[Eeglablist] Filter distortion around boundary events

[Erickson]

Thanks for your responses.

Andreas, I used the "basic FIR filter" on automatic order calculation
filtering from 1 to 55.

Here is a drive link to a zip file with 30s of example data. These data
were imported using fileIO and the event channel (65) was imported as
events (Nothing else has been done to this example data).

> https://drive.google.com/open?id=1KPlpeBCrCnQJHCJOcMYsnNSJVOjj0bTO


Then I plotted the data and cut out one section around a TMS event, and
another section NOT around a TMS event;

> EEG = eeg_eegrej( EEG, [4615 5372;11482 12002]);
> [ALLEEG EEG CURRENTSET] = pop_newset(ALLEEG, EEG, 1,'setname','with
> Rejections','gui','off');


filtered the data using the FIR filter from 1 to 55hz;

> EEG = pop_eegfiltnew(EEG, 1,55,6600,0,[],1);
> [ALLEEG EEG CURRENTSET] = pop_newset(ALLEEG, EEG, 2,'setname','with
> Rejections and Filter','gui','off');
>

then plotting the data, the artifact is visible as a "pinching" around each
boundary, with a faster oscillation riding on top. So basically just reject
any section of data and filter and the artifact appears.

​Makoto, to answer the question about the spectra - if I cut out the TMS
pulses and then filter, the PSD looks pretty normal. Prior to filtering
there is so much DC power I can't visually inspect the data very well.

The room is very noisy, but this artifact appears only wherever I make a
boundary and filter so it seems like an edge effect due to filtering - but
I've never encountered this kind of artifact before since EEGLAB knows not
to filter over boundaries. Perhaps I cannot use the automatic filter order
here due to some noise profile in my data?

Thanks to you both for considering the problem!

Brian


On Thu, Jul 12, 2018 at 8:54 PM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
wrote:

> Dear Erickson
>
> Is this really a distortion introduced by the filter? Rather, isn't it the
> case that that portion of the data was poorly filtered i.e., your original
> data, before filtering, had that noise constantly? Could you please check
> your original data by eyeballing the raw time series and by checking power
> spectral density?
>
> Makoto
>
> On Thu, Jul 12, 2018 at 10:33 AM Erickson <ericksonb.eng at gmail.com> wrote:
>
>> Hello EEGLAB list,
>>
>> We are experiencing a strange artifact on only some of our EEG data. For
>> some subjects, when we cut out portions of data (creating boundary events)
>> and then filter (from 1 to 40 or 50hz) we observe a distortion around the
>> boundary.
>>
>> Specifically, we see a rising or falling wave on either side of the
>> boundary, with a fast oscillation riding on top of it. See here:
>> https://drive.google.com/open?id=1yt6h_WLgI6jWAApVd52nwVLuiduYgXIH
>>
>> Although this data is from concurrent TMS-EEG recording, the distortion
>> has nothing to do with the TMS pulse itself - we get the same
>> boundary-related distortion when we filter after cutting out a random
>> section of continuous data (a section with no TMS in it).
>>
>> I am at a loss - we've tried a few different filters and nothing seems to
>> solve this issue. Meanwhile, on some subjects there is no boundary-related
>> filter distortion at all! There is a lot of noise in the room but I'm
>> not sure how that could create this specific issue.
>>
>> Any perspectives on this issue are appreciated! Thank you,
>>
>> Brian
>>
>> *Brian Erickson**, Ph.D.*
>> *Postdoctoral Researcher, CogNeW Lab*
>>
>> *Drexel University*
>> 3141 Chestnut Street
>> <https://maps.google.com/?q=3141+Chestnut+Street&entry=gmail&source=g>
>> Stratton Hall Room 320
>> Philadelphia, PA 19104
>>
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>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
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