I have one more question if you don't mind

When discussing a new topic, please create a new thread on the forum, otherwise other users won't be able to access the answers easily.

I have used the volume head model to estimate the activity sources using MNI, dSPM. Afterwards, I want to project my sources onto the template grid and compute the connectivity.

Make sure you are doing this using this procedure:

https://neuroimage.usc.edu/brainstorm/Tutorials/CoregisterSubjects#Volume_source_models

However, the source estimation using unscaled dSPM shows very high levels of synchronisation when visualised. I see that this is probably simply because the activation scale is computed for each window.

I'm not sure I understand this.

If you are not sure that your source maps are correct, post some screen captures here (eg. MRI Viewer at the peak of a spike, scout time series around the seizure onset zone).

Therefore, I decided to normalise the results and I computed the scale-average dSPM. The source estimation looks great now as the activation scale is normalised.

The "unscaled" and "scaled" dSPM maps should look exactly the same visually, except for the values in the colorbar. The only difference is a fixed scaling factor applied uniformly to all the sources and all the time points. Therefore I'm not sure what you did is correct.

Do I understand correctly that the resulting scaled dSPM cannot be projected onto the template grid then used for connectivity computation (since it can be used for visualisation and interpretation only)?

I think the message we give in the process is pretty clear: use the unscaled dSPM values for everything except for final plots where you want the range of values to reflect correctly the statistical significance of the results.

https://neuroimage.usc.edu/brainstorm/Tutorials/SourceEstimation#Averaging_normalized_values