[FieldTrip] combining magnetometers and planad gradiometers for analysis

Tony W. Wilson tony.w.wilson at gmail.com
Sat Jun 4 00:09:08 CEST 2011


Dear Michael, everyone,
I believe you are correct.  Most Neuromag sites have SSP vectors computed
for data acquisitions; some groups occasionally recompute these per subject.
 These acquisition SSP vectors are stored in the fif file, but not applied
w/o user intervention. In addition, I believe the tag relating to these
vectors is removed from the fif file if you apply (t)SSS (through
Maxfilter).  However, I am not totally certain because I have never tried to
apply these SSP vectors after applying SSS.  Regardless, during analysis I
treat the motion corrected t(SSS) output files just like raw data files. ...
It is also worth noting here that one can compute new sets of SSP vectors in
the MNE software (not directly related to the data acquisition vectors) and
save these with the fif file for later use.  I am not sure if these SSP
vectors can be used outside of the MNE software.

Tony

On Fri, Jun 3, 2011 at 4:05 PM, <michael.wibral at web.de> wrote:

> dear neuromag / fieldtrip community,
>
> just a small question so I can keep up with the dsicussion:
> I thought that (t)SSS would nor project in a classical sense, i.e. for
> activities from inside the head leadfields would not have to be corrected?
> at least this I have read on the neuromeg list. So in most respcts the data
> should be treated just like rawdata - with standard coil positions after the
> motion correction ?
>
> maybe someone can shed more light on this?
>
> michael
>
> -----Urspr√ľngliche Nachricht-----
> Von: "Alexandre Gramfort" <alexandre.gramfort at inria.fr>
> Gesendet: 03.06.11 15:30:25
> An: "Email discussion list for the FieldTrip project" <
> fieldtrip at donders.ru.nl>
> Betreff: Re: [FieldTrip] combining magnetometers and planad gradiometers
> for analysis
>
> >Hi everyone,
> >
> > > I don't know enough about the elekta-software/mne-software to know what
> is
> > > happening to the data when the ssp vectors are computed, but once we
> know
> > > enough details it should be straightforward to build it into fieldtrip.
> >
> > SSP vectors are stored in fif files and applied to data only if
> requested.
> >
> > In a typical MNE workflow, the SSP vectors are no directly applied to the
> raw
> > file (avoid data duplication on disk) but rather to the noise covariance
> matrix
> > and consequently in the whitening matrix. When computing an inverse
> solution,
> > the whitening matrix is used on the data and the lead field, hence taking
> > into account the SSP vectors.
> >
> > Hope this helps.
> >
> > --
> > Alexandre Gramfort, PhD
> > gramfort at nmr.mgh.harvard.edu
> > Dept. of Radiology MGH Martinos Center / Harvard Medical School
> > http://www-sop.inria.fr/members/Alexandre.Gramfort/
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> > fieldtrip at donders.ru.nl
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> >
>
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