Sensory modulation of movement, posture and locomotion

Cross-frequency modulation of postural fluctuations and scalp EEG in older adults: error amplification feedback for rapid balance adjustments

Virtual error amplification (VEA) in visual feedback enhances attentive control over postural stability, although the neural mechanisms are still debated. This study investigated the distinct cortical control of unsteady stance in older adults using VEA through cross-frequency modulation of postural fluctuations and scalp EEG. Thirty-seven community-dwelling older adults (68.1 ± 3.6 years) maintained an upright stance on a stabilometer while receiving either VEA or real error feedback. Along with postural fluctuation dynamics, phase-amplitude coupling (PAC) and amplitude-amplitude coupling (AAC) were analyzed for postural fluctuations under 2 Hz and EEG sub-bands (theta, alpha, and beta). The results revealed a higher mean frequency of the postural fluctuation phase (p = .005) and a greater root mean square of the postural fluctuation amplitude (p = .003) with VEA compared to the control condition. VEA also reduced PAC between the postural fluctuation phase and beta-band EEG in the left frontal (p = .009), sensorimotor (p = .002), and occipital (p = .018) areas. Conversely, VEA increased the AAC of posture fluctuation amplitude and beta-band EEG in FP2 (p = .027). Neither theta nor alpha band PAC or AAC were affected by VEA. VEA optimizes postural strategies in older adults during stabilometer stance by enhancing visuospatial attentive control of postural responses and facilitating the transition of motor states against postural perturbations through a disinhibitory process. Incorporating VEA into virtual reality technology is advocated as a valuable strategy for optimizing therapeutic interventions in postural therapy, particularly to mitigate the risk of falls among older adults.

Insufficiencies in sensory systems reweighting is associated with walking impairment severity in chronic stroke: an observational cohort study

Background

Walking and balance impairment are common sequelae of stroke and significantly impact functional independence, morbidity, and mortality. Adequate postural stability is needed for walking, which requires sufficient integration of sensory information between the visual, somatosensory, and vestibular centers. "Sensory reweighting" describes the normal physiologic response needed to maintain postural stability in the absence of sufficient visual or somatosensory information and is believed to play a critical role in preserving postural stability after stroke. However, the extent to which sensory reweighting successfully maintains postural stability in the chronic stages of stroke and its potential impact on walking function remains understudied.

Methods

In this cross-sectional study, fifty-eight community-dwelling ambulatory chronic stroke survivors underwent baseline postural stability testing during quiet stance using the modified Clinical test of Sensory Interaction in Balance (mCTSIB) and assessment of spatiotemporal gait parameters.

Results

Seventy-six percent (45/58) of participants showed sufficient sensory reweighting with visual and somatosensory deprivation for maintaining postural stability, albeit with greater postural sway velocity indices than normative data. In contrast, survivors with insufficient reweighting demonstrated markedly slower overground walking speeds, greater spatiotemporal asymmetry, and limited acceleration potential.

Conclusion

Adequate sensory system reweighting is essential for chronic stroke survivors' postural stability and walking independence. Greater emphasis should be placed on rehabilitation strategies incorporating multisensory system integration testing and strengthening as part of walking rehabilitation protocols. Given its potential impact on outcomes, walking rehabilitation trials may benefit from incorporating formal postural stability testing in design and group stratification.

Movement-related tactile gating in blindness

When we perform an action, self-elicited movement induces suppression of somatosensory information to the cortex, requiring a correct motor-sensory and inter-sensory (i.e. cutaneous senses, kinesthesia, and proprioception) integration processes to be successful. However, recent works show that blindness might impact some of these elements. The current study investigates the effect of movement on tactile perception and the role of vision in this process. We measured the velocity discrimination threshold in 18 sighted and 18 blind individuals by having them perceive a sequence of two movements and discriminate the faster one in passive and active touch conditions. Participants' Just Noticeable Difference (JND) was measured to quantify their precision. Results showed a generally worse performance during the active touch condition compared to the passive. In particular, this difference was significant in the blind group, regardless of the blindness duration, but not in the sighted one. These findings suggest that the absence of visual calibration impacts motor-sensory and inter-sensory integration required during movement, diminishing the reliability of tactile signals in blind individuals. Our work spotlights the need for intervention in this population and should be considered in the sensory substitution/reinforcement device design.

Role of selective attention in fatigue in neurological disorders

BACKGROUND

Chronic fatigue is a significant symptom in several diseases including traumatic and degenerative neurological disorders. While several studies have investigated the correlates of chronic fatigue, there is as yet no unifying framework to explain chronic fatigue.

METHODS

In this narrative review, I investigate the role of selective attention in the development of chronic fatigue and discuss results within the framework of the sensory attenuation model of fatigue, which posits that fatigue is the phenomenological output of altered attention to sensory input. Following a short introduction of this framework, I present results from investigations that address attentional mechanisms in fatigue in multiple sclerosis, stroke, traumatic brain injury and Parkinson's disease.

RESULTS

Attention was quantified in all four disease models using a variety of outcome measures, including behavioural, neurophysiological, structural and functional brain connectivity. The range of measures precluded direct comparison of results across disease conditions; however, in all four disease models there was evidence of poor selective attention that explained levels of chronic fatigue, supporting the sensory attenuation model of fatigue as a disease-independent mechanism of fatigue. Evidence was lacking to draw any conclusions about the direction of causality.

CONCLUSION

The role of selective attention in development of fatigue is indicated. Future studies must focus on establishing causality and exploring attentional circuitry as a potential therapeutic target.

Youth with Cerebral Palsy Display Abnormal Somatosensory Cortical Activity During a Haptic Exploration Task

There are numerous clinical reports that youth with cerebral palsy (CP) have proprioceptive, stereognosis and tactile discrimination deficits. The growing consensus is that the altered perceptions in this population are attributable to aberrant somatosensory cortical activity seen during stimulus processing. It has been inferred from these results that youth with CP likely do not adequately process ongoing sensory feedback during motor performance. However, this conjecture has not been tested. Herein, we address this knowledge gap using magnetoencephalographic (MEG) brain imaging by applying electrical stimulation to the median nerve of youth with CP (N = 15, Age = 15.8 ± 0.83 yrs, Males = 12, MACS levels I-III) and neurotypical (NT) controls (N = 18, Age = 14.1 ± 2.4 yrs, Males = 9) while at rest (i.e., passive) and during a haptic exploration task. The results illustrated that the somatosensory cortical activity was reduced in the group with CP compared to controls during the passive and haptic conditions. Furthermore, the strength of the somatosensory cortical responses during the passive condition were positively associated with the strength of somatosensory cortical responses during the haptic condition (r = 0.75, P = 0.004). This indicates that the aberrant somatosensory cortical responses seen in youth with CP during rest are a good predictor of the extent of somatosensory cortical dysfunction during the performance of motor actions. These data provide novel evidence that aberrations in somatosensory cortical function in youth with CP likely contribute to the difficulties in sensorimotor integration and the ability to effectively plan and execute motor actions.

Aberrant movement-related somatosensory cortical activity mediates the extent of the mobility impairments in persons with cerebral palsy

KEY POINTS

Persons with cerebral palsy (CP) have reduced somatosensory cortical responses at rest and during movement. The somatosensory cortical responses during movement mediate the relationship between the somatosensory cortical responses at rest and mobility. Persons with CP may have altered sensorimotor feedback that ultimately contributes to impaired mobility.

ABSTRACT

There are numerous clinical reports that persons with cerebral palsy (CP) have proprioceptive, stereognosis and tactile discrimination deficits. The current consensus is that these altered perceptions are attributable to aberrant somatosensory cortical activity. It has been inferred from these data that persons with CP do not adequately process ongoing sensory feedback during motor actions, which accentuates the extent of their mobility impairments. However, this hypothesis has yet to be directly tested. We used magnetoencephalographic (MEG) brain imaging to address this knowledge gap by quantifying the somatosensory dynamics evoked by applying electrical stimulation to the tibial nerve in 22 persons with CP and 25 neurotypical (NT) controls while at rest and during an ankle plantarflexion isometric force motor task. We also quantified the spatiotemporal gait biomechanics of participants outside the scanner. Consistent with the literature, our results confirmed that the strength of somatosensory cortical activity was weaker in the persons with CP compared to the NT controls. Our results also showed that the strength of the somatosensory cortical responses were significantly weaker during the isometric ankle force task than at rest. Most importantly, our results showed that the strength of somatosensory cortical activity during the ankle plantarflexion force production task mediated the relationship between somatosensory cortical activity at rest and both walking velocity and step length. These results suggest that youth with CP have aberrant somatosensory cortical activity during isometric force generation, which ultimately contributes to the extent of mobility impairments seen in this patient population. Abstract figure legend Magnetoencephalographic brain imaging was used to determine the effect of sensory feedback during movement on mobility in persons with cerebral palsy. Persons with cerebral palsy had reduced somatosensory cortical activity at rest and during movement compared with their neurotypical peers. Further, the somatosensory cortical activity during movement mediated the relationship between somatosensory cortical activity at rest and mobility. These results indicate that difficulties in sensorimotor integration may contribute to the mobility impairments seen in this patient population. This article is protected by copyright. All rights reserved.

Predictive attenuation of touch and tactile gating are distinct perceptual phenomena

In recent decades, research on somatosensory perception has led to two important observations. First, self-generated touches that are predicted by voluntary movements become attenuated compared with externally generated touches of the same intensity (attenuation). Second, externally generated touches feel weaker and are more difficult to detect during movement than at rest (gating). At present, researchers often consider gating and attenuation the same suppression process; however, this assumption is unwarranted because, despite more than 40 years of research, no study has combined them in a single paradigm. We quantified how people perceive self-generated and externally generated touches during movement and rest. We show that whereas voluntary movement gates the precision of both self-generated and externally generated touch, the amplitude of self-generated touch is robustly attenuated compared with externally generated touch. Furthermore, attenuation and gating do not interact and are not correlated, and we conclude that they represent distinct perceptual phenomena.

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We tested the perception of self-generated and external touch during movement and rest

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The intensity of self-generated touch is reduced during movement and rest (attenuation)

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The precision of self-generated and external touch is reduced during movement (gating)

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Attenuation and gating neither interact nor correlate, and are distinct phenomena

The implicit sense of agency is not a perceptual effect but is a judgment effect

The sense of agency (SoA) is characterized as the sense of being the causal agent of one's own actions, and it is measured in two forms: explicit and implicit. In the explicit SoA experiments, the participants explicitly report whether they have a sense of control over their actions or whether they or somebody else is the causal agent of seen actions; the implicit SoA experiments study how do participants' agentive or voluntary actions modify perceptual processes (like time, vision, tactility, and audition) without directly asking the participants to explicitly think about their causal agency or sense of control. However, recent implicit SoA literature reported contradictory findings of the relationship between implicit SoA reports and agency states. Thus, I argue that the purported implicit SoA reports are not agency-driven perceptual effects per se but are judgment effects, by showing that (a) the typical operationalizations in implicit SoA domain lead to perceptual uncertainty on the part of the participants, (b) under uncertainty, participants' implicit SoA reports are due to heuristic judgments which are independent of agency states, and (c) under perceptual certainty, the typical implicit SoA reports might not have occurred at all. Thus, I conclude that the instances of implicit SoA are judgments (or response biases)-under uncertainty-rather than perceptual effects.

Assessing kinesthetic proprioceptive function of the upper limb: a novel dynamic movement reproduction task using a robotic arm

Background

Proprioception refers to the perception of motion and position of the body or body segments in space. A wide range of proprioceptive tests exists, although tests dynamically evaluating sensorimotor integration during upper limb movement are scarce. We introduce a novel task to evaluate kinesthetic proprioceptive function during complex upper limb movements using a robotic device. We aimed to evaluate the test–retest reliability of this newly developed Dynamic Movement Reproduction (DMR) task. Furthermore, we assessed reliability of the commonly used Joint Reposition (JR) task of the elbow, evaluated the association between both tasks, and explored the influence of visual information (viewing arm movement or not) on performance during both tasks.

Methods

During the DMR task, participants actively reproduced movement patterns while holding a handle attached to the robotic arm, with the device encoding actual position throughout movement. In the JR task, participants actively reproduced forearm positions; with the final arm position evaluated using an angle measurement tool. The difference between target movement pattern/position and reproduced movement pattern/position served as measures of accuracy. In study 1 (N = 23), pain-free participants performed both tasks at two test sessions, 24-h apart, both with and without visual information available (i.e., vision occluded using a blindfold). In study 2 (N = 64), an independent sample of pain-free participants performed the same tasks in a single session to replicate findings regarding the association between both tasks and the influence of visual information.

Results

The DMR task accuracy showed good-to-excellent test–retest reliability, while JR task reliability was poor: measurements did not remain sufficiently stable over testing days. The DMR and JR tasks were only weakly associated. Adding visual information (i.e., watching arm movement) had different performance effects on the tasks: it increased JR accuracy but decreased DMR accuracy, though only when the DMR task started with visual information available (i.e., an order effect).

Discussion

The DMR task’s highly standardized protocol (i.e., largely automated), precise measurement and involvement of the entire upper limb kinetic chain (i.e., shoulder, elbow and wrist joints) make it a promising tool. Moreover, the poor association between the JR and DMR tasks indicates that they likely capture unique aspects of proprioceptive function. While the former mainly captures position sense, the latter appears to capture sensorimotor integration processes underlying kinesthesia, largely independent of position sense. Finally, our results show that the integration of visual and proprioceptive information is not straightforward: additional visual information of arm movement does not necessarily make active movement reproduction more accurate, on the contrary, when movement is complex, vision appears to make it worse.

Sensory Attenuation in Sport and Rehabilitation: Perspective from Research in Parkinson's Disease

People with Parkinson's disease (PD) experience motor symptoms that are affected by sensory information in the environment. Sensory attenuation describes the modulation of sensory input caused by motor intent. This appears to be altered in PD and may index important sensorimotor processes underpinning PD symptoms. We review recent findings investigating sensory attenuation and reconcile seemingly disparate results with an emphasis on task-relevance in the modulation of sensory input. Sensory attenuation paradigms, across different sensory modalities, capture how two identical stimuli can elicit markedly different perceptual experiences depending on our predictions of the event, but also the context in which the event occurs. In particular, it appears as though contextual information may be used to suppress or facilitate a response to a stimulus on the basis of task-relevance. We support this viewpoint by considering the role of the basal ganglia in task-relevant sensory filtering and the use of contextual signals in complex environments to shape action and perception. This perspective highlights the dual effect of basal ganglia dysfunction in PD, whereby a reduced capacity to filter task-relevant signals harms the ability to integrate contextual cues, just when such cues are required to effectively navigate and interact with our environment. Finally, we suggest how this framework might be used to establish principles for effective rehabilitation in the treatment of PD.

Hyperhomocysteinemia alters cytokine gene expression, cytochrome c oxidase activity and oxidative stress in striatum and cerebellum of rodents

AIMS

Homocysteine has been linked to neurodegeneration and motor function impairments. In the present study, we evaluate the effect of chronic mild hyperhomocysteinemia on the motor behavior (motor coordination, functional performance, and muscular force) and biochemical parameters (oxidative stress, energy metabolism, gene expression and/or protein abundance of cytokine related to the inflammatory pathways and acetylcholinesterase) in the striatum and cerebellum of Wistar male rats.

MAIN METHODS

Rodents were submitted to one injection of homocysteine (0.03 μmol Hcy/g of body weight) between 30th and 60th postnatal days twice a day. After hyperhomocysteinemia induction, rats were submitted to horizontal ladder walking, beam balance, suspension, and vertical pole tests and/or euthanized to brain dissection for biochemical and molecular assays.

KEY FINDINGS

Chronic mild hyperhomocysteinemia did not alter motor function, but induced oxidative stress and impaired mitochondrial complex IV activity in both structures. In the striatum, hyperhomocysteinemia decreased TNF-α gene expression and increased IL-1β gene expression and acetylcholinesterase activity. In the cerebellum, hyperhomocysteinemia increased gene expression of TNF-α, IL-1β, IL-10, and TGF-β, while the acetylcholinesterase activity was decreased. In both structures, hyperhomocysteinemia decreased acetylcholinesterase protein abundance without altering total p-NF-κB, NF-κB, Nrf-2, and cleaved caspase-3.

SIGNIFICANCE

Chronic mild hyperhomocysteinemia compromises several biochemical/molecular parameters, signaling pathways, oxidative stress, and chronic inflammation in the striatum and cerebellum of rats without impairing motor function. These alterations may be related to the mechanisms in which hyperhomocysteinemia has been linked to movement disorders later in life and neurodegeneration.

Movement-induced hypoalgesia: behavioral characteristics and neural mechanisms

Pain is essential for our survival because it helps to protect us from severe injuries. Nociceptive signals may be exacerbated by continued physical activities but can also be interrupted or overridden by physical movements, a process called movement-induced hypoalgesia. Several neural mechanisms have been proposed to account for this effect, including the reafference principle, non-nociceptive interference, and top-down descending modulation. Given that the hypoalgesic effects of these mechanisms temporally overlap during movement execution, it is unclear whether movement-induced hypoalgesia results from a single neural mechanism or from the joint action of multiple neural mechanisms. To address this question, we conducted five experiments on 129 healthy humans by assessing the hypoalgesic effect after movement execution. Combining psychophysics and electroencephalographic recordings, we quantified the relationship between the strength of voluntary movement and the hypoalgesic effect, as well as the temporal and spatial characteristics of the hypoalgesic effect. Our findings demonstrated that movement-induced hypoalgesia results from the joint action of multiple neural mechanisms. This investigation is the first to disentangle the distinct contributions of different neural mechanisms to the hypoalgesic effect of voluntary movement, which extends our understanding of sensory attenuation arising from voluntary movement and may prove instrumental in developing new strategies for pain management.

Similar effects of fatigue induced by a repetitive pointing task on local and remote light touch and pain perception in men and women

Background

Women involved in repetitive, fatiguing, jobs develop more neck and/or shoulder musculoskeletal disorders than men. Sex differences in the pain response to exercise could contribute to the higher prevalence of neck/shoulder musculoskeletal disorders in women. The objective of this study was to assess sex differences in pain sensitivity following a fatiguing upper limb task. Relationships between measures of fatigue and of the sensitivity to nociceptive and to non-nociceptive stimulations were also explored.

Methods

Thirty healthy adults (15 women) performed a fatiguing repetitive pointing task with their dominant arm. Upper limb electromyography was recorded from the dominant upper trapezius, anterior deltoid and bicep brachii and from the contralateral tibialis anterior. Before and immediately after the repetitive pointing task, pressure pain and light touch sensitivity thresholds were measured over the same muscles.

Results

Electromyographic signs of fatigue were observed only in the anterior deltoid and biceps brachii muscles. Pressure pain thresholds over both muscles increased slightly (effect size ≤ 0.34), but no changes occurred over the upper trapezius and the tibialis anterior. Light touch thresholds increased moderately to importantly after the repetitive pointing task over all four muscles (effect sizes = 0.58 to 0.87). No sex differences were observed in any sensory variable. Moreover, no or weak correlations (r = -0.27 to 0.39) were observed between electromyographical signs of fatigue, light touch threshold and pressure pain threshold variables.

Conclusions

We observed sex-independent effects of a repetitive upper limb task on the sensitivity to painful and to nonpainful stimuli. Moreover, the hypoalgesia induced by the repetitive pointing task was weak and localized, and did not directly correlate with the induced muscle fatigue. Results suggest that fatigue-related changes in the sensitivity to noxious and innocuous stimuli could not explain women’s greater prevalence of neck/shoulder musculoskeletal disorders.

Gamma somatosensory cortical oscillations are attenuated during the stance phase of human walking

It is well appreciated that processing of peripheral feedback by the somatosensory cortices plays a prominent role in the control of human motor actions like walking. However, very few studies have actually quantified the somatosensory cortical activity during walking. In this investigation, we used electroencephalography (EEG) and beamforming source reconstruction methods to quantify the frequency specific neural oscillations that are induced by an electrical stimulation that is applied to the right tibial nerve under the following experimental conditions: 1) sitting, 2) standing in place, and 3) treadmill walking. Our experimental results revealed that the peripheral stimulation induced a transient increase in theta-alpha (4-12 Hz; 50-350 ms) and gamma (40-80 Hz; 40-100 ms) activity in the leg region of the contralateral somatosensory cortices. The strength of the gamma oscillations were similar while sitting and standing, but were markedly attenuated while walking. Conversely, the strength of the theta-alpha oscillations were not different across the respective experimental conditions. Prior research suggests the afferent feedback from the Ia sensory fibers are likely attenuated during walking, while afferent feedback from the β polysynaptic sensory fibers are not. We suggest that the attenuated gamma oscillations seen during walking reflect the gating of the Ia afferents, while the similarity of theta-alpha oscillations across the experimental conditions is associated with the afferent information from the type II (Aα and β) polysynaptic sensory fibers.

Ankle proprioception during gait in individuals with incomplete spinal cord injury

INTRODUCTION

Proprioception is known to be affected after a spinal cord injury (SCI). However, it is currently assessed during simple tasks that do not reflect activities of daily living. To better understand how proprioception affects movement, assessing it during a functional sensorimotor task such as walking is therefore of primary importance. Therefore, the objectives of this study were as follows: (a) measure the protocol reliability of a new robotic test in nondisabled controls; (b) evaluate the effect nonlesion-related factors such as sex, age, pain, and gait speed on ankle proprioception; and (c) assess ankle proprioception during walking in individuals with SCI.

METHODS

In the current study, ankle proprioception was assessed during gait in individuals with an incomplete spinal cord injury (iSCI; n = 15) using an electrohydraulic robotized ankle-foot orthosis (rAFO). Ankle proprioceptive threshold was quantified as the participants' ability to detect torque perturbations of varied amplitude applied during swing by the rAFO. In addition, test-retest reliability and the potential effect of nonlesion-related factors (sex, age, pain, and gait speed) were evaluated in nondisabled (ND; n = 65) participants.

RESULTS

During gait, individuals with iSCI had a 53% poorer proprioceptive threshold than ND controls (p < .05). Test-retest reliability was good (ICC = 0.78), and only gait speed affected proprioceptive threshold (p = .018).

CONCLUSION

This study is the first to show that ankle proprioception assessed during gait is impaired in individuals with an iSCI. The developed test can now be used to better characterize proprioception in population with other neurological conditions and has potential to maximize functional recovery during gait training in those populations.

Does the motor system contribute to the perception of changes in objects visual attributes? The neural dynamics of sensory binding by action

The contribution of the motor system to perception has been highlighted in research investigating the effect of performing an action on the conscious processing of information received from the sensory systems. For example, the perceptual temporal asynchrony observed when passively reporting changes in visual object attributes (e.g., colour and position) was found to disappear virtually when the changes resulted from a voluntary motor action. Although the spatio-temporal constraints of sensory binding by action have been broadly investigated, the underlying neural correlates are still largely unknown. In the present study, participants performed temporal order judgments of changes in the colour and position of a visual stimulus, while adapting to a 750 ms delay between a sound (perceptual condition) or the end of a manual reaching action (motor condition), and the visual changes. Behavioural observations indicated that temporal asynchrony (-30.2 ms) decreased in the motor condition (2.7 ms), as a result of sensorimotor adaptation, but not in the perceptual condition (-29.6 ms). EEG-evoked potentials on posterior visual regions showed that early components were altered by sensorimotor adaptation, with in particular a broad reduction in the amplitude of the early P1 component. Furthermore, time-frequency analysis of EEG signals during the 350 ms period preceding the visual changes revealed an increase of the 15-25 Hz frequency band amplitude in the central region and a decrease of the 8-12 Hz frequency band amplitude in the posterior region. Overall the results suggest that sensory binding by action depends on an early top-down modulation of the visual regions by the motor system - in agreement with the pre-activation theory of action-perception coupling - associated with an increase of attentional resources.

Auditory cues for somatosensory targets invoke visuomotor transformations: Behavioral and electrophysiological evidence

Prior to goal-directed actions, somatosensory target positions can be localized using either an exteroceptive or an interoceptive body representation. The goal of the present study was to investigate if the body representation selected to plan reaches to somatosensory targets is influenced by the sensory modality of the cue indicating the target’s location. In the first experiment, participants reached to somatosensory targets prompted by either an auditory or a vibrotactile cue. As a baseline condition, participants also performed reaches to visual targets prompted by an auditory cue. Gaze-dependent reaching errors were measured to determine the contribution of the exteroceptive representation to motor planning processes. The results showed that reaches to both auditory-cued somatosensory targets and auditory-cued visual targets exhibited larger gaze-dependent reaching errors than reaches to vibrotactile-cued somatosensory targets. Thus, an exteroceptive body representation was likely used to plan reaches to auditory-cued somatosensory targets but not to vibrotactile-cued somatosensory targets. The second experiment examined the influence of using an exteroceptive body representation to plan movements to somatosensory targets on pre-movement neural activations. Cortical responses to a task-irrelevant visual flash were measured as participants planned movements to either auditory-cued somatosensory or auditory-cued visual targets. Larger responses (i.e., visual-evoked potentials) were found when participants planned movements to somatosensory vs. visual targets, and source analyses revealed that these activities were localized to the left occipital and left posterior parietal areas. These results suggest that visual and visuomotor processing networks were more engaged when using the exteroceptive body representation to plan movements to somatosensory targets, than when planning movements to external visual targets.

Effect of standing posture on inhibitory postsynaptic potentials in gastrocnemius motoneurons

This study examined the task dependence of sensory inputs on motoneuron excitability by comparing the inhibitory postsynaptic potential (IPSP) evoked by stimulation of the sural nerve between a standing postural task (Free Standing) and a comparable voluntary isometric contraction performed in a supine position (Lying Supine). We hypothesized that there would be a smaller IPSP in standing than in the supine position, based on the task dependence of the ankle plantarflexor activity on the standing task. Ten healthy participants participated in a total of 15 experiments. Single motor unit (MU) firings were recorded with both intramuscular fine-wire electrodes and high-density surface electromyography. Participants maintained the MU discharge at 6-8 Hz in Free Standing or Lying Supine while the right sural nerve was stimulated at random intervals between 1 and 3 s. To evaluate the reflex response, the firing times of the discriminated MUs were used to construct peristimulus time histograms and peristimulus frequencygrams. The sural nerve stimulation resulted in weaker inhibition in Free Standing than in Lying Supine. This finding is discussed in relation to the putative activation of persistent inward currents in standing posture and the task-dependent advantages of overriding inhibitory synaptic inputs to the plantarflexors to maintain the standing posture. NEW & NOTEWORTHY The task-dependent modulation of sensory inputs on motoneuron excitability in standing is not well understood. Evoking an inhibitory postsynaptic potential (IPSP) resulted in a smaller IPSP in gastrocnemius motoneurons in standing than in the supine position. Mildly painful sensory inputs produced weaker motoneuron inhibition in standing, suggesting an imperative to maintain ankle plantarflexion activity for the task of upright stance.

Haptic Cues for Balance: Use of a Cane Provides Immediate Body Stabilization

Haptic cues are important for balance. Knowledge of the temporal features of their effect may be crucial for the design of neural prostheses. Touching a stable surface with a fingertip reduces body sway in standing subjects eyes closed (EC), and removal of haptic cue reinstates a large sway pattern. Changes in sway occur rapidly on changing haptic conditions. Here, we describe the effects and time-course of stabilization produced by a haptic cue derived from a walking cane. We intended to confirm that cane use reduces body sway, to evaluate the effect of vision on stabilization by a cane, and to estimate the delay of the changes in body sway after addition and withdrawal of haptic input. Seventeen healthy young subjects stood in tandem position on a force platform, with eyes closed or open (EO). They gently lowered the cane onto and lifted it from a second force platform. Sixty trials per direction of haptic shift (Touch → NoTouch, T-NT; NoTouch → Touch, NT-T) and visual condition (EC-EO) were acquired. Traces of Center of foot Pressure (CoP) and the force exerted by cane were filtered, rectified, and averaged. The position in space of a reflective marker positioned on the cane tip was also acquired by an optoelectronic device. Cross-correlation (CC) analysis was performed between traces of cane tip and CoP displacement. Latencies of changes in CoP oscillation in the frontal plane EC following the T-NT and NT-T haptic shift were statistically estimated. The CoP oscillations were larger in EC than EO under both T and NT (p < 0.001) and larger during NT than T conditions (p < 0.001). Haptic-induced effect under EC (Romberg quotient NT/T ~ 1.2) was less effective than that of vision under NT condition (EC/EO ~ 1.5) (p < 0.001). With EO cane had little effect. Cane displacement lagged CoP displacement under both EC and EO. Latencies to changes in CoP oscillations were longer after addition (NT-T, about 1.6 s) than withdrawal (T-NT, about 0.9 s) of haptic input (p < 0.001). These latencies were similar to those occurring on fingertip touch, as previously shown. Overall, data speak in favor of substantial equivalence of the haptic information derived from both “direct” fingertip contact and “indirect” contact with the floor mediated by the cane. Cane, finger and visual inputs would be similarly integrated in the same neural centers for balance control. Haptic input from a walking aid and its processing time should be considered when designing prostheses for locomotion.

Development and reliability of a measure evaluating dynamic proprioception during walking with a robotized ankle-foot orthosis, and its relation to dynamic postural control

BACKGROUND

Proprioception is important for proper motor control. As the central nervous system modulates how sensory information is processed during movement (sensory gating), proprioceptive tests performed at rest do not correlate well with performance during dynamic tasks such as walking. Proprioception therefore needs to be assessed during movement execution.

OBJECTIVES

1) To develop a test evaluating the ability to detect movement errors during walking, and its test-retest reliability; 2) to quantify the relationship between proprioceptive threshold (obtained with this new test) and performance in a standardized dynamic balance task.

METHOD

Thirty healthy subjects walked on a treadmill while wearing a robotized ankle-foot orthosis (rAFO) for 2 bouts of 6min on 2 evaluation sessions (test-retest reliability). Force perturbations resisting ankle dorsiflexion during swing were applied to the ankle via the rAFO (150ms duration, variable amplitude). Participants pushed a button when they detected the perturbations. The Star Excursion Balance Test (SEBT) was used to evaluate dynamic balance.

ANALYSIS

Angular differences between perturbed and non-perturbed gait cycles were used to quantify movement error. Detection threshold was defined as the minimal movement error at which 50% of the perturbations were perceived. Intraclass correlation coefficients (ICCs) estimated test-retest reliability, and Pearson coefficients were used to determine the correlation between detection threshold and SEBT.

RESULTS

Detection threshold was 5.31±2.12°. Good reliability (ICC=0.70) and a moderate to strong correlation to SEBT (r=-0.57 to -0.76) were found.

CONCLUSION

Force perturbations produced by the robotized AFO provides a reliable way of evaluating proprioception during walking.

Impaired Automatic Arm Movements in Obstetric Brachial Plexus Palsy Suggest a Central Disorder

The authors aimed to find evidence for a central component of the impairment of movement of the affected arm in children with obstetric brachial plexus palsy. The authors performed a cross-sectional study in 19 children (median age 5 years) with obstetric brachial plexus palsy who were able to voluntarily abduct their affected arm beyond 90 degrees. They were asked to perform 4 tasks designed to provoke automatic arm movements to maintain balance. The authors assumed automatic motor programming to be impaired when 2 of 3 investigators agreed using video recordings that the affected arm did not abduct beyond 90 degrees while the unaffected arm did. Children abducted the affected arm less often than the healthy one (generalized binary logistic model of all 4 tasks, P = .001). The deficit during automatic arm abduction was not observed during voluntary movements and therefore cannot be explained by a peripheral deficit, suggesting a central component.