beamformer on yokogawa data, megplanar.m

Vladimir Litvak v.litvak at ION.UCL.AC.UK
Tue Feb 2 19:07:41 CET 2010


Dear Sangi,

There is no need to convert your data to planar gradient. The
assumption is that the relation between coils and channels is
described by the grad.tra matrix. You can look at it and make sure it
is correct for your system (write back if not). The megplanar function
as apparent from the error message has explicit support for some
particular MEG systems and Yokogawa is not one of those. I'm not sure
how easy it would be to support it generically as there might be
several variants of Yokogawa systems which can be quite hard to
distinguish. But for your particular system you can try to implement
it yourself.

Best,

Vladimir

On Tue, Feb 2, 2010 at 5:22 PM, Sangita Dandekar
<sangita.dandekar at gmail.com> wrote:
> Hi,
> Am hoping to apply beamforming based source localization to MEG data from a
> Yokogawa system.   Think I've managed to coregister MRI and sensor
> coordinate systems, so that part of the problem is pretty much under
> control.
> What I'm wondering about is what the assumptions are of the
> prepare_leadfield and other source localization scripts about the input
> gradiometer data.  Haven't looked at it too closely yet, but does it assume
> that the input sensor data is planar gradient data?  If so am assuming that
> inputting the raw data from the Yokogawa system (axial gradiometers) is
> incorrect?   Or does fieldtrip distinguish between different types of
> gradiometers using the input .grad structure?
> I tried to convert the axial gradiometer data from the yokogawa system to
> planar gradient data by using the megplanar function as shown below, and
> receive the following error:
> (Even if it isn't necessary for source localization, it would be nice to be
> able to view the data as planar gradient data)
>>> cfg=[];
>>> cfg.planarmethod='sincos';
>>> megplanar(cfg, righttrials);
> the input is raw data with 156 channels and 46 trials
> ??? Error using ==> checkdata at 478
> This function requires ctf151, ctf275, bti148 or bti248 data as input, but
> you are giving meg data.
> Error in ==> megplanar at 228
> data  = checkdata(data, 'datatype', {'raw' 'freq'}, 'feedback', 'yes',
> 'ismeg', 'yes', 'senstype', {'ctf151', 'ctf275',
> 'bti148', 'bti248'});
>>>
> Some background information:  used the ft yokogawa2grad.m function (stored
> in private FT directory) to create the gradient structure.  Here is what
> data structure for
> one set of trials looks like:
>>> righttrials
> righttrials =
>       trial: {1x46 cell}
>       label: {1x156 cell}
>        time: {1x46 cell}
>     fsample: 500
>        grad: [1x1 struct]
>      offset: [46x46 double]
>         cfg: [1x1 struct]
>>> righttrials.grad
> ans =
>       pnt: [314x3 double]
>       ori: [314x3 double]
>       tra: [157x314 double]
>     label: {157x1 cell}
>      unit: 'cm'
>>>
>
>
> Thanks in advance for any help!
> Sangi
>
>
>
> ----------------------------------
>
> 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.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> 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/neuroimaging/fieldtrip.



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