Task-dependent intermanual coupling of 8-Hz discontinuities during slow finger movements

Neural Filtering of Physiological Tremor Oscillations to Spinal Motor Neurons Mediates Short-Term Acquisition of a Skill Learning Task

The acquisition of a motor skill involves adaptations of spinal and supraspinal pathways to alpha motoneurons. In this study, we estimated the shared synaptic contributions of these pathways to understand the neural mechanisms underlying the short-term acquisition of a new force-matching task. High-density surface electromyography (HDsEMG) was acquired from the first dorsal interosseous (FDI; 7 males and 6 females) and tibialis anterior (TA; 7 males and 4 females) during 15 trials of an isometric force-matching task. For two selected trials (pre- and post-skill acquisition), we decomposed the HDsEMG into motor unit spike trains, tracked motor units between trials, and calculated the mean discharge rate and the coefficient of variation of interspike interval (COVISI). We also quantified the post/pre ratio of motor units' coherence within delta, alpha, and beta bands. Force-matching improvements were accompanied by increased mean discharge rate and decreased COVISI for both muscles. Moreover, the area under the curve within alpha band decreased by ∼22% (TA) and ∼13% (FDI), with no delta or beta bands changes. These reductions correlated significantly with increased coupling between force/neural drive and target oscillations. These results suggest that short-term force-matching skill acquisition is mediated by attenuation of physiological tremor oscillations in the shared synaptic inputs. Supported by simulations, a plausible mechanism for alpha band reductions may involve spinal interneuron phase-cancelling descending oscillations. Therefore, during skill learning, the central nervous system acts as a matched filter, adjusting synaptic weights of shared inputs to suppress neural components unrelated to the specific task.

Spatial and Temporal Arrangement of Recurrent Inhibition in the Primate Upper Limb

Renshaw cells mediate recurrent inhibition between motoneurons within the spinal cord. The function of this circuit is not clear; we previously suggested based on computational modeling that it may cancel oscillations in muscle activity around 10 Hz, thereby reducing physiological tremor. Such tremor is especially problematic for dexterous hand movements, yet knowledge of recurrent inhibitory function is sparse for the control of the primate upper limb, where no direct measurements have been made to date. In this study, we made intracellular penetrations into 89 motoneurons in the cervical enlargement of four terminally anesthetized female macaque monkeys, and recorded recurrent IPSPs in response to antidromic stimulation of motor axons. Recurrent inhibition was strongest to motoneurons innervating shoulder muscles and elbow extensors, weak to wrist and digit extensors, and almost absent to the intrinsic muscles of the hand. Recurrent inhibitory connections often spanned joints, for example from motoneurons innervating wrist and digit muscles to those controlling the shoulder and elbow. Wrist and digit flexor motoneurons sometimes inhibited the corresponding extensors, and vice versa. This complex connectivity presumably reflects the flexible usage of the primate upper limb. Using trains of stimuli to motor nerves timed as a Poisson process and coherence analysis, we also examined the temporal properties of recurrent inhibition. The recurrent feedback loop effectively carried frequencies up to 100 Hz, with a coherence peak around 20 Hz. The coherence phase validated predictions from our previous computational model, supporting the idea that recurrent inhibition may function to reduce tremor.

SIGNIFICANCE STATEMENT We present the first direct measurements of recurrent inhibition in primate upper limb motoneurons, revealing that it is more flexibly organized than previous observations in cat. Recurrent inhibitory connections were relatively common between motoneurons controlling muscles that act at different joints, and between flexors and extensors. As in the cat, connections were minimal for motoneurons innervating the most distal intrinsic hand muscles. Empirical data are consistent with previous modeling: temporal properties of the recurrent inhibitory feedback loop are compatible with a role in reducing physiological tremor by suppressing oscillations around 10 Hz.

Parkinson's Disease Exhibits Amplified Intermuscular Coherence During Dynamic Voluntary Action

Parkinson's disease (PD) is typically diagnosed and evaluated on the basis of overt motor dysfunction, however, subtle changes in the frequency spectrum of neural drive to muscles have been reported as well. During dynamic actions, coactive muscles of healthy adults often share a common source of 6-15 Hz (alpha-band) neural drive, creating synchronous alpha-band activity in their EMG signals. Individuals with PD commonly exhibit kinetic action tremor at similar frequencies, but the potential relationship between the intermuscular alpha-band neural drive seen in healthy adults and the action tremor associated with PD is not well-understood. A close relationship is most tenable during voluntary dynamic tasks where alpha-band neural drive is strongest in healthy adults, and where neural circuits affected by PD are most engaged. In this study, we characterized the frequency spectrum of EMG synchronization (intermuscular coherence) in 16 participants with PD and 15 age-matched controls during two dynamic motor tasks: (1) rotation of a dial between the thumb and index finger, and (2) dynamic scaling of isometric precision pinch force. These tasks produce different profiles of coherence between the first dorsal interosseous and abductor pollicis brevis muscles. We sought to determine if alpha-band intermuscular coherence would be amplified in participants with PD relative to controls, if such differences would be task-specific, and if they would correlate with symptom severity. We found that relative to controls, the PD group displayed amplified, but similarly task-dependent, coherence in the alpha-band. The magnitude of coherence during the rotation task correlated with overall symptom severity as per the UPDRS rating scale. Finally, we explored the potential for our coherence measures, with no additional information, to discriminate individuals with PD from controls. The area under the Receiver Operating Characteristic curve (AUC) indicated a clear separation between groups (AUC = 0.96), even though participants with PD were on their typical medication and displayed only mild-moderate symptoms. We conclude that a task-dependent, intermuscular neural drive within the alpha-band is amplified in PD. Its quantification via intermuscular coherence analysis may provide a useful tool for detecting the presence of PD, or assessing its progression.

Human 8- to 10-Hz pulsatile motor output during active exploration of textured surfaces reflects the textures’ frictional properties

Active sensing in biological system consists of emitting/receiving a periodic signal to explore the environment. The signal can be emitted toward distant objects, as in echolocation, or in direct contact with the object, for example, whisking in rodents. We explored the hypothesis that a similar mechanism exists in humans. Humans generate periodic signals at ~10 Hz during voluntary finger movements, which reflects a pulsatile motor command in the central nervous system. In the present study, we tested whether the ~10-Hz signal persists during the active exploration of textures and whether the textures’ features can modulate the signal. Our results confirm our assumptions. The ~10-Hz signal persisted during active touch, and its amplitude increased with textures of higher friction. These findings support the idea that the ~10-Hz periodic signal generated during voluntary finger movements is part of an active sensing mechanism acting in a pulse-amplitude modulation fashion to convey relevant tactile information to the brain.

NEW & NOTEWORTHY For the first time, we show that pulsatile motor output during voluntary movement of a finger persists during active exploration of a surface. We propose that this is part of an active sensing system in humans, with generation of an ~10-Hz signal during active touch that reinforces extraction of information about features of the touched surface.

Aging and Strength Training Influence Knee Extensor Intermuscular Coherence During Low- and High-Force Isometric Contractions

Aging is associated with reduced maximum force production and force steadiness during low-force tasks, but both can be improved by training. Intermuscular coherence measures coupling between two peripheral surface electromyography (EMG) signals in the frequency domain. It is thought to represent the presence of common input to alpha-motoneurons, but the functional meaning of intermuscular coherence, particularly regarding aging and training, remain unclear. This study investigated knee extensor intermuscular coherence in previously sedentary young (18–30 years) and older (67–73 years) subjects before and after a 14-week strength training intervention. YOUNG and OLDER groups performed maximum unilateral isometric knee extensions [100% maximum voluntary contraction (MVC)], as well as force steadiness tests at 20 and 70% MVC, pre- and post-training. Intermuscular (i.e., EMG-EMG) coherence analyses were performed for all (three) contraction intensities in vastus lateralis and medialis muscles. Pre-training coefficient of force variation (i.e., force steadiness) and MVC (i.e., maximum torque) were similar between groups. Both groups improved MVC through training, but YOUNG improved more than OLDER (42 ± 27 Nm versus 18 ± 16 Nm, P = 0.022). Force steadiness did not change during 20% MVC trials in either group, but YOUNG demonstrated increased coefficient of force variation during 70% MVC trials (1.28 ± 0.46 to 1.57 ± 0.70, P = 0.01). YOUNG demonstrated greater pre-training coherence during 20% and 70% MVC trials, particularly within the 8–14 Hz (e.g., 20%: 0.105 ± 0.119 versus 0.016 ± 0.009, P = 0.001) and 16–30 Hz (20%: 0.063 ± 0.078 versus 0.012 ± 0.007, P = 0.002) bands, but not during 100% MVC trials. Strength training led to increases in intermuscular coherence within the 40–60 Hz band during 70% MVC trials in YOUNG only, while OLDER decreased within the 8–14 Hz band during 100% MVC trials. Age-related differences in intermuscular coherence were observed between young and older individuals, even when neuromuscular performance levels were similar. The functional significance of intermuscular coherence remains unclear, since coherence within different frequency bands did not explain any of the variance in the regression models for maximum strength or force steadiness during 20 and 70% MVC trials.

Spasms after spinal cord injury show low-frequency intermuscular coherence

Intermuscular coherence allows the investigation of common input to muscle groups. Although beta-band (15–30 Hz) intermuscular coherence is well understood as originating from the cortex, the source of intermuscular coherence at lower frequencies is still unclear. We used a wearable device that recorded electromyographic (EMG) signals during a 24-h period in four lower limb muscles of seven spinal cord injury patients (American Spinal Cord Injury Association impairment scale: A, 6 subjects; B, 1 subject) while they went about their normal daily life activities. We detected natural spasms occurring during these long-lasting recordings and calculated intermuscular coherence between all six possible combinations of muscle pairs. There was significant intermuscular coherence at low frequencies, between 2 and 13 Hz. The most likely source for this was the spinal cord and its peripheral feedback loops, because the spinal lesions in these patients had interrupted connections to supraspinal structures. This is the first report to demonstrate that the spinal cord is capable of producing low-frequency intermuscular coherence with severely reduced or abolished descending drive.

NEW & NOTEWORTHY This is the first report to demonstrate that intermuscular coherence between lower limb muscles at low frequencies can be produced by the spinal cord with severely reduced or abolished descending drive.

Abnormal Blink Reflex and Intermuscular Coherence in Writer's Cramp

Background: Writer's cramp (WC) is a task-specific focal hand dystonia presenting with pain, stiffness and/or tremor while writing. We explored the involvement of cortical and brainstem circuits by measuring intermuscular coherence (IMC) and pre-pulse inhibition (PPI) of the blink reflex. Methods: IMC was measured in 10 healthy controls and 20 WC patients (10 with associated tremor) while they performed a precision grip task at different force levels. Blink responses were evaluated in 9 healthy controls and 10 WC patients by stimulating the right supraorbital nerve and recording surface EMG from the orbicularis oculi muscles bilaterally. PPI involved conditioning this stimulation with a prior shock to the right median nerve (100 ms interval), and measuring the reduction in the R2 component of the blink reflex. Results: Significant IMC at 3-7 Hz was present in WC patients, but not in healthy controls. Compared to healthy controls, in WC patients the R2 component of the blink reflex showed significantly less PPI. IMC at 3-7 Hz could reliably discriminate WC patients from healthy controls. Conclusion: Cortical or sub-cortical circuits generating theta (3-7 Hz) oscillations might play an important role in the pathogenesis of WC. Moreover, the lack of PPI implicates abnormalities in brainstem inhibition in the emergence of WC. IMC may merit further development as an electrodiagnostic test for focal dystonia.

Age-Related Declines in the Ability to Modulate Common Input to Bilateral and Unilateral Plantar Flexors During Forward Postural Lean

Aging can impair an ability to lean the body forward to the edge of the base of support. Here, we investigated, using a coherence analysis, common inputs to bilateral and unilateral plantar flexor muscles to test a hypothesis that the age-related impairment would be related to strong synchronous bilateral activation and reduced cortical control of these muscles. Healthy young (n = 14) and elderly adults (n = 19), who were all right-foot dominant, performed quiet standing task and tasks that required the subjects to lean their body forward to 35 and 75% of the maximum lean distance. The electromyogram was recorded from the bilateral medial gastrocnemius (MG) and soleus (SL) muscles. We analyzed delta-band coherence, that reflects comodulation of muscle activity, between the bilateral homologous muscles (MG-MG and SL-SL pairs). The origin of this bilateral comodulation is suggested to be the subcortical system. Also, we examined beta-band coherence, that is related to the corticospinal drive, between the unilateral muscles (MG-SL pair) in the right leg. Results indicated that the bilateral delta-band coherence for the MG-MG pair was significantly smaller in the 75% forward lean than quiet standing and 35% forward lean tasks for the young adults (quiet: p = 0.036; 35%: p = 0.0011). The bilateral delta-band coherence for the SL-SL pair was significantly smaller in the 75% forward lean than 35% forward lean task for the young adults (p = 0.027). Furthermore, the unilateral beta-band coherence was larger in the forward lean than quiet standing task for the young adults (35%: p < 0.001; 75%: p = 0.029). Contrarily, the elderly adults did not demonstrate such changes. These findings suggest the importance of decreasing the synchronous bilateral activation and increasing the unilateral cortical control of the plantar flexor muscles for the successful forward postural lean performance, and that aging impairs this modulatory ability.

Neural coupling between homologous muscles during bimanual tasks: effects of visual and somatosensory feedback

While the effects of sensory feedback on bimanual tasks have been studied extensively at two ends of the motor control hierarchy, the cortical and behavioral levels, much less is known about how it affects the intermediate levels, including neural control of homologous muscle groups. We investigated the effects of somatosensory input on the neural coupling between homologous arm muscles during bimanual tasks. Twelve subjects performed symmetric elbow flexion/extension tasks under different types of sensory feedback. The first two types involve visual feedback, with one imposing stricter force symmetry than the other. The third incorporated somatosensory feedback via a balancing apparatus that forced the two limbs to produce equal force levels. Although the force error did not differ between feedback conditions, the somatosensory feedback significantly increased temporal coupling of bilateral force production, indicated by a high correlation between left/right force profiles (P < 0.001). More importantly, intermuscular coherence between biceps brachii muscles was significantly higher with somatosensory feedback than others (P = 0.001). Coherence values also significantly differed between tasks (flexion/extension). Notably, whereas feedback type mainly modulated coherence in the α- and γ-bands, task type only affected β-band coherence. Similar feedback effects were observed for triceps brachii muscles, but there was also a strong phase effect on the coherence values (P < 0.001) that could have diluted feedback effects. These results suggest that somatosensory feedback can significantly increase neural coupling between homologous muscles. Additionally, the between-task difference in β-band coherence may reflect different neural control strategies for the elbow flexor and extensor muscles.

NEW & NOTEWORTHY

This study investigated the effects of somatosensory feedback during bimanual tasks on the neural coupling between arm muscles, which remains largely unexplored. Somatosensory feedback using a balancing apparatus, compared with visual feedback, significantly increased neural coupling between homologous muscles (indicated by intermuscular coherence values) and improved temporal correlation of bilateral force production. Notably, feedback type modulated coherence in the α- and γ-bands (more subcortical pathways), whereas task type mainly affected β-band coherence (corticospinal pathway).

Functional connectivity in the neuromuscular system underlying bimanual coordination

Neural synchrony has been suggested as a mechanism for integrating distributed sensorimotor systems involved in coordinated movement. To test the role of corticomuscular and intermuscular coherence in bimanual coordination, we experimentally manipulated the degree of coordination between hand muscles by varying the sensitivity of the visual feedback to differences in bilateral force. In 16 healthy participants, cortical activity was measured using EEG and muscle activity of the flexor pollicis brevis of both hands using high-density electromyography (HDsEMG). Using the uncontrolled manifold framework, coordination between bilateral forces was quantified by the synergy index R_V in the time and frequency domain. Functional connectivity was assessed using corticomuscular coherence between muscle activity and cortical source activity and intermuscular coherence between bilateral EMG activity. The synergy index increased in the high coordination condition. R_V was higher in the high coordination condition in frequencies between 0 and 0.5 Hz; for the 0.5- to 2-Hz frequency band, this pattern was inverted. Corticomuscular coherence in the beta band (16-30 Hz) was maximal in the contralateral motor cortex and was reduced in the high coordination condition. In contrast, intermuscular coherence was observed at 5-12 Hz and increased with bimanual coordination. Within-subject comparisons revealed a negative correlation between R_V and corticomuscular coherence and a positive correlation between R_V and intermuscular coherence. Our findings suggest two distinct neural pathways: 1) corticomuscular coherence reflects direct corticospinal projections involved in controlling individual muscles; and 2) intermuscular coherence reflects diverging pathways involved in the coordination of multiple muscles.

Intermuscular Coherence in Normal Adults: Variability and Changes with Age

We investigated beta-band intermuscular coherence (IMC) in 92 healthy adults stratified by decade of age, and analysed variability between and within subjects. In the dominant upper limb, IMC was estimated between extensor digitorum communis and first dorsal interosseous as well as between flexor digitorum superficialis and first dorsal interosseous. In the ipsilateral lower limb, IMC was measured between medial gastrocnemius and extensor digitorum brevis as well as between tibialis anterior and extensor digitorum brevis. Age-related changes in IMC were analysed with age as a continuous variable or binned by decade. Intrasession variance of IMC was examined by dividing sessions into pairs of epochs and comparing coherence estimates between these pairs. Eight volunteers returned for a further session after one year, allowing us to compare intrasession and intersession variance. We found no age-related changes in IMC amplitude across almost six decades of age, allowing us to collate data from all ages into an aggregate normative dataset. Interindividual variability ranged over two orders of magnitude. Intrasession variance was significantly greater than expected from statistical variability alone, and intersession variance was even larger. Potential contributors include fluctuations in task performance, differences in electrode montage and short-term random variation in central coupling. These factors require further exploration and, where possible, minimisation. This study provides evidence that coherence is remarkably robust to senescent changes in the nervous system and provides a large normative dataset for future applications of IMC as a biomarker in disease states.

The extraction of neural strategies from the surface EMG: an update

A surface EMG signal represents the linear transformation of motor neuron discharge times by the compound action potentials of the innervated muscle fibers and is often used as a source of information about neural activation of muscle. However, retrieving the embedded neural code from a surface EMG signal is extremely challenging. Most studies use indirect approaches in which selected features of the signal are interpreted as indicating certain characteristics of the neural code. These indirect associations are constrained by limitations that have been detailed previously (Farina D, Merletti R, Enoka RM. J Appl Physiol 96: 1486-1495, 2004) and are generally difficult to overcome. In an update on these issues, the current review extends the discussion to EMG-based coherence methods for assessing neural connectivity. We focus first on EMG amplitude cancellation, which intrinsically limits the association between EMG amplitude and the intensity of the neural activation and then discuss the limitations of coherence methods (EEG-EMG, EMG-EMG) as a way to assess the strength of the transmission of synaptic inputs into trains of motor unit action potentials. The debated influence of rectification on EMG spectral analysis and coherence measures is also discussed. Alternatively, there have been a number of attempts to identify the neural information directly by decomposing surface EMG signals into the discharge times of motor unit action potentials. The application of this approach is extremely powerful, but validation remains a central issue.

The selective influence of rhythmic cortical versus cerebellar transcranial stimulation on human physiological tremor

The influence of central neuronal oscillators on human physiological tremor is controversial. To address this, transcranial alternating current stimulation (TACS) was delivered at peak tremor frequency to 12 healthy volunteers in a 2 × 2 crossover study. Two sites were stimulated [contralateral primary motor cortex (M1), vs ipsilateral cerebellum] while participants performed two types of tasks designed to probe the different manifestations of physiological tremor of the hand-kinetic and postural tremor. Tremor was measured by accelerometry. Cortical coherence with the accelerometry signal was also calculated in the absence of stimulation. The phase synchronization index, a measure of the phase entrainment of tremor, was calculated between stimulation and tremor waveforms. The amplitude modulation of tremor was similarly assessed. There was significant phase entrainment that was dependent both on tremor type and site of stimulation: M1 stimulation gave rise to phase entrainment of postural, but not kinetic, tremor, whereas cerebellar stimulation increased entrainment in both cases. There was no effect on tremor amplitude. Tremor accelerometry was shown to be coherent with the cortical EEG recorded during postural, but not kinetic, tremor. TACS modulates physiological tremor, and its effects are dependent both on tremor type and stimulation site. Accordingly, central oscillators play a significant role in two of the major manifestations of tremor in health.

Enhanced corticomuscular coherence by external stochastic noise

Noise can have beneficial effects as shown by the stochastic resonance (SR) phenomenon which is characterized by performance improvement when an optimal noise is added. Modern attempts to improve human performance utilize this phenomenon. The purpose of the present study was to investigate whether performance improvement by addition of optimum noise (ON) is related to increased cortical motor spectral power (SP) and increased corticomuscular coherence. Eight subjects performed a visuomotor task requiring to compensate with the right index finger a static force (SF) generated by a manipulandum on which Gaussian noise was applied. The finger position was displayed on-line on a monitor as a small white dot which the subjects had to maintain in the center of a green bigger circle. Electroencephalogram from the contralateral motor area, electromyogram from active muscles and finger position were recorded. The performance was measured by the mean absolute deviation (MAD) of the white dot from the zero position. ON compared to the zero noise condition induced an improvement in motor accuracy together with an enhancement of cortical motor SP and corticomuscular coherence in beta-range. These data suggest that the improved sensorimotor performance via SR is consistent with an increase in the cortical motor SP and in the corticomuscular coherence.

High-density surface electromyography improves the identification of oscillatory synaptic inputs to motoneurons

Many studies have addressed corticomuscular coherence (CMC), but broad applications are limited by low coherence values and the variability across subjects and recordings. Here, we investigated how the use of high-density surface electromyography (HDsEMG) can improve the detection of CMC. Sixteen healthy subjects performed isometric contractions at six low-force levels using a pinch-grip, while HDsEMG of the adductor pollicis transversus and flexor and abductor pollicis brevis and whole-head magnetoencephalography were recorded. Different configurations were constructed from the HDsEMG grid, such as a bipolar and Laplacian montage, as well as a montage based on principal component analysis (PCA). CMC was estimated for each configuration, and the strength of coherence was compared across configurations. As expected, performance of the precision-grip task resulted in significant CMC in the β-frequency band (16-26 Hz). Compared with a bipolar EMG montage, all multichannel configurations obtained from the HDsEMG grid revealed a significant increase in CMC. The configuration, based on PCA, showed the largest (37%) increase. HDsEMG did not reduce the between-subject variability; rather, many configurations showed an increased coefficient of variation. Increased CMC presumably reflects the ability of HDsEMG to counteract inherent EMG signal factors-such as amplitude cancellation-which impact the detection of oscillatory inputs. In contrast, the between-subject variability of CMC most likely has a cortical origin.

Different phase delays of peripheral input to primate motor cortex and spinal cord promote cancellation at physiological tremor frequencies

Neurons in the spinal cord and motor cortex (M1) are partially phase-locked to cycles of physiological tremor, but with opposite phases. Convergence of spinal and cortical activity onto motoneurons may thus produce phase cancellation and a reduction in tremor amplitude. The mechanisms underlying this phase difference are unknown. We investigated coherence between spinal and M1 activity with sensory input. In two anesthetized monkeys, we electrically stimulated the medial, ulnar, deep radial, and superficial radial nerves; stimuli were timed as independent Poisson processes (rate 10 Hz). Single units were recorded from M1 (147 cells) or cervical spinal cord (61 cells). Ninety M1 cells were antidromically identified as pyramidal tract neurons (PTNs); M1 neurons were additionally classified according to M1 subdivision (rostral/caudal, M1r/c). Spike-stimulus coherence analysis revealed significant coupling over a broad range of frequencies, with the strongest coherence at <50 Hz. Delays implied by the slope of the coherence phase-frequency relationship were greater than the response onset latency, reflecting the importance of late response components for the transmission of oscillatory inputs. The spike-stimulus coherence phase over the 6–13 Hz physiological tremor band differed significantly between M1 and spinal cells (phase differences relative to the cord of 2.72 ± 0.29 and 1.72 ± 0.37 radians for PTNs from M1c and M1r, respectively). We conclude that different phases of the response to peripheral input could partially underlie antiphase M1 and spinal cord activity during motor behavior. The coordinated action of spinal and cortical feedback will act to reduce tremulous oscillations, possibly improving the overall stability and precision of motor control.

Discontinuities in slow finger movements in patients with Parkinson's disease

Slow finger movements in healthy humans are characterized by discontinuous rhythmic changes in a low frequency band about 8 Hz. These pulsatile changes in velocity are thought to present the central output of an oscillatory cerebello-thalamo-cortical network in the same frequency. Hypothesizing that patients with Parkinson's disease (PD) in the dopaminergic OFF- and ON-condition show changes in the characteristics of discontinuities compared to healthy humans, we used a 3D-ultrasound device to measure slow finger movements of 16 patients with PD and 12 age-matched controls. We provide evidence that slow finger movements of patients with PD are characterized by discontinuities in acceleration, which are significantly slower in the OFF- but not in the ON-condition compared to healthy controls. Correlation analysis between clinical motor improvement after dopaminergic medication and changes of peak frequencies and peak power of discontinuities was not significant. We conclude that the oscillatory brain network of slow finger movements is affected in PD, presenting in a lower frequency in the OFF-condition. We suggest that one factor of the modulation of this network is a dopaminergic stimulation.

Identification of common synaptic inputs to motor neurons from the rectified electromyogram

Oscillatory common inputs of cortical or peripheral origin can be identified from the motor neuron output with coherence analysis. Linear transmission is possible despite the motor neuron non-linearity because the same input is sent commonly to several neurons. Because of the linear transmission, common input components to motor neurons can be investigated from the surface EMG, for example by EEG-EMG or EMG-EMG coherence. In these studies, there is an open debate on the utility and appropriateness of EMG rectification. The present study addresses this issue using an analytical, simulation and experimental approach. The main novel theoretical contribution that we report is that the spectra of both the rectified and the raw EMG contain input spectral components to motor neurons. However, they differ by the contribution of amplitude cancellation which influences the rectified EMG spectrum when extracting common oscillatory inputs. Therefore, the degree of amplitude cancellation has an impact on the effectiveness of EMG rectification in extracting input spectral peaks. The theoretical predictions were exactly confirmed by realistic simulations of a pool of motor neurons innervating a muscle in a cylindrical volume conductor of EMG generation and by experiments conducted on the first dorsal interosseous and the abductor pollicis brevis muscles of seven healthy subjects during pinching. It was concluded that when the contraction level is relatively low, EMG rectification may be preferable for identifying common inputs to motor neurons, especially when the energy of the action potentials in the low frequency range is low. Nonetheless, different levels of cancellation across conditions influence the relative estimates of the degree of linear transmission of oscillatory inputs to motor neurons when using the rectified EMG.

Beta-adrenergic modulation of tremor and corticomuscular coherence in humans

Coherence between the bioelectric activity of sensorimotor cortex and contralateral muscles can be observed around 20 Hz. By contrast, physiological tremor has a dominant frequency around 10 Hz. Although tremor has multiple sources, it is partly central in origin, reflecting a component of motoneuron discharge at this frequency. The motoneuron response to ~20 Hz descending input could be altered by non-linear interactions with ~10 Hz motoneuron firing. We investigated this further in eight healthy human subjects by testing the effects of the beta-adrenergic agents propranolol (non-selective β-antagonist) and salbutamol (β(2)-agonist), which are known to alter the size of physiological tremor. Corticomuscular coherence was assessed during an auxotonic precision grip task; tremor was quantified using accelerometry during index finger extension. Experiments with propranolol used a double-blind, placebo-controlled crossover design. A single oral dose of propranolol (40 mg) significantly increased beta band (15.3-32.2 Hz) corticomuscular coherence compared with placebo, but reduced tremor in the 6.2-11.9 Hz range. Salbutamol (2.5 mg) was administered by inhalation. Whilst salbutamol significantly increased tremor amplitude as expected, it did not change corticomuscular coherence. The opposite direction of the effects of propranolol on corticomuscular coherence and tremor, and the fact that salbutamol enhances tremor but does not affect coherence, implies that the magnitude of corticomuscular coherence is little influenced by non-linear interactions with 10 Hz oscillations in motoneurons or the periphery. Instead, we suggest that propranolol and salbutamol may affect both tremor and corticomuscular coherence partly via a central site of action.

Beta-band intermuscular coherence: a novel biomarker of upper motor neuron dysfunction in motor neuron disease

In motor neuron disease, the focus of therapy is to prevent or slow neuronal degeneration with neuroprotective pharmacological agents; early diagnosis and treatment are thus essential. Incorporation of needle electromyographic evidence of lower motor neuron degeneration into diagnostic criteria has undoubtedly advanced diagnosis, but even earlier diagnosis might be possible by including tests of subclinical upper motor neuron disease. We hypothesized that beta-band (15–30 Hz) intermuscular coherence could be used as an electrophysiological marker of upper motor neuron integrity in such patients. We measured intermuscular coherence in eight patients who conformed to established diagnostic criteria for primary lateral sclerosis and six patients with progressive muscular atrophy, together with 16 age-matched controls. In the primary lateral sclerosis variant of motor neuron disease, there is selective destruction of motor cortical layer V pyramidal neurons and degeneration of the corticospinal tract, without involvement of anterior horn cells. In progressive muscular atrophy, there is selective degeneration of anterior horn cells but a normal corticospinal tract. All patients with primary lateral sclerosis had abnormal motor-evoked potentials as assessed using transcranial magnetic stimulation, whereas these were similar to controls in progressive muscular atrophy. Upper and lower limb intermuscular coherence was measured during a precision grip and an ankle dorsiflexion task, respectively. Significant beta-band coherence was observed in all control subjects and all patients with progressive muscular atrophy tested, but not in the patients with primary lateral sclerosis. We conclude that intermuscular coherence in the 15–30 Hz range is dependent on an intact corticospinal tract but persists in the face of selective anterior horn cell destruction. Based on the distributions of coherence values measured from patients with primary lateral sclerosis and control subjects, we estimated the likelihood that a given measurement reflects corticospinal tract degeneration. Therefore, intermuscular coherence has potential as a quantitative test of subclinical upper motor neuron involvement in motor neuron disease.

Neural mechanisms of intermuscular coherence: implications for the rectification of surface electromyography

Oscillatory activity plays a crucial role in corticospinal control of muscle synergies and is widely investigated using corticospinal and intermuscular synchronization. However, the neurophysiological mechanisms that translate these rhythmic patterns into surface electromyography (EMG) are not well understood. This is underscored by the ongoing debate on the rectification of surface EMG before spectral analysis. Whereas empirical studies commonly rectify surface EMG, computational approaches have argued against it. In the present study, we employ a computational model to investigate the role of the motor unit action potential (MAUP) on the translation of oscillatory activity. That is, diverse MUAP shapes may distort the transfer of common input into surface EMG. We test this in a computational model consisting of two motor unit pools receiving common input and compare it to empirical results of intermuscular coherence between bilateral leg muscles. The shape of the MUAP was parametrically varied, and power and coherence spectra were investigated with and without rectification. The model shows that the effect of EMG rectification depends on the uniformity of MUAP shapes. When output spikes of different motor units are convolved with identical MUAPs, oscillatory input is evident in both rectified and nonrectified EMG. In contrast, a heterogeneous MAUP distribution distorts common input and oscillatory components are only manifest as periodic amplitude modulations, i.e., in rectified EMG. The experimental data showed that intermuscular coherence was mainly discernable in rectified EMG, hence providing empirical support for a heterogeneous distribution of MUAPs. These findings implicate that the shape of MUAPs is an essential parameter to reconcile experimental and computational approaches.

Modulation and transmission of peripheral inputs in monkey cuneate and external cuneate nuclei

Somatosensory signals undergo substantial modulation in the dorsal column nuclei. We examined transmission of signals from forelimb afferents in primate cuneate and external cuneate nuclei. In anesthetized macaque monkeys, the median, ulnar, deep radial, and superficial radial nerves were electrically stimulated at 1.5-2× motor threshold with independent Poisson trains whereas extracellular recordings were made from 317 cells. Responses to peripheral stimulation included instances of both brief facilitation and long lasting suppression. A high proportion of cells (87%) responded to stimulation of two or more peripheral nerves, suggesting a large amount of convergence. Facilitated cells showed coherence with the peripheral stimulation across a broad frequency range; coherence was especially high in cells that responded with a burst of action potentials. Cells that responded with suppression also showed significant coherence, but this fell rapidly for frequencies above 25 Hz. Similar results were seen in both the main and external cuneate. When stimulation of one nerve was conditioned by a preceding nerve stimulus, the response to the second stimulus was attenuated for around 40 ms. This occurred independently of whether the first stimulus produced an initial facilitation or suppression or whether the same or a different nerve served as a conditioning stimulus. Mechanical stimulation of a receptive field suppressed responses to a second identical mechanical stimulus over a similar timescale. We conclude that the primate cuneate nucleus is capable of transmitting temporal information about stimuli with high fidelity; stimuli interact both temporally and spatially to modulate the onward transmission of information.

Contributions of descending and ascending pathways to corticomuscular coherence in humans

Non-technical summary

Neural activity in parts of the cerebral cortex related to movement oscillates at frequencies around 20 Hz. These oscillations are correlated with similar rhythms in contracting muscles on the opposite side of the body. In this work, we used an analysis method called directed coherence to investigate the direction of oscillatory coupling. We find that oscillations travel not only from cortex to muscle (as expected for a motor command), but also back from muscle to cortex (reflecting sensory input). This oscillatory loop may allow the cortex to measure features of the limb state, integrating sensory inflow with the motor command.

Abstract

Corticomuscular coherence in the beta frequency band (15–30 Hz) has been demonstrated in both humans and monkeys, but its origin and functional role are still unclear. Phase–frequency plots produced by traditional coherence analysis are often complex. Some subjects show a clear linear phase–frequency relationship (indicative of a fixed delay) but give shorter delays than expected; others show a constant phase across frequencies. Recent evidence suggests that oscillations may be travelling around a peripheral sensorimotor loop. We recorded sensorimotor EEGs and EMGs from three intrinsic hand muscles in human subjects performing a precision grip task, and applied directed coherence (Granger causality) analysis to explore this system. Directed coherence was significant in both descending (EEG→EMG) and ascending (EMG→EEG) directions at beta frequencies. Average phase delays of 26.4 ms for the EEG→EMG direction and 29.5 ms for the EMG→EEG direction were closer to the expected conduction times for these pathways than the average delays estimated from coherence phase (7.9 ms). Subjects were sub-divided into different groups, based on the sign of the slope of the linear relation between corticomuscular coherence phase and frequency (positive, negative or zero). Analysis separated by these groups suggested that different relative magnitudes of EEG→EMG and EMG→EEG directed coherence might underlie the observed inter-individual differences in coherence phase. These results confirm the complex nature of corticomuscular coherence with contributions from both descending and ascending pathways.

Developmental profile of slow hand movement oscillation coupling in humans

In adults, slow hand and finger movements are characterized by 6- to 12-Hz discontinuities visible in the raw records and spectra of motion signals such as acceleration. This pulsitile behavior is correlated with motor unit synchronization at 6–12 Hz as shown by significant coherence at these frequencies between pairs of motor units and between the motor units and the acceleration recorded from the limb part controlled by the muscle, suggesting that it has a central origin. In this study, we examined the correlation between this 6- to 12-Hz pulsatile behavior and muscle activity as a function of childhood development. Sixty-eight participants (ages 4–25 yr) performed static wrist extensions against gravity or slow wrist extension and flexion movements while extensor carpi radialis muscle electromyographic (EMG) and wrist acceleration signals were simultaneously recorded. Coherence between EMG and acceleration within the 6- to 12-Hz frequency band was used as an index of the strength of the relation between central drive and the motor output. The main findings of the study are 1) EMG-acceleration coherence increased with increases in age, with the age differences being greater under movement conditions and the difference between conditions increasing with age; 2) the EMG signal showed increases in normalized power with increases in age under both conditions; and 3) coherence under movement conditions was moderately positively correlated with manual dexterity. These findings indicate that the strength of the 6- to 12-Hz central oscillatory drive to the motor output increases through childhood development and may contribute to age-related improvements in motor skills.

The BUMP model of response planning: intermittent predictive control accounts for 10 Hz physiological tremor

Physiological tremor during movement is characterized by ∼10 Hz oscillation observed both in the electromyogram activity and in the velocity profile. We propose that this particular rhythm occurs as the direct consequence of a movement response planning system that acts as an intermittent predictive controller operating at discrete intervals of ∼100 ms. The BUMP model of response planning describes such a system. It forms the kernel of Adaptive Model Theory which defines, in computational terms, a basic unit of motor production or BUMP. Each BUMP consists of three processes: (1) analyzing sensory information, (2) planning a desired optimal response, and (3) execution of that response. These processes operate in parallel across successive sequential BUMPs. The response planning process requires a discrete-time interval in which to generate a minimum acceleration trajectory to connect the actual response with the predicted future state of the target and compensate for executional error. We have shown previously that a response planning time of 100 ms accounts for the intermittency observed experimentally in visual tracking studies and for the psychological refractory period observed in double stimulation reaction time studies. We have also shown that simulations of aimed movement, using this same planning interval, reproduce experimentally observed speed-accuracy tradeoffs and movement velocity profiles. Here we show, by means of a simulation study of constant velocity tracking movements, that employing a 100 ms planning interval closely reproduces the measurement discontinuities and power spectra of electromyograms, joint-angles, and angular velocities of physiological tremor reported experimentally. We conclude that intermittent predictive control through sequential operation of BUMPs is a fundamental mechanism of 10 Hz physiological tremor in movement.

Spinal interneuron circuits reduce approximately 10-Hz movement discontinuities by phase cancellation

Tremor imposes an important limit to the accuracy of fine movements in healthy individuals and can be a disabling feature of neurological disease. Voluntary slow finger movements are not smooth but are characterized by large discontinuities (i.e., steps) in the tremor frequency range (approximately 10 Hz). Previous studies have shown that these discontinuities are coherent with activity in the primary motor cortex (M1), but that other brain areas are probably also involved. We investigated the contribution of three important subcortical areas in two macaque monkeys trained to perform slow finger movements. Local field potential and single-unit activity were recorded from the deep cerebellar nuclei (DCN), medial pontomedullary reticular formation, and the intermediate zone of the spinal cord (SC). Coherence between LFP and acceleration was significant at 6 to 13 Hz for all areas, confirming the highly distributed nature of the central network responsible for this activity. The coherence phase at 6 to 13 Hz for DCN and pontomedullary reticular formation was similar to our previous results in M1. By contrast, for SC the phase differed from M1 by approximately pi rad. Examination of single-unit discharge confirmed that this was a genuine difference in neural spiking and could not be explained by different properties of the local field potential. Convergence of antiphase oscillations from the SC with cortical and subcortical descending inputs will lead to cancellation of approximately 10 Hz oscillations at the motoneuronal level. This could appreciably limit drive to muscle at this frequency, thereby reducing tremor and improving movement precision.

Corticomuscular and bilateral EMG coherence reflect distinct aspects of neural synchronization

Using electroencephalography (EEG) and electromyography (EMG), corticomuscular and bilateral motor unit synchronization have been found in different frequency bands and under different task conditions. These different types of long-range synchrony are hypothesized to originate from distinct mechanisms. We tested this by comparing time-resolved EEG-EMG and EMG-EMG coherence in a bilateral precision-grip task. Bilateral EMG activity was synchronized between 7 and 13Hz for about 1s when force output from both hands changed from an increasing to a stable force production. In contrast, EEG-EMG coherence was statistically significant between 15 and 30Hz during stable force production. The disparities in their time-frequency profiles accord with the existence of distinct underlying processes for corticomuscular and bilateral motor unit synchronization. In addition, the absence of synchronization between cortical activity and common spinal input at 10Hz renders a cortical source unlikely.

Coherence between motor cortical activity and peripheral discontinuities during slow finger movements

Slow finger movements in man are not smooth, but are characterized by 8- to 12-Hz discontinuities in finger acceleration thought to have a central source. We trained two macaque monkeys to track a moving target by performing index finger flexion/extension movements and recorded local field potentials (LFPs) and spike activity from the primary motor cortex (M1); some cells were identified as pyramidal tract neurons by antidromic activation or as corticomotoneuronal cells by spike-triggered averaging. There was significant coherence between finger acceleration in the approximately 10-Hz range and both LFPs and spikes. LFP-acceleration coherence was similar for flexion and extension movements (0.094 at 9.8 Hz and 0.11 at 6.8 Hz, respectively), but substantially smaller during steady holding (0.0067 at 9.35 Hz). The coherence phase showed a significant linear relationship with frequency over the 6- to 13-Hz range, as expected for a constant conduction delay, but the slope indicated that LFP lagged acceleration by 18 +/- 14 or 36 +/- 8 ms for flexion and extension movements, respectively. Directed coherence analysis supported the conclusion that the dominant interaction was in the acceleration to LFP (i.e., sensory) direction. The phase relationships between finger acceleration and both LFPs and spikes shifted by about pi radians in flexion compared with extension trials. However, for a given trial type the phase relationship with acceleration was similar for cells that increased their firing during flexion or during extension trials. We conclude that movement discontinuities during slow finger movements arise from a reciprocally coupled network, which includes M1 and the periphery.

Bilateral motor unit synchronization of leg muscles during a simple dynamic balance task

To handle the rich repertoire of behavioural goals, the CNS has to control the many degrees of freedom of the musculoskeletal system in a flexible manner. This problem can be drastically simplified if muscle synergies serve as the to-be-controlled building blocks of motor performance, instead of the individual degrees of freedom. Muscle synergies have been identified as coherent activation patterns of a group of muscles in space or time, but the neural mechanisms underlying their formation remain largely unknown. Here we evaluated the hypothesis that synergies are reflected in common input to different contributing muscles, and investigated modulations in motor unit (MU) synchronization of homologous muscles during a rhythmic balance task. If common input is related to muscle synergies, the resultant MU synchronization should not be static but task dependent and, in the present context, vary in time. Coherence between surface electromyographic signals of bilateral leg muscles revealed MU synchronization in two distinct frequency bands. MU synchronization was not constant but modulated within a movement cycle, and its time course resembled the activation patterns of the muscles. These results are congruent with a linkage between MU synchronization and muscle synergies, and suggest that MU synchronization provides an expedient method for studying synergy-related neural mechanisms.

Low-Alcohol Doses Reduce Common 10- to 15-Hz Input to Bilateral Leg Muscles During Quiet Standing

The effects of low doses of alcohol on neural synchronization in muscular activity were investigated in ten participants during quiet standing with eyes open or closed. We focused on changes in common input to bilateral motor unit pools as evident in surface electromyographic (EMG) recordings of lower leg extensor and flexor muscles. The extensor muscles exhibited bilateral synchronization in two distinct frequency bands (i.e., 0–5 and 10–15 Hz), whereas synchronization between flexor muscles was minimal. As expected, alcohol ingestion affected postural sway, yielding increased sway at higher blood-alcohol levels. Whereas vision affected bilateral synchronization only at 0–5 Hz, alcohol ingestion resulted in a progressive decrease of synchronization at 10–15 Hz between the EMG activities of the extensor muscles. The decrease in common bilateral input is most likely related to reduced reticulospinal activity with alcohol ingestion.

Asymmetric control mechanisms of bimanual coordination: an application of directed connectivity analysis to kinematic and functional MRI data

Mirror-symmetrical bimanual movement is more stable than parallel bimanual movement. This is well established at the kinematic level. We used functional MRI (fMRI) to evaluate the neural substrates of the stability of mirror-symmetrical bimanual movement. Right-handed participants (n=17) rotated disks with their index fingers bimanually, both in mirror-symmetrical and asymmetrical parallel modes. We applied the Akaike causality model to both kinematic and fMRI time-series data. We hypothesized that kinematic stability is represented by the extent of neural "cross-talk": as the fraction of signals that are common to controlling both hands increases, the stability also increases. The standard deviation of the phase difference for the mirror mode was significantly smaller than that for the parallel mode, confirming that the former was more stable. We used the noise-contribution ratio (NCR), which was computed using a multivariate autoregressive model with latent variables, as a direct measure of the cross-talk between both the two hands and the bilateral primary motor cortices (M1s). The mode-by-direction interaction of the NCR was significant in both the kinematic and fMRI data. Furthermore, in both sets of data, the NCR from the right hand (left M1) to the left (right M1) was more prominent than vice versa during the mirror-symmetrical mode, whereas no difference was observed during parallel movement or rest. The asymmetric interhemispheric interaction from the left M1 to the right M1 during symmetric bimanual movement might represent cortical-level cross-talk, which contributes to the stability of symmetric bimanual movements.

Coherent Motor Unit Rhythms in the 6–10 Hz Range During Time-Varying Voluntary Muscle Contractions: Neural Mechanism and Relation to Rhythmical Motor Control

In quasi-sinusoidal (0.5–3.0 Hz) voluntary muscle contractions, we studied the 6- to 10-Hz motor unit (MU) firing synchrony and muscle force oscillation with emphasis on their neural substrate and relation to rhythmical motor control. Our analyses were performed on data from 121 contractions of a finger muscle in 24 human subjects. They demonstrate that coherent 6- to 10-Hz components of MU discharges coexist with carrier components and coherent modulation components underlying the voluntary force variations. The 6- to 10-Hz synchrony has the frequency of the tremor synchrony in steady contractions and is also widespread and in-phase. Its strength ranges from very small to very large (MU/MU coherence >0.50) among contractions; moreover, it is not related to the contraction parameters, in accord with the notion of a distinct 6- to 10-Hz synaptic input to the MUs. Unlike the coherent MU modulations and the voluntary force variations, the in-phase 6- to 10-Hz MU components are suppressed or even eliminated during ischemia, while the respective force component is drastically reduced. These findings agree with the widely assumed supraspinal origin of the MU modulations, but they also strongly suggest a key role for muscle spindle feedback in the generation of the 6- to 10-Hz synaptic input. They therefore provide important information for the study of generators of the 6- to 10-Hz rhythm which subserves the postulated rhythmical control and is manifested as force and movement components. Moreover, they argue for a participation of oscillating spinal stretch reflex loops in the rhythm generation, possibly in interaction with supraspinal oscillators.

Cells in somatosensory areas show synchrony with beta oscillations in monkey motor cortex

Oscillatory synchronization between somatosensory and motor cortex has previously been reported using field potential recordings, but interpretation of such results can be confounded by volume conduction. We examined coherence between single-unit discharge in somatosensory/parietal areas and local field potential from the same area as the unit, or from the motor cortex, in two macaque monkeys trained to perform a finger movement task. There were clear coherence peaks at approximately 17.5 Hz for cells in the primary somatosensory cortex (both proprioceptive and cutaneous areas) and posterior parietal cortex (area 5). The size of coherence in all areas was comparable to previous reports analysing motor cortical cells and M1 field potentials. Many coherence phases clustered around -pi/2 radians, indicating zero lag synchronization of parietal cells with M1 oscillatory activity. These results indicate that cells in somatosensory and parietal areas have information about the presence of oscillations in the motor system. Such oscillatory coupling across the central sulcus may play an important role in sensorimotor integration of both proprioceptive and cutaneous signals.

Digit displacement, not object compliance, underlies task dependent modulations in human corticomuscular coherence

Human sensorimotor EEG shows oscillatory activity at approximately 10 and approximately 20 Hz; the latter frequency is coherent with contralateral EMG. The functional significance of this activity is obscure. A recent study found that corticomuscular coherence varied systematically with increasing lever compliance during a precision grip task. However, since subjects exerted the same force in all conditions, changes in lever compliance also produced changes in how far the digits moved. In this study, we disambiguated whether corticomuscular coherence modulates with object compliance or digit displacement. Subjects performed a precision grip task. Under computer control, the manipulandum could simulate a load of arbitrary compliance (spring constant). Subjects were required to produce a hold-ramp-hold profile of lever displacement, under visual feedback. Subjects first performed tasks with different sized lever movements, against an isotonic load (zero spring constant). Corticomuscular coherence was calculated between left sensorimotor EEG and EMG from five right hand and forearm muscles during the hold phase of the task. Coherence magnitude showed a clear dependence on the extent of digit displacement. In the next task, lever compliance instantaneously changed at the onset of the second hold phase of the task. Corticomuscular coherence modulated not with lever compliance during the analysed hold phase, but with digit displacement during the preceding ramp movement. These data suggest that human corticomuscular coherence is directly related to digit displacement during the preceding movement and not to object compliance. We speculate that corticomuscular coherence may reflect a sensorimotor recalibration, providing updated information about system state following movement.

Afferent encoding of central oscillations in the monkey arm

We have investigated whether peripheral afferent fibers could encode the central oscillations that are commonly seen in the primate motor system. We analyzed 52 single afferent recordings from the C8/T1 dorsal root ganglia of two monkeys performing an isometric wrist flexion-extension task. Coherence and directed coherence were calculated between the afferent spikes and forearm EMG. Seven of 52 cells were identified as Group Ia afferents by the production of narrow postspike facilitation in spike-triggered averages of rectified EMG. These identified afferents showed significant coherence, and directed coherence, with EMG over a wide frequency range. By contrast, coherence was weak for a population that showed little directional preference for flexion or extension movements during task performance, and probably contained mainly cutaneous afferents. Oscillations are known to appear in muscle activity; their presence in afferent firing as well implies that central oscillations pass around a peripheral feedback loop and may be involved in sensorimotor integration.

Cortico-cerebellar coherence during a precision grip task in the monkey

We studied the synchronization of single units in macaque deep cerebellar nuclei (DCN) with local field potentials (LFPs) in primary motor cortex (M1) bilaterally during performance of a precision grip task. Analysis was restricted to periods of steady holding, during which M1 oscillations are known to be strongest. Significant coherence between DCN units and M1 LFP oscillations bilaterally was seen at approximately 10-40 Hz (contralateral M1: 25/87 units; ipsilateral: 9/87 units). Averaged coherence between DCN units and contralateral M1 LFP showed a prominent approximately 17-Hz coherence peak and an average phase of approximately -pi/2 radians, implying that the DCN units fired around the time of maximal depolarization of M1 cells. The lack of a time delay between DCN and M1 activity suggests that the cerebellum and cortex may form a pair of phase coupled oscillators. Although coherence values were low (mean peak coherence, 0.018), we used a computational model to show that this probably resulted from the nonlinearity of spike generating mechanisms within the DCN. DCN unit discharge and DCN LFPs also showed significant coherence at approximately 10-40 Hz, with similarly low magnitude (mean peak coherence, 0.012). The average coherence phase was -2.5 radians for the 6- to 14-Hz range and -1.1 radians for the 17- to 41-Hz range, suggesting different frequency-specific underlying mechanisms. Finally, 4/40 pairs of simultaneously recorded DCN units showed a significant cross-correlation peak, and 16/40 pairs showed significant unit-unit coherence. The extensive oscillatory synchronization observed between cerebellum and motor cortex may have functional importance in sensorimotor processing.

Is equilibrium point control feasible for fast goal-directed single-joint movements?

Several types of equilibrium point (EP) controllers have been proposed for the control of posture and movement. EP controllers are appealing from a computational perspective because they do not require solving the "inverse dynamic problem" (i.e., computation of the torques required to move a system along a desired trajectory). It has been argued that EP controllers are not capable of controlling fast single-joint movements. To refute this statement, several extensions have been proposed, although these have been tested using models in which only the tendon compliance, force-length-velocity relation, and mechanical interaction between tendon and contractile element were not adequately represented. In the present study, fast elbow-joint movements were measured and an attempt was made to reproduce these using a realistic musculoskeletal model of the human arm. Three types of EP controllers were evaluated: an open-loop alpha-controller, a closed-loop lambda-controller, and a hybrid open- and closed-loop controller. For each controller we considered a continuous version and a version in which the control signals were sent out intermittently. Only the intermittent hybrid EP controller was capable of generating movements that were as fast as those of the subjects. As a result of the nonlinear muscle properties, the hybrid EP controller requires a more detailed representation of static muscle properties than generally assumed in the context of EP control. In sum, this study shows that fast single-joint movements can be realized without explicitly solving the inverse dynamics problem, but in a less straightforward manner than implied by proponents of conventional EP controllers.

Manipulation of peripheral neural feedback loops alters human corticomuscular coherence

Sensorimotor EEG shows approximately 20 Hz coherence with contralateral EMG. This could involve efferent and/or afferent components of the sensorimotor loop. We investigated the pathways responsible for coherence genesis by manipulating nervous conduction delays using cooling. Coherence between left sensorimotor EEG and right EMG from three hand and two forearm muscles was assessed in healthy subjects during the hold phase of a precision grip task. The right arm was then cooled to 10 degrees C for approximately 90 min, increasing peripheral motor conduction time (PMCT) by approximately 35% (assessed by F-wave latency). EEG and EMG recordings were repeated, and coherence recalculated. Control recordings revealed a heterogeneous subject population. In 6/15 subjects (Group A), the corticomuscular coherence phase increased linearly with frequency, as expected if oscillations were propagated along efferent pathways from cortex to muscle. The mean corticomuscular conduction delay for intrinsic hand muscles calculated from the phase-frequency regression slope was 10.4 ms; this is smaller than the delay expected for conduction over fast corticospinal pathways. In 8/15 subjects (Group B), the phase showed no dependence with frequency. One subject showed both Group A and Group B patterns over different frequency ranges. Following cooling, averaged corticomuscular coherence was decreased in Group A subjects, but unchanged for Group B, even though both groups showed comparable slowing of nervous conduction. The delay calculated from the slope of the phase-frequency regression was increased following cooling. However, the size of this increase was around twice the rise in PMCT measured using the F-wave (regression slope 2.33, 95% confidence limits 1.30-3.36). Both afferent and efferent peripheral nerves will be slowed by similar amounts following cooling. The change in delay calculated from the coherence phase therefore better matches the rise in total sensorimotor feedback loop time caused by cooling, rather than just the change in the efferent limb. A model of corticomuscular coherence which assumes that only efferent pathways contribute cannot be reconciled to these results. The data rather suggest that afferent feedback pathways may also play a role in the genesis of corticomuscular coherence.

The effect of carbamazepine on human corticomuscular coherence

EEG recordings from motor cortex show oscillations at approximately 10 and 20 Hz. The 20-Hz oscillations are coherent with contralateral EMG; in most studies those at 10 Hz are not. However, significant 10-Hz coherence has recently been reported in a group of epileptic patients, all of whom were taking the anticonvulsant drug carbamazepine (CBZ). In a double blind study, we investigated the effects of CBZ on corticomuscular coherence in eight healthy human subjects (all male). Subjects performed a precision grip task against an auxotonic load, whilst left sensorimotor EEG and EMGs from five muscles in the right hand and forearm were recorded. CBZ (100 mg) or a placebo was then given orally, and 6 h later subjects were re-tested. One week separated CBZ and placebo experiments in each subject. Coherence averaged across subjects and muscles during the hold phase of the task was maximal at 21 Hz; it increased significantly (P < 0.05, Z-test) by 89% after CBZ administration. This was significantly greater than a much smaller increase following placebo, which itself may reflect an effect of the time of day when experiments were performed. There was no significant approximately 10-Hz coherence either before or after CBZ administration. CBZ did not significantly alter EEG power at either 10 or 20 Hz. Recently, we showed that diazepam markedly increases the power of approximately 20-Hz motor cortical oscillations with little effect on coherence. We show here that CBZ raises coherence without altering EEG power. This pharmacological dissociation may indicate an important role for corticomuscular coherence in motor control.