For the analysis of my MEG data, I would like to calculate the grand mean
over all subjects. However, for technical reasons, data of one subject
contains 1 channel less than the data of the others. Is there a way to
choose the channels for which the grand mean should be calculated? I tried
to crop the data of the remaining subjects. However, after cropping it seems
like 2d channel positions are lost. Hence, if I want to look at the topology
of the cropped data a message appears: "Get 2d positions for these
Looking forward to your suggestions!