Meg planar

[jan-mathijs schoffelen]

Dear Marie,

The planar gradient transformation algorithm is not dependent on
single trial, or averaged data as an input. The transformation of the
data, moving from 151 (in your case) axial gradients to 302 planar
gradients, is just the consequence of applying a spatial
transformation matrix on the data. This spatial transformation matrix
is computed from the gradiometer information, and the method
specified (btw: in my experience specifying cfg.method = 'sincos'
gives the best results). Anyway, your problems are introduced when
recombining the planar-data into 151 'sensors' again. This is done by
combineplanar, which applies pythagoras to the -dH and -dV components
of a given sensor. This means that all values become positive
(agree?). Thus, applying this procedure on the single trials leads to
a 'positive' baseline, because the average of positive valued numbers
can never go to zero.
If you are bothered by this, you could perhaps not already subtract
the baseline prior to calling timelockstatistics, but after having
called combineplanar.

Yours,

Jan-Mathijs


On Oct 24, 2007, at 5:56 PM, Marie Smith wrote:

> Dear all,
>
> I want to use the planar gradient correction computed on individual
> trials as opposed to the average trial and i am getting some
> strange results. When i compute the planar gradient on the average
> of the trials, it seems to make sense wrt the original data (bottom
> plot, chans*time), however when the single trials are taken into
> account the new planar averaged seems to be very smeared into the
> baseline region (top plot). Even though the baseline has already
> been removed from every trial.
>
> Is this just a result of noise in the single trials and is there
> any way to correct for it? Looking at the statistics tutorial data
> this does not seem to be an issue for that data set, could it be
> caused by some sort of artifacts that i have not taken into account?
>
> Thanks,
>
> Marie
>
>
> Script info:
>
> cfg.keeptrials = 'yes';
> cfg.baseline = [-0.2 0];
> avg = timelockanalysis(cfg, data);
> avg = timelockbaseline(cfg,avg);
> pl = megplanar(cfg, avg);
> plcb = combineplanar(cfg, pl);
>
> cfg = [];
> cfg.keeptrials = 'no';
> cfg.baseline = [-0.2 0];
> avg = timelockanalysis(cfg, data);
> avg = timelockbaseline(cfg,avg);
> pl = megplanar(cfg, avg);
> plcb = combineplanar(cfg, pl);
>
>
>
> ----------------------------------
> 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/fcdonders/fieldtrip.
> <Comparison_planar.tiff>

----------------------------------
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/fcdonders/fieldtrip.