[Eeglablist] ERSP on a epoch by epoch basis

[Thomas Ferree]

Dear Eric,The power spectrum of a stochastic signal requires averaging to
get a good statistical
estimate.  There are numerous ways to do this.  EEGLAB relies upon averaging
across
epochs.  See the paper by Bokil et al., 2007 for expressions about the bias
and variance
of the power spectrum estimate as a function of the number of trials.  That
paper uses
short-time Fourier transform not wavelets, but I expect there is a similar
thing going on
with wavelets.  In addition, EEGLAB uses the epochs to conduct statistical
tests about
differences from baseline.  With a single trial you can't do that, at least
not easily.
Within the tools available in EEGLAB, it is possible that a multi-taper
spectral estimate
could give you something interpretable, since in each time window it
averages over
multiple tapers, but only on the order of 10 of them, so that amount of
averaging is
still not great.  If you have large signal-to-noise ratio this might work,
otherwise not.
Best, Tom.

On Wed, Dec 31, 2008 at 6:05 PM, Eric Landsness <landsness at wisc.edu> wrote:

> The ERSP is calculated by subtracting the mean (of a collection of epochs)
> power across time from a mean baseline power.
>
> If I wanted to subtract the power (of a single epoch) across time from a
> mean baseline power could I do this?  In other words, I want to see on an
> individual epoch by epoch basis how the power is changing.
>
> I have tried to do this using the newtimef function and supplying my own
> 'powbase' baseline spectrum, but I am worried that newtimef wasn't meant to
> be used on an epoch by epoch basis.
>
> Thanks in advance,
> Eric
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-- 
Thomas Ferree, PhD
Department of Radiology
UT Southwestern Medical Center
Email: tom.ferree at gmail.com
Voice: (214) 648-9767
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