[FieldTrip] units of lcmv source-localized activity

Lin Wang wanglinsisi at gmail.com
Mon Jun 14 18:09:21 CEST 2021

Hi Jan-Mathijs,

Thank you very much for your reply. Sorry that I missed your response
earlier, but I only saw it today.

You are right that I was looking at the nom field of the source
output without any normalization. The gradiometer positions (and headmodel
and sourcemodel positions) are all expressed in cm, and the sensor data are
expressed in T. Does this give me a unit of A-cm?

The resulting source activation values are on a scale of 10e-5, so the
conversion from A-cm to nA-m (1 Ac-m = 10e11 nA-m) will result in a scale
of 10e6 nA-m. I read from here
<https://neuroimage.usc.edu/forums/t/eeg-units/1499/4> that "evoked
activity generates nominally a 10 nA-m response, and interictal spikes are
nominally 100 nA-m". The values I got seem to be too large. Did I
do something wrong here?

Thank you again for your help!

On Tue, 18 May 2021 at 08:09, Schoffelen, J.M. (Jan Mathijs) <
jan.schoffelen at donders.ru.nl> wrote:

> Hi Lin,
> If the gradiometer positions (and headmodel and sourcemodel positions) are
> expressed in meters, and the sensor data in T, then I would assume that the
> virtual channel units are Am, under the further assumption that you have
> been using a ’normal’ lcmv algorithm without any normalization of the
> weights, and that you are looking at the mom field of the source output.
> Best wishes,
> Jan-Mathijs
> > On 10 May 2021, at 21:11, Lin Wang <wanglinsisi at gmail.com> wrote:
> >
> > Dear FieldTrip experts,
> >
> > I used lcmv to project single-trial MEG signals (obtained with CTF
> system) from sensor space to source space, and I got values at a scale of
> 10e-5. I'm wondering what's the unit for such activity. It doesn't seem to
> match nAm.
> >
> > Thank you for your help!
> >
> > Lin
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