[Eeglablist] Filter distortion around boundary events

[Roy Cox]

hi all,

Just wanted to chime in that I've noticed this (or something similar), too.

I typically epoch continuous sleep data into 30 s "trials" to exclude bad
epochs, before stitching everything back together
using eeg_epoch2continuous.

When I then apply a highpass filter [pop_eegfiltnew(EEG,0.3,0)], I often
see subtle but clear jumps right at the edge of where my epoch boundaries
used to be. Critically, these jumps were there even if no data had been
removed in between. In contrast, just filtering the original continuous
data works fine.

I checked to see if the continuous->epoched and epoched->continuous
transformations somehow resulted in skipping a sample, or some other change
of amplitude, but amplitude values were identical across boundaries in all
cases. It's as if the pop_eegfiltnew function somehow notices when
continuous data previously existed in an epoched form, and alters its
behavior based on that...

My ugly workaround has been to do all my filtering on continuous data (even
if it includes lots that I know will be rejected anyway), but would be good
if this issue were remedied.

Roy



On Tue, Jul 17, 2018 at 6:54 AM Erickson <ericksonb.eng at gmail.com> wrote:

> 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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