We are going to use motor imagery based BCI for stroke rehabilitation and currently I am going through some papers that discuss the EEG-EMG coherence and time lag. While the results of different studies are not quite consistent and you can see values around 50 MS for LFP-EEG time lag in monkeys (Morrow and Miller, 2003; Rivera-Alvidrez et al., 2010), you will see conduction time measured by TMS around 20 ms(Samii et al., 1998), and finally EEG-EMG lag time of around 26 ms (Whitham et al., 2011).
Could someone please let me know how to interpret these differences?
This is not the answer, but just FYI. I remember that Zaaimi et al presented their study tackling this question: they examined the discrepancy between the conduction time and the estimated time delay by combining the peripheral nerve stimulation and the sensory response recording in M1.
http://www.abstractsonline.com/Plan/ViewAbstract.aspx?sKey=caae67ff-b12e-438b-829b-79fcf8f6e816&cKey=b0e1107a-ba0f-4c23-96d3-b98f7536c4c9&mKey=%7bAFEA068D-D012-4520-8E42-10E4D1AF7944%7d
According to the abstract, they showed that the discrepancy existed even when the response was evoked by the electrical simulation, this could exclude the complex factors (bidirectional interaction, interactions in spinal reflex) which should confound the interpretations in normal voluntary movement condition.
In my opinion, the time lag estimated from the coherence or cross-correlations is not necessary to match to the earliest onset latency, but could rather represent the median of the conduction time or the functional (processing) time.
I also look forward the answer.
Dear Tomohiko,
Thank you for your reply and the provided link. I also believe that the value of best fit we get from coherence analysis, doesn't necessarily represent the average conduction time in natural voluntary movement.
However, considering the alternative neural networks that transfer cortical activities to the spinal cord, maybe we can interpret it as follows:
1- As we know, EEG represent the activity of a population of neurons and while each neuron could choose to take a different path to the spinal cord, we could expect to see the average of taken cortico-spinal routes as a best fit for EEG-EMG time delay.
2- As for LFP-EMG time lag, since we record individual neurons activities, we could expect to see the longest latency based on the theory proposed by Morrow and Miller(2003) that motoneurons act like low-pass filter amplifiers with time constants of 50 -100 ms.
3- As for the measurement of conduction time using TMS, since it has been demonstrated that it takes the fastest routes to transfer cortical activity to the muscle and produce MEP, it is likely to have the fastest conduction time, compared to the other paradigms.
To sum up as EEG- EMG time lag represent the average delay among a number of fast and slow routes while TMS , takes the fastest route and individual neurons the slowest routes( Compared to TMS and EEG), it makes sense to have values about 20 ms for TMS, 25 ms for EEG-EMG time lag and around 50 ms for LFP-EMG time lag.
Plz let me know what you think of it and correct me if I am wrong.
Dear Sam
While the motivation of a paper of mine was related to postural control with cross-correlation being the main tool for analysis, the message of the paper I think applies to parts of your question. Closed loop measures such as cross-correlation or coherence are influenced by many variables and therefore, the results (lags, time delays) obtained from them may vary and be different from case to case. The interpretation of such results has to be done carefully, mainly if compared to open loop data or when some information on causality is desired. The link to the paper is http://link.springer.com/article/10.1007/s00422-005-0016-x#page-1
Regards
Andre
I would repeat the above comments, that the TMS latency is the fastest available conduction pathway. The advantage to my mind of coherence type measures is that it records what is actually used physiologically. This can be slower pathways than the fastest possible. The lag may also have contributions from the whole pathway, including the feedback portion.
Hello. The cortex–muscle latency (as measured during intermediate isometric contraction) corresponds very closely to TMS latency from the motor cx to the muscle as has been shown for proximal and distal upper and lower limb muscles. See eg. Salenius S, Portin K, Kajola M, Salmelin R and Hari R: Cortical control of human motoneuron firing during isometric contraction. J Neurophysiol 1997, 77: 3401–3405; Gross J, Tass P, Salenius S, Hari R, Freund H-J and Schnitzler A: Cortico-muscular synchronization during isometric muscle contraction as revealed by MEG. J Physiol 2000, 527: 623–631, and Salenius S and Hari R: Synchronous cortical oscillatory acivity during motor action. Curr Opin Neurobiol 2003, 13: 678–684 (a review). The latency determined during the about 20-Hz cortex–muscle coherence seems to reflect mainly the efferent commands from the cortex to the spinal level (to populations of motor units) whereas corticokinematic coherence (where you correlate movement frequency, measured with e.g. accelerometer, with cortical activity) seems to be dominated by proprioceptive afference (Piitulainen H, Bourguignon M, De Tiège X, Hari R and Jousmäki V: Corticokinematic coherence during active and passive finger movements. Neuroscience 2013, 238: 361–370).
best regards
riitta
The time lag between cortical activity and EMG activity has been calculated from the phase relationship between them at frequencies showing significant coherence. The results have been inconsistent. The reason for this is probably that most investigators have used rectified EMG in the calculation which introduces spurious frequencies and inconsistencies. (see McLelland et al J Neurosci Methods (2012), doi:10.1016/j.jneumeth.2011.11.001). Afferent input also has a profound effect on coherence (see V. M. McClelland, Z. Cvetkovic, and K. R. Mills, ”Modulation of Corticomuscular Co- herence by Peripheral Stimuli,” Experimental Brain Research, Vol. 219, No. 2, June 2012, pp. 275–292.).
Regards,
Kerry Mills
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