On Sat, Nov 21, 2009 at 12:19 PM, Amir H Javadi <[log in to unmask]> wrote:
> Sorry that I take your time a lot. I'm a little bit confused with the
> time-frequency analysis of SPM. I use the function "spm_eeg_tf" for
> time-frequency analysis. What is the output unit of "power" matrix of this
> function? Is it uV? The input is in uV. I use BioSemi to record my data.
If you don't do any rescaling then the units of power are the
original units squared so in your case uV^2. If you do rescaling then
you can get different units such as uV (sqrt), dB (LogR) or % (Rel).
> One other question. What is the suggested value for "Mfactor" for different
> frequency bands? My intuition is that for high-frequencies it is better to
> use high Mfactor and for low-frequencies it is better to use low Mfactor.
> Currently I use 27 for gamma band and 11 for theta band. What is your idea?
Mfactor determines time vs. frequency resolution trade-off. The
higher it is the better frequency resolution you will have at the
expense of the time resolution. So for instance if by theta you mean 5
Hz then one wavelet cycle is 200ms and 11 cycles is therefore 2.2
sec. That sounds like quite a long time to me but I'm not familiar
with your paradigm. The idea is then that power estimate for each
point is effectively an average of what's happening in +-1.1 sec sec
around this point.
Similarly if by gamma your mean 40Hz then one cycle is 25ms and 27
cycles is 675 ms and you are averaging in +- 337.5 ms around your
point of interest.
> Thanks again for your attention.
> Have a good weekend :-)