Meg planar

[Marie Smith]

Dear Jan-Mathijs,

Thanks this does make sense and I now see the solution. I already
tried removing the baseline as an option inside combineplanar, but I
now realise this was only removing the baseline of the average not of
the single trials. If I perform baseline correction on the planar
transformed single trials it will work out as I expect.

Thanks again,

Marie

On 25 Oct 2007, at 10:40, jan-mathijs schoffelen wrote:

> 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.

----------------------------------
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.