Hi everybody,
Jason: I agree with you. Averaging over the blocks and sessions of the
very same subject should work with MaxMove. I use MaxMove precisely for
that purpose, too.
I have read the manual again. It says at page 33:
-------------
On command line you can select coordinate transformations with the
option -trans name, where name is the FIFF Şle deŞning the coordinate
transformation of the reference head position. If name = default, the
head coordinate axes are identical to the device coordinate axes, and the
head coordinate frame origin corresponds to the location (0, 0, 0) of the
device coordinate frame.
-------------
Does this mean that the option '-trans default' assumes that the head
and the device coordinate system are identical?
If so, it would be a shift of about 40mm in vertical direction. Then it
would really be no wonder that one sees the noisy center part.
There is another statement at page 38:
--------------
On command line you can select the option -movecomp to perform the
movement compensation. You can select the reference position with the
option -trans (see above). If the transformation is not speciŞed, the
program uses the transformation stored in the original Şle when acquiring
the data.
---------------
So obviously, I have transformed my data to the position stored in each
file. So far the data was measured in a single session, but consisted of
2 to 6 different blocks (depending on the experiment conducted). Each
block, however, stores the same position. After all blocks have passed
the MaxMove processing, they shall be as being measured at the same
position. For the averaging over sessions, I think, we have to use the
option '-trans <one_file_of_the_first_session>. By this, all data shall
be transformed to the headposition of the first session of this
individual subject. If one takes additional measures to guarantee that
the head position between session does not differ by more than 1
centimeter (ideally :-), then everything should work.
We have attached tic marks to the backrest of the chair, actually to the
none-moving part. The moving part somehow covers some of them nicely so
that one can easily record the height of the chair in this measurement.
I also do not believe that the size of the subject changes between
measurements, but maybe the style of sitting? Another line of tic marks
is attached to the floor. I am pretty confident that recording of these
two positional numbers allows us to approximate about the same head
position across sessions.
Cheers,
Burkhard
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Max Planck Institute for Human Cognitive and Brain Sciences
Dr. Burkhard Maess mail: maess 'at' cbs.mpg.de
Stephanstr. 1a http://www.cbs.mpg.de
04103 Leipzig, Germany phone/fax: +49(3425)8875-26/-11
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