A newbie's question on Fourier using freqanalysis_mtmfft

[Justin Ales]

In my experience with steady-state visual evoked potentials using
time-frequency analysis with just power works poorly. This is because
it doesn't average out phase incoherent noise sources, and the
steady-state response is a driven coherent signal.

We usually cut our data into 2 second long "trials", to get .5 Hz
resolution, and then coherently average(phase locked) fourier
components over these epochs.

Bach & Meigen have written a good primer on analysis choices, such as
windowing and zeropadding, for steady-state potentials:
http://www.uniklinik-freiburg.de/augenklinik/live/homede/mit/bach/ops/fourier_en.html

Justin Ales
Smith-Kettlewell Eye Research Institute

On Thu, Dec 17, 2009 at 12:47 AM, Anette Giani
<anette.giani at tuebingen.mpg.de> wrote:
> Dear all,
>
> I am a beginner on MEG and I have some basic questions on Fourier
> transformation using the function freqanalysis_mtmfft. It would be great if
> you could help me!
>
>
>
> We are interested in Steady State Responses. Therefore we recorded data 60
> seconds trials.
>
> To transform the time domain into a frequency spectrum we used the function
> freqanalysis_mtmfft. However, I am not sure which value cfg.tapsmofrq. Since
> we would like to have the highest frequency resolution possible I set
> cfg.tapsmofrq to 0.25; the lowest value possible. Do you think that was a
> good choice? Further, does this value mean that our lowest possible
> frequency resolution would be 0.25? If that was the case, how could I avoid
> all smoothing?
>
>
>
> Second, do you think that it would be useful to pad the signal with zeros to
> artificially increase the frequency resolution? I guess this could be done
> using cfg.pad?
>
>
>
> Lastly, an even more general question: What do you think would be handier:
> (1) Compute the frequency spectrum over 60 seconds, to get a high frequency
> resolution or (2) to cut the signal into e.g. 4 second pieces and to average
> all spectra to increase the signal to noise ratio?
>
>
>
> Thanks a lot in advance for your help,
>
>  Anette
>
>
>
> ----------------------------------
>
> The aim of this list is to facilitate the discussion between users of the
> FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and
> EEG analysis.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.