[FieldTrip] planar gradients

Frederic Roux f.roux at bcbl.eu
Thu Nov 22 14:19:45 CET 2012

Dear all,

I would like to follow up this post with another question
related to the planar gradients.

I am working on data aquired with a CTF 275 system but at 
the time of the acquisition there was a number of sensors
that were broken.

So I end up with n = 258 channels.

After running the ft_megplanar function I get n = 516 channels.

However, if I try to combine the gradients with

I get an error saying that I am not providing ctf275_planar but
ctf275 data.

Is the fact that I do not have n = 275 sensors but 258 confusing
the ft_combineplanar function, or is this related to something else?



----- Original Message -----
From: "Eelke Spaak" <eelke.spaak at donders.ru.nl>
To: "FieldTrip discussion list" <fieldtrip at science.ru.nl>
Sent: Wednesday, November 21, 2012 12:47:25 PM
Subject: Re: [FieldTrip] ft_freqstatistics tutorial question

Dear Nenad,

Strictly speaking, planar gradient transformation is not necessary, so
you can just skip those steps if you really want to. However, if you
have axial gradiometer data (as I seem to recall from your earlier
posts) and want to do TF-analysis and -statistics on sensor level, I
would strongly recommend applying a planar gradient transformation.

Axial gradiometer data will produce maximal deflections (of opposite
polarity) on both sides of a current dipole, while planar gradiometer
data produces a positive maximum exactly above the source. If you
apply TF-analysis to axial gradiometer data, you will get two
spatially separated 'blobs' where there was only a single oscillating
dipole in the brain. If you look at power (as is typically done), you
will lose the polarity information, and hence interpreting the power
topography in terms of brain is nearly impossible with axial gradient


On 21 November 2012 11:30, Nenad Polomac <polomacnenad at gmail.com> wrote:
> Dear all,
> I have one doubt. I want to calculate ft_freqstatistics on my MEG date
> obtained form  ft_freqanalysis. However, I see now in ft_freqstatistics
> tutorial that you suggest that operation should be done on planar gradient
> data. So, the first  ft_megplanar and then ft_freqanalysis, ft_combineplanar
> and ft_freqstatistics. My question is, does this step of calculating planar
> gradients for time frequency analysis is necessary or not? Could I apply
> statistic on the data from ft_freqanalysis without any involvement of planar
> gradients calculation.
> Thank you in advance!
> All the best!
> Nenad
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> fieldtrip at donders.ru.nl
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