[FieldTrip] Question about the identification of sensor type & proper timing of gradiometer selection

[Jarang Hahm]

Dear fieldtrip user.


I'm newcomer on an MEG analysis and the fieldtrip.


I have MEG data (.fif file) measured by Neuromag 306 channel system.

Recently I got two problems on 1) identification of sensor type and 2)
proper time to separate gradiometer in data processing.



1) First one is about identification of sensor type when using fieldtrip
toolbox (20111204 version)


After trial definition and preprocessing, I got some warnings in
series when doing time-frequency analysis:

Warning: could be Yokogawa system
> In fileio\private\ft_senstype at 271
  In ft_chantype at 71
  In ft_chantype at 477
  In ft_datatype_sens at 124
  In ft_datatype_raw at 95
  In ft_checkdata at 175
  In ft_freqanalysis at 188
Warning: could be Yokogawa system
> In fileio\private\ft_senstype at 271
  In ft_chantype at 166
  In ft_chantype at 477
  In ft_datatype_sens at 124
  In ft_datatype_raw at 95
  In ft_checkdata at 175
  In ft_freqanalysis at 188

(...There were also the other 12 warnings, which were simillar to above
except for the specific line in ft_chantype.)


As warning indicated, I've been ft_senstype at the line 271 and found that
it couldn't recognize the data as a Neuromag 306 system

although the sensor label of my data was matched with that of Neuromag 306.


I wonder whether those warnings might affect overall analysis procedure or
not and wish to fix this problem.




2) The second question is about a proper time for gradiometer selection.

My analysis of interest is only gradiometer channels, not magnetometer
one, so that the magnetometer can be removed at some time.

In data processing, when is the proper time to separate the gradiometer
channel from magnetometer?


My data will be processed like this way:

Trial definition -> preprocessing -> ICA for EOG rejection  -> artifact
rejection -> time-frequency analysis at sensor level & source analysis.


Should I keep apart gradiometer channel from magnetometer after artifact
rejection? or before ICA analysis?



Need your help.





Sincerely,


Jarang Hahm
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