Thank you Francois,
I don't seem to have this option. I can't find the "Compute source : BEST" anywhere.
The only possibility I have so far is to right click on the file and select MEM from there, but this is very inconvenient when you have hundreds of files to process.
I have updated brainstorm and MEM, but this does not seem to solve the issue.
Do you have any further recommendations?
Thanks again for your help