[FieldTrip] Questions on High pass and Dft filtering

["Jörn M. Horschig"]

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:
http://bugzilla.fcdonders.nl/show_bug.cgi?id=1571

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).

Best,
Jörn

-- 
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

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E-Mail: jm.horschig at donders.ru.nl
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