Advice on Neuromag analysis

For some general information on Signal Space Projection (SSP), Signal Space Separation (SSS) and its temporal extension (tSSS), see here: http://www.megwiki.org/index.php?title=MEG_Analysis

• Downsampling your data by factors of 2-5 within Max Filter can save filespace and time (default sampling frequency is 1000Hz, but one is rarely interested in frequencies above 100Hz). One could of course change the default sampling frequency during acquisition, but be careful that some of the (continuous) Head Positioning Coils use frequencies of several hundred Hz.

More to follow here....

Avoid MaxFilter Bugs!

CBU researchers have encountered problems using some options in NeuroMag's MaxFilter program, and in discussing these issues with the NeuroMag software engineers, we have learned of a few other bugs too. These are briefly summarised below and will be expanded upon as we gain more info. See the NeuroMEG mailing list archives for the full discussion (and if you haven't done it already, join the list!): http://www.jiscmail.ac.uk/archives/neuromeg.html

Don't use '-trans default' without setting '-frame' and '-origin'

If you attempt to transform the subject's head position to the 'default' position corresponding to the origin of the device coordinate frame (see p. 33 of MaxFilter User's Guide) without specifying the -frame and -origin options, the transformed data will contain large-amplitude noise at the vertex. This bug is known to NeuroMag and they are working on a solution. Apparently MaxMove works ok when the target position is defined in another file, or with the default option when -frame and -origin are specified, e.g.: '-trans other_file_sss.fif' or '-trans default -frame head -origin 0 0 40'

Don't use '-trans' with head-movement compensation '-movecomp'

According to NeuroMag, -movecomp always returns the head position to the initial position in the recording.

Don't use '-autobad' with head-position estimation '-headpos' or movement compensation '-movecomp'

Just don't. Apparently -autobad is fine alone (but not really recommended anyway), and -headpos and -movecomp are fine without -autobad. Instead of -autobad, it is recommended that you inspect your raw data, note any noisy or otherwise 'bad' sensors, and use the -bad <sensor number(s)>, e.g.: '-bad 521 1421 2413'

Don't use '-lpfilt' to low-pass filter data

According to NeuroMag, -lpfilt uses an IIR filter which introduces a delay in the output signal. Note that downsampling, option -ds, also necessarily low-pass filters data but it does not cause the same delay because it uses an FIR filter.