[FieldTrip] Baseline removal errors in trials preprocessing

Marion Vincent marion.vincent at univ-lille.fr
Thu Jan 17 10:37:20 CET 2019

Dear FIeldtrip users, 

I’m new in using Fieldtrip for EEG processing and I’ve encountred some issues while removing the Baseline of my trials. 

I’m working on EEG signals recorded with the BioSemi system.  My pre-processing method is : 
(1) Re-referecing the channels (necessary with BioSemi)
(2) Trial definition
(3) Baseline removal

I’ve read on the documentation that ft_preprocessing xcan have several cfg parameter for the Baseline removal :

% cfg.demean  = 'yes';%'no' or 'yes', whether to apply baseline correction (default = 'no')
% cfg.baselinewindow = 'all';%[begin end] ;%in seconds, the default is the complete trial (default = 'all')
% cfg.detrend       = 'no'; %'no' or 'yes', remove linear trend from the data (done per trial) (default = 'no')

I wanted to remove a Baseline defined as the average of a given window preceeding my stimulus onset. 

I tried to use : cfg.baselinewindow = [-0.5 0 ]; cfg.baselinewindow = [0 0.1] or the default window .
But the problem is that I always have the same result, no matter the window I use : the first point of my trial is put to zero and substracted to all my trial. 

I’m sure I’m doing something wrong (and that the answer is very simple), but I can’t manage to understand what is the issue.

Do you have any advice ?

Thanks ! 

Eng., PhD , CNRS Research Engineer

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