Far-field potential production by quadrupole generators in cylindrical volume conductors

Adaptive EMG decomposition in dynamic conditions based on online learning metrics with tunable hyperparameters

Objective. Developing neural decoders robust to non-stationary conditions is essential to ensure their long-term accuracy and stability. This is particularly important when decoding the neural drive to muscles during dynamic contractions, which pose significant challenges for stationary decoders.Approach. We propose a novel adaptive electromyography (EMG) decomposition algorithm that builds on blind source separation methods by leveraging the Kullback-Leibler divergence and kurtosis of the signals as metrics for online learning. The proposed approach provides a theoretical framework to tune the adaptation hyperparameters and compensate for non-stationarities in the mixing matrix, such as due to dynamic contractions, and to identify the underlying motor neuron (MN) discharges. The adaptation is performed in real-time (∼22 ms of computational time per 100 ms batches).Main results. The hyperparameters of the proposed adaptation captured anatomical differences between recording locations (forearm vs wrist) and generalised across subjects. Once optimised, the proposed adaptation algorithm significantly improved all decomposition performance metrics with respect to the absence of adaptation in a wide range of motion of the wrist (80∘). The rate of agreement, sensitivity, and precision were⩾90%in⩾80%of the cases in both simulated and experimentally recorded data, according to a two-source validation approach.Significance. The findings demonstrate the suitability of the proposed online learning metrics and hyperparameter optimisation to compensate the induced modulation and accurately decode MN discharges in dynamic conditions. Moreover, the study proposes an experimental validation method for EMG decomposition in dynamic tasks.

Somatosensory-Evoked Potentials as a Marker of Functional Neuroplasticity in Athletes: A Systematic Review

Background

Somatosensory-evoked potentials (SEP) represent a non-invasive tool to assess neural responses elicited by somatosensory stimuli acquired via electrophysiological recordings. To date, there is no comprehensive evaluation of SEPs for the diagnostic investigation of exercise-induced functional neuroplasticity. This systematic review aims at highlighting the potential of SEP measurements as a diagnostic tool to investigate exercise-induced functional neuroplasticity of the sensorimotor system by reviewing studies comparing SEP parameters between athletes and healthy controls who are not involved in organized sports as well as between athlete cohorts of different sport disciplines.

Methods

A systematic literature search was conducted across three electronic databases (PubMed, Web of Science, and SPORTDiscus) by two independent researchers. Three hundred and ninety-seven records were identified, of which 10 cross-sectional studies were considered eligible.

Results

Differences in SEP amplitudes and latencies between athletes and healthy controls or between athletes of different cohorts as well as associations between SEP parameters and demographic/behavioral variables (years of training, hours of training per week & reaction time) were observed in seven out of 10 included studies. In particular, several studies highlight differences in short- and long-latency SEP parameters, as well as high-frequency oscillations (HFO) when comparing athletes and healthy controls. Neuroplastic differences in athletes appear to be modality-specific as well as dependent on training regimens and sport-specific requirements. This is exemplified by differences in SEP parameters of various athlete populations after stimulation of their primarily trained limb.

Conclusion

Taken together, the existing literature suggests that athletes show specific functional neuroplasticity in the somatosensory system. Therefore, this systematic review highlights the potential of SEP measurements as an easy-to-use and inexpensive diagnostic tool to investigate functional neuroplasticity in the sensorimotor system of athletes. However, there are limitations regarding the small sample sizes and inconsistent methodology of SEP measurements in the studies reviewed. Therefore, future intervention studies are needed to verify and extend the conclusions drawn here.

Far-field potentials in the compound muscle action potential

It has long been believed that the compound muscle action potential (CMAP) in motor-nerve conduction studies (MCSs) records the action potential beneath the active electrode over the muscle belly. However, recent studies have revealed the contribution of the reference electrode to the CMAP, most prominent in the tibial nerve, followed by the ulnar nerve. This "reference electrode potential" is recorded when the conventional reference electrode distal to the muscle belly is connected to a proximal reference. It must be a far-field potential (FFP) considering its distribution, although the precise mechanism of its generation has not been clarified. The conventional theory of termination of the action potential at the muscle-tendon junction is insufficient. Regarding the ulnar CMAP, interosseous muscles mostly contribute to the FFPs. New understanding of CMAP based on the FFP theory may provide new insights into the interpretation of MCSs and related techniques, including motor unit number estimation.

The origin of the premotor potential recorded from the second lumbrical muscle in normal man

OBJECTIVE

When recording with a palm electrode, a premotor potential precedes the compound muscle action potential (CMAP), evoked from the second lumbrical (2L) muscle following median nerve stimulation. The purpose of this study was to determine the origin of the premotor potential from the 2L.

METHODS

We recorded potentials with multi-channel electrodes in the palm and finger in a bipolar or referential manner, stimulating the second digit or median nerve at the wrist.

RESULTS

We recorded the traveling nearfield sensory nerve action potential (SNAP) and stationary negative potential in the palm. The peak latency of the stationary negative potential was the same as the one of the near-field potential of the digital sensory fibers at the base of the second finger. The onset of the premotor potential from the 2L muscle is aligned to the palmar SNAP in a bipolar manner by antidromic stimulation.

CONCLUSIONS

We conclude that the premotor potential from the 2L muscle is composed of a SNAP arising from antidromically activated palm sensory branches and a far-field potential generated by the median digital nerve fibers as they pass from the palm into the second finger.

SIGNIFICANCE

Our results might be useful for evaluating the 2L-interossei test for diagnosing carpal tunnel syndrome.

Classification of the extracellular fields produced by activated neural structures

BACKGROUND

Classifying the types of extracellular potentials recorded when neural structures are activated is an important component in understanding nerve pathophysiology. Varying definitions and approaches to understanding the factors that influence the potentials recorded during neural activity have made this issue complex.

METHODS

In this article, many of the factors which influence the distribution of electric potential produced by a traveling action potential are discussed from a theoretical standpoint with illustrative simulations.

RESULTS

For an axon of arbitrary shape, it is shown that a quadrupolar potential is generated by action potentials traveling along a straight axon. However, a dipole moment is generated at any point where an axon bends or its diameter changes. Next, it is shown how asymmetric disturbances in the conductivity of the medium surrounding an axon produce dipolar potentials, even during propagation along a straight axon. Next, by studying the electric fields generated by a dipole source in an insulating cylinder, it is shown that in finite volume conductors, the extracellular potentials can be very different from those in infinite volume conductors. Finally, the effects of impulses propagating along axons with inhomogeneous cable properties are analyzed.

CONCLUSION

Because of the well-defined factors affecting extracellular potentials, the vague terms far-field and near-field potentials should be abandoned in favor of more accurate descriptions of the potentials.

Digit distribution of proper digital nerve action potential

Antidromic sensory nerve action potential testing is well characterized and commonly used to assess the sensory component of the upper limb median and ulnar nerves. The final terminal segments of these nerves are the proper digital nerves. Ring recording electrodes are commonly used to detect the proper digital nerves' antidromic responses. Attempts to record the separate contributions of individual digital nerves along the lateral aspects of each finger, using small surface electrodes, is shown to be unreliable for determining the integrity of a single terminal digital branch. We found between 50% to 77% of the stimulated terminal branch's response amplitude when recorded at electrodes positioned over the nonstimulated branch located 180 degrees from the activated terminal branch. Detecting a single terminal nerve response was achieved by using the fourth digit and the second digit with one of the second digit's branches neurophysiologically blocked by local anesthetic. The volume-conducted response from the opposite side of the finger resulted in this relatively large recorded response, which remains within the range of reference values precluding the simple use of antidromic techniques to assess injury to a single proper digital nerve. Techniques are proposed to avoid such pitfalls and to assess most accurately the desired response.

Supplementary motor area activation preceding voluntary movement is detectable with a whole-scalp magnetoencephalography system

Despite the fact that the knowledge about the structure and the function of the supplementary motor area (SMA) is steadily increasing, the role of the SMA in the human brain, e.g., the contribution of the SMA to the Bereitschaftspotential, still remains unclear and controversial. The goal of this study was to contribute further to this discussion by taking advantage of the increased spatial information of a whole-scalp magnetoencephalography (MEG) system enabling us to record the magnetic equivalent of the Bereitschaftspotential 1, the Bereitschaftsfeld 1 (BF 1) or readiness field 1. Five subjects performed a complex, and one subject a simple, finger-tapping task. It was possible to record the BF 1 for all subjects. The first appearance of the BF 1 was in the range of -1.9 to -1.7 s prior to movement onset, except for the subject performing the simple task (-1 s). Analysis of the development of the magnetic field distribution and the channel waveforms showed the beginning of the Bereitschaftsfeld 2 (BF 2) or readiness field 2 at about -0.5 s prior to movement onset. In the time range of BF 1, dipole source analysis localized the source in the SMA only, whereas dipole source analysis containing also the time range of BF 2 resulted in dipole models, including dipoles in the primary motor area. In summary, with a whole-head MEG system, it was possible for the first time to detect SMA activity in healthy subjects with MEG.

Anatomic origin of P13 and P14 scalp far-field potentials

After median nerve stimulation, noncephalic or earlobe reference montages enable one to record over the scalp a well-defined, positive far-field response, which has been labeled the P14 or P13-P14 complex. It has been ascertained that this wave is generated in the caudal brainstem. Its use is reliable and sometimes mandatory in assessing a number of diseases that affect primarily the brainstem, such as multiple sclerosis or coma. Because of its complex shape as well as discrepant findings in the literature, it is still debated whether this potential is produced by a single or by multiple serial generators. The authors present these different views and summarize the different recording methods, while bearing in mind that some recording techniques are more suitable for routine purposes and others are preferred in selected cases, when more information regarding caudal brainstem function is required.

Near- and far-fields: source characteristics and the conducting medium in neurophysiology

It is possible to appreciate the production of far-field potentials by considering constant current dipolar source voltage distributions in bounded volumes, especially when they are stretched in one direction, e.g., a cylinder. An essentially nondeclining voltage is detected when the recording electrodes are on opposite sides of, and relatively far from, the dipolar source. This voltage maintains its (a) latency, (b) amplitude, (c) morphology, and (d) polarity even if recordings are performed a whole body length away. These four criteria define far-field potentials. A propagating action potential (AP) can be conceptualized as a linear quadrupole or the summation of two dipoles "back-to-back" (+ - - +). The far-field components of the summated dipoles cancel resulting in the anticipated triphasic waveform for APs with only near-field characteristics, not meeting the first three criteria above. Far-field potentials can be transiently generated when any propagating AP constitutes a net "real" or "virtual" dipolar source. "Real" dipolar sources can occur if an AP encounters the termination of excitable tissue, an alteration in conduction velocity, curvature in excitable tissue resulting in a change in propagation direction, or an abrupt change in resistance of the excitable tissue. Virtual dipolar sources may be produced if an AP encounters a change in the size or shape of the extracellular medium or a transition in extracellular conductivity.

Anatomical and electrophysiological determinants of the human thenar compound muscle action potential

Clinical interpretation of the compound muscle action potential (CMAP) requires a precise understanding of its underlying mechanisms. We recorded normal thenar CMAP5 and motor unit action potentials using different electrode configurations and different thumb positions. Computer simulations show that the CMAP has four parts: rising edge, negative phase, positive phase, and tail which correspond to four distinct stages of electrical activity in the muscle: initiation at the end-plate, propagation, termination at the muscle/tendon junctions, and slow repolarization. The shapes of volume-conducted signals recorded beyond the muscle are also explained by these four stages. Changes in CMAP shape associated with thumb abduction are due to changes in termination times resulting from changes in muscle-fiber lengths. These findings demonstrate that the negative and positive phases of the CMAP are due to different mechanisms, and that anatomical factors, particularly muscle-fiber lengths, play an important role in determining CMAP shape.

Generator sources for the early and late ulnar hypothenar premotor potentials: short segment electrophysiologic studies and cadaveric dissection

OBJECTIVE

Determine the generator sources for the ulnar hypothenar premotor potentials (PMPs).

DESIGN

Observational.

SETTING

EMG laboratory.

SUBJECTS

Ten asymptomatic adult volunteers, three cadaver hands.

MAIN OUTCOME MEASURE

Far-field versus near-field characteristics of recorded PMPs as determined by bipolar and referential recording electrode montages. A possible anatomic basis for any observed differences between ulnar PMPs and previously studied median PMPs were explored through cadaveric dissection.

RESULTS

An early PMP (E-PMP) had a latency that varied with changes in the position of G1 only. A late PMP (L-PMP was seen only when G1 and G2 were on different volumes (palm vs fifth digit, or second digit vs fifth digit); its latency did not vary significantly with changes in the position of G1 and G2.

CONCLUSIONS

(1) E-PMP is a near-field potential generated by the ulnar nerve passing near the G1 electrode. (2) L-PMP represents a far-field potential generated by the ulnar digital nerves as they traverse from the hand volume containing G1 to the finger volume containing G2. (3) Greater L-PMP-to-CMAP separation in the median than in the ulnar nerve was explained by cadaveric dissection, which revealed that the motor branch (responsible for the trailing CMAP) is longer in the median nerve than in the ulnar nerve relative to each nerve's corresponding digital sensory branch (responsible for the preceding L-PMP). (4) The PMP that is typically recorded with G1 at the hypothenar motor point and G2 on the fifth digit most likely represents E-PMP. (5) Any proposed diagnostic use of the ulnar PMPs must take into consideration these generator sources.

Generators of the early and late median thenar premotor potentials

The generator sources of the median thenar premotor potentials (PMPs) have remained elusive despite debate in the literature. By studying the median nerve in the hand with a variety of bipolar and referential recording montages, we systematically examined the possible near-field and far-field sources that may determine these potentials. The results suggest that the early PMP is a near-field potential recorded by G1 and generated by the median nerve traversing the distal carpal tunnel. The late PMP represents a far-field potential generated by the median digital nerve fibers as they pass from the palm volume into the thumb volume. Characteristics of the late PMP are explained using the leading/trailing dipole (L/TD) model of far-field potential generation. The diagnostic utility of these PMPs is questionable, since they are recorded from "regions" along the nerve rather than from more clearly defined sites.

Median/ulnar premotor potential identification and localization

A small negative waveform is known to precede the median and ulnar compound muscle action potentials when recorded with surface or concentric needle electrodes. This investigation documents that there are two distinct waveforms preceding the median compound muscle action potential (CMAP) depending upon the type of recording electrodes used (concentric needle versus surface) and their respective locations. The negative waveform originally described with a concentric needle electrode positioned within the substance of the distal thenar eminence and having a restricted zone of detection is referred to as the intramuscular nerve action potential (INAP). This potential is shown to be distinct from the premotor potential (the small negative waveform preceding surface recorded ulnar and median CMAPs). Detection of the median and ulnar premotor potentials at multiple locations about the hand with the same respective onset/peak latencies and amplitudes substantiates that this potential is a far-field potential. The median and ulnar premotor potentials most likely originate from a dipolar moment imbalance generated by digital sensory nerve action potentials as they cross the first and fifth metacarpophalangeal junctions, respectively. Applying far-field principles permits the documentation of additional far-field potentials as they are generated at the second through fourth metacarpophalangeal junctions following median nerve stimulation. Also, because the premotor potential is a far-field potential, caution must be exercised with respect to its diagnostic utility as joint position and other unknown factors may affect amplitude and onset/peak latency. The INAP following median nerve excitation, however, is documented to be a near-field potential distinct from the premotor potential arising from the recurrent branch of the median nerve.(ABSTRACT TRUNCATED AT 250 WORDS)