Correlation analysis of proprioceptive acuity in ipsilateral position-matching and velocity-discrimination

In order to plan and control movements the central nervous system (CNS) needs to continuously keep track of the state of the musculoskeletal system. Therefore the CNS constantly uses sensory input from mechanoreceptors in muscles, joints and skin to update information about body configuration on different levels of the CNS. On the conscious level, such representations constitute proprioception. Different tests for assessment of proprioceptive acuity have been described. However, it is unclear if the proprioceptive acuity measurements in these tests correlate within subjects. By using both uni- and multivariate analysis we compared proprioceptive acuity in different variants of ipsilateral active and passive limb position-matching and ipsilateral passive limb movement velocity-discrimination in a group of healthy subjects. The analysis of the position-matching data revealed a higher acuity of matching for active movements in comparison to passive ones. The acuity of matching was negatively correlated to movement extent. There was a lack of correlation between proprioceptive acuity measurements in position-matching and velocity-discrimination.     

Replication and discrimination of limb movement velocity

It is well known that proprioception is composed of the senses of movement and position. Whereas tests of position sense are quite commonly used, tests of the acuity in perception of movement velocity are scarce. In the present study we examined some novel tests for assessing the sense of limb movement velocity, involving replication and discrimination of single-joint movement velocity. Specifically, we investigated: (1) whether replication of limb movement velocity is more accurate following active criterion movements as compared to passive; (2) whether antagonist muscle contraction during passive limb movement enhances velocity discrimination; (3) how criterion movement velocity influences response accuracy; (4) the relationship between movement velocity and movement extent during velocity replication; and (5) whether subjects really base discrimination of velocities on perceived velocity. Sixteen healthy subjects participated in four tests (I-IV). For each test, horizontal abductions were performed about the right glenohumeral joint from the sagittal plane. The subjects were required to actively replicate the velocity of either an active (Test I) or passive (Test II) criterion movement, or judge whether a passive/semipassive (passive during antagonist muscle contraction) movement was faster or slower than a previous passive/semipassive criterion movement (Test III/IV). The results revealed higher response accuracy for Test I compared to Test II and for slower movements compared to faster, but no difference in response accuracy between Test III and IV. For velocity discrimination, the analysis revealed that the subjects based their judgment on the difference between criterion and comparison velocity rather than time or extent cues.     

Replication and discrimination of limb movement velocity

It is well known that proprioception is composed of the senses of movement and position. Whereas tests of position sense are quite commonly used, tests of the acuity in perception of movement velocity are scarce. In the present study we examined some novel tests for assessing the sense of limb movement velocity, involving replication and discrimination of single-joint movement velocity. Specifically, we investigated: (1) whether replication of limb movement velocity is more accurate following active criterion movements as compared to passive; (2) whether antagonist muscle contraction during passive limb movement enhances velocity discrimination; (3) how criterion movement velocity influences response accuracy; (4) the relationship between movement velocity and movement extent during velocity replication; and (5) whether subjects really base discrimination of velocities on perceived velocity. Sixteen healthy subjects participated in four tests (I-IV). For each test, horizontal abductions were performed about the right glenohumeral joint from the sagittal plane. The subjects were required to actively replicate the velocity of either an active (Test I) or passive (Test II) criterion movement, or judge whether a passive/semipassive (passive during antagonist muscle contraction) movement was faster or slower than a previous passive/semipassive criterion movement (Test III/IV). The results revealed higher response accuracy for Test I compared to Test II and for slower movements compared to faster, but no difference in response accuracy between Test III and IV. For velocity discrimination, the analysis revealed that the subjects based their judgment on the difference between criterion and comparison velocity rather than time or extent cues.     

Method for Qualitative and Quantitative Assessment of Proprioceptive Perception of Single-joint Arm Movements

The phenomenon of reproduction of the series of passive single-joint movements in the tested arm by the contralateral arm just in the course of passive movements with no visual control was studied in 35 healthy subjects and 13 post-stroke patients in order to develop a new method for objective assessment of sense of the arm motion for the detection of proprioceptive deficit and for monitoring of the changes in proprioception during rehabilitation. We examined the reproduction of flexion–extension at the elbow and wrist joints, abduction–adduction at the wrist joint and the forearm pronation–supination in both right and left arms in healthy subjects and in the affected arm in post-stroke patients. Displacements of the angles in the tested joint and a homonymous joint of the other arm were acquired by means of video recording system, goniometers, or 9-DoF inertional-magnetometric sensors. Qualitative and quantitative indicators were evaluated to assess the similarity of the passive and active movements. It has been found that the healthy subjects are able to actively reproduce the repeated passive movements at different joints of either the left or right tested arm almost simultaneously and with quite accurate reproduction of an amplitude and shape of movement. At the same time, most of post-stroke patients reproduce movements either with qualitative errors demonstrating incorrect location or wrong estimation of direction or number of repeated test movements, or with significant reduction of accuracy (increased latency or shape distortion). We proposed a method for the assessment of movement proprioception at individual joints. The procedure is easy and convenient for both physicians and patients. It does not require special heavy equipment and can easily be performed under different conditions in a wide range of patients.     

Similarities in the Neural Control of the Shoulder and Elbow Joints Belie Their Structural Differences

Movement of the hand in three dimensional space is primarily controlled by the orientation of the shoulder and elbow complexes. Due to discrepancies in proprioceptive acuity, overlap in motor cortex representation and grossly different anatomies between these joints, we hypothesized that there would be differences in the accuracy of aimed movements between the two joints. Fifteen healthy young adults were tested under four conditions – shoulder motion with the elbow constrained and unconstrained, and elbow motion with the shoulder constrained and unconstrained. End point target locations for each joint were set to coincide with joint excursions of 10, 20 or 30 degrees of either the shoulder or elbow joint. Targets were presented in a virtual reality environment. For the constrained condition, there were no significant differences in angular errors between the two joints, suggesting that the central nervous system represents linked segment models of the limb in planning and controlling movements. For the unconstrained condition, although angle errors were higher, hand position errors remained the same as those of the constrained trials. These results support the idea that the CNS utilizes abundant degrees of freedom to compensate for the potentially different contributions to end-point errors introduced by each joint.     

Three-dimensional dynamical capabilities of the human masticatory muscles

Many habitual human jaw movements are non-symmetrical. Generally, it is observed that when the lower incisors move to one side the contralateral condyle moves forwards onto the articular eminence, whereas the ipsilateral condyle stays in the mandibular fossa, moving slightly to the ipsilateral side. These jaw movements are the result of contractions of active masticatory muscles and guided by the temporomandibular joints, their ligaments and passive elastic properties of the muscles. It is not known whether the movements are primarily dependent on passive guidance, active muscle control or both. Therefore, the objective of this study was to analyse the interplay between these factors during non-symmetrical jaw movements. A six-degrees-of-freedom dynamical biomechanical model of the human masticatory system was used. The movements were not restricted to a priori defined joint axes. Jaw movement simulations were performed by unilateral activity of the muscles. The ligaments or the passive elastic properties of the muscles could be removed during these simulations. Laterodeviations conform to naturally observed ones could be generated by unilateral muscle contractions. The movement of the lower incisors was hardly affected by the absence of passive elastic muscle properties or temporomandibular ligaments. The latter, however, influenced the movement of the condyles. The movements could be understood by analysing the combination of forces and torques with respect to the centre of gravity of the lower jaw. In addition, the loading of the condyles appeared to be an important determinant for the movement. This analysis emphasizes that the movements of the jaw are primarily dependent on the orientation of the contributing muscles with respect to this centre of gravity and not on the temporomandibular ligaments or passive elastic muscle properties.     

Coarse Electrocorticographic Decoding of Ipsilateral Reach in Patients with Brain Lesions

In patients with unilateral upper limb paralysis from strokes and other brain lesions, strategies for functional recovery may eventually include brain-machine interfaces (BMIs) using control signals from residual sensorimotor systems in the damaged hemisphere. When voluntary movements of the contralateral limb are not possible due to brain pathology, initial training of such a BMI may require use of the unaffected ipsilateral limb. We conducted an offline investigation of the feasibility of decoding ipsilateral upper limb movements from electrocorticographic (ECoG) recordings in three patients with different lesions of sensorimotor systems associated with upper limb control. We found that the first principal component (PC) of unconstrained, naturalistic reaching movements of the upper limb could be decoded from ipsilateral ECoG using a linear model. ECoG signal features yielding the best decoding accuracy were different across subjects. Performance saturated with very few input features. Decoding performances of 0.77, 0.73, and 0.66 (median Pearson''s r between the predicted and actual first PC of movement using nine signal features) were achieved in the three subjects. The performance achieved here with small numbers of electrodes and computationally simple decoding algorithms suggests that it may be possible to control a BMI using ECoG recorded from damaged sensorimotor brain systems.     

Proprioception, gait kinematics, and rate of loading during walking: are they related?

The cyclic nature of walking can lead to repetitive stress and associated complications due to the rate of loading (ROL) experienced by the body at the initial contact of the foot with the ground. An individual's gait kinematics at initial contact has been suggested to give rise to the ROL, and a repetitive, high ROL may lead to several disorders, including osteoarthritis. Additionally, proprioception, the feedback signaling of limb position and movement, may play a role in how the foot strikes the ground and thus, the ROL. Our goal was to explore the relationship between proprioception, gait kinematics and ROL. Thirty-eight women were recruited for gait analysis, and the gait characteristics 50 ms prior to and at initial contact were examined. Two proprioception tests, joint angle reproduction and threshold to detect passive motion were used to examine the subject's proprioceptive acuity. Our results indicate that individuals with a larger knee angle (i.e., greater extension) 50 ms prior to initial contact (IC) experience a higher ROL during gait and have poorer proprioceptive scores. However, it remains unclear whether poor proprioception causes a high ROL or if a high ROL damages the mechanoreceptors involved in proprioception, but the apparent relationship is significant and warrants further investigation.     

A new approach to reaching control for tetraplegic subjects

A simple upper limb control strategy to guide reaching in preparation to grasp for tetraplegic subjects is proposed. The control is based on new studies of self-paced human arm movements involving rotations about the shoulder and elbow joints. An experimental study of reaching, while grasping, by able-bodied humans, allowed us to reduce the dimensionality of the control vector from two to a single variable. This was accomplished by detailed analysis of the synergy between shoulder and elbow joint angles. This study examined only movements in teh horizontal plane. In the experiments we varied: (a) the shape of targets; (b) their position relative to the initial position of the hand; and (c) the speed of reaching. A synergy between shoulder and elbow joint angles was found in most analysed movements, and it was characterized by a scaling parameter between elbow and shoulder angular velocities. The scaling parameter was determined from the target position presented in the visual perceptive field and initial shoulder and elbow angles. The same experimental setup in studies with tetraplegics with retained shoulder movements showed that this natural synergism is preserved even though the motor and sensory components of the upper limb are reduced or absent. Tetraplegics originally showed a very different reaching pattern, but after short training sessions they developed a reaching behaviour which was similar to able-bodied subjects. The results presented can be used in the following way: a tetraplegic subject lacking elbow extension and flexion may be fitted with an assistive system which will be volitionally controlled only from ipsilateral shoulder movements. The assistive system can comprise either a motorized brace, or a functional electrical stimulation system applied to elbow flexors and extensors. With this system volitional movements at the shoulder would bring the hand into the correct position to accomplish an assisted grasping motion.     

Asymmetries and relationships between dynamic loading,muscle strength,and proprioceptive acuity at the knees in symptomatic unilateral hip osteoarthritis

Introduction

High joint loading, knee muscle weakness, and poor proprioceptive acuity are important factors that have been linked to knee osteoarthritis (OA). We previously reported that those with unilateral hip OA and bilateral asymptomatic knees are more predisposed to develop progressive OA in the contralateral knee relative to the ipsilateral knee. In the present study, we evaluate asymmetries in muscle strength and proprioception between the limbs and also evaluate relationships between these factors and joint loading that may be associated with the asymmetric evolution of OA in this group.

Methods

Sixty-two participants with symptomatic unilateral hip OA and asymptomatic knees were evaluated for muscle strength, joint position sense and dynamic joint loads at the knees. Muscle strength and proprioception were compared between limbs and correlations between these factors and dynamic joint loading were evaluated. Subgroup analyses were also performed in only those participants that fulfilled criteria for severe hip OA.

Results

Quadriceps muscle strength was 15% greater, and in the severe subgroup, proprioceptive acuity was 25% worse at the contralateral compared to ipsilateral knee of participants with unilateral hip OA (P <0.05). In addition, at the affected limb, there was an association between decreased proprioceptive acuity and higher knee loading (Spearman’s rho = 0.377, P = 0.007) and between decreased proprioceptive acuity and decreased muscle strength (Spearman’s rho = −0.328, P = 0.016).

Conclusions

This study demonstrated asymmetries in muscle strength and proprioception between the limbs in a unilateral hip OA population. Early alterations in these factors suggest their possible role in the future development of OA at the contralateral ‘OA-predisposed knee’ in this group. Furthermore, the significant association observed between proprioception, loading, and muscle strength at the affected hip limb suggests that these factors may be interrelated.     

Mapping Proprioception across a 2D Horizontal Workspace

Relatively few studies have been reported that document how proprioception varies across the workspace of the human arm. Here we examined proprioceptive function across a horizontal planar workspace, using a new method that avoids active movement and interactions with other sensory modalities. We systematically mapped both proprioceptive acuity (sensitivity to hand position change) and bias (perceived location of the hand), across a horizontal-plane 2D workspace. Proprioception of both the left and right arms was tested at nine workspace locations and in 2 orthogonal directions (left-right and forwards-backwards). Subjects made repeated judgments about the position of their hand with respect to a remembered proprioceptive reference position, while grasping the handle of a robotic linkage that passively moved their hand to each judgement location. To rule out the possibility that the memory component of the proprioceptive testing procedure may have influenced our results, we repeated the procedure in a second experiment using a persistent visual reference position. Both methods resulted in qualitatively similar findings. Proprioception is not uniform across the workspace. Acuity was greater for limb configurations in which the hand was closer to the body, and was greater in a forward-backward direction than in a left-right direction. A robust difference in proprioceptive bias was observed across both experiments. At all workspace locations, the left hand was perceived to be to the left of its actual position, and the right hand was perceived to be to the right of its actual position. Finally, bias was smaller for hand positions closer to the body. The results of this study provide a systematic map of proprioceptive acuity and bias across the workspace of the limb that may be used to augment computational models of sensory-motor control, and to inform clinical assessment of sensory function in patients with sensory-motor deficits.     

Music Attenuates Excessive Visual Guidance of Skilled Reaching in Advanced but Not Mild Parkinson's Disease

Parkinson''s disease (PD) results in movement and sensory impairments that can be reduced by familiar music. At present, it is unclear whether the beneficial effects of music are limited to lessening the bradykinesia of whole body movement or whether beneficial effects also extend to skilled movements of PD subjects. This question was addressed in the present study in which control and PD subjects were given a skilled reaching task that was performed with and without accompanying preferred musical pieces. Eye movements and limb use were monitored with biomechanical measures and limb movements were additionally assessed using a previously described movement element scoring system. Preferred musical pieces did not lessen limb and hand movement impairments as assessed with either the biomechanical measures or movement element scoring. Nevertheless, the PD patients with more severe motor symptoms as assessed by Hoehn and Yahr (HY) scores displayed enhanced visual engagement of the target and this impairment was reduced during trials performed in association with accompanying preferred musical pieces. The results are discussed in relation to the idea that preferred musical pieces, although not generally beneficial in lessening skilled reaching impairments, may normalize the balance between visual and proprioceptive guidance of skilled reaching.     

Joint position reproduction and joint position discrimination at the ankle are not related

Abstract

Purpose: Limited data in current literature can be found on the relation between the two commonly-used active proprioception assessment methods –active joint position reproduction (JPR) and active movement extent discrimination assessment (AMEDA). The current study compared the two active methods, JPR and AMEDA, to investigate their interrelationship over two studies that differed in task difficulty, using active ankle inversion movements made in weight-bearing to maximise ecological validity.

Methods: 50 participants volunteered in this research, 20 of whom on a harder protocol and the other 30 on an easier protocol, were tested by both methods, JPR and AMEDA. Proprioceptive acuity was represented by absolute error (AE) and variable error (VE) for JPR and by AE and the area under the curve (AUC) for AMEDA.

Results: Proprioceptive acuity scores are found to be significantly correlated within test methods but not between methods for either hard or easy tasks, where JPR AE and VE scores were not correlated with either AMEDA AE or AUC. Further, proprioceptive acuity scores were significantly higher on the easy task when tested with the AMEDA method, but not with JPR method.

Conclusion: Scores obtained from the two active movement proprioception tests, movement extent discrimination and joint position reproduction, were not significantly correlated. Taken together with previous findings, these results show that for proprioception, scores from the three classical psychophysical methods for measuring sensitivity (adjustment, limits and constant stimuli) are not correlated with each other. This suggests that each proprioception measurement system assesses a different aspect of proprioception.     

Effect of stochastic resonance on proprioception and kinesthesia in anterior cruciate ligament reconstructed patients

Low amplitude mechanical noise vibration has been shown to improve somatosensory acuity in various clinical groups with comparable deficiencies through a phenomenon known as Stochastic Resonance (SR). This technology showed promising outcomes in improving somatosensory acuity in other clinical patients (e.g., Parkinson’s disease and osteoarthritis). Some degree of chronic somatosensory deficiency in the knee has been reported following anterior cruciate ligament (ACL) reconstruction surgery. In this study, the effect of the SR phenomenon on improving knee somatosensory acuity (proprioception and kinesthesia) in female ACL reconstructed (ACLR) participants (n = 19) was tested at three months post-surgery, and the results were compared to healthy controls (n = 28). Proprioception was quantified by the measure of joint position sense (JPS) and kinesthesia with the threshold to detection of passive movement (TDPM).The results based on the statistical analysis demonstrated an overall difference between the somatosensory acuity in the ACLR limb compared to healthy controls (p = 0.007). A larger TDPM was observed in the ACLR limb compared to the healthy controls (p = 0.002). However, the JPS between the ACLR and healthy limbs were not statistically significantly different (p = 0.365). SR significantly improved JPS (p = 0.006) while the effect was more pronounced in the ACLR cohort. The effect on the TDPM did not reach statistical significance (p = 0.681) in either group.In conclusion, deficient kinesthesia in the ACLR limb was observed at three months post-surgery. Also, the positive effects of SR on somatosensory acuity in the ACL reconstructed group warrant further investigation into the use of this phenomenon to improve proprioception in ACLR and healthy groups.     

Proprioceptive Feedback and Brain Computer Interface (BCI) Based Neuroprostheses

Brain computer interface (BCI) technology has been proposed for motor neurorehabilitation, motor replacement and assistive technologies. It is an open question whether proprioceptive feedback affects the regulation of brain oscillations and therefore BCI control. We developed a BCI coupled on-line with a robotic hand exoskeleton for flexing and extending the fingers. 24 healthy participants performed five different tasks of closing and opening the hand: (1) motor imagery of the hand movement without any overt movement and without feedback, (2) motor imagery with movement as online feedback (participants see and feel their hand, with the exoskeleton moving according to their brain signals, (3) passive (the orthosis passively opens and closes the hand without imagery) and (4) active (overt) movement of the hand and rest. Performance was defined as the difference in power of the sensorimotor rhythm during motor task and rest and calculated offline for different tasks. Participants were divided in three groups depending on the feedback receiving during task 2 (the other tasks were the same for all participants). Group 1 (n = 9) received contingent positive feedback (participants'' sensorimotor rhythm (SMR) desynchronization was directly linked to hand orthosis movements), group 2 (n = 8) contingent “negative” feedback (participants'' sensorimotor rhythm synchronization was directly linked to hand orthosis movements) and group 3 (n = 7) sham feedback (no link between brain oscillations and orthosis movements). We observed that proprioceptive feedback (feeling and seeing hand movements) improved BCI performance significantly. Furthermore, in the contingent positive group only a significant motor learning effect was observed enhancing SMR desynchronization during motor imagery without feedback in time. Furthermore, we observed a significantly stronger SMR desynchronization in the contingent positive group compared to the other groups during active and passive movements. To summarize, we demonstrated that the use of contingent positive proprioceptive feedback BCI enhanced SMR desynchronization during motor tasks.     

Effects of passive flexion of the forepaw on the response gain of limb extensors to sinusoidal stimulation of labyrinth receptors

The main aim of the present study was to find out whether the dynamic characteristics of responses of limb extensor muscles to labyrinth stimulation were modified by the proprioceptive input elicited by appropriate displacements of the corresponding limb extremity. In cats decerebrated at precollicular or intercollicular level, the multiunit EMG activity of the medial head of the triceps brachii was recorded during roll tilt of the animal at the frequency of 0.15 Hz, +/- 10 degrees leading to selective stimulation of labyrinth receptors. This stimulation was then tested several times at regular intervals of 2 to 6 min for several hours while maintaining the ipsilateral forelimb in the horizontal extended position, i.e. with the plantar surface of the foot lying on the tilting table, or during passive flexion of the forepaw in plantar or dorsal direction. In all the experiments in which the forelimb was in the control position, the multiunit EMG responses of the triceps brachii were characterized by an increased activity during side-down tilt of the animal and a decreased activity during side up tilt. These responses were related to animal position and not to the velocity of animal displacement, thus being attributed to stimulation of macular, utricular receptors. Static displacement of limb extremities following plantar flexion of the forepaw greatly decreased the amplitude of the EMG modulation and thus the gain of the first harmonic component of the multiunit EMG responses of the ipsilateral triceps brachii to animal tilt. This reduced gain was due not only to a reduced number of motor units recruited during labyrinth stimulation, but also to a reduced modulation of firing rate of the active motor units, as shown by recording the activity of individual motor units. On the other hand, displacement of the same extremity in the opposite direction, i.e. following dorsiflexion of the forepaw, enhanced the amplitude of the EMG modulation and thus the gain of the multiunit EMG responses of the ipsilateral triceps brachii to animal tilt. This finding was mainly due to an increased recruitment of motor units during side-down tilt, although an increased modulation of the firing rate of individual motor units could not be excluded. In both instances, no changes in the phase angle to the responses were observed. The changes in response gain described above depended on the amount of passive displacement of the forepaw and persisted unmodified throughout the new maintained position.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dispersion of helical axes during shoulder movements in young and elderly subjects

The shoulder complex (SC) consists of joints with little congruence and its active and passive structures ensure its stability. Stability of the SC rotation centre during upper arm movements can be estimated through the analysis of Helical Axes (HAs) dispersion.The aim of this study was to describe shoulder HAs dispersion during upper limb movements performed with dominant and non-dominant arms by young and elderly subjects. Forty subjects participated in the study (20 young: age 24.8 ± 2.8 years and 20 elderly: age 71.7 ± 6.3 years). Subjects were asked to perform four cycles of 15 rotations, flexions, elevations and abductions with one arm at a time at constant speed. Reflective markers were placed on participants’ arms and trunk in order to detect movements and the HAs dispersion with an optoelectronic system. Mean Distance (MD) from the HAs barycenter and Mean Angle (MA) were used as HAs dispersion indexes. Young subjects showed significant lower MD compared to the elderly during all motion ranges of rotation, flexion and elevation (p < 0.001). Moreover, the MD was lower in the dominant arm compared to the contralateral for rotation (p = 0.049) and flexion (p = 0.019). The results may be due to joint degeneration described in elderly subjects and differences in neuromuscular control of SC stability.     

Identification of passive elastic joint moments in the lower extremities.

Musculotendon actuators produce active and passive moments at the joints they span. Due to the existence of bi-articular muscles, the passive elastic joint moments are influenced by the angular positions of adjacent joints. To obtain quantitative information about this passive elastic coupling between lower limb joints, we examined the passive elastic joint properties of the hip, knee, and ankle joint of ten healthy subjects. Passive elastic joint moments were found to considerably depend on the adjacent joint angles. We present a simple mathematical model that describes these properties on the basis of a double-exponential expression. The model can be implemented in biomechanical models of the lower extremities, which are generally used for the simulation of multi-joint movements such as standing-up, walking, running, or jumping.     

Homologous Muscle Contraction during Unilateral Movement Does Not Show a Dominant Effect on Leg Representation of the Ipsilateral Primary Motor Cortex

Co-activation of homo- and heterotopic representations in the primary motor cortex (M1) ipsilateral to a unilateral motor task has been observed in neuroimaging studies. Further analysis showed that the ipsilateral M1 is involved in motor execution along with the contralateral M1 in humans. Additionally, transcranial magnetic stimulation (TMS) studies have revealed that the size of the co-activation in the ipsilateral M1 has a muscle-dominant effect in the upper limbs, with a prominent decline of inhibition within the ipsilateral M1 occurring when a homologous muscle contracts. However, the homologous muscle-dominant effect in the ipsilateral M1 is less clear in the lower limbs. The present study investigates the response of corticospinal output and intracortical inhibition in the leg representation of the ipsilateral M1 during a unilateral motor task, with homo- or heterogeneous muscles. We assessed functional changes within the ipsilateral M1 and in corticospinal outputs associated with different contracting muscles in 15 right-handed healthy subjects. Motor tasks were performed with the right-side limb, including movements of the upper and lower limbs. TMS paradigms were measured, consisting of short-interval intracortical inhibition (SICI) and recruitment curves (RCs) of motor evoked potentials (MEPs) in the right M1, and responses were recorded from the left rectus femoris (RF) and left tibialis anterior (TA) muscles. TMS results showed that significant declines in SICI and prominent increases in MEPs of the left TA and left RF during unilateral movements. Cortical activations were associated with the muscles contracting during the movements. The present data demonstrate that activation of the ipsilateral M1 on leg representation could be increased during unilateral movement. However, no homologous muscle-dominant effect was evident in the leg muscles. The results may reflect that functional coupling of bilateral leg muscles is a reciprocal movement.     

Role of uncertainty in sensorimotor control

Neural signals are corrupted by noise and this places limits on information processing. We review the processes involved in goal-directed movements and how neural noise and uncertainty determine aspects of our behaviour. First, noise in sensory signals limits perception. We show that, when localizing our hand, the central nervous system (CNS) integrates visual and proprioceptive information, each with different noise properties, in a way that minimizes the uncertainty in the overall estimate. Second, noise in motor commands leads to inaccurate movements. We review an optimal-control framework, known as 'task optimization in the presence of signal-dependent noise', which assumes that movements are planned so as to minimize the deleterious consequences of noise and thereby minimize inaccuracy. Third, during movement, sensory and motor signals have to be integrated to allow estimation of the body's state. Models are presented that show how these signals are optimally combined. Finally, we review how the CNS deals with noise at the neural and network levels. In all of these processes, the CNS carries out the tasks in such a way that the detrimental effects of noise are minimized. This shows that it is important to consider effects at the neural level in order to understand performance at the behavioural level.