[FieldTrip] Gradiometer definition after ft_appenddata

["Jens Klinzing, Universität Tübingen"]

Dear all,
I have a question concerning ft_appenddata dropping the gradiometer 
definition to make sure one is not using a grad definition that does not 
fit the data (see [1] for an earlier quick discussion).

I have an MEG data set in which some subjects had a short break during 
recording. The fiducial markers remained in place during the break. For 
each of these subjects, I would like to define trials in both data parts 
and then append them before doing further analysis. Is it valid to use 
the first part's grad definition for the combined data set?

The grad definitions in the two parts are not identical. I assume this 
is because the subjects moved during the break and the grad definition 
is in relation to the fiducial markers at recording onset?

Thanks in advance for your help!

All the best,
Jens


P.S.: Sorry in case there is information on that on the fieldtrip 
website. I could only find an earlier quick exchange on this mailinglist:
[1]
http://mailman.science.ru.nl/pipermail/fieldtrip/2013-April/006405.html
Here, Eelke Spaak said one can attach gradiometer data as long as one is 
sure it's the correct one. But how do I know it is correct?

...and the general info about the grad definitions:
[2]
http://fieldtrip.fcdonders.nl/faq/how_are_electrodes_magnetometers_or_gradiometers_described