[Mne_analysis] Lead field spikes using sample BEM - bug or explainable?

[Taylor Williams]

        External Email - Use Caution        

I've found something unusual in the lead fields computed using MNE-Python
that I can't explain. When arrays containing a large number of magnetometer
sensors are used with the 'sample' subject BEM
(sample-5120-5120-5120-bem.fif) and the ico4 source space to create a
forward model, there are some unusual strong spikes in the lead field.
These spikes are not present when I use a spherical BEM model instead.

To demonstrate, I created three different theoretical arrays of point
magnetometers (coil_id: 2000) measuring roughly the normal component of the
magnetic field with respect to the scalp surface. Each model contains an
increasing number of total sensors (102 sensors, 1632 sensors, and 3672
sensors).

Figure 1 shows the lead field for a particular dipole (#364 using ico4
source space) for each array using a spherical BEM model. A smooth dipolar
response is shown for each that is roughly equivalent over the arrays. The
peak response for each array doesn't change appreciably, but there is an
improvement in detail of the lead field. All evoked topomap plots are fixed
to the same color bar limits (400 fT).

Figure 2 shows the same set of lead fields but with the 'sample' subject
BEM instead. It shows curiously strong spikes present in the two high
density arrays. These spikes vary in their location and magnitude. The main
dipolar response is still present, and appears to be fairly consistent.
However, due to the spikes, the peak response for each array is vastly
different. Again, evoked topomap plots use a fixed color bar limit at 400
fT.

Are these lead field patterns in Figure 2 explainable? They certainly seem
like erroneous values, but perhaps something about the BEM model creates a
focusing effect for the magnetic field response when we sample at different
spatial frequencies? I'm happy to provide some sample forward models for
inspection.

Thanks,
Taylor Williams
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