I have a dataset with MEPs elicited in a forearm muscle (the FDS). In around 50% of participants the TMS artifact is very large and still present for some of the 10-40ms window in which I look for MEPs. In these cases there are visible MEPs, but I'm not sure to what extent they are distorted by the artifact. My question is: are these data salvageable? Is there a way (using ICA perhaps) to filter out the artifact? Or is it a problem as the latency and magnitude of the artifact varies between participants?
In case it is important: I was eliciting MEPs in the hand simultaneously and those data are fine (i.e. the TMS artifact is of the 'usual' small amplitude and short latency).
I think you have to improve the recording quality by acting on the ground, or on filters or on the stimulus in order to remove the artifact. For off-line analysis of your data the easiest thing is to try to filter out the low frequences, otherwise you have to proceed as indicated by the previous speaker.
It is very surprising to hear that single pulse TMS on forearm muscle surface EMG is producing a huge artifact. First you need to assure whether it really is a TMS-induced artifact (I doubt that the TMS pulse disturb such a remote recording source on the forearm?). Second option is that you got a mechanical (stimulator) artifact, as the forespeakers point out it is important to check cable-crossing, grounding, maybe change the socket. Third point might be that the surface EMG is not only recording FDS but more muscle at the same time that is then generating a artifact-like signal, might be likely because your recording on hand muscles are fine. For filtering I refer to the forespeakers, another option I propose is a linear interpolation filter. Good luck!
I should not emphasize enough to check the grounding if you have a bigger-than-normal artifact every time. We had this problem once in a while with no apparent reason and one day I was able to figure it out: the wet (ECG type) electrodes we were using for ground on clavicle; we opened the pouch of 4 electrodes and leave the pouch opened for a couple days for the next experiment, so they were getting dried.
Thanks again for these tips.
I have since been running studies measuring from hand muscles only, with the ground on the wrist. With this arrangement I am still sometimes getting large artifacts in the hand (similar size/shape as the artifacts I was getting in the forearm). I am using Biopac. Supposedly the ground can be placed anywhere with this system, but I have been advised that with stimulation the ground location might affect the artifact. I tried moving the ground to the elbow so that is roughly halfway between stimulating coil and recording electrodes; this seemed to help a little but the problem is still there.
I'm using self-adhesive electrodes - I guess they sometimes are open for a while between subjects but I always seal the pack with tape and I've never noticed any difference in the electrodes so I would be surprised if that was a cause... I will keep an eye on it though!
Katherine,
Try placing their self adhasive (ECG type?) ground electrode on clavicle or acromion process. As you may very well know, the ground should be placed on anywhere between the two sites (TMS and EMG recording) but we have had the best recordings with those two locations.
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