[Eeglablist] Correcting skin potential artifact for ERP studies

[周云晖]

Hello Volodymyr,


Thank you for the reply. I think the artifact is spontaneous (or maybe somehow evoked by our task) skin potential response, as the shape is very similar to the canonical function in the article you mentioned. I may send some example data to you later. However I am not sure whether the GLM can remove this kind of artifact as it is not strictly event-related in our data. Also the skin potential response did sometimes overlap. I guess some kind of "blind identification" method should be used with very little prior information about the magnitude and the timing of the artifact?



Best,


Zhou



-----Original Messages-----
From:"Vladimir Bogdanov" <vlabogd at yahoo.com>
Sent Time:2019-12-04 23:47:33 (Wednesday)
To: "周云晖" <yhzhou17 at fudan.edu.cn>
Cc:
Subject: Re: [Eeglablist] Correcting skin potential artifact for ERP studies


Hello Zhou,


The variation in skin potential across non-glabrous skin are about 5 mV for skin potential level. The dynamics of this would be few mV per hour or few uV per second, this should not be a problem. Moreover, it should be pretty slow and be well filtered-out by 0.1 Hz high pass. 


Other culprit can be polarization of the electrodes the Ag/AgCl coating is compromised, than it can give larger drifts but it will have nothing to do with sweating.


Other possibility is that you somehow record good spontaneous (or task related) skin potential responses across the scalp. If it has a certain topography, and a standard wave-shape, you can try ICA for isolation of those responses. At the first hand it might be interesting to see their topography and if it is the same across subjects.


Other option to identify skin potential response one can also try GLM,  though it can be more complex with multiphasic skin potential https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877881/


If you send me time series one of your typical recordings I would try to characterize this "electrodermal activity" to see if this is indeed electrodermal activity or something else. Electrodermal activity has a set of very specific waveshapes which is hard to confuse with anything else.


Sincerely,
Volodymyr






On Wednesday, December 4, 2019, 02:47:43 PM GMT+1, 周云晖 <yhzhou17 at fudan.edu.cn> wrote:




Hello Volodymyr,


Do you mean re-reference to average reference? I have already done it. The artifact is caused by changing skin potential (such as sweating), and it is not uniform across scalp electrodes.


I am also checking the book by Steven J. Luck (2014), but up to now I can't find a good way to correct these artifacts.



Best,


Zhou



-----Original Messages-----
From:"Vladimir Bogdanov" <vlabogd at yahoo.com>
Sent Time:2019-12-04 18:45:06 (Wednesday)
To: "周云晖" <yhzhou17 at fudan.edu.cn>
Cc:
Subject: Re: [Eeglablist] Correcting skin potential artifact for ERP studies


Dear Zou,


Did you mean-reference?


This must remove most of the drift, since it might be pretty uniform across scalp electrodes.


if the drift is just in few electrodes, I would exclude them.


Sincerely yours,
Volodymyr


On Wednesday, December 4, 2019, 09:00:59 AM GMT+1, 周云晖 <yhzhou17 at fudan.edu.cn> wrote:




Dear colleagues,



I am trying to analyze visual event related potentials (ERP) from some EEG data, but a few subjects showed quite long period of skin potential artifacts throughout the experiment. We previously removed

most of these artifacts by high-pass filtering at 1 Hz, but now for ERP analysis I set the cut-off frequency at 0.1 Hz. After epoching (900 ms long) I see lots of epochs with baseline drift.



I found that many epochs with such drift can be corrected by linear detrending, and some may need further correction by parabolic detrending, but I am not sure how this will affect the ERP shape. Or

I can reject all the epochs with such drift, but that means significant data loss for these subjects.



The data was recorded by actiCHAMP (Brain Products) with 64 active electrodes, and we lowered the impedance of all electrodes to below 10 kOhm. The experiment was performed recently in winter and the room

was cool and dry. I hope some experts in ERP can give me some suggestions on dealing with skin potentials.



Thanks,



Zhou











_______________________________________________

Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html

To unsubscribe, send an empty email to eeglablist-unsubscribe at sccn.ucsd.edu

For digest mode, send an email with the subject "set digest mime" to eeglablist-request at sccn.ucsd.edu