Pasma JH, Boonstra TA, Campfens SF, Schouten AC, Van der Kooij H. Sensory reweighting of proprioceptive information of the left and right leg during human balance control. J Neurophysiol 108: 1138-1148, 2012. First published May 23, 2012 doi:10.1152/jn.01008.2011.-To keep balance, information from different sensory systems is integrated to generate corrective torques. Current literature suggests that this information is combined according to the sensory reweighting hypothesis, i.e., more reliable information is weighted more strongly than less reliable information. In this approach, no distinction has been made between the contributions of both legs. In this study, we investigated how proprioceptive information from both legs is combined to maintain upright stance. Healthy subjects maintained balance with eyes closed while proprioceptive information of each leg was perturbed independently by continuous rotations of the support surfaces (SS) and the human body by platform translation. Two conditions were tested: perturbation amplitude of one SS was increased over trials while the other SS 1) did not move or 2) was perturbed with constant amplitude. With the use of system identification techniques, the response of the ankle torques to the perturbation amplitudes (i.e., the torque sensitivity functions) was determined and how much each leg contributed to stabilize stance (i.e., stabilizing mechanisms) was estimated. Increased amplitude of one SS resulted in a decreased torque sensitivity. The torque sensitivity to the constant perturbed SS showed no significant differences. The properties of the stabilizing mechanisms remained constant during perturbations of each SS. This study demonstrates that proprioceptive information from each leg is weighted independently and that the weight decreases with perturbation amplitude. Weighting of proprioceptive information of one leg has no influence on the weight of the proprioceptive information of the other leg. According to the sensory reweighting hypothesis, vestibular information must be up-weighted, because closing the eyes eliminates visual information. system identification; system identification; posture; asymmetry BALANCE IS DESCRIBED as the ability to maintain upright posture in a gravitational field (Niam et al. 1999) and is involved in many daily life activities, such as bipedal stance, walking, and cycling. For small deviations, the gravitational pull effectively is a negative stiffness; a deviation from a perfect upright position results in a torque that accelerates the body further away from this position. External mechanical disturbances, such as a misstep or a slip, and conflicting information of the sensory systems can disturb the equilibrium of the balance system. The central nervous system (CNS) has to cope with these disturbances to maintain the body in upright position.The CNS receives feedback about the body orientation from three main sensory systems: the visual, proprioceptive, and vestibular systems. The CNS integrates this sensory feedback and subseq...

BackgroundNeuroplasticity drives recovery of walking after a lesion of the descending tract. Intramuscular coherence analysis provides a way to quantify corticomotor drive during a functional task, like walking and changes in coherence serve as a marker for neuroplasticity. Although intramuscular coherence analysis is already applied and rapidly growing in interest, the reproducibility of variables derived from coherence is largely unknown. The purpose of this study was to determine the test-retest reliability and agreement of intramuscular coherence variables obtained during walking in healthy subjects.Methodology/Principal FindingsTen healthy participants walked on a treadmill at a slow and normal speed in three sessions. Area of coherence and peak coherence were derived from the intramuscular coherence spectra calculated using rectified and non-rectified M. tibialis anterior Electromyography (EMG). Reliability, defined as the ability of a measurement to differentiate between subjects and established by the intra-class correlation coefficient, was on the limit of good for area of coherence and peak coherence when derived from rectified EMG during slow walking. Yet, the agreement, defined as the degree to which repeated measures are identical, was low as the measurement error was relatively large. The smallest change to exceed the measurement error between two repeated measures was 66% of the average value. For normal walking and/or other EMG-processing settings, not rectifying the EMG and/or high-pass filtering with a high cutoff frequency (100 Hz) the reliability was only moderate to poor and the agreement was considerably lower.Conclusions/significanceOnly for specific conditions and EMG-processing settings, the derived coherence variables can be considered to be reliable measures. However, large changes (>66%) are needed to indicate a real difference. So, although intramuscular coherence is an easy to use and a sufficiently reliable tool to quantify intervention-induced neuroplasticity, the large effects needed to reveal a real change limit its practical use.

The possibility to regain motor function after stroke depends on the intactness of motor and sensory pathways. In this study, we evaluated afferent sensory pathway information transfer and processing after stroke with the coherence between cortical activity and a position perturbation (position-cortical coherence, PCC). Eleven subacute stroke survivors participated in this study. Subjects performed a motor task with the affected and non-affected arm while continuous wrist position perturbations were applied. Cortical activity was measured using EEG. PCC was calculated between position perturbation and EEG at the contralateral and ipsilateral sensorimotor area. The presence of PCC was quantified as the number of frequencies where PCC is larger than zero across the sensorimotor area. All subjects showed significant contralateral PCC in affected and non-affected wrist tasks. Subjects with poor motor function had a reduced presence of contralateral PCC compared with subjects with good motor function in the affected wrist tasks. Amplitude of significant PCC did not differ between subjects with good and poor motor function. Our results show that poor motor function is associated with reduced sensory pathway information transfer and processing in subacute stroke subjects. Position-cortical coherence may provide additional insight into mechanisms of recovery of motor function after stroke.

The applicability of corticomuscular coherence (CMC) as a connectivity measure is limited since only 40-50 % of the healthy population presents significant CMC. In this study, we applied continuous joint position perturbations to obtain a more reliable measure of connectivity in motor control. We evaluated the coherence between joint position perturbations and EEG (position-cortical coherence, PCC) and CMC. Healthy subjects performed two isotonic force tasks against the handle of a wrist manipulator. The baseline task was isometric; in the perturbed task, the handle moved continuously with small amplitude. The position perturbation signal covered frequencies between 5 and 29 Hz. In the perturbed task, all subjects had significant PCC and 86 % of the subjects had significant CMC, on both stimulus and non-stimulus frequencies. In the baseline task, CMC was present in only 45 % of the subjects, mostly on beta-band frequencies. The position perturbations during an isotonic force task elicited PCC in all subjects and elicited CMC in most subjects on both stimulus and non-stimulus frequencies. Perturbed CMC possibly arises by two separate processes: an intrinsic process, similar to the process in an unperturbed task, involving both efferent and afferent pathways; and a process related to the excitation of the afferent and efferent pathways by the perturbation. These processes cannot be separated. PCC, however, reflects connectivity via the afferent pathways only. As PCC was present in all healthy subjects, we propose this coherence as a reliable measure for connectivity in motor control via the afferent pathways.

Coherency phase is often interpreted as a time delay reflecting a transmission delay between spatially separated neural populations. However, time delays estimated from corticomuscular coherency are conflicting and often shorter than expected physiologically. Recent work suggests that corticomuscular coherence is influenced by afferent sensory feedback and bidirectional interactions. We investigated how bidirectional interaction affects time delay estimated from coherency, using a feedback model of the corticomuscular system. We also evaluated the effect of bidirectional interaction on two popular directed connectivity measures: directed transfer function (DTF) and partial directed coherence (PDC). The model is able to reproduce the range of time delays found experimentally from coherency phase by varying the strengths of the efferent and afferent pathways and the recording of sensory feedback in the cortical signal. Both coherency phase and DTF phase were affected by sensory feedback, resulting in an underestimation of the transmission delay. Coherency phase was altered by the recording of sensory feedback in the cortical signals and both measures were affected by the presence of a closed loop feedback system. Only PDC phase led to the correct estimation of efferent transmission delay in all simulated model configurations. Coherency and DTF phase should not be used to estimate transmission delays in neural networks as the estimated time delays are meaningless in the presence of sensory feedback and closed feedback loops.

Abstract-Sensory feedback is of vital importance in motor control, yet rarely assessed in diseases with impaired motor function like stroke. Muscle stretch evoked potentials (StrEPs) may serve as a measure of cortical sensorimotor activation in response to proprioceptive input. The aim of this study is: 1) to determine early and late features of the StrEP and 2) to explore whether StrEP waveform and features can be measured after stroke.Consistency of StrEP waveforms and features was evaluated in 22 normal subjects. StrEP features and similarity between hemispheres were evaluated in eight subacute stroke subjects.StrEPs of normal subjects had a consistent shape across conditions and sessions (mean cross correlation waveforms 0.75). Stroke subjects showed heterogeneous StrEP waveforms. Stroke subjects presented a normal early peak (40 ms after movement onset) but later peaks had abnormal amplitudes and latencies. No significant differences between stroke subjects with good and poor motor function were found . With the consistent responses of normal subjects the StrEP meets a prerequisite for potential clinical value. Recording of StrEPs is feasible even in subacute stroke survivors with poor motor function. How StrEP features relate to clinical phenotypes and recovery needs further investigation.

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The ankles play an important role in human balance. In most studies investigating balance control the contribution of the left and right leg is not separated. However, in certain pathologies such as stroke and Parkinson's disease, balance control can be asymmetric. Here, a bilateral ankle perturbator (BAP) is presented, which applies support surface rotations to both ankles independently. The device consists of two small foot-size support surfaces, which are independently actuated. The BAP device can operate in either angle or torque control mode. The device is able to apply support surface rotations up to 8.6° with a bandwidth of 42 Hz. Additionally the platforms can be replaced by 6-DoF force plates to measure the center of pressure underneath each foot. With the optional force plates the bandwidth decreases to 16 Hz as a result of the additional weight. Two possible applications of the device to investigate human balance control are demonstrated: ankle stiffness by applying minimum jerk profiles and sensory reweighting of the proprioceptive information. In conclusion, we developed a bilateral ankle perturbator which is able to apply support surface rotations to both ankles independently. The major application of the device will be to investigate the contribution of both ankles to human balance control, and the interactions in balance control between both legs.