[FieldTrip] Time Frequency parameters for <5hz with a 3 second epoch

[Schoffelen, J.M. (Jan Mathijs)]

Hi Ketan,

If you’re interested in the low frequencies, and not necessarily in the time dimension (i.e. how the amplitude of band-limited components vary across time), you probably would want to use ft_freqanalysis with the cfg.method = ‘mtmfft’.

Low frequencies require longer segments of data in order to be estimated in a meaningful way.

With wavelet convolution, I am also getting a lot of smoothing/stretching in the frequency domain at  frequencies above ~3 Hz and Im not sure what the right parameters are to avoid this.

This is an inherent property of wavelet convolution, and cannot be avoided.

I have also attached example figures from both implementations (MTM and Wavelet convolution) for your reference.

Here are my questions:
1) Is there any way to avoid the loss of the data at the edges with mtmconvol? Also, do the parameters that I have used seem to be okay for what I am trying to do?

See above. I would use cfg.method = ‘mtmfft’ to get a decent estimate of the lower frequencies. A 0.5 second time window only starts making sense for frequencies at which the time window is filled with a few full cycles worth of oscillation to begin with (I’d say 6 Hz (but possibly more) and up).


2) With wavelet convolution, is it just that my epochs are not long enough to get at these low frequencies? Or am I getting the parameters wrong? Any help would be greatly appreciated.​

Nothing wrong with the parameters as such: it’s an interaction between the length of your data epochs and the wavelet parameters.


3) How can I avoid the smearing that I am getting for the higher frequencies using wavelet convolution?

You cannot.


and finally,

4) At the moment, I am only doing the baseline normalisation when plotting the output of ft_freqanalysis. Is there any way to perform this baseline normalisation using ft_freqanalysis​ itself?



No. You’d either use ft_freqbaseline (when you have a time frequency represenation, and want to use some basline defined for the TFR), or use a direct contrast.

Best wishes,
Jan-Mathijs



Thank you in advance for your help!

Regards,

Ketan Jaltare, M.Sc. | PhD Student
Research Group Health Psychology
KU Leuven, Belgium
<Figure_mtm.jpg><Figure_wavelet.jpg>_______________________________________________
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