[FieldTrip] MEG lead field units

Marc Lalancette marc.lalancette at sickkids.ca
Wed Mar 26 21:33:43 CET 2014

Hello all,
I am not (yet) a common user of Fieldtrip, but I am using it now to investigate some basic properties of our MEG system.  I only needed a realistic forward solution so I chose Nolte's "singleshell" model with mostly default parameters.  I just spent quite a bit of time going through the code trying to figure out the units of the lead field I obtained.  As it was not too obvious, I would appreciate some confirmation/correction.

By default, my volume conduction and sensor structures were in units of 'cm'.  That seems to be also what Nolte's method requires.  Nowhere by default are the units specified for the (meg) channels or the source dipoles when simply calling ft_prepare_leadfield, and looking at the spherical computation in Nolte's code, I concluded that it would work with the channels in T and the dipole in A*cm.  Thus the lead field units would be T/(A*cm).  This also seems consistent with the lead field amplitude I am getting for our system (in the order of 10^-9).  Is this correct?

Thanks for your help!
Marc Lalancette
Lab Research Project Manager
The Hospital for Sick Children, Toronto, Canada


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