Concentric-ring electrode systems for noninvasive detection of single motor unit activity

Comparison of Two Types of Electrodes for Measuring Submental Muscle Activity During Swallowing

PURPOSE

This study aimed to investigate the potential of a newly developed small electrode to accurately record muscle activity during swallowing.

MATERIAL AND METHODS

This study included 31 healthy participants. The participants underwent swallowing trials with three types of material. The recordings involved the following conditions: 1) swallowing saliva, 2) swallowing 3 mL water, and 3) swallowing 5 mL water. Two types of electrodes, a conventional electrode (CE) and a newly developed small electrode (NE), were symmetrically positioned on the skin over the suprahyoid muscle group, starting from the center. From the surface electromyography data, the swallowing duration (s), peak amplitude, and rising time (duration from swallowing onset to peak amplitude: s) were measured. Additionally, the equivalence of characteristics of the waveform of muscle activities was calculated by using the variance in both the upper and lower confidence limits in duration and rising time.

RESULTS

No significant differences in baseline, swallowing duration or rising time between the CE and NE were observed for any swallowing material. The peak amplitude was significantly higher for the NE than for the CE for all swallowing materials. The CE and NE displayed no significant difference in the equivalence of characteristics of the waveform of muscle activities for any swallowing material.

CONCLUSIONS

The gold-plated small electrodes utilized in this study indicated the ability to record the same characteristics of muscle activity as conventional electrodes. Moreover, it was able to capture the muscle activity of each muscle group with improved sensitivity in a narrow area, such as under the submandibular region, with more precision than that of conventional electrodes.

Multiphysics and multiscale modeling of uterine contractions: integrating electrical dynamics and soft tissue deformation with fiber orientation

The development of a comprehensive uterine model that seamlessly integrates the intricate interactions between the electrical and mechanical aspects of uterine activity could potentially facilitate the prediction and management of labor complications. Such a model has the potential to enhance our understanding of the initiation and synchronization mechanisms involved in uterine contractions, providing a more profound comprehension of the factors associated with labor complications, including preterm labor. Consequently, it has the capacity to assist in more effective preparation and intervention strategies for managing such complications. In this study, we present a computational model that effectively integrates the electrical and mechanical components of uterine contractions. By combining a state-of-the-art electrical model with the Hyperelastic Mass-Spring Model (HyperMSM), we adopt a multiphysics and multiscale approach to capture the electrical and mechanical activities within the uterus. The electrical model incorporates the generation and propagation of action potentials, while the HyperMSM simulates the mechanical behavior and deformations of the uterine tissue. Notably, our model takes into account the orientation of muscle fibers, ensuring that the simulated contractions align with their inherent directional characteristics. One noteworthy aspect of our contraction model is its novel approach to scaling the rest state of the mesh elements, as opposed to the conventional method of applying mechanical loads. By doing so, we eliminate artificial strain energy resulting from the resistance of soft tissues' elastic properties during contractions. We validated our proposed model through test simulations, demonstrating its feasibility and its ability to reproduce expected contraction patterns across different mesh resolutions and configurations. Moving forward, future research efforts should prioritize the validation of our model using robust clinical data. Additionally, it is crucial to refine the model by incorporating a more realistic uterus model derived from medical imaging. Furthermore, applying the model to simulate the entire childbirth process holds immense potential for gaining deeper insights into the intricate dynamics of labor.

Study Regarding the Monitoring of Nocturnal Bruxism in Children and Adolescents Using Bruxoff Device

Bruxism is a parafunctional activity represented by the gnashing and clenching of one's teeth. The aim of this study was to determine the utility of screening and monitoring with a Bruxoff device during nocturnal bruxism in 51 children and adolescents (36 with bruxism and 15 without bruxism) by assessing the variations in the intensity and duration of parafunctional activity in each patient. Bruxoff measurements were recorded for at least 60 min for three consecutive nights for each subject. All the parameters recorded using Bruxoff in the control and the study groups showed a statistically significant difference (p < 0.05). The differences found by comparing the values recorded in the male and female study groups are significant for heart rate, the number of masseter muscle contractions during one night, and mixed contractions. The Bruxoff device proved to be important in diagnosing patients with bruxism in our practice.

Identification of atrial fibrillation drivers by means of concentric ring electrodes

BACKGROUND AND OBJECTIVE

The prevalence of atrial fibrillation (AF) has tripled in the last 50 years due to population aging. High-frequency (DFdriver) activated atrial regions lead the activation of the rest of the atria, disrupting the propagation wavefront. Fourier based spectral analysis of body surface potential maps have been proposed for DFdriver identification, although these approaches present serious drawbacks due to their limited spectral resolution for short AF epochs and the blurring effect of the volume conductor. Laplacian signals (BC-ECG) from bipolar concentric ring electrodes (CRE) have been shown to outperform the spatial resolution achieved with conventional unipolar recordings. Our aimed was to determine the best DFdriver estimator in endocardial electrograms and to assess the BC-ECG capacity of CRE to quantify AF activity non-invasively.

METHODS

31 AF episodes were simulated using realistic tridimensional models of the atria electrical activity and torso. Periodogram and autoregressive (AR) spectral estimators were computed and the percentile (P90^th, P95^th and P98^th) to impose on the dominant frequencies (DFs) across whole atria to define the best DFdriver estimator evaluated. The identification of DFdriver on DFs from BC-ECG and unipolar surface signals with conventional disc electrodes was compared.

RESULTS

The best DFdriver estimator was P95^th and AR order 100. BC-ECG signals allowed better detection of AF activity than unipolar signals, with a significantly greater percentage of electrode locations in which DFdriver was identified (p-value 0.0095).

CONCLUSIONS

The use of BC-ECG signals for body surface Laplacian potential mapping with CRE could be helpful for better AF diagnosis, prognosis and ablation procedures than those with conventional disk electrodes.

Variations in athletic performance with occlusal splint in track and field athletes: a randomized clinical trial

BACKGROUND

Several studies have been proposed with the aim to demonstrate correlations between the dento-mandibular apparatus and the skeletal muscle system of the human body even in regions distant from the oral cavity. However, a definite conclusion cannot be drawn. The aim of this paper is to demonstrate a possible correlation between dental occlusion and sport performances in track and field athletes.

METHODS

16 track and field athletes were enrolled for the study and were randomly divided in three groups: Untreated control group, Placebo group (with a lower plaque without occlusal coverage) and Treated group (with occlusal splint). Changes in sprint and jump performance were assessed on a weekly basis for 5 consecutive weeks, during which athletes had to wear oral devices, except for the first week of baseline, for at least 3 trainings lasting 2 hours per week. All participants performed the countermovement jump (CMJ), the drop jump (DJ), the 10m and 30m sprint tests, always on the same day on the week.

RESULTS

No statistically significant difference resulted between Control group and Placebo group and between Control group and Treated group. However it was possible to observe a clinical improvement of measurements obtained, especially for CMJ, 10m and 30m sprint tests. No variation neither statistical neither clinical was observed in DJ test analysis.

CONCLUSIONS

Even if statistically it was not possible to demonstrate an improvement in sport performance, most of the athlete analyzed showed better results during training session with occlusal splint compared to athlete without occlusal splint, in countermovement jump, in 10m and 30m sprint tests.

Are occlusal splints effective in reducing myofascial pain in patients with muscle-related temporomandibular disorders? A randomized-controlled trial

Objectives

This study aims to evaluate the effectiveness of upper Michigan occlusal splint (OS) compared to mandibular OS in terms of pain, range of motion (ROM), and muscle activity as assessed by surface electromyography (sEMG) in patients affected by muscle-related temporomandibular disorders (TMD).

Patients and methods

In this randomized-controlled trial, a total of 40 adult patients (13 males, 27 females; mean age: 47.2±12.8 years; range, 22 to 56 years) with a diagnosis of myofascial pain, lasting from at least three months on at least one masseter muscle. The patients were randomly allocated into two groups: Group 1 (n=20) using upper Michigan OS and Group 2 (n=20) using mandibular OS. At baseline (T0), at one (T1), three (T2), and six months (T3), the following outcomes were assessed: myofascial pain by Visual Analog Scale (VAS) and ROM of mandible movements, activity of the main masticatory muscles through sEMG.

Results

There were no significant intra-group differences in the outcome measures assessed in both groups. However, Group 2 had a significantly higher right lateral mandibular ROM at T2 (7.1±3.1 vs. 9.8±2.3, respectively; p<0.05) and a significantly higher left lateral mandibular ROM at T3 (7.6±3.5 vs. 10.5±2.1, respectively; p<0.05). We found no significant difference in none of the sEMG parameters.

Conclusion

Our study results suggest that OS, independently from being built on the upper or lower arch, seems to not have significant effects in reducing pain over a six-month period in TMD patients.

Evaluation of Swallowing Related Muscle Activity by Means of Concentric Ring Electrodes

Surface electromyography (sEMG) can be helpful for evaluating swallowing related muscle activity. Conventional recordings with disc electrodes suffer from significant crosstalk from adjacent muscles and electrode-to-muscle fiber orientation problems, while concentric ring electrodes (CREs) offer enhanced spatial selectivity and axial isotropy. The aim of this work was to evaluate CRE performance in sEMG recordings of the swallowing muscles. Bipolar recordings were taken from 21 healthy young volunteers when swallowing saliva, water and yogurt, first with a conventional disc and then with a CRE. The signals were characterized by the root-mean-square amplitude, signal-to-noise ratio, myopulse, zero-crossings, median frequency, bandwidth and bilateral muscle cross-correlations. The results showed that CREs have advantages in the sEMG analysis of swallowing muscles, including enhanced spatial selectivity and the associated reduction in crosstalk, the ability to pick up a wider range of EMG frequency components and easier electrode placement thanks to its radial symmetry. However, technical changes are recommended in the future to ensure that the lower CRE signal amplitude does not significantly affect its quality. CREs show great potential for improving the clinical monitoring and evaluation of swallowing muscle activity. Future work on pathological subjects will assess the possible advantages of CREs in dysphagia monitoring and diagnosis.

Evaluation of Respiratory Muscle Activity by Means of Concentric Ring Electrodes

Surface electromyography (sEMG) can be used for the evaluation of respiratory muscle activity. Recording sEMG involves the use of surface electrodes in a bipolar configuration. However, electrocardiographic (ECG) interference and electrode orientation represent considerable drawbacks to bipolar acquisition. As an alternative, concentric ring electrodes (CREs) can be used for sEMG acquisition and offer great potential for the evaluation of respiratory muscle activity due to their enhanced spatial resolution and simple placement protocol, which does not depend on muscle fiber orientation. The aim of this work was to analyze the performance of CREs during respiratory sEMG acquisitions. Respiratory muscle sEMG was applied to the diaphragm and sternocleidomastoid muscles using a bipolar and a CRE configuration. Thirty-two subjects underwent four inspiratory load spontaneous breathing tests which was repeated after interchanging the electrode positions. We calculated parameters such as (1) spectral power and (2) median frequency during inspiration, and power ratios of inspiratory sEMG without ECG in relation to (3) basal sEMG without ECG (R_ins/noise), (4) basal sEMG with ECG (R_ins/cardio) and (5) expiratory sEMG without ECG (R_ins/exp). Spectral power, R_ins/noise and R_ins/cardio increased with the inspiratory load. Significantly higher values (p < 0.05) of R_ins/cardio and significantly higher median frequencies were obtained for CREs. R_ins/noise and R_ins/exp were higher for the bipolar configuration only in diaphragm sEMG recordings, whereas no significant differences were found in the sternocleidomastoid recordings. Our results suggest that the evaluation of respiratory muscle activity by means of sEMG can benefit from the remarkably reduced influence of cardiac activity, the enhanced detection of the shift in frequency content and the axial isotropy of CREs which facilitates its placement.

Tripolar concentric EEG electrodes reduce noise

OBJECTIVE

To assay EEG signal quality recorded with tripolar concentric ring electrodes (TCREs) compared to regular EEG electrodes.

METHODS

EEG segments were recorded simultaneously by TCREs and regular electrodes, low-pass filtered at 35 Hz (REG35) and 70 Hz (REG70). Clips were rated blindly by nine electroencephalographers for presence or absence of key EEG features, relative to the "gold-standard" of the clinical report.

RESULTS

TCRE showed less EMG artifact (F = 15.4, p < 0.0001). Overall quality rankings were not significantly different. Focal slowing was better detected by TCRE and spikes were better detected by regular electrodes. Seizures (n = 85) were detected by TCRE in 64 cases (75.3%), by REG70 in 75 (88.2%) and REG35 in 69 (81.2%) electrodes. TCRE detected 9 (10.6%) seizures not detected by one of the other 2 methods. In contrast, 14 seizures (16.5%) were not detected by TCRE, but were by REG35 electrodes. Each electrode detected interictal spikes when the other did not.

CONCLUSIONS

TCRE produced similar overall quality and confidence ratings versus regular electrodes, but less muscle artifact. TCRE recordings detected seizures in 7% of instances where regular electrodes did not.

SIGNIFICANCE

The combination of the two types increased detection of epileptiform events compared to either alone.

Analysis of variance to assess statistical significance of Laplacian estimation accuracy improvement due to novel variable inter-ring distances concentric ring electrodes

Concentric ring electrodes have shown promise in non-invasive electrophysiological measurement demonstrating their superiority to conventional disc electrodes, in particular, in accuracy of Laplacian estimation. Recently, we have proposed novel variable inter-ring distances concentric ring electrodes. Analytic and finite element method modeling results for linearly increasing distances electrode configurations suggested they may decrease the truncation error resulting in more accurate Laplacian estimates compared to currently used constant inter-ring distances configurations. This study assesses statistical significance of Laplacian estimation accuracy improvement due to novel variable inter-ring distances concentric ring electrodes. Full factorial design of analysis of variance was used with one categorical and two numerical factors: the inter-ring distances, the electrode diameter, and the number of concentric rings in the electrode. The response variables were the Relative Error and the Maximum Error of Laplacian estimation computed using a finite element method model for each of the combinations of levels of three factors. Effects of the main factors and their interactions on Relative Error and Maximum Error were assessed and the obtained results suggest that all three factors have statistically significant effects in the model confirming the potential of using inter-ring distances as a means of improving accuracy of Laplacian estimation.

Computation of Surface Laplacian for tri-polar ring electrodes on high-density realistic geometry head model

Neural activity inside the human brain generate electrical signals that can be detected on the scalp. Electroencephalograph (EEG) is one of the most widely utilized techniques helping physicians and researchers to diagnose and understand various brain diseases. Due to its nature, EEG signals have very high temporal resolution but poor spatial resolution. To achieve higher spatial resolution, a novel tri-polar concentric ring electrode (TCRE) has been developed to directly measure Surface Laplacian (SL). The objective of the present study is to accurately calculate SL for TCRE based on a realistic geometry head model. A locally dense mesh was proposed to represent the head surface, where the local dense parts were to match the small structural components in TCRE. Other areas without dense mesh were used for the purpose of reducing computational load. We conducted computer simulations to evaluate the performance of the proposed mesh and evaluated possible numerical errors as compared with a low-density model. Finally, with achieved accuracy, we presented the computed forward lead field of SL for TCRE for the first time in a realistic geometry head model and demonstrated that it has better spatial resolution than computed SL from classic EEG recordings.

Proof of concept Laplacian estimate derived for noninvasive tripolar concentric ring electrode with incorporated radius of the central disc and the widths of the concentric rings

Tripolar concentric ring electrodes are showing great promise in a range of applications including braincomputer interface and seizure onset detection due to their superiority to conventional disc electrodes, in particular, in accuracy of surface Laplacian estimation. Recently, we proposed a general approach to estimation of the Laplacian for an (n + 1)-polar electrode with n rings using the (4n + 1)-point method for n ≥ 2 that allows cancellation of all the truncation terms up to the order of 2n. This approach has been used to introduce novel multipolar and variable inter-ring distances concentric ring electrode configurations verified using finite element method. The obtained results suggest their potential to improve Laplacian estimation compared to currently used constant interring distances tripolar concentric ring electrodes. One of the main limitations of the proposed (4n + 1)-point method is that the radius of the central disc and the widths of the concentric rings are not included and therefore cannot be optimized. This study incorporates these two parameters by representing the central disc and both concentric rings as clusters of points with specific radius and widths respectively as opposed to the currently used single point and concentric circles. A proof of concept Laplacian estimate is derived for a tripolar concentric ring electrode with non-negligible radius of the central disc and non-negligible widths of the concentric rings clearly demonstrating how both of these parameters can be incorporated into the (4n + 1)-point method.

Core Muscle Activation in Suspension Training Exercises

A quantitative observational laboratory study was conducted to characterize and classify core training exercises executed in a suspension modality on the base of muscle activation. In a prospective single-group repeated measures design, seventeen active male participants performed four suspension exercises typically associated with core training (roll-out, bodysaw, pike and knee-tuck). Surface electromyographic signals were recorded from lower and upper parts of rectus abdominis, external oblique, internal oblique, lower and upper parts of erector spinae muscles using concentric bipolar electrodes. The average rectified values of electromyographic signals were normalized with respect to individual maximum voluntary isometric contraction of each muscle. Roll-out exercise showed the highest activation of rectus abdominis and oblique muscles compared to the other exercises. The rectus abdominis and external oblique reached an activation higher than 60% of the maximal voluntary contraction (or very close to that threshold, 55%) in roll-out and bodysaw exercises. Findings from this study allow the selection of suspension core training exercises on the basis of quantitative information about the activation of muscles of interest. Roll-out and bodysaw exercises can be considered as suitable for strength training of rectus abdominis and external oblique muscles.

Analytic assessment of Laplacian estimates via novel variable interring distances concentric ring electrodes

Noninvasive concentric ring electrodes are a promising alternative to conventional disc electrodes. Currently, superiority of tripolar concentric ring electrodes over disc electrodes, in particular, in accuracy of Laplacian estimation has been demonstrated in a range of applications. In our recent work we have shown that accuracy of Laplacian estimation can be improved with multipolar concentric ring electrodes using a general approach to estimation of the Laplacian for an (n + 1)-polar electrode with n rings using the (4n + 1)-point method for n ≥ 2. This paper takes the next step toward further improving the Laplacian estimate by proposing novel variable inter-ring distances concentric ring electrodes. Derived using a modified (4n + 1)-point method, linearly increasing inter-ring distances tripolar (n = 2) and quadripolar (n = 3) electrode configurations are analytically compared to their constant inter-ring distances counterparts using coefficients of the Taylor series truncation terms. Obtained results suggest that increasing inter-ring distances electrode configurations may decrease the truncation error of the Laplacian estimation resulting in more accurate Laplacian estimates compared to respective constant inter-ring distances configurations. For currently used tripolar electrode configuration the truncation error may be decreased more than two-fold while for the quadripolar more than seven-fold decrease is expected.

Time-frequency representations of the sternocleidomastoid muscle electromyographic signal recorded with concentric ring electrodes

The use of non-invasive methods for the study of respiratory muscle signals can provide clinical information for the evaluation of the respiratory muscle function. The aim of this study was to evaluate time-frequency characteristics of the electrical activity of the sternocleidomastoid muscle recorded superficially by means of concentric ring electrodes (CREs) in a bipolar configuration. The CREs enhance the spatial resolution, attenuate interferences, as the cardiac activity, and also simplify the orientation problem associated to the electrode location. Five healthy subjects underwent a respiratory load test in which an inspiratory load was imposed during the inspiratory phase. During the test, the electromyographic signal of the sternocleidomastoid muscle (EMGsc) and the inspiratory mouth pressure (Pmouth) were acquired. Time-frequency characteristics of the EMGsc signal were analyzed by means of eight time-frequency representations (TFRs): the spectrogram (SPEC), the Morlet scalogram (SCAL), the Wigner-Ville distribution (WVD), the Choi-Williams distribution (CHWD), two generalized exponential distributions (GED1 and GED2), the Born-Jordan distribution (BJD) and the Cone-Kernel distribution (CKD). The instantaneous central frequency of the EMGsc showed an increasing behavior during the inspiratory cycle and with the increase of the inspiratory load. The bilinear TFRs (WVD, CHWD, GEDs and BJD) were less sensitive to cardiac activity interference than classical TFRs (SPEC and SCAL). The GED2 was the TFR that shown the best results for the characterization of the instantaneous central frequency of the EMGsc.

Finite element method modeling to assess Laplacian estimates via novel variable inter-ring distances concentric ring electrodes

Noninvasive concentric ring electrodes are a promising alternative to conventional disc electrodes. Currently, superiority of tripolar concentric ring electrodes over disc electrodes, in particular, in accuracy of Laplacian estimation has been demonstrated in a range of applications. In our recent work we have shown that accuracy of Laplacian estimation can be improved with multipolar concentric ring electrodes using a general approach to estimation of the Laplacian for an (n + 1)-polar electrode with n rings using the (4n + 1)-point method for n ≥ 2. This paper takes the next step toward further improving the Laplacian estimate by proposing novel variable inter-ring distances concentric ring electrodes. Derived using a modified (4n + 1)-point method, linearly increasing and decreasing inter-ring distances tripolar (n = 2) and quadripolar (n = 3) electrode configurations are compared to their constant inter-ring distances counterparts using finite element method modeling. Obtained results suggest that increasing inter-ring distances electrode configurations may decrease the estimation error resulting in more accurate Laplacian estimates compared to respective constant inter-ring distances configurations. For currently used tripolar electrode configuration the estimation error may be decreased more than two-fold while for the quadripolar configuration more than six-fold decrease is expected.

Improving the Accuracy of Laplacian Estimation with Novel Variable Inter-Ring Distances Concentric Ring Electrodes

Noninvasive concentric ring electrodes are a promising alternative to conventional disc electrodes. Currently, the superiority of tripolar concentric ring electrodes over disc electrodes, in particular, in accuracy of Laplacian estimation, has been demonstrated in a range of applications. In our recent work, we have shown that accuracy of Laplacian estimation can be improved with multipolar concentric ring electrodes using a general approach to estimation of the Laplacian for an (n + 1)-polar electrode with n rings using the (4n + 1)-point method for n ≥ 2. This paper takes the next step toward further improving the Laplacian estimate by proposing novel variable inter-ring distances concentric ring electrodes. Derived using a modified (4n + 1)-point method, linearly increasing and decreasing inter-ring distances tripolar (n = 2) and quadripolar (n = 3) electrode configurations are compared to their constant inter-ring distances counterparts. Finite element method modeling and analytic results are consistent and suggest that increasing inter-ring distances electrode configurations may decrease the truncation error resulting in more accurate Laplacian estimates compared to respective constant inter-ring distances configurations. For currently used tripolar electrode configuration, the truncation error may be decreased more than two-fold, while for the quadripolar configuration more than a six-fold decrease is expected.

A Spatially Focused Method for High Density Electrode-Based Functional Brain Mapping Applications

Mapping the electric field of the brain with electrodes benefits from its superior temporal resolution but is prone to low spatial resolution property comparing with other modalities such as fMRI, which can directly impact the precision of clinical diagnosis. Simulations show that dense arrays with straightforwardly miniaturized electrodes in terms of size and pitch may not improve the spatial resolution but only strengthen the cross coupling between adjacent channels due to volume conduction. We present a new spatially focused method to improve the electrode spatial selectivity and consequently suppress the neural signal coupling from the sources in the vicinity. Compared with existing spatial filtering methods with fixed coefficients, the proposed method is adaptively optimized for the geometric parameters of the recording electrode arrays, including electrode size, pitch and source depth. The effective spatial bandwidth, characterized as Radius of Half Power, can be reduced by about 70% for ECoG and the case of distant sources scenarios. The proposed method has been applied to the analysis of high-frequency oscillations (HFOs) in seizures to study the ictal pathway in the epileptogenic region. The results reveal lucid HFO wavefront propagation in both preictal and ictal stages due to a 75% reduction in the coupling effect. The results also show that a specific power threshold of preictal HFOs is needed in order to initiate an epileptic seizure. This demonstrates that our method indeed facilitates the investigation of complex neurobiological signals preprocessing applications.

Improving the accuracy of Laplacian estimation with novel multipolar concentric ring electrodes

Conventional electroencephalography with disc electrodes has major drawbacks including poor spatial resolution, selectivity and low signal-to-noise ratio that are critically limiting its use. Concentric ring electrodes, consisting of several elements including the central disc and a number of concentric rings, are a promising alternative with potential to improve all of the aforementioned aspects significantly. In our previous work, the tripolar concentric ring electrode was successfully used in a wide range of applications demonstrating its superiority to conventional disc electrode, in particular, in accuracy of Laplacian estimation. This paper takes the next step toward further improving the Laplacian estimation with novel multipolar concentric ring electrodes by completing and validating a general approach to estimation of the Laplacian for an (n + 1)-polar electrode with n rings using the (4n + 1)-point method for n ≥ 2 that allows cancellation of all the truncation terms up to the order of 2n. An explicit formula based on inversion of a square Vandermonde matrix is derived to make computation of multipolar Laplacian more efficient. To confirm the analytic result of the accuracy of Laplacian estimate increasing with the increase of n and to assess the significance of this gain in accuracy for practical applications finite element method model analysis has been performed. Multipolar concentric ring electrode configurations with n ranging from 1 ring (bipolar electrode configuration) to 6 rings (septapolar electrode configuration) were directly compared and obtained results suggest the significance of the increase in Laplacian accuracy caused by increase of n.

Toward improving the Laplacian estimation with novel multipolar concentric ring electrodes

Conventional electroencephalography with disc electrodes has major drawbacks including poor spatial resolution, selectivity and low signal-to-noise ratio that are critically limiting its use. Concentric ring electrodes are a promising alternative with potential to improve all of the aforementioned aspects significantly. In our previous work, the tripolar concentric ring electrode was successfully used in a wide range of applications demonstrating its superiority to conventional disc electrode, in particular, in accuracy of Laplacian estimation. This paper takes the first fundamental step toward further improving the Laplacian estimation of the novel multipolar concentric ring electrodes by proposing a general approach to estimation of the Laplacian for an (n + 1)-polar electrode with n rings using the (4n + 1)-point method for n ≥ 2 that allows cancellation of all the truncation terms up to the order of 2n. Examples of using the proposed approach to estimate the Laplacian for the cases of tripolar and, for the first time, quadripolar concentric ring electrode are presented.

Feasibility and analysis of bipolar concentric recording of electrohysterogram with flexible active electrode

The conduction velocity and propagation patterns of the electrohysterogram (EHG) provide fundamental information on the electrophysiological condition of the uterus. However, the accuracy of these measurements can be impaired by both the poor spatial selectivity and sensitivity to the relative direction of the contraction propagation associated with conventional disc electrodes. Concentric ring electrodes could overcome these limitations. The aim of this study was to examine the feasibility of picking up surface EHG signals using a new flexible tripolar concentric ring electrode (TCRE), and to compare these signals with conventional bipolar recordings. Simultaneous recording of conventional bipolar signals and bipolar concentric EHG (BC-EHG) were carried out on 22 pregnant women. Signal bursts were characterized and compared. No significant differences were found between the channels in either duration or dominant frequency in the Fast Wave High frequency range. Nonetheless, the high pass filtering effect of the BC-EHG recordings gave lower frequency content between 0.1 and 0.2 Hz. Although the BC-EHG signal amplitude was about 5-7 times smaller than that of bipolar recordings, a similar signal-to-noise ratio was obtained. These results suggest that the flexible TCRE is able to pick up uterine electrical activity and could provide additional information for deducing the uterine electrophysiological condition.

Evaluation of Laplacian diaphragm electromyographic recording in a dynamic inspiratory maneuver

The analysis of the electromyographic signal of the diaphragm muscle (EMGdi) can provide important information for evaluating the respiratory muscular function. The EMGdi can be recorded using surface Ag/AgCl disc electrodes in monopolar or bipolar configuration. However, these non-invasive EMGdi recordings are usually contaminated by the electrocardiographic (ECG) signal. EMGdi signal can also be noninvasively recorded using concentric ring electrodes in bipolar configuration (CRE) that estimate Laplacian surface potential. Laplacian recordings increase spatial resolution and attenuate distant bioelectric interferences, such as the ECG. Thus, the objective of this work is to compare and to evaluate CRE and traditional bipolar EMGdi recordings in a healthy subject during a dynamic inspiratory maneuver with incremental inspiratory loads. In the conducted study, it was calculated the cumulative percentage of power spectrum of EMGdi recordings to determine the signal bandwidth, and the power ratio between the EMGdi signal segments with and without cardiac activity. The results of this study suggest that EMGdi acquired with CRE electrodes is less affected by the ECG interference, achieves a wider bandwidth and a higher power ratio between segments without cardiac activity and with cardiac activity.

Equivalency between emulated disc electrodes and conventional disc electrode human electroencephalography

We have previously shown that tripolar concentric ring electrode (TCRE) Laplacian electroencephalography (tEEG) has significantly better signal-to-noise ratio, spatial resolution, and mutual information than disc electrode electroencephalography (EEG). This paper compares the EEG signals acquired simultaneously from the outer ring of the TCRE (oTCRE), shorting all three elements of the TCRE (sTCRE) and disc electrode (disc) concurrently from nearly the same location on the human scalp. We calculated the average correlation for the time series between each pair of signals and average coherence over the pass-band frequencies between all pairs of signals as well. All the correlations and coherences were above 0.99. The results suggest that the oTCRE can be used to record EEG concurrently with tEEG from the same sensor at the same location.

Comparison of electromyographic signals from monopolar current and potential amplifiers derived from a penniform muscle, the gastrocnemius medialis

Electromyograms (EMGs) are measured by bipolar surface electrodes that quantify potential differences. Bipolar potentials over penniform muscles may be associated with errors. Our assumption was that muscle activity can be quantified more reliably and with a higher spatial resolution using current measurements. The purpose of this work is: (a) to introduce the concept of current measurements to detect muscle activity, (b) to show the coherences observed over a segment of a typical penniform muscle, the gastrocnemius medialis where one would expect a synchronicity of the activation, and (c) to show the amount of mixing that is caused by the finite inter electrode resistance. A current amplifier was developed. EMGs were recorded at 40% of maximum voluntary contraction during isometric contractions of the gastrocnemius medialis. EMGs of twelve persons were recorded with an array of four peripheral and one central electrode. Monopolar EMGs were recorded for "all-potential", "center at current" and "all-current" conditions. Coherence revealed the similarity of signals recorded from neighboring electrodes. Coherence was high for the "all-potential", significant for the "current at center" condition and disappeared in the "all-current" condition. It was concluded that EMG array recordings strongly depends on the measurement configuration. The proposed current amplifier significantly improves spatial resolution of EMG array recordings because the inter-electrode cross talk is reduced.

Active concentric ring electrode for non-invasive detection of intestinal myoelectric signals

Although the surface electroenterogram (EEnG) is a weak signal contaminated by strong physiological interference, such as ECG and respiration, abdominal surface recordings of the EEnG could provide a non-invasive method of studying intestinal activity. The goal of this work was to develop a modular, active, low-cost and easy-to-use sensor to obtain a direct estimation of the Laplacian of the EEnG on the abdominal surface in order to enhance the quality of bipolar surface monitoring of intestinal activity. The sensor is made up of a set of 3 concentric dry Ag/AgCl ring electrodes and a battery-powered signal-conditioning circuit. Each section is etched on a different printed circuit board (PCB) and the sections are joined to each other by surface mount technology connectors. This means the sensing electrodes can be treated independently for purposes of maintenance and replacement and the signal conditioning circuit can be re-used. A total of ten recording sessions were carried out on humans. The results show that the surface recordings of the EEnG obtained by the active sensor present significantly less ECG and respiration interference than those obtained by bipolar recordings. In addition, bioelectrical sources whose frequency fitted with the slow wave component of the EEnG (SW) were identified by parametric spectral analysis in the surface signals picked up by the active sensors.

Improved separability of dipole sources by tripolar versus conventional disk electrodes: a modeling study using independent component analysis

Tripolar electrodes have been shown to have less mutual information and higher spatial resolution than disc electrodes. In this work, a four-layer anisotropic concentric spherical head computer model was programmed, then four configurations of time-varying dipole signals were used to generate the scalp surface signals that would be obtained with tripolar and disc electrodes, and four important EEG artifacts were tested: eye blinking, cheek movements, jaw movements, and talking. Finally, a fast fixed-point algorithm was used for signal independent component analysis (ICA). The results show that signals from tripolar electrodes generated better ICA separation results than from disc electrodes for EEG signals with these four types of artifacts.

Characterization of the sensitivity of a TCB laplacian sensor for surface EEnG recordings

The improvement of the quality of electroenterogram (EEnG) recordings on abdominal surface could lead to a non-invasive technique to diagnose intestinal motility dysfunctions. In this context, the use of coaxial active electrodes, which permit to record the laplacian potential, can help to achieve such signal enhancement. In this paper, we present a methodology to obtain the maps of sensitivity of this kind of electrodes to pick up the activity of electric dipoles of different orientations. The proposed methodology employs mathematical models, as well as experimental studies (phantoms) to check the theoretical results. The mathematical model of the electrode, and of the human abdomen is developed by means of ANSYS. A simplified physical model is formed by real ring electrodes, a methacrylate tank of size 50 x 50 x 50 cm filled with a saltwater mixture of 2.5 g/l concentration, and moving electric dipoles made by wires of 0.3 mm in diameter. Sensitivity of the sensor is obtained for different depths and different axial distances of vertical and horizontal dipoles. Preliminary results of tripolar ring electrodes in bipolar configuration (TCB) are shown. The obtained results prove the agreement between mathematical and experimental results. The validated model will allow us to study the behavior of laplacian ring electrodes of different dimensions and materials to record the EEnG activity and to analyze the influence of the abdominal layers.

Spatial filtering improves EMG classification accuracy following targeted muscle reinnervation

The combination of targeted muscle reinnervation (TMR) and pattern classification of electromyography (EMG) has shown great promise for multifunctional myoelectric prosthesis control. In this study, we hypothesized that surface EMG recordings with high spatial resolution over reinnervated muscles could capture focal muscle activity and improve the classification accuracy of identifying intended movements. To test this hypothesis, TMR subjects with transhumeral or shoulder disarticulation amputations were recruited. Spatial filters such as single differential filters, double differential filters, and various two-dimensional, high-order spatial filters were used, and the classification accuracies for fifteen different movements were calculated. Compared with monopolar recordings, spatially localized EMG signals produced increased accuracy in identifying the TMR patients' movement intents, especially for hand movements. When the number of EMG signals was constrained to 12, the double differential filters gave 5-15% higher classification accuracies than the filters with lower spatial resolution, but resulted in comparable accuracies to the filters with higher spatial resolution. These results suggest that double differential EMG recordings may further improve the TMR-based neural interface for robust, multifunctional control of artificial arms.

Technology and instrumentation for detection and conditioning of the surface electromyographic signal: state of the art

The aim of this review is to present the state of the art of the technology of detection and conditioning systems for surface electromyography (sEMG). The first part of the manuscript focuses on the sEMG electrode system technology: the electrode classification, impedance, noise, transfer function, the spatial filtering effect of surface electrode configurations, the effects of electrode geometry, and location on the recorded sEMG signal. Examples of experimental sEMG signals are provided to show the potential value of high-density sEMG electrode grids and multichannel amplifiers that allow to add spatial information to the temporal information content of the sEMG signal. Furthermore, the results of a simple simulation are reported, in order to emphasize the effects of the subcutaneous tissue layers and of the detection volume on the recorded sEMG signal. The second part of the manuscript focuses on the sEMG amplifier technology: the front end amplifier characteristics for signal conditioning, the methods for stimulation artifact reduction, filtering methods, safety requirements, and the methods for analog-to-digital conversion of the sEMG signal.

Application of tripolar concentric electrodes and prefeature selection algorithm for brain-computer interface

For persons with severe disabilities, a brain-computer interface (BCI) may be a viable means of communication. Lapalacian electroencephalogram (EEG) has been shown to improve classification in EEG recognition. In this work, the effectiveness of signals from tripolar concentric electrodes and disc electrodes were compared for use as a BCI. Two sets of left/right hand motor imagery EEG signals were acquired. An autoregressive (AR) model was developed for feature extraction with a Mahalanobis distance based linear classifier for classification. An exhaust selection algorithm was employed to analyze three factors before feature extraction. The factors analyzed were 1) length of data in each trial to be used, 2) start position of data, and 3) the order of the AR model. The results showed that tripolar concentric electrodes generated significantly higher classification accuracy than disc electrodes.

Transcranial current stimulation focality using disc and ring electrode configurations: FEM analysis

We calculated the electric fields induced in the brain during transcranial current stimulation (TCS) using a finite-element concentric spheres human head model. A range of disc electrode configurations were simulated: (1) distant-bipolar; (2) adjacent-bipolar; (3) tripolar; and three ring designs, (4) belt, (5) concentric ring, and (6) double concentric ring. We compared the focality of each configuration targeting cortical structures oriented normal to the surface ('surface-radial' and 'cross-section radial'), cortical structures oriented along the brain surface ('surface-tangential' and 'cross-section tangential') and non-oriented cortical surface structures ('surface-magnitude' and 'cross-section magnitude'). For surface-radial fields, we further considered the 'polarity' of modulation (e.g. superficial cortical neuron soma hyper/depolarizing). The distant-bipolar configuration, which is comparable with commonly used TCS protocols, resulted in diffuse (un-focal) modulation with bi-directional radial modulation under each electrode and tangential modulation between electrodes. Increasing the proximity of the two electrodes (adjacent-bipolar electrode configuration) increased focality, at the cost of more surface current. At similar electrode distances, the tripolar-electrodes configuration produced comparable peak focality, but reduced radial bi-directionality. The concentric-ring configuration resulted in the highest spatial focality and uni-directional radial modulation, at the expense of increased total surface current. Changing ring dimensions, or use of two concentric rings, allow titration of this balance. The concentric-ring design may thus provide an optimized configuration for targeted modulation of superficial cortical neurons.

A simulation study for a surface EMG sensor that detects distinguishable motor unit action potentials

An advanced volume conductor model was used to simulate the surface-detected motor unit action potentials (MUAPs) due to current sources located at different depths within the muscle tissue of the biceps brachii. Seven different spatial filters were investigated by linear summation of the monopolarly detected surface MUAPs on a square array of nine electrodes. The criterion of the relative energy-of-difference (EOD) between the MUAPs was used to rank spatial filters for their ability to distinguish two motor units located at different depths. Using the same criterion pair wise combinations of spatial filters were ranked for their ability to generate different MUAP shape representations of the same motor unit. In both analyses, the bi-transversal double-differential (BiTDD) configurations and pair wise combinations involving a BiTDD configuration consistently ranked highest. Varying electrode spacing did not change the results in a relevant way. Based on the EOD calculations, a four-channel detection system using all available electrodes of the array is proposed. The implications of using only six electrodes, effectively reducing contact area of the sensor in half, are discussed.

A modified cardiac stabilizer used for recording electrograms

Originally, tissue stabilizer has been developed by Medtronic, Inc. It is used to prevent a certain part of myocardium from moving. For example, such an approach is utilized during Coronary Artery Bypass Grafting (CABG). Moreover, medical procedures during such operations involve blood arrest, thus, potentially may involve ischemia of myocardium. A construction of the modified tissue stabilizer used during surgery on a beating heart is presented in the paper. We have developed a stabilizer that aside preventing part of myocardium from moving enables recording of epicardial electrograms. Four electrograms are recorded simultaneously together with one lead of standard ECG as a reference signal. Time relations between electrograms, recorded at four different points surrounding potentially ischemic area, are strongly modified by processes triggered by myocardium ischemia. Aside a construction of the stabilizer the associated measurement system is also presented. Measurement properties of the developed stabilizer and measurement system have been examined during tank studies. Finally, the waveforms obtained during in vivo study are also presented.

Improvement of spatial selectivity and decrease of mutual information of tri-polar concentric ring electrodes

Electroencephalography (EEG) signals are spatio-temporal in nature. EEG has very good temporal resolution but typically does not possess high spatial resolution. The surface Laplacian enhances the spatial resolution and selectivity of the surface electrical activity recording. Concentric ring electrodes have been shown to estimate the surface Laplacian directly with significantly better spatial resolution than conventional electrodes. For this report movement-related potentials (MRP) signals were analyzed. The signals were recorded using tri-polar ring electrodes in the original configuration as well as in bipolar and unipolar configurations achieved by excluding or shorting recording surfaces of the tri-polar version, respectively. The electrodes were placed in an array scheme of 35, encompassing the area between Fz-Cz-Pz-P3-T5-T3-F7-F3 centered on C3. Data were measured in five steps sequentially using only seven electrodes at a time, displaced after each step and aligned during evaluation later. Subjects were cued to press a micro-switch. The signal-to-noise ratio (SNR), spatial selectivity, and mutual information (MI) of the MRP signals recorded with the different electrode systems were compared. The MRP signals recorded with the tri-polar concentric ring electrode system have significantly higher SNR than from bipolar concentric ring electrode and conventional disc electrode emulations. The tri-polar electrodes have also shown significantly higher spatial selectivity as well as significantly less mutual information between locations than the other two electrode configurations tested. These characteristics should make tri-polar concentric electrodes beneficial for EEG applications.

Clinical applications of high-density surface EMG: A systematic review

High density-surface EMG (HD-sEMG) is a non-invasive technique to measure electrical muscle activity with multiple (more than two) closely spaced electrodes overlying a restricted area of the skin. Besides temporal activity HD-sEMG also allows spatial EMG activity to be recorded, thus expanding the possibilities to detect new muscle characteristics. Especially muscle fiber conduction velocity (MFCV) measurements and the evaluation of single motor unit (MU) characteristics come into view. This systematic review of the literature evaluates the clinical applications of HD-sEMG. Although beyond the scope of the present review, the search yielded a large number of "non-clinical" papers demonstrating that a considerable amount of work has been done and that significant technical progress has been made concerning the feasibility and optimization of HD-sEMG techniques. Twenty-nine clinical studies and four reviews of clinical applications of HD-sEMG were considered. The clinical studies concerned muscle fatigue, motor neuron diseases (MND), neuropathies, myopathies (mainly in patients with channelopathies), spontaneous muscle activity and MU firing rates. In principle, HD-sEMG allows pathological changes at the MU level to be detected, especially changes in neurogenic disorders and channelopathies. We additionally discuss several bioengineering aspects and future clinical applications of the technique and provide recommendations for further development and implementation of HD-sEMG as a clinical diagnostic tool.

Tri-polar concentric ring electrode development for Laplacian electroencephalography

Brain activity generates electrical potentials that are spatio-temporal in nature. Electroencephalography (EEG) is the least costly and most widely used noninvasive technique for diagnosing many brain problems. It has high temporal resolution, but lacks high spatial resolution. In an attempt to increase the spatial selectivity, researchers introduced a bipolar electrode configuration utilizing a five-point finite difference method (FPM) and others applied a quasi-bipolar (tri-polar with two elements shorted) concentric electrode configuration. To further increase the spatial resolution, the authors report on a tri-polar concentric electrode configuration for approximating the analytical Laplacian based on a nine-point finite difference method (NPM). For direct comparison, the FPM, quasi-bipolar method (a hybrid NPM), and NPM were calculated over a 400 x 400 mesh with 1/400 spacing using a computer model. A closed-form analytical computer model was also developed to evaluate and compare the properties of concentric bipolar, quasi-bipolar, and tri-polar electrode configurations, and the results were verified with tank experiments. The tri-polar configuration and the NPM were found to have significantly improved accuracy in Laplacian estimation and localization. Movement-related potential (MRP) signals were recorded from the left prefrontal lobes on the scalp of human subjects while they performed fast repetitive movements. Disc, bipolar, quasi-bipolar, and tri-polar electrodes were used. MRP signals were plotted for all four electrode configurations. The signal-to-noise ratio and spatial selectivity of the MRP signals acquired with the tri-polar electrode configuration were significantly better than the other configurations.

Mutual information of tri-polar concentric ring electrodes

Electroencephalography (EEG) signals are spatio-temporal. EEG has very good temporal resolution but typically doesn't possess high spatial resolution. The surface Laplacian enhances the spatial resolution and selectivity of the surface electrical activity. Concentric ring electrodes have been shown to estimate the surface Laplacian directly with significantly better spatial resolution than conventional electrodes and possess spatial filtering characteristics. Movement Related potentials (MRP) were recorded using tri-polar and bipolar concentric ring electrodes as well as conventional disc EEG electrodes while the subjects were pressing a micro-switch. The electrodes were placed in an array of 35 encompassing the area between Fz-Cz-Pz-P3-T5-T3-T7-F3. Mutual information (MI) of the MRP signals recorded with the different electrode systems was compared. The MRP signals recorded with the tri-polar concentric ring electrode system have significantly less MI between locations than the other two electrode configurations tested. The decrease in MI should increase the total information available by pooling of information from independent tri-polar concentric ring electrodes. These characteristics should make tri-polar concentric electrodes beneficial for EEG applications.

Application of Higher Order Statistics Techniques to EMG Signals to Characterize the Motor Unit Action Potential

The electromyographic (EMG) signal provides information about the performance of muscles and nerves. At any instant, the shape of the muscle signal, motor unit action potential (MUAP), is constant unless there is movement of the position of the electrode or biochemical changes in the muscle due to changes in contraction level. The rate of neuron pulses, whose exact times of occurrence are random in nature, is related to the time duration and force of a muscle contraction. The EMG signal can be modeled as the output signal of a filtered impulse process where the neuron firing pulses are assumed to be the input of a system whose transfer function is the motor unit action potential. Representing the neuron pulses as a point process with random times of occurrence, the higher order statistics based system reconstruction algorithm can be applied to the EMG signal to characterize the motor unit action potential. In this paper, we report results from applying a cepstrum of bispectrum based system reconstruction algorithm to real wired-EMG (wEMG) and surface-EMG (sEMG) signals to estimate the appearance of MUAPs in the Rectus Femoris and Vastus Lateralis muscles while the muscles are at rest and in six other contraction positions. It is observed that the appearance of MUAPs estimated from any EMG (wEMG or sEMG) signal clearly shows evidence of motor unit recruitment and crosstalk, if any, due to activity in neighboring muscles. It is also found that the shape of MUAPs remains the same on loading.