[Eeglablist] How do pauses (boundary events) influence filtering of EEG data?

Andreas Widmann widmann at uni-leipzig.de
Fri Apr 16 14:04:26 PDT 2021


> First of all, thank you @Andreas for your reply - this helps a lot. I could not find any information about this in the documentation, maybe it would also help others in the future if this information would be added.
Where would you expect to find this kind of information? At least in the EEGLAB wiki it is documented (https://urldefense.com/v3/__https://eeglab.org/tutorials/05_Preprocess/Filtering.html__;!!Mih3wA!UsYlfO9n3dtymN4Gb5lVAzn8QGlpLlbvGMOfgX7A2uLCCbOnUqIKFSmClVJPJvhyQhz_3A$ ): "Note that removing data portions containing major artifacts (by visual inspection), such as large spikes in the data, before filtering can be preferable since filtering can “spread” the artifact out over “good” data, requiring more data to be rejected after filtering. When you remove major artifacts, and “boundary” event replaces the removed data. Filtering is only applied to continuous data segments, not across boundaries."

Best,
Andreas

> With respect to your point @Malte, I assume that it would be beneficial to insert boundary events and then apply the pop_eegfiltnew or pop_firws functions, for example. 
> 
> Best wishes, 
> Pia
> 
> 
> -----Original Message-----
> From: Malte Anders <malteanders at gmail.com>
> To: Andreas Widmann <widmann at uni-leipzig.de>
> Cc: piabinkmann at aol.com; EEGLAB List <eeglablist at sccn.ucsd.edu>; p.brinkmann at maastrichtuniversity.nl
> Sent: Fri, Apr 16, 2021 7:04 pm
> Subject: Re: [Eeglablist] How do pauses (boundary events) influence filtering of EEG data?
> 
> Hi Andreas and Pia,
> I just saw your discussion and out of curiosity I wanted to ask: During long EEG recordings, I usually allow my participants to go to the toilet. During that time, the recording is not stopped but I plug out the headbox from the EEG machine and the subject leaves the room with the headbox in his or her hand. Plugging the EEG headbox back in and the recording goes from flat back to normal.
> This does not introduce a boundary event. Do I need to consider those events during filtering? When I think about this, I cannot come up with an answer.
> 
> Best wishes and thank you all for your time!Malte
> Andreas Widmann via eeglablist <eeglablist at sccn.ucsd.edu> schrieb am Fr., 16. Apr. 2021, 18:55:
> 
> Dear Pia,
> 
> Filtering must be done on the continuous data before epoching. The FIR filter functions („Basic FIR filter (new, default)“/pop_eegfiltnew, „Windowed sinc FIR filter“/pop_firws, pop_firpm, and pop_firpm) will automatically take „boundary" events into account, that is, filter both segments of your continuous data separately.
> 
> Caveat: Not all data import functions/plugins do actually import and/or correctly rename (all) DC offset events to „boundary“ (e.g., bdfimport). So, check whether you actually have an event of type „boundary“ where expected.
> 
> Hope this helps! Best,
> Andreas 
> 
>> Am 16.04.2021 um 15:52 schrieb piabinkmann--- via eeglablist <eeglablist at sccn.ucsd.edu>:
>> 
>> Dear all,  I have a question regarding filtering EEG data where therewas a pause during the recording. To be more precise, we are pausing therecording in the middle of our sequence to give the participant some rest andthen continue with the recording. In the end we have one data file with anadditional boundary event in the middle (thus, no need to merge datasets). Theepoch containing the boundary event will be excluded for the ERP analysis.  Now, I read in Widmann et al. 2015 that ‘filters must not beapplied across signal discontinuities’. I assume that the reasoning behind thisis that the DC offset will change. How can I solve this? Is itsufficient to exclude the epochs with boundary events, or shall I split thedata and process it separately and combine it again only after ICAdecomposition and artifact rejection? Thank you already for your input! Best wishes, Pia Pia BrinkmannPhD CandidateMaastricht University | Faculty of Psychology and Neuroscience |Department of Neuropsychology & Psychopharmacology
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