[Eeglablist] Power Spectrum vs. Power Spectral Density

Ole Traupe ole.traupe at tu-berlin.de
Wed Jun 4 02:33:54 PDT 2014

Hi Brian, hi Makoto,

as far as I know, and notwithstanding any considerations regarding a 
particular software implementation, the PS yields the power of a 
discrete, predefined set of frequencies interpretable as individual 
values. In contrast, similar to the probability density of the 
(standard) normal distribution, the PSD is a density function the 
(partial) area under which can be interpreted in terms of power. The 
transformation '10*log10' rescales to dB not affecting the units (i.e. 
unit^2 and unit^2/Hz, respectively).

Therefore, the choice between PS and PSD depends on the question you 
want to answer. In EEG matters you are typically interested in the power 
of continuous frequency bands and therefore should consider the PSD (and 
determine the area within certain limits of interest).


Am 03.06.2014 23:26, schrieb eeglablist-request at sccn.ucsd.edu:
> Dear Brian,
> Actualy I did not know the difference. According to wikipedia, 
> spectral density is something like spectra represented as cumulative 
> distribution function.
> http://en.wikipedia.org/wiki/Spectral_density
> I don't know though when it is more appropriate to use power spectra 
> and spectral density... someone in the list please help us.
> Makoto
> On Wed, May 28, 2014 at 8:50 AM, Erickson <ericksonb.eng at gmail.com 
> <mailto:ericksonb.eng at gmail.com>> wrote:
>     List,
>     The function "spectopo" produces output in units of 10log_10
>     (uV^2/Hz). This is power spectral density, as opposed to a power
>     spectrum. Could anyone comment on the implications of
>     interpreting, physiologically, the PS vs. the PSD of a neural
>     signal? Thank you,
>     - Brian
>     Applied Brain and Cognitive Sciences Program
>     Drexel University
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