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I'm trying to analyze the electrocorticograms (ECoG) of opening and closing the fist. I have the data of a glove to capture when the hand moves. I'm struggling with finding the best epoch or segment that represents the ECoG signals that are mostly related to opening and closing the fist. So, I'm wondering if there are works that already being done to calculate the latency between muscles response and the actual "brain order".

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Short answer
Reported motor delays between brain activity and motor response are in the order of 6 - 9 ms.

Background
I found a study where the authors used deep brain stimulation electrodes implanted in the subthalamic nucleus (STN, Fig. 1) in patients with Parkinson's disease to activate motor tracts at a subcortical level. The STN is not part of the pyramidal motor system, but is thought to keep things in check. Stimulation in Parkinson's is used to control involuntary movements. In this study, the authors activated motor tracts at a subcortical level via the STN (Costa et al, 2007)

The authors recorded facial, cervical and upper limb muscle responses bilaterally by means of motor-evoked potentials (MEPs) in the trapezius, deltoid, biceps and thenar muscles as well as the orbicularis oculi, orbicularis oris, masseter and sternocleidomastoid muscles. They report mean latencies in the range 6.0 - 9.1 ms. MEP latencies in the orbicularis oculi and orbicularis oris were significantly longer than in the masseter and sternocleidomastoid.

In your experiments, there was direct, voluntary activation of the motor pathway via the pyramidal system in (healthy?) subjects. In contrast, the cited study recorded latencies in Parkinson's sufferers after direct electrical activation of a subcortical structure. Hence, the pathway is different between the two studies and the latencies observed in the cited study may not apply fully to your data set. Nonetheless, it may help you to narrow the time window in which you might search for the brain activity relative to the motor output from the experimental glove.

STN
Fig. 1. Overview of a brain slice showing the position of the STN. source: Wikipedia

Reference
- Costa et al., Brain. (2007); 130(1): 245-55

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