[FieldTrip] axial gradiometers vs planar gradient

Jose joseluisblues at gmail.com
Wed Sep 28 17:02:39 CEST 2016

dear fieldtrip community,

I'm working with CTF MEG data,
I have a confusion regarding the use of the (pure) axial gradiometers and
the synthetic planar gradients,

>From what I have read "the planar field gradient simplifies the
interpretation of the sensor-level data because the maximal signal power is
located above the source". In practice, this means that the topography
would resemble more the sources? Is that correct? Would be meaningless to
do this if one intend to do source analyses anyway?

However is not clear for me if the planar gradient is used only for
visualization purposes, or if is intended to replace the use of axial
gradiometers for data analysis. Some papers do mention the aforementioned
transformation but then they do not specify which data is used to run
statistical analysis so I assume they do it with planar gradients. Others
they clearly perform statistical analyses such as non-parametric cluster
permutation tests with planar gradient data. So, the second question would
be if one should run statistical analyses in planar gradient or axial
gradiometers data?. What is the criteria to choose one or the other?

If one apply cluster-based permutation tests to either axial gradiometers
or the planar gradient one will find distinct results because the activity
is distributed in different sensors, so distinct clusters will be observed,
right? Does make sense to find different results depending on whether we
analyze gradiometer or planar data?

Some recommend use planar gradient data to perform statistics (
while others other advise against it (

Is there a consensus at the moment?

I would really appreciate some directions here,
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