amplitude modulation on planar MEG

[Giovanni Piantoni]

Dear Fieldtrip users,

I do not know how to find the time points in which the amplitude in a
frequency band (say, alpha, 8-12 Hz) is higher than a threshold. I am
using axial (CTF) MEG data, but I want to find the amplitude modulation on
the planar gradient (as it makes more physiological sense).

On EEG data, I would
1_ bandpass the data between 8-12 Hz,
2_ take the envelope (f.e. with a hilbert transform),
3_ find all the time points that are above the threshold.

On MEG data, I do not know what to do, considering that I cannot really
bandpass planar data, as they are always positive.

Which option is the best?
1_ bandpass on axial, then megplanar, combineplanar and take envelope on
the combined planar, and then find the time points above the threshold.
2_ bandpass on axial, then megplanar, and take the envelope for the two
components and use those to determine when the amplitude is above the
threshold (but how can I combine the two then?)

Or any other option? I am very confused about this issue.

Any help is greatly appreciated!

Thanks,

Giovanni

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Giovanni Piantoni, Ph.D. student
Dept. Sleep & Cognition
Netherlands Institute for Neuroscience
Meibergdreef 47
1105 BA Amsterdam (NL)

+31 (0)20 5665492
g.piantoni at nin.knaw.nl
www.nin.knaw.nl/research_teams/van_someren_group

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