I wanted to compare OS and BEM forward models, but I got much larger differences than expected (median around 20%, but up to 1000%, and this is for the field vector amplitude, not power). Then I compared BEM with 1, 2 and 3 compartments and again the differences are much larger than I think they should be (10-100%). Here are some figures:
Those upper tails go above 100% but these sources [edit: after correcting figure mistake] are where you'd expect more error (near the surface boundaries and radial sources). This is comparing BEM with 1 (brain) or 2 (+skull) compartments:
BEM with 2 vs 3 compartments, or OS vs BEM look roughly similar. I got the same when selecting "adaptive integration" or not.
I hope it's something I did wrong, but maybe this is "normal" and just much worse than I expected. Can someone else reproduce this?
No, the integration points was something I did before this. It shouldn't be relevant since all this was with the default 4 points per coil. Again, unless I unknowingly did something wrong, I just compared BEM against OS and against itself with 1, 2 or 3 layers.
I would really not expect the small currents in the skull to change the observed field by 10-20% (but maybe I'm wrong?).
I've corrected my first post. In the cortex figure, I had "absolute values" turned on (and it's a plot of log(error)), so actually, the large errors are where we'd expect: superficial and radial sources. Sorry for that mistake.
I also tested a concentric sphere conductor. Here are the results, which look ok (though the sources are relatively far from the inner-skull surface in this spherical construction):
One thing I still don't understand is why 2 layers is much worse in the spherical case, while 1 and 3 layers are almost identical.
But did I just have wrong expectations? Is the error from ignoring the outer layers in a realistic head model really on the order of 10% for MEG?
We must be thinking different things:
1 layer: brain (air), conductivity goes from 1 (arbitrary units) to 0.
2 layers: brain, skull (air), conductivity goes from 1 to 0.0125 to 0 (so smoother transition, no?).
With 3 layers, the currents in the scalp are probably on the same order as those in the skull? But again the conductivity jumps back to 1 in the scalp, so I'm not sure what you mean by a smoother transition in conductivity.