megrealign

[Robert Oostenveld]

Hi Tom,

On 19-okt-2005, at 0:47, Tom Holroyd wrote:
> Is it possible, working with CTF data, for megrealign to also
> interpolate the reference channels?

You mean the channels that are far away from the head (Rxx etc.)? In
theory, yes, but in practice no. There is little MEG information
around those channels and if you rotate the helmet around the
subjects head, the shift will be the largest in those channels.
Therefore, after rotation of the helmet those channels might even
move completely outside of the range that they cover by themselves.
Furthermore, the signal that is picked up by those sensors is poorly
modelled by the dipoles in the brain that MEGREALIGN uses. That is
the actual purpose of them, that they pick up mostly noise and little
brain signal.

If you want to incorporate those channels in realigning, the best
that I can imagine is to do the inverse estimation towards the brain
dipoles using MEG+REF on 3rd order balanced data, and then do the
forward calculation using (rotated+shifted) MEG alone. However,
support for the 3rd order balanced MEG signals is still not
implemented in FieldTrip. It is a recurring topic, which I am not too
enthousiastic about to put much effort in, since it relates to
incomplete specified file formats and it is more engineering than
science.

best
Robert