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

[Taylor Williams]

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Eric,

Please find the forward model and script for the 1632 magnetometer array in
the following folder. Script is also attached.
https://drive.google.com/drive/folders/1dX7YZSCa4RAcn_b4JH3I0CADe2ZOQIM-?usp=sharing

A bit more context on the forward model: It places point magnetometers on
102 simulated rectangular 24mm x 24mm chips that are projected towards the
scalp from the standard Neuromag sensor locations. The center point of each
sensor cluster is 1.5mm above the nearest outer skin surface from the BEM.
All sensors are sensitive in the z-direction and preserve the orientation
vector from the derived Neuromag magnetometer.

This script generates the full set of lead fields in the Evoked object Y0.
There are plenty of other instances where this occurs, not just dipole
#364. Let me know if you have any questions about the construction of the
forward model that I can answer for context. I'm curious to see what you
think.

Thanks,
Taylor

On Thu, Feb 6, 2020 at 4:12 PM Eric Larson <larson.eric.d at gmail.com> wrote:

>         External Email - Use Caution
>
> 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:
>
>>         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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