[FieldTrip] no coil pos for planar combined data

[Kaelasha Tyler]

Hi Eelke,

Partly I was just following the tutorials suggestion.
I am using a Neuromag system. Up until now I have been working primarily with the magnetometers. 
I was interested in what difference I would find with data from the planar gradiometers. 

I had understood that  planar gradiometer sensor pairs measure planar magnetic fields perpendicular to each other, at a similar point near the scalp. That being the case, I would have thought that using original planar gradiometer data in source analysis without combining it would produce in nonsensical results. Wouldn't source data at one grid point would then be incongruously trying to represent two different sets of information?

Kaelasha

________________________________________
From: fieldtrip-bounces at science.ru.nl [fieldtrip-bounces at science.ru.nl] on behalf of Eelke Spaak [eelke.spaak at donders.ru.nl]
Sent: Wednesday, 22 October 2014 2:05 AM
To: FieldTrip discussion list
Subject: Re: [FieldTrip] no coil pos for planar combined data

Hi Kaelasha,

Why do you want to do source analysis on combined planar gradiometer
data? In general, I would say this does not make sense. Instead, I
would use the original (axial or planar, depending on your system)
gradiometer data.

Best,
Eelke

On 21 October 2014 15:57, Kaelasha Tyler <ktyler at swin.edu.au> wrote:
> Hi all,
>
> I am having problems using ft_sourceanalysis with planar gradiometer data.
>
> I am following a tutorial for beamforming with lcmv method (which I believe is a work in progress) found here:
> http://fieldtrip.fcdonders.nl/tutorial/beamformer_lcmv?s[]=covariancewindow
> Using planar gradiometer data is mentioned in the tutorial, although not in detail.
>
> My problem:
> The .avg field produced by time locking, is lost when combining planar gradiometers using 'ft_combineplanar'.
> In this case, I am unable to to use 'ft_sourceanalysis' on combined planar gradiometer data.
> I get the error: Reference to non-existent field 'avg'.
>
> I reasoned that I may be able to keep the .avg filed if I combine the planar gradiometer data at the outset, prior to timelocking. But in this case, the gradiometer positions in data.grad does not include 'coilpos'. Eg, no data.grad.coilpos.
> When calling 'ft_sourceanalysis' in this case, I get the error: Reference to non-existent field 'coilpos'.
>
> Any suggestions?
>
> If the .grad.coilpos was retained when I combined the planar gradiometer data at the outset, (after preprocessesing and prior to timelokcing) then I would be able to use ft_sourceanalysis on planar gradiometer combined data successfully.
> Is that possible?
>
> Thanks in advance! (and thank you to all previous very helpful responses on this list).
>
> Kaelasha Tyler
>
> PhD Candidate
> Brain and Psychological Science Research Centre
> Swinburne University of Technology
> Melbourne
> Australia
>
>
>
>
>
>
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