bug in megrealign

Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Tue Feb 13 17:01:18 CET 2007

Dear FT-MEG users,

I just noticed a bug in the MEGREALIGN function. I don't know how
long it has been there, nor whether it affects all configurations or
only whis particular one.

The problem could be recognized from the output that MEGREALIGN
prints on the screen: the buggy version shows
   original -> template             RV 4128.84 %
whereas the corrected version shows
   original -> template             RV 10.32 %
Also the scalp topography of the realigned ERF looked completely
messy in the buggy version. The problem relates to the shift of the
dipole layer on which the scalp data is projected: instead of
shifting it inward with 2.5 cm, it was shifted outward and the
dipoles bumped into the MEG helmet, causing weird interpolation effects.

I noticed the problem when using a single-sphere headmodel fitted to
the skin surface. It may be that the problem would not have as
obvious for other head models, but am not sure. Maybe someone who
recently used MEGREALIGN with singleshell headmodels could comment...

I have made an attempt to fix it (per direct on fcdc/common, in this
evening version on the ftp). Furthermore, I have now removed the
default of 2.5 cm for cfg.inwardshift, since probably a lot of people
are not using single-sphere headmodel fitted to the skin surface
anymore. See the help of MEGREALIGN for more details.

If you are unsure of wheteher it affects your results, I suggest that
you rerun the megrealign part and also use the option
cfg.feedback=yes (besides the default cfg.verify=yes). Note that from
now on you must explicitely specify cfg.inwardshift.

sorry for the inconvenience,

More information about the fieldtrip mailing list