Detection of Motor-Evoked Potentials Below the Noise Floor: Rethinking the Motor Threshold

Background: Motor-evoked potentials (MEP) are one of the most prominent responses to brain stimulation, such as supra-threshold transcranial magnetic stimulation (TMS) and electrical stimulation. Understanding of the neurophysiology and the determination of the lowest stimulation strength that evokes responses requires the detection of even smaller responses, e.g., from single motor units. However, available detection and quantization methods suffer from a large noise floor. Objective: This paper develops a detection method that extracts MEPs hidden below the noise floor. With this method, we aim to estimate excitatory activations of the corticospinal pathways well below the conventional detection level. Methods: The presented MEP detection method presents a self-learning matched-filter approach for improved robustness against noise. The filter is adaptively generated per subject through iterative learning. For responses that are reliably detected by conventional detection, the new approach is fully compatible with established peak-to-peak readings and provides the same results but extends the dynamic range below the conventional noise floor. Results: In contrast to the conventional peak-to-peak measure, the proposed method increases the signal-to-noise ratio by more than a factor of 5. The first detectable responses appear to be substantially lower than the conventional threshold definition of 50 μV median peak-to-peak amplitude. Conclusion: The proposed method shows that stimuli well below the conventional 50 μV threshold definition can consistently and repeatably evoke muscular responses and thus activate excitable neuron populations in the brain. As a consequence, the IO curve is extended at the lower end, and the noise cut-off is shifted. Importantly, the IO curve extends so far that the 50 μV point turns out to be closer to the center of the logarithmic sigmoid curve rather than close to the first detectable responses. The underlying method is applicable to a wide range of evoked potentials and other biosignals, such as in electroencephalography.

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Output of Human Motoneuron Pools to Corticospinal Inputs During Voluntary Contractions

Journal of Neurophysiology ◽

10.1152/jn.01230.2005 ◽

2006 ◽

Vol 95 (6) ◽

pp. 3512-3518 ◽

Cited By ~ 89

Author(s):

P. G. Martin ◽

S. C. Gandevia ◽

J. L. Taylor

Keyword(s):

Evoked Potentials ◽

Biceps Brachii ◽

Motor Evoked Potentials ◽

Voluntary Contraction ◽

Flexor Muscle ◽

Spinal Level ◽

Motoneuron Pool ◽

Wide Range ◽

Voluntary Contractions ◽

Stimulation Of

This study investigated transmission of corticospinal output through motoneurons over a wide range of voluntary contraction strengths in humans. During voluntary contraction of biceps brachii, motor evoked potentials (MEPs) to transcranial magnetic stimulation of the motor cortex grow up to about 50% maximal force and then decrease. To determine whether the decrease reflects events at a cortical or spinal level, responses to stimulation of the cortex and corticospinal tract (cervicomedullary motor evoked potentials, CMEPs) as well as maximal M-waves (Mmax) were recorded during strong contractions at 50 to 100% maximum. In biceps and brachioradialis, MEPs and CMEPs (normalized to Mmax) evoked by strong stimuli decreased during strong elbow flexions. Responses were largest during contractions at 75% maximum and both potentials decreased by about 25% Mmax during maximal efforts ( P < 0.001). Reductions were smaller with weaker stimuli, but again similar for MEPs and CMEPs. Thus the reduction in MEPs during strong voluntary contractions can be accounted for by reduced responsiveness of the motoneuron pool to stimulation. During strong contractions of the first dorsal interosseous, a muscle that increases voluntary force largely by frequency modulation, MEPs declined more than in either elbow flexor muscle (35% Mmax, P < 0.001). This suggests that motoneuron firing rates are important determinants of evoked output from the motoneuron pool. However, motor cortical output does not appear to be limited at high contraction strengths.

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Recruitment and Discharge Patterns of Single Motor Units During Speech Production

Journal of Speech and Hearing Research ◽

10.1044/jshr.2004.613 ◽

1977 ◽

Vol 20 (4) ◽

pp. 613-630 ◽

Cited By ~ 9

Author(s):

Harvey M. Sussman ◽

Peter F. MacNeilage ◽

Randall K. Powers

Keyword(s):

Dynamic Range ◽

Isometric Force ◽

Discharge Rate ◽

Motor Units ◽

Size Principle ◽

Single Motor ◽

Single Motor Units ◽

Movement Dynamics ◽

Discharge Patterns ◽

Anterior Belly

Recruitment and discharge patterns of single motor units (MUs) in the anterior belly of digastric were studied during speech in three subjects, using electrodes facilitating selective recording at high force levels. Fixed recruitment order was observed in over 99% of all comparisons. Later recruited units invariably possessed muscle action potentials of higher amplitude, suggesting that units were activated in accordance with the “size principle.” Additional evidence for this was that later recruited units, of a set of three studied during speech, motor unit training, and isometric force ramps, showed greater sensitivity to input, and greater dynamic range than earlier recruited units. Units in this set were much more sensitive to rapid changes in input associated with speech gestures than to static activation even at high force levels. Several significant relations between discharge characteristics and aspects of movement dynamics were observed, including relations between (1) recruitment interval (MU1 to MU3) and latency of mandibular lowering, (2) onset of initial discharge of MU1 and relative mechanical advantage of the mandible, (3) number of MUs active and velocity and displacement of the mandible, and (4) discharge rate of MU3 and velocity and displacement of the mandible.

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The efficacy of somatosensory evoked potentials in evaluating new neurological deficits after spinal thoracic fusion and decompression

Journal of Neurosurgery Spine ◽

10.3171/2019.12.spine191157 ◽

2020 ◽

Vol 33 (1) ◽

pp. 35-40

Author(s):

Samyuktha R. Melachuri ◽

Carolyn Stopera ◽

Manasa K. Melachuri ◽

Katherine Anetakis ◽

Donald J. Crammond ◽

...

Keyword(s):

Evoked Potentials ◽

Somatosensory Evoked Potentials ◽

Odds Ratio ◽

Motor Evoked Potentials ◽

Univariate Analysis ◽

Neurological Deficits ◽

Neurophysiological Monitoring ◽

Fusion Surgery ◽

Wide Range ◽

The Impact

OBJECTIVEPosterior thoracic fusion (PTF) is used as a surgical treatment for a wide range of pathologies. The monitoring of somatosensory evoked potentials (SSEPs) is used to detect and prevent injury during many neurological surgeries. The authors conducted a study to evaluate the efficacy of SSEPs in predicting perioperative lower-extremity (LE) neurological deficits during spinal thoracic fusion surgery.METHODSThe authors included patients who underwent PTF with SSEP monitoring performed throughout the entire surgery from 2010 to 2015 at the University of Pittsburgh Medical Center (UPMC). The sensitivity, specificity, odds ratio, and receiver operating characteristic curve were calculated to evaluate the diagnostic accuracy of SSEP changes in predicting postoperative deficits. Univariate analysis was completed to determine the impact of age exceeding 65 years, sex, obesity, abnormal baseline testing, surgery type, and neurological deficits on the development of intraoperative changes.RESULTSFrom 2010 to 2015, 771 eligible patients underwent SSEP monitoring during PTF at UPMC. Univariate and linear regression analyses showed that LE SSEP changes significantly predicted LE neurological deficits. Significant changes in LE SSEPs had a sensitivity and specificity of 19% and 96%, respectively, in predicting LE neurological deficits. The diagnostic odds ratio for patients with new LE neurological deficits who had significant changes in LE SSEPs was 5.86 (95% CI 2.74–12.5). However, the results showed that a loss of LE waveforms had a poor predictive value for perioperative LE deficits (diagnostic OR 1.58 [95% CI 0.19–12.83]).CONCLUSIONSPatients with new postoperative LE neurological deficits are 5.9 times more likely to have significant changes in LE SSEPs during PTF. Surgeon awareness of an LE SSEP loss may alter surgical strategy and positively impact rates of postoperative LE neurological deficit status. The relatively poor sensitivity of LE SSEP monitoring may indicate a need for multimodal neurophysiological monitoring, including motor evoked potentials, in thoracic fusion surgery.

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EEG-triggered TMS reveals stronger brain state-dependent modulation of motor evoked potentials at weaker stimulation intensities

10.1101/251363 ◽

2018 ◽

Cited By ~ 3

Author(s):

Natalie Schaworonkow ◽

Jochen Triesch ◽

Ulf Ziemann ◽

Christoph Zrenner

Keyword(s):

Evoked Potentials ◽

Real Time ◽

Motor Evoked Potentials ◽

Corticospinal Excitability ◽

Brain State ◽

Stimulation Intensity ◽

Negative Peak ◽

State Dependent ◽

Wide Range ◽

Brain States

AbstractBackgroundCorticospinal excitability depends on the current brain state. The recent development of real-time EEG-triggered transcranial magnetic stimulation (EEG-TMS) allows studying this relationship in a causal fashion. Specifically, it has been shown that corticospinal excitability is higher during the scalp surface negative EEG peak compared to the positive peak of µ-oscillations in sensorimotor cortex, as indexed by larger motor evoked potentials (MEPs) for fixed stimulation intensity.ObjectiveWe further characterize the effect of µ-rhythm phase on the MEP input-output (IO) curve by measuring the degree of excitability modulation across a range of stimulation intensities. We furthermore seek to optimize stimulation parameters to enable discrimination of functionally relevant EEG-defined brain states.MethodsA real-time EEG-TMS system was used to trigger MEPs during instantaneous brain-states corresponding to µ-rhythm surface positive and negative peaks with five different stimulation intensities covering an individually calibrated MEP IO curve in 15 healthy participants.ResultsMEP amplitude is modulated by µ-phase across a wide range of stimulation intensities, with larger MEPs at the surface negative peak. The largest relative MEP-modulation was observed for weak intensities, the largest absolute MEP-modulation for intermediate intensities. These results indicate a leftward shift of the MEP IO curve during the µ-rhythm negative peak.ConclusionThe choice of stimulation intensity influences the observed degree of corticospinal excitability modulation by µ-phase. Lower stimulation intensities enable more efficient differentiation of EEG µ-phase-defined brain states.

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The spectral features of EEG responses to transcranial magnetic stimulation of the primary motor cortex depend on the amplitude of the motor evoked potentials

10.1101/133769 ◽

2017 ◽

Author(s):

Matteo Fecchio ◽

Andrea Pigorini ◽

Angela Comanducci ◽

Simone Sarasso ◽

Silvia Casarotto ◽

...

Keyword(s):

Transcranial Magnetic Stimulation ◽

Motor Cortex ◽

Evoked Potentials ◽

Magnetic Stimulation ◽

Primary Motor Cortex ◽

Motor Evoked Potentials ◽

Spectral Features ◽

Time Frequency ◽

Resting Motor Threshold ◽

Wide Range

ABSTRACTTranscranial magnetic stimulation (TMS) of the primary motor cortex (M1) can excite both cortico-cortical and cortico-spinal axons resulting in TMS-evoked potentials (TEPs) and motor-evoked potentials (MEPs), respectively. Despite this remarkable difference with other cortical areas, the influence of motor output and its amplitude on TEPs is largely unknown. Here we studied TEPs resulting from M1 stimulation and assessed whether their waveform and spectral features depend on the MEP amplitude. To this aim, we performed two separate experiments. In experiment 1, single-pulse TMS was applied at the same supra-threshold intensity on primary motor, prefrontal, premotor and parietal cortices and the corresponding TEPs were compared by means of local mean field power and time-frequency spectral analysis. In experiment 2 we stimulated M1 at resting motor threshold in order to elicit MEPs characterized by a wide range of amplitudes. TEPs computed from high-MEP and low-MEP trials were then compared using the same methods applied in experiment 1. In line with previous studies, TMS of M1 produced larger TEPs compared to other cortical stimulations. Notably, we found that only TEPs produced by M1 stimulation were accompanied by a late (∼300 ms after TMS) event-related desynchronization (ERD), whose magnitude was strongly dependent on the amplitude of MEPs. Overall, these results suggest that M1 produces peculiar responses to TMS possibly reflecting specific anatomo-functional properties, such as the re-entry of proprioceptive feedback associated with target muscle activation.

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