multitapers and high gamma band activity

[Pascal Fries]

Hi Christine,

The parameters that you specified are OK and can be used.

In general, you should make sure that the cfg.foi contains integer multiples
of the Rayleigh frequency (1/cfg.t_ftimwin) and the spectral concentration
should also be small integer multiples of the Rayleigh frequencies, e.g.
between 3 and 11. Cfg.tapsmofrq specifies half the spectral concentration
and thus, your configuration gives a spectral concentration of 4 times the
Rayleigh frequency - approximately, because two tapers (with poor
concentration properties) are dismissed.

Thus: You're configuration is fully OK. But: I generally discourage to use
window lengths and spectral concentrations that change as a function of
frequency. Such an approach is often inspired by Wavelet analyses. But it
precludes any intepretation of a spectral pattern, because the analysis at
different frequencies is based on different data segments and different
degrees of freedom.

This is what we typically use for the gamma frequencies:

T_ftimwin of 0.25 s
-> Rayleigh frequency of 4 Hz
Spectral concentration (tapsmofrq) of plus/minus 12 Hz (up to 20 Hz).

All the best, Pascal


Pascal Fries
Principal Investigator
F.C. Donders Centre for Cognitive Neuroimaging
Kapittelweg 29, 6525 EN Nijmegen, Netherlands
E-mail:      pascal.fries at fcdonders.ru.nl
Website:    www.ru.nl/fcdonders/
Phone:       (+31) (0)24 36 10657
Fax:           (+31) (0)24 36 10989



> -----Original Message-----
> From: FieldTrip discussion list
> [mailto:FIELDTRIP at NIC.SURFNET.NL] On Behalf Of Christine Tillmann
> Sent: Tuesday, August 21, 2007 4:50 PM
> To: FIELDTRIP at NIC.SURFNET.NL
> Subject: [FIELDTRIP] multitapers and high gamma band activity
>
> Hi all,
>
> I'm doing time-frequency analysis based on multitapers. Do
> you have any suggestions how many tapers should optimally be
> used when analysing activity in the higher gamma band
> (30-150Hz)and how I would implement this in the freqanalysis function?
>
> Currently I'm using the following parameters:
> cfg.t_ftimwin = 5./cfg.foi and
> cfg.tapsmofrq = 0.4*cfg.foi
>
> Thanks in advance,
> Christine
>
> ----------------------------------
> 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/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/fcdonders/fieldtrip.