quiestions about planar transformation

jan-mathijs schoffelen jan.schoffelen at DONDERS.RU.NL
Fri Mar 5 11:12:03 CET 2010

Dear Mark,

There may be some confusion here, relating to whether we are talking
about frequency domain data (=power, always a positive value), or
whether we are talking about time domain data (=amplitude, can be
positive and negative).

Single trial combination of planar gradient transformed axial gradient
(or magnetometer) data is not the best thing to do for time domain
Reason: combination takes place by applying Pythagoras' rule to the
_dV and _dH pairs, and when the individual _dH and _dV components are
noisy, the noise is also squared, added, and squar-rooted, which leads
to an 'amplification' of noise. Not good. It works after trial-
averaging because you squeeze out the noise first and Pythagoras the
reduced-noise signals.

For frequency domain data this is not an issue.
Reason: combination usually takes place by just adding the _dV and _dH
pairs (power is a squared value already). This is just a linear step
(just like averaging) and the order of performing the linear steps
does not matter for the result. So single trial combination prior to
averaging or vice versa should not make a difference here.

In general I would advise against trying to do statistics on the
combined planar gradient and use it for visualization purposes only.
Alternatively, one could come up with a non-parametric statistical
test (permutation), in which I could think of a way of extracting a p-
value from a single _dH/_dV pair combined. But this would be a
different story...



> Hi all
> I am currently a little confused as to how to properly use megplanar
> and combineplanar for my 4D 148 axial gadiometer data. I would
> ideally like to use planar gradients as it will make subsequent ERF
> and frequency analysis easier to interpret. My question is when
> should I combine the v and h components?
> Looking  at some previous posts it is suggested that you should
> combine the gradients on an individual basis, but when I do this i
> get an almost flat amplitude across all sensors which doesn't look
> right (see attached image, 'bad_planar'). When i use combineplanar
> after averaging, the data the data looks ok (good_planar). I get the
> same result doing both megplanar and combineplanar on the averaged
> data.
> I just want to double check that the 'bad' planar fields i am
> getting are correct and not due to a bug in field trip. Intuitively,
> I would expect combining the components on the individual level
> world be more accurate as it would prevent fields from opposeingly
> orientated dipoles cancelling out when averaged.
> If it is the case that you should use combineplanar after trial
> averaging, how do you go about combining after statistical tests?
> i.e. when you have a p-value or a test statistic for the v and h
> components for each sensor, how would you combine these together?
> Any help or advice would be appreciated.
> Many thanks
> Mark

Dr. J.M. (Jan-Mathijs) Schoffelen
Donders Institute for Brain, Cognition and Behaviour,
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands
J.Schoffelen at donders.ru.nl
Telephone: 0031-24-3668063

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