learning may not be applicable. New and NoteworthyMinimal, adaptive binary feedback can effectively support the acquisition and transfer of spatially complex motor skills. While use-dependent and static reinforcement learning failed to enhance performance, adaptive reinforcement and error-based feedback significantly improved accuracy and reduced movement variability. Notably, these gains transferred to the untrained hand, highlighting the potential of reinforcement-based strategies in contexts where error-based learning is limited or unavailable, offering important implications for rehabilitation and motor training design.

BackgroundTraumatic brachial plexus injury (TBPI) causes a sensorimotor deficit in upper limb (UL) movements. ObjectiveOur aim was to investigate the arm-forearm coordination of both the injured and uninjured UL of TBPI subjects. MethodsTBPI participants (n=13) and controls (n=10) matched in age, gender, and anthropometric characteristics were recruited. Kinematics from the shoulder, elbow, wrist and index finger markers were collected while upstanding participants transported a cup to mouth and returned the UL to a starting position. The UL coordination was measured through the relative phase (RP) between arm and forearm phase angles and analyzed as a function of the hand kinematics. ResultsFor all participants, the hand transport had a shorter time to peak velocity (p<0.01) compared to the return. Also, for the control and the uninjured TBPI UL, the RP showed a coordination pattern that favored forearm movements in the peak velocity of the transport phase (p<0.001). TBPI participants injured UL showed a longer movement duration in comparison to controls (p<0.05), but no differences in peak velocity, time to peak velocity and trajectory length, indicating preserved hand kinematics. The RP of the injured UL revealed altered coordination in favor of arm movements compared to controls and to the uninjured UL (p<0.001). Finally, TBPI participants uninjured UL showed altered control of arm and forearm phase angles during the deceleration of hand movements compared to controls (p<0.05). ConclusionThese results suggest that UL coordination is reorganized after a TBPI so as to preserve hand kinematics.

Hand choice is an unconscious decision frequently made in daily life. A correlation has been found between the state of brain activity before target presentation and hand choice for the target, around which the hand choice probability is in equilibrium. However, whether the neural state before target presentation affects hand choice remains unknown. Therefore, this study aimed to examine whether instantaneous somatosensory electrical stimulation administered to the unilateral wrist at 0, 300, or 600 ms before the target presentation facilitates or inhibits stimulated hand choice for targets around the hand selection equilibrium point. A single electrical stimulation comprised five trains of 1 ms electrical pulses, with a 20 ms inter-pulse interval. The stimulus intensity was set at 80% of the motor threshold. Fourteen right- handed healthy adults (five females, nine males; mean age, 25.1 {+/-} 4.64 years) participated. Unilateral wrist stimulation significantly increased the probability of choosing the stimulated hand and led to a faster reaction time than bilateral wrist stimulation and no-stimulation conditions. The results suggest that prior somatosensory stimulation significantly affects the hand-choice process, effectively promoting selection of the stimulated hand. These findings highlight the potential application of this stimulation method in stroke rehabilitation to facilitate use of the paretic hand. Impact StatementPrior neural stimulation significantly affected the hand-selection process by promoting the selection of the stimulated hand, thus potentially beneficial in rehabilitating the paretic hand of patients with stroke.

While the embodiment of non-human body parts is well-established, the precise conditions that facilitate this remain incompletely understood. One critical yet underexplored factor is whether the non-human body part affords greater or lesser functionality than its biological counterpart. This study investigated how the sensorimotor capabilities of a dynamic virtual arm modulate the experience of embodiment. Given that childrens body representations are widely considered to be more flexible than those of adults, they may be especially suited to embodying functionally altered virtual bodies. To test this, both child and adult participants engaged in goal-directed forward reaching movements with a virtual arm to feed animals within an immersive virtual environment. Reaching functionality was systematically manipulated via changes in visual gain, adjusting the arms length and functionality from a normal (100%) condition, to be slightly reduced (80%); slightly increased (120%); or markedly increased (400%). Our findings reveal that extreme alterations in reaching functionality (400% visual gain) significantly reduced subjective ratings of limb ownership, an effect most evident in adult participants. Despite these perceptual disruptions, participants across all ages adjusted their reach kinematics in ways that reflected an integration of the virtual arms perceived capabilities with the physical limitations of their own bodies. Interestingly, children responded to the altered embodiment with more cautious and less refined movement strategies than adults, suggesting developmental differences in adaptive motor control over non-human bodies. Moreover, across both age groups, exposure to functionally enhanced virtual arms led to increased subjective estimates of reaching affordances, highlighting the influence of altered sensorimotor feedback on perceived action capabilities. Collectively, these results demonstrate that the sense of body ownership, the accuracy of body representations, and the properties of sensorimotor control are closely associated with bodily function. Moreover, while children may exhibit greater tolerance to functional alterations in terms of perceived ownership, they do not show superior motor control. These findings reveal that sensorimotor function and developmental factors interact to shape the boundaries of embodiment in virtual contexts.

Motor adaptation arises from multiple learning mechanisms, including use-dependent learning (UDL) driven by repetition and implicit error-based learning (EBL) driven by motor prediction errors. Although both mechanisms contribute implicitly to shaping movement execution, whether these two mechanisms interact remains unclear. The present study used an error-clamp (EC) task to isolate implicit EBL and directly examined whether UDL affects subsequent implicit adaptation. Participants first performed extensive single-target repetitive reaches to induce use-dependent biases and then immediately transitioned to an EC task. We manipulated whether the UDL bias was aligned with, opposed to, or neutral toward the to-be-adapted direction in three separate groups of participants. Results demonstrate that UDL robustly shifted the initial state of adaptation, but did not alter the learning dynamics: all groups showed comparable trial-by-trial adaptation and converged to similar asymptotic levels despite their different initial states. These findings support an independent rather than an interactive relationship between UDL and implicit EBL, highlighting that execution-level motor learning components are additive.

Motor skill learning is the process of developing new movements with practice until they can be performed automatically. This necessitates the interaction of high-level cognitive processes with low-level sensorimotor mechanisms. Skill learning involves not only changes in the motor system but also proprioception (position sense). Proprioceptive deficits increase variability and decrease accuracy of movement. The somatosensory cortex, where low-level proprioception is processed, is known to play a role in motor skill learning, but the involvement of higher-level proprioceptive regions is unclear. Dorsolateral prefrontal cortex (DLPFC) has been linked to the high-level early stages of motor learning, and indirectly to proprioception. Supramarginal gyrus (SMG), an interface area between motor and sensory cortices, has been linked to higher-order proprioceptive processing. In this study, we asked how activity in DLPFC and SMG influences motor skill learning. Participants learned an upper limb motor skill designed to be spatially complex and dependent on proprioception: tracing a two-dimensional maze as accurately as possible within the desired speed range, using a KINARM Endpoint robotic manipulandum. Proprioceptive acuity (sensitivity and bias) was assessed before and after continuous theta burst transcranial magnetic stimulation (cTBS) was applied to inhibit activity in DLPFC, SMG, or Sham. To measure motor skill, movement accuracy and variability were examined at the trained speed as well as at a faster (more difficult) and a slower (easier) speed. Skill was assessed before and after cTBS, after 40 training trials (early learning) and after another 80 trials (late learning). All three groups showed improvements in movement accuracy and variance, indicating they learned the maze tracing skill. However, the Sham group improved movement variability at the faster speed significantly more than the DLPFC or SMG groups did. This suggests that DLPFC and SMG are important for the more challenging aspects of motor skill learning, consistent with their association with higher-level proprioceptive function.

Dyad practice of complex motor skills, characterized by two learners alternating between physical and observational practice, can yield better motor outcomes and reduce practice time compared to physical practice alone. It is unknown if the superior effects of dyad practice on motor learning extend to proprioceptive learning. Forty-two healthy participants (18-35 years) were randomized into three groups (n=14 each): Dyad practice, physical practice with rest (PP-rest), and physical practice without rest (PP-no rest). Participants practiced a 2 degree-of-freedom gamified wrist movement task for 20 minutes using a custom-made wrist robotic device. Wrist position sense acuity was assessed before (baseline) and 24 hours after the end of training (retention), using the Just-noticeable-difference (JND) threshold and Uncertainty. Only the PP-no rest group exhibited significantly lower JND thresholds at retention compared to baseline (t(13)=2.44; p= 0.03, Hedges g=0.70). There were no differences in position sense Uncertainty within or between groups. Dyad practice may yield superior gains in motor performance, but this did not translate into comparable gains in proprioceptive acuity. A possible explanation for these findings is that the recruitment of explicit motor learning mechanisms during dyad motor skill practice does not enhance the implicit learning mechanisms underlying proprioceptive learning. HighlightsO_LIDyad practice (DP) may yield superior motor gains compared to physical practice C_LIO_LIDP does not yield superior proprioceptive gains compared to physical practice C_LIO_LIIntensive physical practice yields the largest gains in position sense acuity C_LI

Reward can increase the speed and accuracy of movements in both simple and sequential reaching tasks. Two mechanisms are thought to be responsible for this: an increase in maximum velocity, due to increased muscle stiffness, resulting in faster, but energetically inefficient, individual movements; or coarticulation - the blending of sub-movements into single, smoother, more energetically efficient movements. Older adults have shown reduced sensitivity to reward in decision paradigms, but there is little research relating reward and motor performance in older adults. Using a novel online sequential reaching task, we compared the effects of reward on motor performance between young (18-23 years) and older (65-79 years) participants. We found that movement time decreased across training in all groups, and reward invigorated this decrease in both age groups. This suggests that reward is a viable facilitator of motor performance to compensate for age-related motor decline and has the potential for use in the design of rehabilitation programmes for age-related motor deficits or disease.

Considerable resources are being invested to provide bidirectional control of substitutive and augmentative motor interfaces through artificial somatosensory feedback. Here, we investigated whether intrinsic somatosensory information, from body part(s) proportionally controlling an augmentation device, can be utilised to infer the devices state and position, to better support motor control and learning. In a placebo-controlled design, we used local anaesthetic to attenuate somatosensory inputs to the big toes while participants learned to operate a toe-controlled robotic extra finger (Third Thumb) using pressure sensors. Motor learning outcomes were compared against a control group who received sham anaesthetic. The availability of somatosensory cues about the amount of exerted pressure generally facilitated acquisition, retention and transfer of motor skills, and performance under cognitive load. Motor performance was not impaired by anaesthesia when tasks involved close collaboration with the biological fingers, indicating that the brain could close the gap of the missing pressure signals by alternative means, including feedback from other body parts involved in the motor task. Together, our findings demonstrate that there are intrinsic natural avenues to provide surrogate position information to support motor control of an artificial body part, beyond artificial extrinsic signalling.

A subset of bilateral tasks requires one arm to perform a stabilizing role while the other completes a movement, such as slicing a loaf of bread. Visual attention during bilateral tasks has previously been examined with bilateral reaching tasks, demonstrating that visual attention switches between the two target locations. The goal of this study was to characterize visual attention during a cooperative mechanically coupled bilateral "stabilizing and reach" task to determine how visual attention is divided between the two limbs when one limb plays a stabilizing role. Twenty-six healthy young adults completed a robotic task in which the hands were coupled with a haptic spring. Participants were instructed to keep one hand stationary in a target while they reached for a target with the other hand, thus stretching the spring and applying a force to both arms. We found that individuals primarily fixated their gaze on the reaching target, only fixating on the stabilizing target for approximately 10% of the reaching time. Longer fixations on the reaching target were associated with faster reaching times, while longer fixations on the stabilizing target were associated with slower reaching times. While the performance of the stabilizing hand differed between the dominant and non-dominant limbs, visual strategies did not vary based on which hand was stabilizing. These results demonstrate that unlike bilateral reaching tasks in which the eyes frequently saccade between the two targets, visual guidance is primarily used for the reaching hand while minimal overt visual attention is directed to the stabilizing hand.

Cybersickness is a major barrier to the widespread adoption of virtual reality (VR), yet its underlying neurophysiological mechanisms remain poorly understood. This study investigated the relationship between vestibulomotor weighting and cybersickness. Vestibulomotor weighting was quantified using electrical vestibular stimulation (EVS), with coherence and gain between the EVS input and medial-lateral center-of-pressure (ML-CoP) responses indexing the contribution of vestibular input to postural control. Thirty-eight healthy young adults (females n=21, males n=17) completed a standing VR rollercoaster task while receiving continuous stochastic EVS (0-25 Hz; {+/-}4.5 mA), with ML-CoP responses recorded using a force plate. Cybersickness was assessed using the Fast Motion Sickness Scale (FMS) and Simulator Sickness Questionnaire, and participants were classified as non-sick (FMS < 5), medium-sick (FMS [&ge;] 5), or high-sick (terminated the VR exposure early due to intolerance). Baseline EVS-ML-CoP coherence across 2.5-8 Hz was significantly greater in high-sick than in non-sick participants, indicating elevated vestibulomotor weighting in individuals who developed symptoms. During VR exposure, coherence declined over time in symptomatic groups (mean slope = -0.0027 for medium-sick), whereas non-sick participants maintained consistently low coherence (mean slope = -0.0005). Despite this reduction in vestibular coupling, postural sway increased in the high-sick group relative to the medium-and non-sick groups (+29% vs. -7% and -30% change in ML-CoP RMS, respectively), while vestibular-evoked response amplitude decreased (gain reduced by 64% across 2.5-3.5 Hz). These findings indicate that greater baseline vestibulomotor weighting was associated with increased susceptibility to cybersickness, whereas reductions in vestibular contributions during VR with EVS reflected adaptive reweighting that was insufficient to prevent instability and symptom progression. Together, the results highlight baseline sensory reliance as a key determinant of cybersickness vulnerability and suggest that reweighting during exposure plays a secondary, mitigating role. New and NoteworthyWe provide the first evidence that baseline vestibulomotor weighting predicts susceptibility to cybersickness in virtual reality and is dynamically reduced during exposure. Using electrical vestibular stimulation, we show that symptomatic individuals begin with greater reliance on vestibular input for postural control and progressively downweight these signals in response to sensory conflict.

Stabilizing the upright posture of the trunk relies on vestibular and proprioceptive afference. Previous studies found that the feedback responses to sensory afference vary between postures and tasks. We investigated whether and how vestibular and proprioceptive afference contribute to trunk stabilization during different postural tasks, and during walking at different speeds. Twelve healthy adults performed tasks in a random order: sitting, standing on the right foot or both feet, and treadmill walking at five speeds: 0.8, 2.0, 3.2, 4.3 and 5.5 km/h, while exposed to unilateral muscle vibration on the right paraspinal muscles at the level of the second lumbar vertebra, or to a step-like electrical vestibular stimulation (EVS) with the anode behind the left ear. The mediolateral displacements of markers at the sixth thoracic level and sacrum in the global coordinate system were used to evaluate the responses to sensory stimulation. No significant responses to EVS at T6 and sacrum level were found in sitting and standing. Responses to muscle vibration were significant and differed between unipedal standing compared to sitting and bipedal standing. The latter suggests a different interpretation of the sensation of muscle lengthening in these postures. During walking, the magnitude of the responses to both stimuli increased from very slow speeds to moderate speeds. From moderate to higher speeds, responses to muscle vibration decreased, whereas responses to EVS plateaued. These findings suggest speed-dependent modulation of vestibular and proprioceptive contributions in trunk stabilization during walking. Summary statementBy applying electrical vestibular stimulation and muscle vibration, we found that how vestibular and proprioceptive signals are used for trunk stabilization differs between postural tasks and walking speeds.

Tactile sensitivity drops in a moving than static limb due to a combination of central, predictive mechanisms and peripheral effects. This suppression is dynamically modulated during movement, as shown during upper-limb actions, yet little is known about its implication during complex lower-limb movements. We investigated tactile sensitivity during naturalistic kicking by delivering vibrotactile probe stimuli to the balancing and kicking feet at different movement phases. In Experiment 1, participants kicked a suspended ball while tactile sensitivity was probed at movement onset, mid-swing, ball contact, and after-contact. Results revealed distinct modulation patterns in each foot. When transitioning from bipedal to unipedal stance, tactile processing at the balancing foot was particularly suppressed but at the kicking foot it improved, suggesting concurrent modulation across the two legs depending on their motor function. Tactile sensitivity remained rather invariant at other time points, but was strongly suppressed on the kicking foot at the moment of ball contact. The strength of this suppression correlated with kicking speed, which could reflect either stronger predictive control or stronger peripheral processes that mask the vibrotactile probe. To test these, a new set of participants held their foot still while a ball collided with it at high or low speed. Suppression was greater with faster ball contacts, revealing that peripheral processes can modulate tactile processing. These findings show that lower-limb tactile sensitivity during goal-directed leg movements can be concurrently modulated across the legs, presumably reflecting an interplay between central sensorimotor processes guiding the movement and peripheral processes affecting sensitivity. Significance StatementTactile sensitivity is known to fluctuate during movement, but little is understood about how it is tuned during complex lower-limb actions. Using a naturalistic ball-kicking task, we reveal distinct modulation patterns in the balancing and kicking feet, showing that postural and guiding demands dynamically shape tactile sensitivity. We further demonstrate that the strength of tactile modulation is influenced by peripheral processes, such as tactile masking. These findings highlight that lower-limb tactile processing can be flexibly and concurrently modulated in the two legs during state transitions that impose different sensorimotor demands for complex natural behavior.