Whether the leadfield matrix is fixed

Hello,

I wonder whether leadfield matrix (forward solution) is different by individual differences and trials.
Or is it different by APIs (MNE etc.)?

If not, is leadfield matrix always the same no matter who or when?

Hello Jaeun,

The leadfield matrix is a matrix that is dependent on the technique you use to make it (Boundary element method or Finite Elements) and is dependent on the individual head model.

It represents how possible sources effect the EEG or MEG electrodes. Saying that, for the same API and the same head model with its conductivities and solving method the leadfield matrix should be more or less the same.

It should, for one person, not change over trials, except if the electrodes are moved.

Kind regards,
Steven

The forward model explains how the brain sources influence the scalp potentials (or fields).
If you modify the source space (the cortex surface or the volume grid on which you plan to estimate the sources) or the position of the sensors, you need to compute a different forward model.
It is however independent from the recordings.

thanks for your reply SBeumer,

Do individuals have different head models? Even if they have different shapes of head, aren't they mapped as a general head model and get the same model?

Also I wonder if there is an electrode at a specific location called A B on the cap, can I get the same leadfield for anyone who wears the same cap?

Jaeun

Hi Jaeun,

You can align and transform them more or less to a general head model, though never perfect. But that would be useful when you want to compare results for many different individuals. In that case you map the results from the individual case via that transformations again to the general head model.

The leadfield however still would have to be on the original head model, because the brain topology, cortical orientation, skull thickness, possible pathologies etc. have a big influence on the leadfield matrix.

I do not fully understand this part of the question, though using the same cap for different people will get different locations of the electrodes on the scalp, thus different lead fields.

You can use the same standard head model for everyone with standard position electrodes, though that will give quite an approximation error depending on what you want to do with your data.

Kind regards,
Steven

Ps: A nice paper on the effects of errors in forward modelling is the following:
Akalin Acar, Z., Makeig, S., 2013. Effects of forward model errors on EEG source localization. Brain Topogr. 26, 378–396. https://doi.org/10.1007/s10548-012-0274-6

No, the geometry of the head is important for modelling accurately the currents. If you have individual head shapes and electrodes positions, the leadfield is computed again for each spatial configuration, it does not use any generic head model.

If you want, or if you don't have individual anatomical scans or electrode positions, you can use the same head model (MNI ICMB152 template or other) and the same electrodes positions. But expect a lower spatial accuracy.

Thanks for your help!