[FieldTrip] Structure of 4D-Fourierspectrum with Multi-Taper

jan-mathijs schoffelen jan.schoffelen at donders.ru.nl
Wed Jul 4 09:19:30 CEST 2012

Hi Andreas,

The tapers per trial are grouped together, i.e. it is according to the first scenario you sketch.
Note that averaging fourier-coefficients across tapers is not correct, because each of the tapers induces a non-trivial phase shift. Phase differences however can be averaged across tapers, but for this you need a reference signal.



On Jul 3, 2012, at 11:44 PM, Wutz, Andreas wrote:

> Dear all,
> in requesting from a ft_freqanalysis with output ='fourier' and 2 multitapers, I figured out that in the 4D matrix in the field 'fourierspctrm' the first dimension refers to the number of tapers that are calculated (Number of trials -times- number of tapers). My question refers to the organization of this dimension. Are in the example with 2 tapers the two tapers from the same trial next to each other? Or do we have first, the outputs from the first taper for all the trials and then, all the trials for the 2nd taper? I would like to calculate the circular mean of the phase angles across trials and therefore would need to combine the outputs of the two tapers per trial. Is there already some fieldtrip function to do this? How should it be done? 
> Thank you very much.
> Best
> Andreas
> Andreas Wutz
> PhD Student 
> CIMeC - Center for Mind/Brain Sciences
> Università degli studi di Trento
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> fieldtrip at donders.ru.nl
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Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

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