Virtual Channel

[J.M. Schoffelen]

Dear Bin,

Here are some answers to your questions:

1. How to read virtual channels data with FieldTrip codes?

Provided you have your virtual-channel data in a file-format that fieldtrip
can handle, it would not be too difficult to read them in. As far as I know,
for example in CTF-datasets, virtual channels are saved along with the raw
data, and they are given a specific label, which you can use when you read
in the data in preprocessing:

cfg.channel = {'name of virtual channel'};

On the other hand, you can create your own virtual channels within fieldtrip
by using the 'lcmv'-option in the sourceanalysis-function.


2. Why there are different extreme value localizations
with axial gradiometer and planar gradiometer
respectively for a given magetic dipole source?   As I
understand, the localizations of extreme values are
also fixed in space if the source is fixed no matter
what sensor one uses to pick up the output signals
(just like EEG).   But why the extreme are at each
side of the fixed dipole for axial gradiometer, but is
just above the dipole source for planar gradiometer.

There is actually a very big difference between what a planar gradiometer
picks up as compared to an axial gradiometer! Imagine a horizontal wire
through which is passed an electrical current. This generates a cilindrical
magnetic field around the wire, according to the right hand rule. A
gradiometer-array above the wire picks up this magnetic field.
An axial gradiometer array is most sensitive to a magnetic field
perpendicular to the surface, so it will 'give the highest signal' on both
sides of the wire (=dipole), which are of opposite sign. On the other hand,
a planar gradiometer is most sensitive to a magnetic field parallel to the
surface, so it will 'give the highest signal' right above the wire.

More info might be found on:
http://butler.cc.tut.fi/~malmivuo/bem/bembook/
and
http://butler.cc.tut.fi/~malmivuo/bem/bembook/14/14.htm#02/

Yours,

Jan-Mathijs