[FieldTrip] When to detrend/demean

Vitória Magalhães Piai vitoria.piai at gmail.com
Sat Jan 5 11:21:34 CET 2013

Dear ftrippers,

I'm having a discussion with a colleague on something that is still a 
bit unclear to us. Since I trust the knowledge going around here a lot, 
I thought it would be my best chance to get a good answer: When should 
we demean/detrend?

As relevant background, our EEG datasets involve speech production on 
every trial.
We read in the data, use ft_databrowser to mark the artefacts and then 
do complete artefact rejection with ft_rejectartifact. The trials often 
include speech (onset).
I see in the tutorial that the cfg for preprocessing is pretty simple, 
and ft_preprocessing default has no detrend/demean.
But in the FT example 'Reading and pre-processing EEG data', the cfg is

cfg.demean           = 'yes';
cfg.baselinewindow  = [-0.2 0];

In my data, I used cfg.demean = 'yes'; with no cfg for the baseline 
window because I don't want to correct the signal with a specific 
interval (and I assume this will take the whole segment then).
Our concern is that, given that people speak during part of the trial 
(always towards the end), using demean here is not a good idea (the 
signal changes induced by moving the jaws, etc., are included in the 
calculation). Is this necessarily the case or can it be fixed with 
subsequent computations (see below)? Do I need to go through artefact 
rejection again? My guess would be that the damage caused by having 
demean here doesn't change that much where the eyeblinks are and I 
always take quite broad windows to mark the artefacts, so at least for 
the AR I should be safe, but I'd like to check that with you guys.

Then, when calculating ERPs, I had both demean and detrend before 
But for the TFRs, I didn't do any of these (dunno why). I'm using the 
ft_freqanalysis after the 2011 change (removing the first order linear 
trend from the time domain data).
Do I need to redo my TFRs or is it enough if I do sanity checks and 
everything is in place (like visual alpha and gamma, etc.)?

And my last question, for once and for all, so that I get it right next 
time from the start (assuming that I'll always have EEG speech 
production data with ERPs and TFRs analysed). Is this the best way to do it?
- preprocess with default (so NO detrend and NO demean)
- then demean and detrend for ft_timelockanalysis and ft_freqanalysis

Thanx a lot, and (keeping to the Dutch tradition) all the best for 2013!

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