How do you load Neuromag Averaged files into Fieldtrip?

[Robert Oostenveld]

Dear Mark

On 16 Nov 2006, at 1:54, Mark Andermann wrote:
> I'd like to use Fieldtrip to fit dipoles to data acquired on a 306
> channel neuromag MEG system. While Fieldtrip offers instructions
> for loading raw data, I have not found a simple script to load in
> Neuromag data that's already been averaged in MATLAB, and converted
> back to an averaged .fif file.

Fieldtrip usually reads the data from disk itself, since the data is
accopanied with auxiliary information (e.g. events and channel
locations). So there is no default option to read in data from a
matlab file, or from a matlab variable.

The fif reading functions are nott included by default in fieldtrip
(since they are closed source), but if you have the meg_pd toolbox on
your matlab path then fieldtrip will detect that and use those
functions (seee http://www2.ru.nl/fcdonders/fieldtrip/doku.php?
id=fieldtrip:documentation:dataformats). The only issue with Neuromag
data that I am aware about is that the stimulus channel is noisy, and
hence the event detection might not be flawless.

The alternative is to take the variables that you already have in
your matlab session and convert them (manually) into a data structure
taht is compatible with fieldtrip. You can find instructions on how
to do that in the frequently asked questions list (see http://
www2.ru.nl/fcdonders/fieldtrip/doku.php?
id=fieldtrip:documentation:frequently_asked_questions#how_can_i_import_m
y_own_dataformat).

> Also, can MEG dipole fitting (and time-series estimation) in
> Fieldtrip be done with a spherical head model in the absence of any
> subject MRI data?

Yes, but then you have to define the spherical head model yourself.
For the DIPOLEFITTING function you would specify

cfg.vol.r = radius, single number
cfg.vol.o = origin, [x y z]

Both these values should be expressed in the same units (e.g. meters,
centimeters or milimeters) as the channel locations. At the moment I
do not recall what the default units are for Neuromag, but you can
have a look at the "grad.pnt" field (i.e. the position of all the
gradiometer coils) and you will probably easily recognize the units.

best regards,
Robert