Dear brainstorm team and users,

I am working on a study using EEG and time-frequency analysis and I would like your opinion on a method I used for source reconstruction.

Without going into detail of my study design, I examined the differences between Condition 1 (C1) and 2 (C2) on EEG signal in different frequency bands, on 65 electrodes. I found a difference in the form of a statistically significant cluster in the alpha frequency band, during a specific time window and involving several electrodes. To better interpret my result, I tried to perform source reconstruction for this cluster.

Here are the steps I undertook with Brainstorm (v 2021-03-17) for this:

- First, I imported my clean (ICA-preprocessed and artefacted trials rejected) EEG signal previously filtered in the alpha frequency band (time x chanel x trial) for both C1 and C2.
- I computed my BEMhead model with OpenMEEG based on a template MRI normalized in the MNI system (MNI/Colin27; since I don’t have individual MRI) with the 65 electrodes.
- For C1, I computed my covariance matrix using the baseline period of all corresponding trials. Then I kept in my dataset only the electrodes that are included in my sensor-level cluster and I computed a source reconstruction (inverse solution) per trial with the covariance matrice computed just before with all the electrodes. I used minimum norm imaging method with current density map measure and unconstrained dipole (in total 45000 dipoles). Finally, I averaged the sources across trials and then across the time window of my cluster.
- I applied the same procedure for C2.
- At the end, I contrasted the 2 conditions (C1-C2) without statistical test to avoid the problem of double dipping.

I obtained a result without any error and that seems to be consistent and should represent the source of the significant cluster I observed at the sensor level. Yet, I would like to check: Is it OK to “filter” (that is, to restrict) the source reconstruction computation based on some electrodes, and do you think the method I used is valid?

Your feedback on this question will be really appreciated. Thank you in advance for your help.

Kind regards,

Théophile Bieth