[Eeglablist] Power spectrum too high?

[Arnaud Delorme]

Michael and Tom,

just so you know, the spectral estimates returned by EEGLAB are the same scale as the spectral estimates returned by the Matlab pwelch function.
Best,

Arno

On Feb 11, 2011, at 11:20 AM, Thomas Ferree wrote:

> Dear Michael,
> 
> There are a variety of normalizations for power spectra as matters of definition.  In addition, tapering, zero-padding, and using one-sided versus two-sided conventions can change apparent values further.   Numerical Recipes in C has a nice summary of these issues.  You can access freely by following this link http://apps.nrbook.com/c/index.html, then go to Section 13.4, page 550.
> 
> Consequently, comparing numerical values of power spectra across studies is difficult and normally avoided.  Instead, most researchers restrict their efforts to test for group differences, task differences, etc, within whatever normalization conventions they are using within their lab or study.
> 
> In the past I have scrutinized the calculation of power spectra in EEGLAB and everything is done correctly, so you need not worry that something is wrong based upon the numerical values you are seeing.
> 
> Best, Tom.
> 
> On Sat, Feb 5, 2011 at 11:26 AM, Michael Gandal <mgandal at mail.med.upenn.edu> wrote:
> Hi,
> 
> I use the FFT power spectrum function for a single channel and it outputs a nice spectrum in dB that looks qualitatively very much like it should.
> However, when I compare with the values that I calculate by hand with matlab or Spike2 it seems that the EEGlab power spectral values are always too high.
> 
> Does anyone know why this is and how to correct for it?
> 
> Thanks
> Mike
> 
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> -- 
> Thomas Ferree, PhD
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