Synchronization in monkey motor cortex during a precision grip task. II. effect of oscillatory activity on corticospinal output

Revealing neuronal functional organization through the relation between multi-scale oscillatory extracellular signals

The spatial organization of neuronal elements and their connectivity make up the substrate underlying the information processing carried out in the networks they form. Conventionally, anatomical findings make the initial structure which later combines with superimposed neurophysiological information to create a functional organization map. The most common neurophysiological measure is the single neuron spike train extracted from an extracellular recording. This single neuron firing pattern provides valuable clues on information processing in a given brain area; however, it only gives a sparse and focal view of this process. Even with the increase in number of simultaneously recorded neurons, inference on their large-scale functional organization remains problematic. We propose a method of utilizing additional information derived from the same extracellular recording to generate a more comprehensive picture of neuronal functional organization. This analysis is based on the relationship between the oscillatory activity of single neurons and their neighboring neuronal populations. Two signals that reflect the multiple scales of neuronal populations are used to complement the single neuron spike train: (1) the high-frequency background unit activity representing the spiking activity of small localized sub-populations and (2) the low-frequency local field potential that represents the synaptic input to a larger global population. The three coherences calculated between pairs of these three signals arising from a single source of extracellular recording are then used to infer mosaic representations of the functional neuronal organization. We demonstrate this methodology on experimental data and on simulated leaky integrate-and-fire neurons.

Neocortical networks entrain neuronal circuits in cerebellar cortex

Activity in neocortex is often characterized by synchronized oscillations of neurons and networks, resulting in the generation of a local field potential (LFP) and electroencephalogram. Do the neuronal networks of the cerebellum also generate synchronized oscillations and are they under the influence of those in the neocortex? Here we show that, in the absence of any overt external stimulus, the cerebellar cortex generates a slow oscillation that is correlated with that of the neocortex. Disruption of the neocortical slow oscillation abolishes the cerebellar slow oscillation, whereas blocking cerebellar activity has no overt effect on the neocortex. We provide evidence that the cerebellar slow oscillation results in part from the activation of granule, Golgi, and Purkinje neurons. In particular, we show that granule and Golgi cells discharge trains of single spikes, and Purkinje cells generate complex spikes, during the "up" state of the slow oscillation. Purkinje cell simple spiking is weakly related to the cerebellar and neocortical slow oscillation in a minority of cells. Our results indicate that the cerebellum generates rhythmic network activity that can be recorded as an LFP in the anesthetized animal, which is driven by synchronized oscillations of the neocortex. Furthermore, we show that correlations between neocortical and cerebellar LFPs persist in the awake animal, indicating that neocortical circuits modulate cerebellar neurons in a similar manner in natural behavioral states. Thus, the projection neurons of the neocortex collectively exert a driving and modulatory influence on cerebellar network activity.

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.

Renshaw cell recurrent inhibition improves physiological tremor by reducing corticomuscular coupling at 10 Hz

Corticomuscular coherence between the primary motor cortex (M1) and hand muscle electromyograms (EMG) occurs at approximately 20 Hz but is rarely seen at approximately 10 Hz. This is unexpected, because M1 has oscillations at both frequencies, which are effectively transmitted to the spinal cord via the corticospinal tract. We have previously speculated that a specific "neural filter" may selectively reduce coherence at approximately 10 Hz. This would have functional utility in minimizing physiological tremor, which often has a dominant component around this frequency. Recurrent inhibition via Renshaw cells in the spinal cord is a putative neural substrate for such a filter. Here we investigate this system in more detail with a biophysically based computational model. Renshaw cell recurrent inhibition reduced EMG oscillations at approximately 10 Hz, and also reduced corticomuscular coherence at this frequency (from 0.038 to 0.014). Renshaw cell inhibitory feedback also generated synchronous oscillations in the motoneuron pool at approximately 30 Hz. We show that the effects at 10 Hz and 30 Hz can both be understood from the dynamics of the inhibitory feedback loop. We conclude that recurrent inhibition certainly plays an important role in reducing 10 Hz oscillations in muscle, thereby decreasing tremor amplitude. However, our quantitative results suggest it is unlikely to be the only system for tremor reduction, and probably acts in concert with other neural circuits which remain to be elucidated.

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.

Application of matched filtering to identify behavioral modulation of brain oscillations

Brain oscillations modulated by motor behaviors are coupled to steady-state and other potentially unrelated to movement oscillations, with energy in the same frequency bands as the signals of interest. We applied matched filtering, a quasi-optimum signal detection technique, to decouple and extract movement-related signals from local field potentials (LFPs) recorded in monkey motor cortical areas during the execution of a visually instructed reach-out task. Using a matched-filterbank, we examined coupling and interference of pre-movement and initial steady-state oscillations with movement-induced signals. Once these signal contributions were eliminated, we were able to identify significant correlations of the residual signals with behavioral parameters, which appeared attenuated by pre-movement signal interference in the raw LFPs. Specifically, the maximum and minimum amplitudes of filtered LFPs were directly modulated by peak movement velocity and micro-movements, respectively, identified in recorded hand velocity profiles. In addition, we identified phase correlations between signals during the delay (when the instructional cue was presented) and movement intervals, as well as modulation of LFP phase by movement direction. For pairs of orthogonal movement directions, corresponding LFP signals were consistently out of phase. Finally, β-band energy which is typically reduced during movement execution, possibly partly due to destructive interference between the modulated by behavior signal and unrelated oscillations, appeared to be recovered in the filtered signals.

Bridging the brain to the world: a perspective on neural interface systems

Neural interface (NI) systems hold the potential to return lost functions to persons with paralysis. Impressive progress has been made, including evaluation of neural control signals, sensor testing in humans, signal decoding advances, and proof-of-concept validation. Most importantly, the field has demonstrated that persons with paralysis can use prototype systems for spelling, "point and click," and robot control. Human and animal NI research is advancing knowledge about neural information processing and plasticity in healthy, diseased, and injured nervous systems. This emerging field promises a range of neurotechnologies able to return communication, independence, and control to people with movement limitations.

Selectivity for grasp in local field potential and single neuron activity recorded simultaneously from M1 and F5 in the awake macaque monkey

The selectivity for object-specific grasp in local field potentials (LFPs) was investigated in two awake macaque monkeys trained to observe, reach out, grasp and hold one of six objects presented in a pseudorandom order. Simultaneous, multiple electrode recordings were made from the hand representations of primary motor cortex (M1) and ventral premotor cortex (area F5). LFP activity was well developed during the observation and hold periods of the task, especially in the beta-frequency range (15-30 Hz). Selectivity of LFP activity for upcoming grasp was rare in the observation period, but common during stable grasp. The majority of M1 (90 of 92) and F5 (81 of 97) sites showed selectivity for at least one frequency, which was maximal in the beta range but also present at higher frequencies (30-50 Hz). When the LFP power associated with grasp of a specific object was large in the beta-frequency range, it was usually of low power in the higher 30-50 Hz range, and vice-versa. Simple hook grips involving flexion of one or more fingers were associated with large beta power, whereas more complex grips involving the thumb (e.g., precision grip) were associated with small beta power. At many M1 sites, there was a highly significant inverse relationship between the tuning of spikes (including those of identified pyramidal tract neurons) and beta-range LFP for different grasps, whereas a positive correlation was found at higher frequencies (30-50 Hz). High levels of beta LFP and low pyramidal cell spike rate may reflect a common mechanism used to control motor set during different types of grasp.

Circuits generating corticomuscular coherence investigated using a biophysically based computational model. I. Descending systems

Recordings of motor cortical activity typically show oscillations around 10 and 20 Hz; only those at 20 Hz are coherent with electromyograms (EMGs) of contralateral muscles. Experimental measurements of the phase difference between approximately 20-Hz oscillations in cortex and muscle are often difficult to reconcile with the known corticomuscular conduction delays. We investigated the generation of corticomuscular coherence further using a biophysically based computational model, which included a pool of motoneurons connected to motor units that generated EMGs. Delays estimated from the coherence phase–frequency relationship were sensitive to the width of the motor unit action potentials. In addition, the nonlinear properties of the motoneurons could produce complex, oscillatory phase–frequency relationships. This was due to the interaction of cortical inputs to the motoneuron pool with the intrinsic rhythmicity of the motoneurons; the response appeared more linear if the firing rate of motoneurons varied widely across the pool, such as during a strong contraction. The model was able to reproduce the smaller than expected delays between cortex and muscles seen in experiments. However, the model could not reproduce the constant phase over a frequency band sometimes seen in experiments, nor the lack of around 10-Hz coherence. Simple propagation of oscillations from cortex to muscle thus cannot completely explain the observed corticomuscular coherence.

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.

Study on connectivity between coherent central rhythm and electromyographic activities

Whether afferent feedback contributes to the generation of cortico-muscular coherence (CMCoh) remains an open question. In the present study, a multivariate autoregressive (MVAR) model and partial directed coherence (PDC) were applied to investigate the causal influences between the central rhythm and electromyographic (EMG) signals in the process of CMCoh. The system modeling included activities from the contralateral and ipsilateral primary sensorimotor cortex (M1/S1), supplementary motor area (SMA) and the time series from extensor carpi radialis (ECR) muscles. The results showed that afferent sensory feedback could also play an important role for the generation of CMCoh. Meanwhile, significant coherence between the EMG signals and the activities in the SMA was found in two subjects out of five. Connectivity analysis revealed a significant descending information flow which possibly reflected direct recruitment on the motoneurons from the SMA to facilitate motor control.

Oscillatory interactions between sensorimotor cortex and the periphery

Field potential recordings from motor cortex show oscillations in the beta-band (∼20 Hz), which are coherent with similar oscillations in the activity of contralateral contracting muscles. Recent findings have revised concepts of how this activity might be generated in the cortex, suggesting it could achieve useful computation. Other evidence shows that these oscillations engage not just motor structures, but also return from muscle to the central nervous system via feedback afferent pathways. Somatosensory cortex has strong beta-band oscillations, which are synchronised with those in motor cortex, allowing oscillatory sensory reafference to be interpreted in the context of the oscillatory motor command which produced it.

Spinomuscular coherence in monkeys performing a precision grip task

We recorded local field potentials (LFPs) from cervical spinal cord (C(5)-C(8)) in monkeys performing a precision grip task and examined their coherence with electromyographic (EMG) activities (spinomuscular coherence) recorded from hand and arm muscles. Among 164 LFP-EMG pairs, significant coherence was found in 34 pairs (21%). We classified the coherence into two groups based on its frequency range, narrowband coherence, and broadband coherence. The narrowband coherence was restricted to discrete frequencies in the range of 14-55 Hz and was widespread throughout the superficial and deep gray matter. In contrast, the broadband coherence distributed between 10 and 95 Hz and was found only in the ventral half of the spinal cord. The narrowband coherence suggests that oscillations, which have been described in many motor control areas of the brain, could also pass though spinal interneurons to affect motor output and sensorimotor integration. On the other hand, the broadband coherence could be a unique feature of spinal motoneuron-muscle physiology.

The oscillation score: an efficient method for estimating oscillation strength in neuronal activity

We present a method that estimates the strength of neuronal oscillations at the cellular level, relying on autocorrelation histograms computed on spike trains. The method delivers a number, termed oscillation score, that estimates the degree to which a neuron is oscillating in a given frequency band. Moreover, it can also reliably identify the oscillation frequency and strength in the given band, independently of the oscillation in other frequency bands, and thus it can handle superimposed oscillations on multiple scales (theta, alpha, beta, gamma, etc.). The method is relatively simple and fast. It can cope with a low number of spikes, converging exponentially fast with the number of spikes, to a stable estimation of the oscillation strength. It thus lends itself to the analysis of spike-sorted single-unit activity from electrophysiological recordings. We show that the method performs well on experimental data recorded from cat visual cortex and also compares favorably to other methods. In addition, we provide a measure, termed confidence score, that determines the stability of the oscillation score estimate over trials.

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.

Cortical asymmetry: catching an object in free fall

The main goal of the present study was to analyze theta asymmetry through quantitative electroencephalography (qEEG) when individuals were exposed to a sequential motor task, i.e. catching a ball. The sample was composed of 23 healthy subjects, male and female, between 25 and 40 years of age. A two-way factor Anova was applied to compare pre and post moments related to the balls' drop and scalp regions (i.e., frontal and parieto-occipital cortices). The first analysis of the frontal region compared electrodes in the left, right and left/right hemispheres combined, with the frontal midline electrode (FZ) included in the analysis. The results showed moment and region main effects. The second analysis compared left versus right hemisphere without the FZ site. The findings demonstrated an interaction effect between moment and region. The first parieto-occipital analysis, comparing left, right and central regions, with PZ included in all regions, showed main effects of moment and region. The second analysis, comparing left, right (without Pz) and central regions strictly demonstrated a region main effect. Thus, we observed an asymmetric pattern in the frontal cortex (i.e., planning and response selection) when the subjects were waiting for the balls' drop. Moreover, the left hemisphere seems to engage differently from the other regions when the central nervous system needs to prepare for a motor action. On the other hand, the parieto-occipital cortex, which is related to attentive processes, demonstrated a more asymmetric activity towards the right region which implies a participation of this area in cognitive strategies in this particular task. Taken together, we concluded that the adopted experimental approach can be useful to explore several others directions combining sensorimotor integration tasks with different pathologies, such as depression, Alzheimer's and Parkinson's diseases.

Muscle responses to transcranial stimulation in man depend on background oscillatory activity

Muscle responses to transcranial stimulation show high sweep-to-sweep variability, which may reflect an underlying noise process in the motor system. We examined whether response amplitude correlated with the level of prestimulus background EMG, and network oscillations. Transcranial magnetic or electrical stimulation was delivered to primary motor cortex whilst human subjects performed a precision grip task known to promote beta-band ( approximately 20 Hz) cortical oscillations. Responses were recorded from two intrinsic hand muscles. Response magnitude correlated significantly with the level of background EMG (mean r(2) = 0.20). Using a novel wavelet method, we quantified the amplitude and phase of oscillations in prestimulus sensorimotor EEG. Surprisingly, response magnitude showed no significant correlation with EEG oscillations at any frequency. However, oscillations in the prestimulus EMG were significantly correlated with response size; the correlation coefficient had peaks around 20 Hz. When oscillations in one muscle were used to predict response amplitude in a different muscle, correlations were substantially smaller. Finally, for each recording, we calculated the best possible prediction of response size obtainable from up to 20 measures of prestimulus EEG and EMG oscillations. Such optimal predictions had low correlation coefficients (mean r(2) = 0.2; 76% were below 0.3). We conclude that prestimulus oscillations, mainly in the beta-band, do explain some of the variability in responses to transcranial stimulation. Oscillations may likewise increase the noise of natural motor processing, explaining why this form of network activity is usually suppressed prior to dynamic movements. However, the majority of the variation is determined by other factors, which are not accessible by noninvasive recordings.

Different contributions of the corpus callosum and cerebellum to motor coordination in monkey

We investigated the different contribution of the corpus callosum (CC) and cerebellum to motor control in two macaque monkeys trained to perform a precision grip task with one or both hands. Recordings were made from antidromically identified CC cells and nearby unidentified neurons (UIDs) in the hand representation of the supplementary motor area (SMA) and compared with cells from the deep cerebellar nuclei (DCN). All cells showed their greatest modulation in activity (rate change locked to particular task event) during the movement epochs of the task (CC, 21.3 +/- 22.2; UIDs, 36.2 +/- 30.1 spike/s for contralateral trials; DCN, 63 +/- 56.4 for ipsilateral trials; mean +/- SD). Surprisingly, CC cells fired at very low basal rates compared with UIDs (3.9 +/- 4.9 vs. 10 +/- 9.1 spike/s) or DCN neurons (50.8 +/- 23.8 spike/s). However, SMA cells had the greatest rate modulation to baseline ratio (CC: 12.1 +/- 13.7; UID: 5.3 +/- 5.4; DCN: 1.7 +/- 2.0). This would allow them to code the timing of a behavioral event with better fidelity than DCN cells. A multivariate regression analysis between cell firing and EMG measured cells' representation of moment-by-moment modulations in muscle activity. CC neurons coded these real-time behavioral parameters significantly less well than the other cells types, using both linear and nonlinear models. Basal firing rate substantially constrains cell function. CC cells with low basal rates have restricted dynamic range for coding continuous parameters, but efficiently code the time of discrete behavioral events. DCN neurons with higher basal rates are better suited to control continuously variable parameters of movement.

Thumb and finger forces produced by motor units in the long flexor of the human thumb

The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers (P < 0.001). On average, these motor units produced small but significant loading forces on the index finger ( approximately 5.3% of their force on the thumb) with the same time-to-peak force as the thumb ( approximately 50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2-10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.

Neural networks a century after Cajal

At the time of Golgi and Cajal's reception of the Nobel Prize in 1906 most scientists had accepted the notion that neurons are independent units. Although neuroscientists today still believe that neurons are independent anatomical units, functionally, it is thought that some sort of population coding occurs. Throughout this essay, we provide evidence that suggests that populations of neurons can code information through the synchronization of their responses. This synchronization occurs at several levels in the brain. Whereas spike synchrony refers to the correlation between spikes of different neurons' spike trains, oscillatory synchrony refers to the synchronization of oscillatory responses, generally among large groups of neurons. In the first section of this essay we describe the dependence of the brain's developmental processes on synchronous firing and how these processes form a brain that supports and is sensitive to synchronous spikes. Data are then presented that suggest that spike and oscillatory synchrony may serve as useful neural codes. Examples from sensory (auditory, olfactory and somatosensory), motor and higher cognitive (attention, memory) systems are then presented to illustrate potential roles for these synchronous codes in normal brain function. Results from these studies collectively suggest that spike synchrony in sensory and motor systems may provide detail information not available from changes in firing rate. Oscillatory synchrony, on the other hand, may be globally involved in the coordination of long-distance neuronal communication during higher cognitive processes. These concepts represent a dramatic shift in direction since the times of Golgi and Cajal.

Input-Driven Oscillations in Networks with Excitatory and Inhibitory Neurons with Dynamic Synapses

Previous work has shown that networks of neurons with two coupled layers of excitatory and inhibitory neurons can reveal oscillatory activity. For example, Börgers and Kopell (2003) have shown that oscillations occur when the excitatory neurons receive a sufficiently large input. A constant drive to the excitatory neurons is sufficient for oscillatory activity. Other studies (Doiron, Chacron, Maler, Longtin, & Bastian, 2003; Doiron, Lindner, Longtin, Maler, & Bastian, 2004) have shown that networks of neurons with two coupled layers of excitatory and inhibitory neurons reveal oscillatory activity only if the excitatory neurons receive correlated input, regardless of the amount of excitatory input. In this study, we show that these apparently contradictory results can be explained by the behavior of a single model operating in different regimes of parameter space. Moreover, we show that adding dynamic synapses in the inhibitory feedback loop provides a robust network behavior over a broad range of stimulus intensities, contrary to that of previous models. A remarkable property of the introduction of dynamic synapses is that the activity of the network reveals synchronized oscillatory components in the case of correlated input, but also reflects the temporal behavior of the input signal to the excitatory neurons. This allows the network to encode both the temporal characteristics of the input and the presence of spatial correlations in the input simultaneously.

Cortical Involvement in the Generation of Essential Tremor

Conflicting results on the existence of tremor-related cortical activity in essential tremor (ET) have raised questions on the role of the cortex in tremor generation. Here we attempt to address these issues. We recorded 64 channel surface EEGs and EMGs from forearm muscles in 15 patients with definite ET. EEG and EMG power spectra, relative power of the rhythmic EMG activity, relative EEG power at the tremor frequency, and EEG–EMG and EEG–EEG coherence were calculated and their dynamics over time explored. Corticomuscular delay was studied using a new method for narrow-band coherent signals. Corticomuscular coherence in the contralateral central region at the tremor frequency was present in all patients in recordings with a relative tremor EMG power exceeding a certain level. However, the coherence was lost intermittently even with tremors far above this level. Physiological 15- to 30-Hz coherence was found consistently in 11 patients with significantly weaker EMG activity in this frequency range. A more frontal (mesial) hot spot was also intermittently coupled with the tremor and the central hot spot in five patients. Corticomuscular delays were compatible with transmission in fast corticospinal pathways and feedback of the tremor signal. Thus the tremor rhythm is intermittently relayed only in different cortical motor areas. We hypothesize that tremor oscillations build up in different subcortical and subcortico-cortical circuits only temporarily entraining each other.

Muscle temperature has a different effect on force fluctuations in young and older women

OBJECTIVE

To investigate the effect of muscle temperature on force fluctuations during isometric contractions in young and older females.

METHODS

Fifteen young and 11 older subjects performed 3x30-s long submaximal isometric ankle dorsi-flexions (5%, 10%, and 15% of the maximal force). Tibialis anterior muscle temperature was monitored with an intramuscular probe and manipulated to obtain a cold, control, and warm condition. The coefficient of variation (CofV) and the relative power in the frequency bands 0-3Hz (low), 4-6Hz (middle), and 8-12Hz (high) of the force signal were computed to characterise steadiness. Intramuscular EMG signals were recorded from the tibialis anterior muscle to assess motor unit discharge pattern.

RESULTS

CofV was higher in the older than in the young subjects (P<0.001) in all conditions. In the older group only, CofV increased with cooling with respect to control temperature (P<0.001), whereas in the young group only, relative power of force fluctuations at high frequency decreased with cooling. Motor unit discharge rate and inter-pulse interval variability were not different between groups and across temperatures.

CONCLUSIONS

The findings indicate a different effect of temperature on the ability to maintain constant force in young and older subjects.

SIGNIFICANCE

These results highlight the risk of further impairment to the motor control of older individuals with varying temperature.

Network oscillations and intrinsic spiking rhythmicity do not covary in monkey sensorimotor areas

We investigated the relationship between local field potential (LFP) oscillations and intrinsic spiking rhythmicity in the sensorimotor system, because intrinsic rhythmicity has the potential to enhance network oscillations. LFPs and 918 single units were recorded from primary motor cortex (M1), primary somatosensory cortex (S1, areas 3a and 2), posterior parietal cortex (area 5) and the deep cerebellar nuclei (DCN). Some cells were antidromically identified as pyramidal tract neurons (PTNs). In each area the power of approximately 20 Hz LFP oscillations was assessed during periods of steady holding, when such oscillations have previously been shown to be maximal in M1. Oscillations were strongest in area 5 and weakest in the DCN. Using a previously developed method, the postspike distance-to-threshold trajectory was determined from the interspike interval histogram for each cell. Many cells had significant peaks, suggesting an intrinsic tendency towards rhythmic firing. Surprisingly, trajectory peaks were most common for M1 PTNs (115/146 cells) and rarest for area 5 neurons (12/82 cells). The extent of intrinsic spiking rhythmicity is not therefore simply related to the strength of 20 Hz oscillations in the sensorimotor system. These results suggest that intrinsic rhythmicity is not required for the generation and maintenance of oscillatory activity.

Correlated neural activity as the driving force for functional changes in auditory cortex

The functional role of neural synchrony is reflected in cortical tonotopic map reorganization and in the emergence of pathological phenomena such as tinnitus. First of all experimenter-centered and subject-centered views of neural activity will be contrasted; this argues against the use of stimulus-correction procedures and favors the use of a correction procedure based on neural activity without reference to stimulus timing. Within a cortical column neurons fired synchronously with on average about 6% of their spikes in a 1 ms bin and occasionally showing 30% or more of such coincident spikes. For electrode separations exceeding 200 microm the average peak correlation strength only occasionally reached 3%. The experimental evidence for coincidence of neural activity, neural correlation and neural synchrony shows that horizontal fibers activity can induce strong neural correlations. Cortico-cortical connections for a large part connect cell groups with characteristic frequencies differing by more than one octave. Such neurons have generally non-overlapping receptive fields but still can have sizeable peak cross-correlations. Correlated neural activity and heterotopic neural interconnections are presented as the substrates for cortical reorganization; increased neural synchrony and tonotopic map reorganization go hand in hand. This links cortical reorganization with hypersynchrony that can be considered as an important driving force underlying tinnitus.

Cortical functional network organization from autoregressive modeling of local field potential oscillations

A framework is presented for quantifying functional network organization in the brain by spectral analysis based on autoregressive modeling. Local field potentials (LFPs), simultaneously recorded from distributed sites in the cerebral cortex of monkeys, are treated as signals generated by local neuronal assemblies. During the delay period of a visual pattern discrimination task, oscillatory assembly activity is manifested in the LFPs in the beta-frequency range (14-30 Hz). Coherence analysis has shown that these oscillations are phase synchronized in functional networks in the sensorimotor cortex in relation to maintenance of contralateral hand position, and in the visual cortex in relation to anticipation of the visual stimulus. Granger causality analysis has revealed information flow in the sensorimotor network that is consistent with a peripheral sensorimotor feedback loop, and in the visual network that is consistent with top-down anticipatory modulation of assemblies in the primary visual cortex.

Direct comparison of the task-dependent discharge of M1 in hand space and muscle space

Since its introduction in the early 1980s, the concept of a "preferred direction" for neuronal discharge has proven to be a powerful means of studying motor areas of the brain. In the current paper, we introduce the concept of a "muscle-space"-preferred direction (PD(M)) that is analogous to the familiar hand-space-preferred direction (PD(H)). PD(M) reflects the similarity between the discharge of a given neuron and the activity of each muscle in much the way that PD(H) reflects the similarity of discharge with motion along each of the three Cartesian coordinate axes. We used PD(M) to analyze the data recorded from neurons in the primary motor cortex (M1) of three different monkeys. The monkeys performed center-out movements within two different cubical workspaces centered either to the left or right of the monkey's shoulder while we simultaneously recorded neuronal discharge, muscle activity, and limb orientation. We calculated preferred directions in both hand space and muscle space, and computed the angles between these vectors under a variety of conditions. PDs for different neurons were broadly distributed throughout both hand space and muscle space, but the muscle-space vectors appeared to form clusters of functionally similar neurons. In general, repeated estimates of PD(M) were more stable over time than were similar estimates of PD(H). Likewise, there was less change in PD(M) than in PD(H) for data recorded from the two different workspaces. However, although a majority of neurons had this muscle-like property, a significant minority was more stable in Cartesian hand space, reflecting a heterogeneity of function within M1.

Cortical muscle coupling in Parkinson's disease (PD) bradykinesia

OBJECTIVES

To determine if novel methods establishing patterns in EEG-EMG coupling can infer subcortical influences on the motor cortex, and the relationship between these subcortical rhythms and bradykinesia.

BACKGROUND

Previous work has suggested that bradykinesia may be a result of inappropriate oscillatory drive to the muscles. Typically, the signal processing method of coherence is used to infer coupling between a single channel of EEG and a single channel of rectified EMG, which demonstrates 2 peaks during sustained contraction: one, approximately 10 Hz, which is pathologically increased in PD, and a approximately 30 Hz peak which is decreased in PD, and influenced by pharmacological manipulation of GABAA receptors in normal subjects.

MATERIALS AND METHODS

We employed a novel multiperiodic squeezing paradigm which also required simultaneous movements. Seven PD subjects (on and off L-Dopa) and five normal subjects were recruited. Extent of bradykinesia was inferred by reduced relative performance of the higher frequencies of the squeezing paradigm and UPDRS scores. We employed Independent Component Analysis (ICA) and Empirical Mode Decomposition (EMD) to determine EEG/EMG coupling.

RESULTS

Corticomuscular coupling was detected during the continually changing force levels. Different components included those over the primary motor cortex (ipsilaterally and contralaterally) and over the midline. Subjects with greater bradykinesia had a tendency towards increased approximately 10 Hz coupling and reduced approximately 30 Hz coupling that was erratically reversed with L-dopa.

CONCLUSIONS

These results suggest that lower approximately 10 Hz peak may represent pathological oscillations within the basal ganglia which may be a contributing factor to bradykinesia in PD.

Precise rhythmicity in activity of neocortical, thalamic and brain stem neurons in behaving cats and rabbits

Rhythmic discharges of neurons are believed to be involved in information processing in both sensory and motor systems. However their fine structure and functional role need further elucidation. We employed a pattern-based approach to search for episodes of precisely rhythmic activity of single neurons recorded in different brain structures in behaving cats and rabbits. We defined discharge patterns using an algorithmic description, which is different from the previously suggested template methods. We detected episodes of precisely rhythmic discharges, specifically, triads of constant (precision +/-2.5%) inter-spike intervals in the 10-70 ms range. In 54% (67/125) of neurons tested, these patterns could not be explained by random occurrences or by steady or slowly changing input. Rhythmic patterns occurred at a wide range of inter-spike intervals, and were imbedded in non-rhythmic activity. In many neurons, timing of these precisely rhythmic patterns was related to different locomotion tasks or to respiration.

Assessing Neuronal Coherence with Single-Unit, Multi-Unit, and Local Field Potentials

The purpose of this study was to obtain a better understanding of neuronal responses to correlated input, in particular focusing on the aspect of synchronization of neuronal activity. The first aim was to obtain an analytical expression for the coherence between the output spike train and correlated input and for the coherence between output spike trains of neurons with correlated input. For Poisson neurons, we could derive that the peak of the coherence between the correlated input and multi-unit activity increases proportionally with the square root of the number of neurons in the multi-unit recording. The coherence between two typical multi-unit recordings (2 to 10 single units) with partially correlated input increases proportionally with the number of units in the multi-unit recordings. The second aim of this study was to investigate to what extent the amplitude and signal-to-noise ratio of the coherence between input and output varied for single-unit versus multi-unit activity and how they are affected by the duration of the recording. The same problem was addressed for the coherence between two single-unit spike series and between two multi-unit spike series. The analytical results for the Poisson neuron and numerical simulations for the conductance-based leaky integrate-and-fire neuron and for the conductance-based Hodgkin-Huxley neuron show that the expectation value of the coherence function does not increase for a longer duration of the recording. The only effect of a longer duration of the spike recording is a reduction of the noise in the coherence function. The results of analytical derivations and computer simulations for model neurons show that the coherence for multi-unit activity is larger than that for single-unit activity. This is in agreement with the results of experimental data obtained from monkey visual cortex (V4). Finally, we show that multitaper techniques greatly contribute to a more accurate estimate of the coherence by reducing the bias and variance in the coherence estimate.

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.

Measurement of time-dependent changes in the irregularity of neural spiking

Irregularity of firing in spike trains has been associated with coding processes and information transfer or alternatively treated as noise. Previous studies of irregularity have mainly used the coefficient of variation (CV) of the interspike interval distribution. Proper estimation of CV requires a constant underlying firing rate, a condition that most experimental situations do not fulfill either within or across trials. Here we introduce a novel irregularity metric based on the ratio of adjacent intervals in the spike train. The new metric is not affected by firing rate and is very localized in time so that it can be used to examine the time course of irregularity relative to an alignment marker. We characterized properties of the new metric with simulated spike trains of known characteristics and then applied it to data recorded from 108 single neurons in the motor cortex of two monkeys during performance of a precision grip task. Fifty-six cells were antidromically identified as pyramidal tract neurons (PTNs). Sixty-one cells (30 PTNs) exhibited significant temporal modulation of their irregularity during task performance with the contralateral hand. The irregularity modulations generally differed in sign and latency from the modulations of firing rate. High irregularity tended to occur during the task phases requiring the most detailed control of movement, whereas neural firing became more regular during the steady hold phase. Such irregularity modulation could have important consequences for the response of downstream neurons and may provide insight into the nature of the cortical code.

Existing motor state is favored at the expense of new movement during 13-35 Hz oscillatory synchrony in the human corticospinal system

Oscillations in local field potentials in the beta-frequency band (13-35 Hz) are a pervasive feature of human and nonhuman primate motor cortical areas. However, the function of such synchronous activity across populations of neurons remains unknown. Here, we test the hypothesis that beta activity may promote existing motor set and posture while compromising processing related to new movements. Three experiments were performed. First, healthy subjects were instructed to make reaction time movements of the outstretched index finger in response to imperative cues triggered by transient increases in corticospinal synchrony, as evidenced by phasic elevations of beta-frequency band microtremor and intermuscular synchrony. Second, healthy subjects were instructed to resist a stretch to the index finger triggered in the same way. Finger acceleration in the reaction time task and transcortical components of the stretch reflex were measured and compared with those elicited by random cue or stretch presentation. Finally, we sought a correlation between finger acceleration in the reaction time task and cortical synchrony directly measured from the electrocorticogram in two patients undergoing functional neurosurgery. We demonstrate that movements are slowed and transcortical responses to stretch are potentiated during periods of elevated beta-band cortical synchrony. The results suggest that physiological periods of beta synchrony are associated with a cortical state in which postural set is reinforced, but the speed of new movements impaired. The findings are of relevance to Parkinson's disease, in which subcortical and cortical beta-band synchronization is exaggerated in the setting of increased tone and slowed movements.

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.

Finger movements during reach-to-grasp in the monkey: amplitude scaling of a temporal synergy

To reduce the complexity of controlling hand-shaping, recent evidence suggests that the central nervous system uses synergies. In this study, two Rhesus monkeys reached-to-grasp 15 objects, varying in geometric properties, at five grasp force levels. Hand kinematics were recorded using a video-based tracking system. Individual finger movements were described as vectors varying in length and angle. Inflection points (i.e., stereotypic minima/maxima in the temporal profile of each finger vector) exhibited a temporal synchrony for individual fingers and in the coupling across fingers. Inflection point amplitudes varied significantly across objects grasped, scaling linearly with the object grasp dimension. Thus, differences in the vectors as a function of the objects were in the relative scaling of the vector parameters over time rather than a change in the temporal structure. Mahalanobis distance analysis of the inflection points confirmed that changes in inflection point amplitude as a function of objects were greater than changes in timing. Inflection points were independent of the grasp force, consistent with the observation that reach-to-grasp kinematics and grasp force are controlled independently. In summary, the shaping of the hand during reach-to-grasp involves scaling the amplitude of highly stereotypic temporal movements of the fingers.

Parallel neuronal mechanisms underlying physiological force tremor in steady muscle contractions of humans

We present results from a study of the 6-to 12-Hz force tremor in relation to motor unit (MU) firing synchrony. Our experimental observations from 32 subjects, 321 contractions, and 427 recorded MUs reveal that tremor is accompanied by corresponding, in-phase MU rhythms that are additional to the ones at the MU intrinsic firing rates. This rhythmical synchrony is widespread and has a uniform strength that ranges from near zero to very large (MU/MU coherence > 0.50) in different contractions. Both the synchrony and the tremor are suppressed during ischemia, and this strongly suggests an involvement of spindle feedback in their generation. Furthermore, in the presence of substantial synchrony, the tremor enhancement, relative to the minimal tremor of ischemia, reflects the strength of the synchrony. Theoretical considerations based on these observations indicate that the muscle force signal is expected to show 1) frequency components in the band of the firing rates of the last-recruited, large MUs, and 2) because of the synchronized MU rhythms, an additional, distinct component with a size reflecting the strength of synchrony. Furthermore, synchronized MU rhythms, with frequencies in the 6- to 12-Hz range, are expected to arise from self-oscillations in the monosynaptic stretch reflex loop, due primarily to the associated muscle delay (several tens of milliseconds). Our results therefore reveal the parallel action of two tremor mechanisms, one of which involves MU synchrony probably caused by loop action. Clearly, the results on the synchrony and its impact also apply to other possible generators of tremor synchrony, including supraspinal ones.

Coherence at 16-32 Hz Can Be Caused by Short-Term Synchrony of Motor Units

Time- and frequency-domain measures of discharge times for pairs of motor units are used to infer the proportion of common synaptic input received by motor neurons. The physiological mechanisms that can produce the experimentally observed peaks in the cross-correlation histogram and the coherence spectrum are uncertain. The present study used a computational model to impose synchronization on the discharge times of motor units. Randomly selected discharge times of a unit that was being synchronized to a reference unit were aligned with some of the discharge times of the reference unit, provided the original discharge time was within 30 ms of the discharge by the reference unit. All time-domain measures (indexes CIS, E, and k′) were sensitive to changes in the level of imposed motor-unit synchronization ( P < 0.01). In addition, synchronization caused a peak between 16 and 32 Hz in the coherence spectrum. The shape of the cross-correlogram determined the frequency at which the peak occurred in the coherence spectrum. Further, the magnitude of the coherence peak was highly correlated with the time-domain measures of motor-unit synchronization ( r2 > 0.80), with the highest correlation occurring for index E ( r2 = 0.98). Thus the peak in the 16- to 32-Hz band of the coherence spectrum can be caused by the time that individual discharges are advanced or delayed to produce synchrony. Although the in vivo processes that adjust the timing of motor-unit discharges are not fully understood, these results suggest that they may not depend entirely on an oscillatory drive by the CNS.

Medidas eletrencefalográficas durante a aprendizagem de tarefa motora sob efeito do bromazepam

Neuromoduladores alteram constantemente as relações neurais pré-existentes no sistema nervoso. O bromazepam é utilizado com freqüência na prática clínica para diminuir padrões de ansiedade. Poucos são os experimentos correlacionando este ansiolítico às tarefas motoras. Neste contexto, o presente experimento visa analisar as alterações motoras e eletrocorticais decorrentes da administração de diferentes doses de bromazepam mediante a prática motora, e relacionar o efeito da droga a performance motora mão-dominante versus não dominante. Sujeitos saudáveis (39), de ambos os sexos, entre 20 a 30 anos compuseram a amostra. Os grupos controle (placebo) e experimental (bromazepam de 3mg e 6mg) foram treinados na tarefa de datilografia num modelo duplo-cego randomizado. Resultados do teste Stroop (atenção) não demonstraram diferenças no escore bruto e no tempo de execução do mesmo. Em contrapartida, nos resultados comportamentais foram observados um efeito principal entre blocos nas variáveis tempo de execução e erros cometidos durante a pratica motora. Os dados eletrofisiológicos evidenciaram interações significantes para: lateralidade/condição/momento; lateralidade/condição; lateralidade/momento; condição/momento; condição/setor.

Effects of Aging on the Regularity of Physiological Tremor

The purpose of this investigation was to determine the effects of healthy aging on the regularity of physiological tremor under rest and postural conditions. Additionally, we examined the contribution of mechanical reflex factors to age-related changes in postural physiological tremor. Tremor regularity, tremor–electromyographic (EMG) coherence, tremor amplitude, and tremor modal frequency were calculated for 4 age groups (young: 20–30 yr, young-old: 60–69 yr, old: 70–79 yr, and old-old: 80–94 yr) under resting and loaded postural conditions. There were 6 important findings from this study: 1) there were no differences between the young and elderly subjects for any of the dependent variables measured under the rest condition; 2) postural physiological tremor regularity was increased in the elderly; 3) postural physiological tremor-EMG coherence was also increased in the elderly, and there was a strong linear relation between peak tremor-EMG coherence in the 1- to 8-Hz frequency band and regularity of tremor. This relation was primarily driven by the increased magnitude of tremor-EMG coherence at 5.85 and 6.83 Hz; 4) enhanced mechanical reflex properties were not responsible for the increased magnitude of tremor-EMG coherence in the elderly subjects; 5) tremor amplitude was not different between the 4 age groups, but there was a slight decline in tremor modal frequency in the oldest age group in the unloaded condition; and 6) despite the increases in postural physiological tremor regularity and the magnitude of low frequency tremor-EMG coherence with age, there was a clear demarcation between healthy aging and previously published findings related to tremor pathology.

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.

Temporal evolution of oscillations and synchrony in GPi/muscle pairs in Parkinson's disease

Both standard spectral analysis and time-dependent phase correlation techniques were applied to 27 pairs of tremor-related single units in the globus pallidus internus (GPi) and EMG of patients with Parkinson's disease (PD) undergoing stereotactic neurosurgery. Over long time-scales (approximately 60 s), GPi tremor-related units were statistically coherent with restricted regions of the peripheral musculature displaying tremor. The distribution of pooled coherence across all pairs supports a classification of GPi cell/EMG oscillatory pairs into coherent or noncoherent. Analysis using approximately 2-s sliding windows shows that oscillatory activity in both GPi tremor units and muscles occurs intermittently over time. For brain/muscle pairs that are coherent, there is partial overlap in the times of oscillatory activity but, in most cases, no significant correlation between the times of oscillatory subepisodes in the two signals. Phase locking between coherent pairs occurs transiently; however, the phase delay is similar for different phase-locking subepisodes. Noncoherent pairs also show episodes of transient phase locking, but they occurred less frequently, and no preferred phase delay was seen across subepisodes. Tremor oscillations in pallidum and EMGs are punctuated by phase slips, which were classified as synchronizing or desynchronizing depending on their effect on phase locking. In coherent pairs, the incidence of synchronizing slips is higher than desynchronizing slips, whereas no significant difference was seen for noncoherent pairs. The results of this quantitative characterization of parkinsonian tremor provide a foundation for hypotheses about the structure and dynamical functioning of basal ganglia motor control networks involved in tremor generation.

Behavioral context and coherent oscillations in the supplementary motor area

Movements with similar physical characteristics can occur in various behavioral contexts, as when they are embedded in different sequences or when the expected outcomes of movements vary. Similarly, neurons in various sensory and motor structures in the brain commonly display modulations in their activity according to contextual factors, such as expected reward. Although these contextual signals must be combined with incoming sensory inputs to generate appropriate behaviors according to the animal's motivational state, the mechanisms by which these two signals are integrated remain poorly understood. The present study examined the effects of contextual factors on the magnitude of coherent oscillations in the activity of individual neurons recorded in the supplementary motor area (SMA) of monkeys during a serial reaction time task. In this task, the animal produced a predictable sequence of hand movements repeatedly according to visual instructions. The performance of the animal was influenced by the location of the rewarded target as well as the ordinal position of the movement. In contrast, the level of coherent oscillations in the activity of SMA neurons was affected only by the rewarded target location but not by the ordinal position of the movement sequence. In addition, changes in coherent oscillations were not accounted for by systematic changes in the mean firing rates. These results are consistent with the proposal that synchronous spikes might be used to control the flow of information and suggest that coherent oscillations in the SMA might encode contextual variables, such as expected reward.

Fluctuations in plantar flexion force are reduced after prolonged tendon vibration

The purpose of the study was to examine the effect of prolonged vibration on the force fluctuations during a force-matching task performed at low-force levels. Fourteen young healthy men performed a submaximal force-matching task of isometric plantar flexion before and after Achilles tendon vibration (n = 8, vibration subjects) or lying without vibration (n = 6, control subjects) for 30 min. The target forces were 2.5-10% of the previbration maximal voluntary contraction force. The standard deviation of force decreased by a mean of 29 +/- 20% across target forces after vibration, whereas it did not decrease significantly in control subjects (-5 +/- 12%). This change was significantly greater compared with control subjects (P < 0.01 for both). Power spectral density of the force was predominantly composed of signals of low-frequency bandwidth (<or =5 Hz) with few higher frequency components. In vibration subjects, there was a significant decrease in power in the frequency range < or =2 Hz after vibration. The decrease in power at this frequency range was linearly related to the decrease in the force fluctuations (r = 0.96, P < 0.001). The results indicate that prolonged Achilles tendon vibration reduces the fluctuations in plantar flexion force in the frequency range < or =2 Hz during low-level contractions. It suggests that Ia afferent inputs contribute to the low-frequency force fluctuations in plantar flexion.

The 1- to 2-Hz oscillations in muscle force are exacerbated by stress, especially in older adults

Although force fluctuations during a steady contraction are often heightened in old adults compared with young adults and are enhanced in young adults during the stress response, the mechanisms underlying the augmentation are uncertain. The purpose of the study was to compare the effect of a stressor on the plasma concentrations of selected stress hormones and on the force fluctuations experienced by young and old adults during the performance of a precision grip. Thirty-six men and women (19–86 yr) participated in a protocol that comprised anticipatory (30 min), stressor (15 min), and recovery periods (25 min). The stressor was a series of noxious electrical stimuli applied to the dorsal surface of the left hand. Subjects sustained a pinch-grip force with the right hand at 2% of the maximal voluntary contraction force. The fluctuations in pinch-grip force, the interference electromyogram (EMG) of six muscles, and the spectra for the force and EMG were quantified across the 70-min protocol. The stressor increased the force fluctuations, largely due to an enhancement of the power at 1–2 Hz in the force spectrum ( r2 = 0.46). The effect was greatest for the old adults compared with young and middle-aged adults. The plasma concentrations of the stress hormones (adrenocorticotropin, epinephrine, and norepinephrine) were elevated to similar levels for all three age groups, and the changes were not associated with modulation of the force fluctuations. Furthermore, the heightened EMG activity exhibited by the old adults during all periods was not related to the changes in the force fluctuations or the 1- to 2-Hz force oscillations. The absence of a change in the mean pinch-grip force during the protocol and the lack of an association between elevation of the plasma concentrations for the stress hormones and modulation of the force fluctuations suggest that the enhanced force fluctuations caused by the stressor was due to an increase in the low-frequency output of the spinal motor neurons.

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.

Monkey hand postural synergies during reach-to-grasp in the absence of vision of the hand and object

Understanding how the CNS controls reach-to-grasp will require behavioral and neurophysiological studies of reach-to-grasp in the monkey, including the evaluation of whole-hand grasp with explicit force requirements. In this study, monkeys performed a reach-to-grasp task in which the size, shape, and orientation of the objects were varied. The monkeys were required to grasp each object at five force levels based on visual feedback. Seventeen positions on the wrist and hand were monitored to quantify kinematics. Hand shaping began with initiation of reach and continued throughout the reach, matching object properties even without vision of the hand or object. Grasp aperture scaled to object size. Singular value decomposition analysis of the marker positions identified two dominant hand postures. The first eigenvector or "eigenposture" consisted of an open hand configuration midway between flexion and extension that explained >93% of the variance. The second eigenposture consisted of hyperextension of all joints that accounted for another 4-5% of the variance. The two eigenpostures were similar across force levels and between monkeys. Reach kinematics consisted of a U-shaped hand path with a bell-shaped velocity profile. Trajectory and speed were independent of grasp force and object properties. In summary, hand shaping during the reach occurred without vision of the hand or object, and hand kinematics were not dependent on grasp force. Furthermore, the reach was independent of grasp force and object properties. These observations imply that the kinematics of reach-to-grasp and grasp force are controlled independently. Similar to humans, monkeys may use a simplifying strategy to reduce the degrees of freedom of the hand during reach-to-grasp.

Synchronous cortical oscillatory activity during motor action

Oscillations of the motor cortex interact with similar activity of the spinal motoneuron pool in the 15-30 Hertz frequency range. Recent observations have demonstrated how this interaction affects the firing of single corticospinal neurons. The interaction, reflected as corticomuscular coherence, occurs for both distal and proximal muscles and it constitutes one connection in a larger web of oscillatory interactions, including several other motor areas in the cortex, thalamus, and cerebellum. New results cast light on the possible functional significance of this interaction. The rhythmic interaction may reveal interesting information in several motor disorders, including essential tremor, Parkinson's disease, myoclonus epilepsy, and mirror movements.