question regarding topoplotER

[Robert Oostenveld]

Dear Jim,

The interpretation of the topoplot is usually the least ambiguous for
EEG data, in which you know that the electrodes are positioned
symmetric relative to the anatomical landmarks on the head, i.e. the
nose and ears and where all electrodes are attached to the skin. For
MEG data there is indeed the problem that the position of the head
relative to the sensor (or vice versa) and the distances are not
guaranteed.

If the subjects head is rotated in the helmet, then the field of
symmetrically located cortical areas will not be picked up by
symemtically positioned sensors. If the subject is closer to one side
of the helmet, then on that side the fields will be stronger. This
problem of strength remains and is not solved by the topoplotting
(although ft_megrealign can be used to solve it).

Whether the topoplot is correct depends on how you use it: the
template layouts (i.e. the fieldtrip/templaye/*.lay files) have all
been constructed to be reasonably symmetric. If you are worried about
the relative position of the head and helmet, then you should _not_
use the template layout. Instead you should create a custom layout for
that single dataset, in which the gradiometer positions -- which are
expressed relative to the head -- are used to interpolate the data to
create the topography. See ft_prepare_layout. If you don't specify a
cfg.layout in the topoplot function, it will create one from the
gradiometer positions that are present in the data data, which by
construct is a custom one. The triangle indicating the nose and the
schematic location of the ears are remain an accurate representation,
because the position of the sensors is expressed relative to those
(*), not the other way around.

For example, on the page
http://fieldtrip.fcdonders.nl/tutorial/layout
there is one layout
http://fieldtrip.fcdonders.nl/_detail/tutorial/layout/easycap32ch-avg.png?id=tutorial%3Alayout
which was measured with a Polhemus tracker on a single subject. In
this subject, the EEG cap is positioned rather asymmetric, which you
can see by the position of the electrodes over the midline and towards
O1 and O2.

Of course you can test the validity of the topoplot by doing two ERF
measurements of e.g. a simple sensory stimulus, one in which the
subject is sitting straight and one in which he rotates his/her head.
If you record the same ERF for the two locations, you can compare.
With the correct (custom) layouts the topoplots should look similar,
although the field strength can be different (because of the different
distances from brain to sensors in the two measurements). And you
should see in the topoplots (with the cfg.marker option) that the
channels are shifted relative to the head.

best,
Robert

*) this applies to most MEG systems, but cannot be guaranteed to
apply. Local procedures in your MEG lab may differ, so ask your local
MEG experts to be sure.





On 24 Sep 2010, at 0:49, Jim Li wrote:

> Dear all,
>
> I have a question about the topoplotER:
>
> I like the fact that the cartoon image of the nose and ears are
> drawn to
> help visualize the relative position of the sensor and the head. For
> data
> collection done in supine position, this works well when the
> subject's head
> position is centered in the sensor. But if the head is tilted to the
> left or
> right quite a bit (say a patient who can not cooperate), can I still
> count
> on such a plot to tell the relative  position between patient head and
> sensor-level activity? My experience seems to tell me "no", but I
> just want
> to confirm...
>
> Thanks,
>
> Jim
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users
> of the FieldTrip  toolbox, to share experiences and to discuss new
> ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html
>  and http://www.ru.nl/neuroimaging/fieldtrip.
>

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.