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

Andreas Widmann widmann at uni-leipzig.de
Fri Apr 16 13:52:43 PDT 2021


Hi Malte,

> 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.
You must never filter over large artifacts (I assume un/plugging produces). This might potentially smear the artifacts over wide time ranges. I would at least manually place a boundary event before and after the un/plug artifacts so that filtering will not smear them into adjacent data sections. Personally, I would manually reject/cut the segment from before plugging out until after plugging back in. If you do so with EEGLAB tools a boundary event should automatically be placed at the cutting point.

Hope this helps! Best,
Andreas

> Am 16.04.2021 um 19:04 schrieb Malte Anders <malteanders at gmail.com>:
> 
> 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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