[FieldTrip] Questions on High pass and Dft filtering

"Jörn M. Horschig" jm.horschig at donders.ru.nl
Wed Jul 4 15:35:52 CEST 2012

HI Philipp,

> The issue with the continuous data is the following: I tried using
>       cfg                     = [];
>       cfg.channel      = {'EEG'};
>       cfg.datafile       = dataset;
>       cfg.headerfile   = dataset;
>       cfg.dataset       = dataset;
>       cfg.continuous = 'yes';
>       (cfg.padding         = 10;)
>       cfg.dftfilter           = 'yes';
>        filteredContinuousData  = ft_preprocessing(cfg);
>   but looking at averaged timelocked data for the subject, the data looks
>   identical to the result I get using no filter at all, i.e., contaminated by
>   strong line noise. Including padding makes no difference either, therefore
>   the brackets.

I tested this with my own data as well with as synthetic signal. The 
line noise is reduced by a factor of ~8 (I looked at the Fourier 
spectrum of the filtered and the unfiltered signal).The problem you 
encounter is either caused by nonstationarity of the 50Hz component 
(i.e. a drift in power, see FAQ), or maybe your acquisition system is 
misspecifying the sampling frequency a bit - Robert says this might 
happen, see:

In any case, it might help for you if you specify cfg.dftfreq = 
[49.6:0.2:50.4 100 150] or similar (depends on your padding parameter). 
That might help to filter a bit 'better' (but maybe a bandstop filter 
works better here).


Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group

P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

E-Mail: jm.horschig at donders.ru.nl
Tel:    +31-(0)24-36-68493
Web: http://www.ru.nl/donders

Visiting address:
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