I'm currently working with EEG data which I wanted to analyse in brainstorm to obtain the source activity. Right now, I'm struggling to understand what brainstorm is doing and I couldn't find the information needed in the tutorials or other forum posts.
Basically, I have calculated the dspm source currents for a group of subjects for different regions of interest and now I would like to calculate the group average of the absolute activity following the tutorial of Stropahl et al. 2018.
The problem is that I do not understand how brainstorm does the preprocessing step 'absolute values'. I first calculate the group average, then I add the process 'absolute values'. Naively I would think, it just takes the absolute value of the averaged dspm source activity. But in the attached picture one clearly sees that this cannot be the case. Could you explain, how brainstorm creates the plots on the right from the ones on the left?
Similarly, I would like to extract the single subject data in order to perform some statistical analyses in R. But I am unsure, which values to extract. Should I take the dspm source activity or the absolute values of the source activity? I would like to compare the mean absolute activities between two groups and since I don't know, how brainstorm calculates the mean on the right hand side of the attached plot, I am not sure, how to calculate the mean manually with the extracted values.
Thanks in advance for your help.