Lead Field Matrix, the unit of GridLoc

1

Hi,
Now I have computed the LeadField matrix with brainstorm, I want to use it to do forward modeling to compute the MEG signal, but I have a question, does every row of the LeadField matrix calculated by brainstrom match every row of the MEG data channel in the order? And the location coordinates of the volume that brainstrom traced back to, what are their units? Is it meter or Amper meter?
Very Thankful!

2

Yes, all the channels are labeled as MEG sensors.
You also have to ensure that MEG has two different sensors, the Magnetometers and the gradiometers. The computed/measured values are not the same.

Brainstorm uses the SCS coordinate system, and distance units are meter. When computing the LF the dipole is set to 1A.m

3

Very thankful! Yes, I ' m sure that MEG has two different sensors: the Magnetometers and the gradiometers. Then is every row of LF in the same unit? Because as we know, the MEG signal has different unit in Magnetometers(unit:T) and the gradiometers(unit:T/m), if I want to compute the MEG with LF, do I need to transform the unit?

4

Every row in the lead field matrix (aka gain), located in the Gain field in the HeadModel structure, has the proper units for the channels: [T] for magnetometers, [T/m] for gradiometers and [V] for electrodes, as it was mentioned: Compute MEG signal with Lead Field matrix - #2 by Sylvain

Yes, those wo sensors have different units. Signals in the gradiometers are converted from [T/m] top [T] ONLY for visualization (for example when plotting time series). Check these links:

No, there is no need of scaling. The lead field matrix has already the proper units.
You can prove this by yourself:

  1. Import in Brainstorm MEG recordings with magnetometers and gradiometers

  2. Export those recordings to Matlab and plot them with plot(). You will notice that multiplying the gradiometers signals by 0.04 will give you the same plot as in Brainstorm.

  3. Compute a head model and compute sources for that recording.

  4. In Matlab, simulated MEG (mag and grad) from the sources by multiplying the lead filed by the sources.

  5. Plot the simulated signals as in Step 2. Note that the signals are in their proper units.

:bulb: The function process_simulate_recordings.m contains the code to simulate sensor recordings from sources, you can use it as reference: brainstorm3/toolbox/process/functions/process_simulate_recordings.m at master · brainstorm-tools/brainstorm3 · GitHub

1 Like