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

Williams ER, Baker SN - J. Neurophysiol. (2008)

Bottom Line: The model was able to reproduce the smaller than expected delays between cortex and muscles seen in experiments.However, the model could not reproduce the constant phase over a frequency band sometimes seen in experiments, nor the lack of around 10-Hz coherence.Simple propagation of oscillations from cortex to muscle thus cannot completely explain the observed corticomuscular coherence.

View Article: PubMed Central - PubMed

Affiliation: Institute of Neuroscience, Newcastle University, Henry Wellcome Building, Newcastle upon Tyne, NE2 4HH, UK.

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

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A: spike-triggered average (STA) of the rectified EMG compiled using spikes from all motoneurons in the pool. Horizontal lines show the pretrigger baseline ±2SDs. The center of gravity (CoG) was calculated for the primary STA peak (shaded).
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f5: A: spike-triggered average (STA) of the rectified EMG compiled using spikes from all motoneurons in the pool. Horizontal lines show the pretrigger baseline ±2SDs. The center of gravity (CoG) was calculated for the primary STA peak (shaded).

Mentions: The model was constructed to include a minimum conduction delay from motoneurons to the muscle of 11 ms, with a maximum of 2-ms dispersion across the different motor axons. By contrast, the delay measured from the coherence phase between motoneuron spikes and EMG was around 18 ms (Fig. 3E). To understand this discrepancy, a spike-triggered average (STA) of the rectified EMG was constructed, triggered by the motoneuron spikes (Fig. 5). The onset latency of the peak in the STA was 13.6 ms, close to the expected neural conduction time. However, coherence analysis will not be sensitive to the onset latency, but to an “average” latency of the whole response. The center of gravity of the peak in Fig. 5 (shaded area) had a latency of 20.4 ms, close to the delay estimated from the coherence phase. The peripheral delay estimated using coherence is thus sensitive not only to the neural conduction time, but also to the duration of the motor unit action potentials. This was previously suggested by Riddle and Baker (2005).


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

Williams ER, Baker SN - J. Neurophysiol. (2008)

A: spike-triggered average (STA) of the rectified EMG compiled using spikes from all motoneurons in the pool. Horizontal lines show the pretrigger baseline ±2SDs. The center of gravity (CoG) was calculated for the primary STA peak (shaded).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2637020&req=5

f5: A: spike-triggered average (STA) of the rectified EMG compiled using spikes from all motoneurons in the pool. Horizontal lines show the pretrigger baseline ±2SDs. The center of gravity (CoG) was calculated for the primary STA peak (shaded).
Mentions: The model was constructed to include a minimum conduction delay from motoneurons to the muscle of 11 ms, with a maximum of 2-ms dispersion across the different motor axons. By contrast, the delay measured from the coherence phase between motoneuron spikes and EMG was around 18 ms (Fig. 3E). To understand this discrepancy, a spike-triggered average (STA) of the rectified EMG was constructed, triggered by the motoneuron spikes (Fig. 5). The onset latency of the peak in the STA was 13.6 ms, close to the expected neural conduction time. However, coherence analysis will not be sensitive to the onset latency, but to an “average” latency of the whole response. The center of gravity of the peak in Fig. 5 (shaded area) had a latency of 20.4 ms, close to the delay estimated from the coherence phase. The peripheral delay estimated using coherence is thus sensitive not only to the neural conduction time, but also to the duration of the motor unit action potentials. This was previously suggested by Riddle and Baker (2005).

Bottom Line: The model was able to reproduce the smaller than expected delays between cortex and muscles seen in experiments.However, the model could not reproduce the constant phase over a frequency band sometimes seen in experiments, nor the lack of around 10-Hz coherence.Simple propagation of oscillations from cortex to muscle thus cannot completely explain the observed corticomuscular coherence.

View Article: PubMed Central - PubMed

Affiliation: Institute of Neuroscience, Newcastle University, Henry Wellcome Building, Newcastle upon Tyne, NE2 4HH, UK.

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

Show MeSH
Related in: MedlinePlus