[FieldTrip] multitaper and NaN

[Rodrigo Montefusco]

Hi Silvia,

As I see in your config, you are using 5 cycles per frequency
(cfg.t_ftimwin  = 5./cfg.foi;), that means that for 2 Hz you need at least
500ms * 5 = 2.5 secs epochs to resolve that frequency.
1.- you can try with longer epochs...(eg 5 secs)
2.- using less cycles per freq.

please someone correct me if I'm wrong

cheers

Y

On Tue, Jun 26, 2012 at 12:04 PM, Silvia Pagano <silvia.pagano at unitn.it>wrote:

> Dear list subscribers,
>
> I am an ERPer looking for the first time into the world of time-frequency
> analysis. I am trying to perform some multitaper analysis on a data set.
> Ideally I would like to perform similar analysis as described in a paper by
> Gould et al (2011) on Journal of Neurophysiology. (i.e. in their own words
> "Time-frequency analysis was performed on
> all epochs using a multitaper approach (2–30 Hz, 0.5-Hz steps, 500-ms
> temporal smoothing window, 2-Hz frequency resolution)" )
>
> I have epochs from -1100 ms to 100 time locked to the stimulus onset and I
> am trying to replicate the analysis they did. The problem is that I got
> boundary effects on all my conditions, namely I obtain NaN in the extreme
> 100 ms of the time window for all frequencies and for very low frequencies
> (e.g. 2-4Hz) I have no data in the whole time window. I thought I had time
> windows large enough to avoid boundary effects, however there must be
> something I am missing in my code.
>
> Here's what I did following the tutorial:
>
> cfg = [];
> cfg.output     = 'pow';
> cfg.channel    = 'EEG';
> cfg.method     = 'mtmconvol';
> cfg.foi        = 2:2:30;
> cfg.t_ftimwin  = 5./cfg.foi;
> cfg.tapsmofrq  = 0.4*cfg.foi;
> cfg.toi        = -1.1:0.02:0.1;
> cfg.pad        = 'maxperlen';
> TFRmult = ft_freqanalysis(cfg, data);
>
>
> Does anyone have a clue on this?
>
>
> Thank you very much
>
> Silvia
> --
> __________________________________
>
> Silvia Pagano PhD student
> CIMeC - Center for Mind/Brain Science
> Corso Bettini, 31 38068 Rovereto (TN)
> Tel. +39 0464 808709
>
>
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