Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Fri Oct 21 14:13:38 CEST 2005

On 19-okt-2005, at 13:35, Tom Holroyd wrote:

> Hmm?  The reference channel positions and orientations are
> specified in the res4 file, just like all the other channels.

Yes, but not the linear weighting of all channels that is used to
construct the offline synthetic gradient. That is also stored in the
res4, but I do not know how to get it out of there.

The synthetic 3rd order gradient computation is
   MEG_3rd = T03 * MEG_0th
where MEG_3rd is a Nchan by Ntimepoints matrix, just like MEG_0th,
and T03 is the linear transformation matrix from 0th (unbalanced 1st
order gradients) to 3rd order gradients. Similarly, the res4 should
include teh information for
   MEG_1rd = T01 * MEG_0th
   MEG_2rd = T02 * MEG_0th
   MEG_3rd = T03 * MEG_0th
Using those, you can also go back between different synthetic
gradients of course (using the inverse of the Txy matrices). The
simplest example is
   MEG_0th = T00 * MEG_0th
which means that T00 is eye(Nchans). The Txy matrices should be used
to premultiply the grad.tra matrix which is used in the forward model
(i.e. the transformation of the field at all coils to the channel

> By the way, I have a .oct file for meg_leadfield1.m so it works
> fast in Octave (like 800 times faster), in case anybody is interested.

Do you want me to put it in the release version? If so, send it to me
and I'll include it.


More information about the fieldtrip mailing list