attention: default changed for MEG leadfields

Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Wed Oct 4 10:37:32 CEST 2006

Dear fieldtrippers,

I want to bring to your attention that I just changed the default
value for the option cfg.reducerank in the SOURCANALYSIS and the
PREPARE_LEADFIELD functions. The old default was 'no', the new
default depends on the type of data that you are working with.

In case of EEG, the new default is 3. This basically is the same as
'no', since the intrinsic rank of the EEG leadfield for a dipole is
3. So effectively nothing changed here.

In case of MEG, the new default is 2. This was the value that I
always recommend when someone would ask me for it. To prevent people
from accidentally forgetting to set it to 2, we now decided to change
the default.

The change is immediate for people within the Donders, and for the
external users it will become visible in the daily fieldtrip version
on the FTP server this evening.

Background: The cfg.reducerank option relates to how the leadfields
are computed, and is only relevant for MEG in combination with
realistic (localspheres, singleshell) volume conduction models for
the head. In Fieldtrip, we always work with dipoles that can be
oriented in x, y, and z direction. The dipole orientation and
strenghth together form the dipole mooment. There is one particular
[x,y,z] direction for the dipole for which it points to the surface
of the head, i.e. that is the radial component. The magnetic field
for that radial component is zero (in a single sphere model), or
almost zero (in a realistic model). Since sources that are almost
zero pick up a lot of noise in the beamformer method, it is desirable
to explicitely set the leadfield in that direction to exactly zero.
That is done by the reducerank option, which ensures that the
leadfield will only contain the two tangential components and which
makes the radial component zero.

best regards,

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