Hallo,

I have never used the -trans option so far. Until now, I have not seen 
the noisy stuff in my current data.  But  it reminds me to something I 
have seen with other software and another MEG device as well:

It looks as if the distance between helmet and head is changed too 
heavily during the transformation. 
Our own home-made software has produced a similar output whenever the 
two head positions (before and after transformation) were too distant to 
each other.
We have used the home-made software to allow for averaging across blocks 
measured from one subject in one or even several sessions. There is no 
problem to be expected if the head position stays the same within 1 to 2 
centimeters. Nowadays, I use maxmove for the same purpose as well.

Computation of a grand average is an interesting step in MEG analysis. 
If your are lucky you may already get a glimpse to your effects. 
Although the neuromag manual recommends the use of MaxMove for much 
crisper grandaverages, I think, this really overemphasizes the 
importance of the field grand average to the MEG community. We have the 
possibilities to do individual source localization. So our grand average 
should be based on those localization results and not magnetic field 
measurements. Therefore, I think, it is a waste of effort to recompute a 
complete MEG experiment to a virtual position at which none of the 
participants was placed during a recording. Sorry, if this statement was 
too harsh, but I stick to the opinion that maxmove has to fail if we try 
to recompute the field distribution for a very distant head position 
(say, e.g. more than 2 centimeters distance between measurement and 
transformed head position).

I regularly use the option '-origin -2.4 1.9 50.4' in the command line. 
I always used this option '-frame head', too. But, I have not checked 
which position the output file stores.

Finally, I would like to say, that Jukka Nenonen has recommended to work 
with manual bad channel detection whenever you analyze data from 
continuous HPI measurements (which I have done exclusively). Use 
'-autobad off -bad 533 2222' when you like to declare MEG0533 and 
MEG2222 bad.
The manual bad channel detection results in a more stable behaviour when 
estimating the current head position from the HPI coil signals. With 
"autobad on" the head position had with some subjects/data sets a 
bistable appearance. The head appeared as moving between two close 
positions (approx. 2mm away from each other). Nothing serious all in 
all, but different from real behaviour of the subject, I guess.

Cheers,
Burkhard



-- 
Dr. Burkhard Maess
Max Planck Institute for Human Cognitive and Brain Sciences
Stephanstr. 1a, P.O. Box 500355, D-04303 Leipzig
Aussenstelle Bennewitz, phone/fax: +49(3425)8875-26/-11   
mail: maess 'at' cbs.mpg.de,           http://www.cbs.mpg.de