Characterization of spatiotemporal dynamics in EEG data during picture naming with optical flow patterns.

In this study, we investigate the spatiotemporal dynamics of the neural oscillations by analyzing the electric potential that arises from neural activity. We identify two types of dynamics based on the frequency and phase of oscillations: standing waves or as out-of-phase and modulated waves, which represent a combination of standing and moving waves. To characterize these dynamics, we use optical flow patterns such as sources, sinks, spirals and saddles. We compare analytical and numerical solutions with real EEG data acquired during a picture-naming task. Analytical approximation of standing waves helps us to establish some properties of pattern location and number. Specifically, sources and sinks are mainly located in the same location, while saddles are positioned between them. The number of saddles correlates with the sum of all the other patterns. These properties are confirmed in both the simulated and real EEG data. In particular, source and sink clusters in the EEG data overlap with each other with median percentages around 60%, and hence have high spatial correlation, while source/sink clusters overlap with saddle clusters in less than 1%, and have different locations. Our statistical analysis showed that saddles account for about 45% of all patterns, while the remaining patterns are present in similar proportions.

Nonlinear analysis of periodic waves in a neural field model.

Various types of brain activity, including motor, visual, and language, are accompanied by the propagation of periodic waves of electric potential in the cortex, possibly providing the synchronization of the epicenters involved in these activities. One example is cortical electrical activity propagating during sleep and described as traveling waves [Massimini et al., J. Neurosci. 24, 6862-6870 (2004)]. These waves modulate cortical excitability as they progress. Clinically related examples include cortical spreading depression in which a wave of depolarization propagates not only in migraine but also in stroke, hemorrhage, or traumatic brain injury [Whalen et al., Sci. Rep. 8, 1-9 (2018)]. Here, we consider the possible role of epicenters and explore a neural field model with two nonlinear integrodifferential equations for the distributions of activating and inhibiting signals. It is studied with symmetric connectivity functions characterizing signal exchange between two populations of neurons, excitatory and inhibitory. Bifurcation analysis is used to investigate the emergence of periodic traveling waves and of standing oscillations from the stationary, spatially homogeneous solutions, and the stability of these solutions. Both types of solutions can be started by local oscillations indicating a possible role of epicenters in the initiation of wave propagation.

Modeling of post-stroke stimulation of cortical tissue.

Following a stroke, cortical networks in the penumbra area become fragmented and partly deactivated. We develop a model to study the propagation of waves of electric potential in the cortical tissue with integro-differential equations arising in neural field models. The wave speed is characterized by the tissue excitability and connectivity determined through parameters of the model. Post-stroke tissue damage in the penumbra area creates a hypoconnectivity and decreases the speed of wave propagation. It is proposed that external stimulation could restore the wave speed in the penumbra area under certain conditions of the parameters. Model guided cortical stimulation could be used to improve the functioning of cortical networks.

Neurophysiological and behavioral effects of a 60 Hz, 1,800 µT magnetic field in humans.

The effects of time-varying magnetic fields (MF) on humans have been actively investigated for the past three decades. One important unanswered question is the potential for MF exposure to have acute effects on human biology. Different strategies have been used to tackle this question using various physiological, neurophysiological and behavioral indicators. For example, researchers investigating electroencephalography (EEG) have reported that extremely low frequency (ELF, <300 Hz) MF can increase resting occipital alpha rhythm (8-12 Hz). Interestingly, other studies have demonstrated that human motricity can be modulated by ELF MF: a reduction of anteroposterior standing balance or a decrease of physiological tremor intensity have been reported as consequences of exposure. However, the main limitation in this domain lies in the lack of results replication, possibly originating from the large variety of experimental approaches employed. Therefore, the present study aimed to investigate the effects of a 60 Hz, 1,800 µT MF exposure on neurophysiological (EEG) and neuromotor (standing balance, voluntary motor function, and physiological tremor) aspects in humans using a single experimental procedure. Though results from this study suggest a reduction of human standing balance with MF exposure, as well as an increase of physiological tremor amplitude within the frequency range associated with central nervous system contribution, no exposure effect appeared on other investigated parameters (e.g., EEG or voluntary motor control). These results suggest that 1 h of 60 Hz, 1,800 µT MF exposure may modulate human involuntary motor control without being detected in the cortical electrical activity.

Effects of acute hypoxia on postural and kinetic tremor.

Human physiological tremor is a complex phenomenon that is modulated by numerous mechanical, neurophysiological, and environmental conditions. Researchers investigating tremor have suggested that acute hypoxia increases tremor amplitude. Based on the results of prior studies, we hypothesized that human participants exposed to a simulated altitude of 4,500 m would display an increased tremor amplitude within the 6-12 Hz frequency range. Postural and kinetic tremors were recorded with a laser system in 23 healthy male participants before, during, and after 1 h of altitude-induced hypoxia. A large panel of tremor characteristics was used to investigate the effect of hypoxia. Acute hypoxia increased tremor frequency content between 6 and 12 Hz during both postural and kinetic tremor tasks (P < 0.05, F = 6.142, Eta(2) = 0.24 and P < 0.05, F = 3.767 Eta(2) = 0.14, respectively). Although the physiological mechanisms underlying the observed changes in tremor are not completely elucidated yet, this study confirms that acute hypoxia increases tremor frequency in the 6-12 Hz range. Furthermore, this study indicates that changes in physiological tremor can be detected at lower hypoxemic levels than previously reported (blood saturation in oxygen = 80.9%). The effects of hypoxia mainly result from a cascade of events starting with the activation of the hypothalamic-pituitary-adrenal axis causing in turn an increase in catecholamine release, leading to an augmentation of tremor amplitude in the 6- to 12-Hz interval and heart rate increase.

Dynamics of the subthalamo-pallidal complex in Parkinson's disease during deep brain stimulation.

The dynamics of the subthalamo-pallidal complex in Parkinson's disease during deep brain stimulation (DBS) were studied using two models, a simple firing-rate model and a population-based model. We extended the simple firing-rate model of the complex formed by the subthalamic nucleus (STN) and the external segment of the Globus Pallidus (GPe) to explore its dynamical regime during DBS. More specifically, the modulation of neuronal activity (i.e., pattern and amplitude) during DBS was studied. A similar approach was used with the population-based model. Simulation results revealed a gradual decrease in bursting activity in STN cells when the DBS frequency increased. In addition, the contribution of the stimulation current type (mono- or biphasic) to the results was also examined. A comparison of the two models indicated that the population-based model was more biologically realistic and more appropriate for exploring DBS mechanisms. Understanding the underlying mechanisms of DBS is a prerequisite for developing new stimulation protocols.

New insights offered by a computational model of deep brain stimulation.

Deep brain stimulation (DBS) is a standard neurosurgical procedure used to treat motor symptoms in about 5% of patients with Parkinson's disease (PD). Despite the indisputable success of this procedure, the biological mechanisms underlying the clinical benefits of DBS have not yet been fully elucidated. The paper starts with a brief review on the use of DBS to treat PD symptoms. The second section introduces a computational model based on the population density approach and the Izhikevich neuron model. We explain why this model is appropriate for investigating macroscopic network effects and exploring the physiological mechanisms which respond to this treatment strategy (i.e., DBS). Finally, we present new insights into the ways this computational model may help to elucidate the dynamic network effects produced in a cerebral structure when DBS is applied.

Using wavelet analysis to detect the influence of low frequency magnetic fields on human physiological tremor.

The influence of extremely low frequency magnetic fields (ELF-MFs) on human physiological processes and, in particular, on motor activity is still not established with certainty. Using the wavelet-transform approach, changes in the characteristics of human finger micromovement are studied in the presence of a low intensity MF centred at the level of the head. Different approaches to nonstationary signal analysis involving real as well as complex wavelet functions are considered. We find evidence that ELF-MFs lead to more regular postural tremor and more homogeneous energy distribution.

Neurologic signs of ciguatera disease: evidence of their persistence.

Ciguatoxins exert their effect on the voltage-sensitive sodium channels of the cellular membranes of all excitable tissues. This effect confers to ciguatera disease (CD) its neurologic hallmarks. A prospective study among French Polynesian adults over a two-month period was conducted to characterize and determine the persistence of neurologic symptoms of CD. We compared 47 patients with CD with 125 controls. In the acute phase of the disease, patients had mainly sensory disturbances as detected by an hypoesthesia on the palm of the hand and poorer sway performance compared with controls. Follow-up two months showed improvement of sway performance that eventually reaching control levels. However, for light-touch threshold, even if we observed a decrease threshold towards normal values, more than 50% of patients did not reach normal values 60 days after disease onset. Our results support the existence of neurologic impairments of CD and suggest their persistence for at least two months after onset.

Sensorimotor mapping affects movement correction deficits in early Huntington's disease.

Huntington's disease (HD) is associated with early voluntary movement problems linked to striatal dysfunction. In pointing movements, HD increases the irregularity of the terminal part of movements, suggesting a dysfunction in error feedback control. We tested this hypothesis in movements requiring continuous feedback control. Patients in the early stages of HD and controls traced as fast and accurately as possible circles within a 5-mm annulus on a digitizing tablet when visual feedback of the hand and the circle was direct or indirect (through a monitor). Patients deviated more often from the annulus and showed larger corrections toward the circle than controls when using indirect visual feedback but not with direct visual feedback. When velocity requirements were removed, patients showed little change in these control problems. These results suggest that HD does not affect error feedback control in all movements and that the striatal contribution to voluntary movement is sensitive to sensorimotor mapping.

Accelerometric measurement of involuntary movements during pallidal deep brain stimulation of patients with generalized dystonia.

Accelerometric activity during rest and posture was quantified in the upper dominant limb of 14 patients with primary or secondary dystonia and five healthy control subjects. Data were recorded before and after bilateral implantation of the stimulating electrodes in the Globus Pallidus internus. Clinical evaluation was based on the Burke-Marsden-Fahn's Dystonia Rating Scale (BMFDRS). For the patient group, I(t), the integral (i.e. area) of the acceleration power spectrum over the total frequency range (0.6-16 Hz) decreased as the clinical state of the patients improved following deep brain stimulation (p < 0.01) during rest and posture. Ten days after surgery, there were no I(t) differences between control subjects and patients (p > 0.05). A significant correlation was found between the global BMFDRS scores and I(t) for rest (p < 0.01) but not for posture. No significant correlation was found between I(t) and a partial BMFDRS score for the right arm for rest or posture. The integral I(t) provides a valid indicator of the motor activity generated by the arm of the patient but further analyses are needed to monitor patients' progress not only during their hospitalization but also after they are released from the hospital, and to understand why this measure does not correlate with partial BMFDRS scores.

[Kinematic evaluation of dystonic syndromes in patients treated with deep brain stimulation]. / La cinématique dans l'évaluation des syndromes dystoniques sous stimulation cérébrale profonde.

INTRODUCTION: Quantification of motor functions of patients with dystonic syndromes treated by chronic high frequency stimulation of the internal globulus pallidus is a challenge. OBJECTIVE: Through a series of clinical examples this paper shows that kinematic analysis of movements in dystonic syndromes treated by deep brain stimulation (DBS) is a complement to clinical evaluation. In addition, it provides valuable information for early detection of improvement or impairment of movements associated with modifications of stimulation parameters. METHOD: Thirteen dystonic patients and eleven reference subjects completed three tests (i.e., rest: lying supine; posture: standing with arms held in front (at shoulder height); and alternative movements: bimanual finger-to-nose test). These tests were recorded with an electromagnetic system quantifying movement kinematics (position) in three-dimensional space. RESULTS: From the recorded data, several indices were developed and provided a quantitative evaluation of movements during each test. In addition, a clinical evaluation (BMFDRS) was also completed. No correlation between clinical and kinematic evaluations was found. CONCLUSION: It is shown that kinematic analysis is a useful complement of clinical evaluation and can assist clinicians in monitoring the evolution of movements in dystonic patients treated by DBS in a simple, reliable and valid fashion.

[Characterization and discrimination of kinetic tremor in Parkinson's disease]. / Caractérisation et discrimination du tremblement cinétique dans la maladie de Parkinson.

Kinetic tremor recorded with a laser system during a compensatory tracking task of the index finger was analyzed in 21 patients with PD whose tremor amplitude was between low and moderate, and 30 control subjects. Nine characteristics quantifying the tracking task and tremor including mean tracking error, reaction time, peakedness, harmonicity, median frequency, proportional power in the 3-4 Hz, 4-6 Hz and 7-12 Hz ranges, and power at 0.25 Hz, were applied to the processed signal. The discriminating power of each characteristic was evaluated using differences between group means (p values), maximum percentage discrimination, and number of outliers in the patient group using z-score and 96.7(th) percentile of the control group. All nine characteristics showed significant differences between means of the two groups using Welch-modified t-tests for unequal variances. The most discriminating characteristics reflected differences in the frequency distribution of the movement and did not correlate highly with postural tremor amplitude nor with clinical ratings of tremor. Discrimination methods classified correctly up to 66.7 p.cent of the patients. Combining representative information about proportional power during posture and tracking gave a much higher discrimination (90 p.cent) with respect to the 96.7th percentile of the control group. These results suggest that by combining information coming from postural and kinetic tremors it is possible to isolate a specific aspect of PD symptomatology which could be used to reevaluate the classic distinction made between the akineto-rigid and tremulous forms of PD independently of tremor amplitude.

Standardization of a neuromotor test battery: the CATSYS system.

Interindividual and intraindividual variability in neuromotor behaviors is expected and normal. Early changes in neuromotor behaviors associated with neurodegenerative disorders or neurotoxic effects are often subtle and fluctuating in their characteristics. Therefore, their detection at an early stage is particularly difficult without precise recording instruments. The CATSYS system developed by Danish Product Development (DPD) is a portable device recording four measures of neuromotor control including tremor, reaction time, hand coordination and postural sway. The aim of this study is to develop a set of normative data. One hundred and fifty healthy men and women were divided into five age groups: (1) 20 to 29 years (n=30); (2) 30 to 39 years (n=30); (3) 40 to 49 years (n=30); (4) 50 to 59 years (n=30); (5) 60 to 70 years (n=30). All participants were free of neurological deficits at the time of testing and they were tested individually for approximately 30 min. Hand coordination was measured with prono-supination and finger-tapping movements executed at constant and accelerated rhythms. Reaction time was assessed in both hands using a hand held switch activated by the thumb. Postural tremor was quantified in both hands during 24.6 sec. by asking the subject to hold a stylus horizontally at 10 cm in front of his/her navel. The stylus contained a biaxial accelerometer. Postural sway was tested by asking the subject to stand on a force platform for 75 sec. under four conditions: (1) eyes open; (2) eyes closed; (3) eyes open standing on a foam pad; and (4) eyes closed standing on a foam pad. ANOVAs and multiple comparison tests were performed and the results were examined taking into account age, gender and experimental condition effects.

Neuromotor profiles: what are they and what can we learn from them?

Eye movements, alternating movements, rapid pointing movements, and various tremors were measured on patients with Parkinson's disease (n = 21), on Cree subjects exposed to methylmercury (n = 36), and on healthy control subjects (n = 30). Neuromotor profiles were created according to thirty characteristics extracted from test results of four subgroups matched for age and composed of six subjects each. Z scores were calculated with respect to the mean and standard deviation of the control group for each of the 30 characteristics. The subgroup with the lower methylmercury blood level had larger z scores than the control subgroup and with a few positive values above one standard deviation. The subgroup with the higher methylmercury blood level had several z scores above two standard deviations. Interestingly, the abnormal values for the subgroup with Parkinson's disease were mostly limited to static tremor recorded with no visual feedback and reached up to 5 standard deviations. These results indicate that neuromotor profiles can be used to summarize information extracted from different neuromotor tests and to differentiate neurological conditions.

Neuromotor microalterations in subjects with spasmophilia symptoms: a preliminary study.

Spasmophilia is a relatively unknown condition characterized by perturbations of the neuromuscular system. We hypothesized that spasmophilia may negatively affect neuromotor functions in subtle ways. Three tests including tremor, rapid pointing movements, and alternating movements were quantified in a group of subjects with spasmophilia symptoms (SS) (n = 10) and a healthy control group (n = 10). Most of the characteristics used to evaluate motor functions in these three tests revealed no significant differences between the two groups except for two characteristics in alternating movements and two characteristics in rapid pointing movements. In terms of variances, a dissociation between voluntary movements and involuntary movements was observed for the two groups. Control subjects had significantly higher variances in involuntary movements such as tremor, while subjects with SS had significantly higher variances in voluntary movements such as alternating and rapid pointing movements. A significant increase in asymmetry in hand laterality was also noted for some characteristics in subjects with SS.

A possible approach for discrimination between normal and pathological signals using the WFLC algorithm.

The discrimination between normal and pathological tremor is difficult when amplitude is relatively small. The WFLC algorithm, a time domain adaptive Fourier transform, is designed to control physiological tremor and to improve precision during microsurgery. We added two iterative optimization processes to initialize the following parameters: initial frequency weight (omega0), amplitude adaptation rate (mu) and frequency adaptation rate (mu0). Then, we applied the methods on data sets recorded on patients with different tremors (control, parkinsonian, cerebellar, and essential) sampled at 200 Hz. After filtering the data, the WFLC algorithm tracked the time-varying dominant frequencies and amplitudes of the transformed data sets. Our results illustrate the potential of using this algorithm as an approach to discriminate between normal and pathological signals even when amplitude is not a significant discriminating factor.

Physiological tremor: does handedness make a difference?

The purpose of this study was to quantify with precision the characteristics of normal physiological tremor in the dominant and nondominant hand of a group of right-handed females. Twenty-two right-handed females (aged 20 to 40) gave informed consent to participate. Manual dominance determined with a questionnaire had to be equal to, or above 90%, for subjects to be accepted in the study. While the subjects kept their eyes closed, tremor was recorded simultaneously in the two extended index fingers during 40 s. Preceding the recording of postural tremor there was a period during which the subjects were asked to relax (control condition); to exert a pressure with the two index fingers on two strain gauges (static condition); or to press on and off with both index fingers on two strain gauges (dynamic condition). Each condition was recorded twice with two helium neon velocity lasers. This procedure was repeated after permutation of the lasers, thus a total of twelve sweeps of data were recorded. Seven parameters were used to analyze the tremor signal including measures of harmonicity, median frequency, proportional power in the 7-12 Hz range, amplitude, waveform morphology, entropy, and nonlinearity. Significant differences between hands were found in three parameters (median frequency, power in 7-12 Hz and entropy) but not in tremor amplitude (RMS). In addition, significant differences were found in five out of seven characteristics between conditions. These results suggest that neuromotor activity preceding the recording of tremor has a significant influence on tremor characteristics and that some of these characteristics are significantly different between the right and left hand in right-handed females.

Using time domain characteristics to discriminate physiologic and parkinsonian tremors.

Tremor amplitude and frequency do not always clearly differentiate subjects with particular pathologies from control subjects or from subjects with other pathologies, especially in early stages of a disease. For patients with early stages of Parkinson's disease (PD) the discriminative power of amplitude was compared with that of other time domain characteristics of tremor recordings that are probably not evident clinically. Postural tremor with and without visual feedback and rest tremor were recorded in both hands of a group of patients with Parkinson's disease (n = 21) and a group of healthy control subjects (n = 30) using displacement lasers. Velocity and acceleration data were derived from displacement data. Twelve time domain characteristics were calculated on each recording and the discriminating power of each was evaluated using the worse hand in each case. Postural tremor with no visual feedback separates the two groups of subjects most efficiently, especially in velocity and acceleration. Tremor in Parkinson's disease (in comparison to normal physiologic tremor) has a specific morphology, has a distinctive histogram, is more periodic, and contains indications of nonlinearity in the underlying dynamics. There may also be greater difference in amplitude between the two hands and time asymmetry in tremor of patients with PD. A series of finger flexions seems to enhance normal tremor but not tremor in PD and may thus aid in discrimination. Discrimination of tremor attributable to PD from normal physiologic tremor can be enhanced by measuring time domain characteristics subtler than amplitude, particularly when amplitude itself is not large. Tremor measurement should not be limited to acceleration data because some information is more visible in other variables.

The measurement of tremor using a velocity transducer: comparison to simultaneous recordings using transducers of displacement, acceleration and muscle activity.

Precise kinematic measurements of tremor have historically been obtained using accelerometers. However, current technology permits precise measurements in velocity and displacement. The primary advantage of velocity recording is that only one step of integration or differentiation is required for either displacement or acceleration. A method is presented of measuring finger tremor using a laser system that transduces velocity precisely. Measurements of postural finger tremor thus obtained were compared to those simultaneously obtained from a laser system that transduces displacement, from an accelerometer and from surface electromyography (EMG) of the extensor digitorum communis. A range of amplitude and frequency content was obtained by testing control subjects and subjects with Parkinson's disease. The velocity transducer showed excellent correspondence of amplitude and frequency measurement with the displacement transducer. Measures of absolute and relative amplitude correlated well (r > or = 0.96 in amplitude measures in displacement, velocity and acceleration), and high coherence was found throughout the frequency range of interest. Measurements by the accelerometer generally showed poorer correspondence with those of the other instruments. EMG measurements showed good correspondence in some trials but poorer correspondence in others, attributed to the low level of muscle activity required in the task. Precise kinematic measurements appear to be highly sensitive to neuromotor impairment.