[FieldTrip] frequency analysis

Johanna Zumer johanna.zumer at gmail.com
Thu Dec 19 17:18:50 CET 2013

Dear Raghavan,

For your 3rd question, I would add to Jorn's answer that you can, but not
are required to, use combineplanar of your planar gradiometer pairs if you
want one answer (of ERF or TFR) per channel location rather than 2, prior
to grandaveraging.   For statistics, be sure to use the correct neighbours
template file:  neuromag306planar_neighb.mat if you have not combined,
and neuromag306cmb_neighb.mat if you have combined.


2013/12/12 "Jörn M. Horschig" <jm.horschig at donders.ru.nl>

> Dear Raghavan,
> 1)  Sure is it possible, z-scoring is nothing else then subtracting the
> mean and dividing by the standard deviation. The time-period from which you
> obtain the mean and standard deviation depend on what you want and how you
> define it.
> 2) Usually we do not use megrealign at all and assume that subjects are
> approximately similarly seated in the helmet for computing the
> grandaverage. MEG sensor-level analysis is not suited very well for making
> spatial inferences anyway, I would advise to go to source space for such a
> thing.
> 3) For neuromag data, you already have planar gradiometers, so you do not
> need megplanar and combineplanar. We, on the other hand, only have axial
> gradiometers with our CTF system, thus we need a planar transformation to
> get a planar representation.
> 4) I cannot help with this one, sorry.
> Best,
> Jörn
> Raghavan Gopalakrishnan wrote:
>> I have a few questions regarding frequency analysis using wavelets.
>> 1. Is it possible to z-score the TF spectrograms? Is so how and where
>> should I specify the baseline?
>> 2. Before grand averaging ERFs, the pipeline has 'megrealign' to make
>> sure all sensors from multi subject are matched. However, why is this not
>> performed before grand averaging time-frequency data since the powspctrm is
>> specific to each channel?
>> 3. Is combineplanar a necessary step before any grand averaging i.e. ERF
>> or TFR? I am using neuromag data, so can I perform grand averaging and
>> statistics without combineplanar?
>> 4. Fieldtrip only reads neuromag FIF files that has been processed
>> through maxfilter. There are times when there is a need not to use
>> maxfilter, is there a work around?
>> Thanks very much.
>> Raghavan
>> _______________________________________________
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>> fieldtrip at donders.ru.nl
>> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
> --
> Jörn M. Horschig
> PhD Student
> Donders Institute for Brain, Cognition and Behaviour
> Centre for Cognitive Neuroimaging
> Radboud University Nijmegen
> Neuronal Oscillations Group
> FieldTrip Development Team
> P.O. Box 9101
> NL-6500 HB Nijmegen
> The Netherlands
> Contact:
> E-Mail: jm.horschig at donders.ru.nl
> Tel:    +31-(0)24-36-68493
> Web: http://www.ru.nl/donders
> Visiting address:
> Trigon, room 2.30
> Kapittelweg 29
> NL-6525 EN Nijmegen
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