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

Eric Larson larson.eric.d at gmail.com
Thu Feb 6 19:10:59 EST 2020
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I'm not sure offhand what would cause this. Can you share a script to
produce the bottom plot in the source 364, 1632-point-magnetometer case?
That would help us look into what specifically what (at least: where in the
code) is causing the value to become so large.

Eric


On Thu, Feb 6, 2020 at 3:51 PM Taylor Williams <williams.taylor at gmail.com>
wrote:

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>
> 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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