# [Eeglablist] PSD for single subjects vs. STUDY function

Roxanne Sopp roxannesopp at gmail.com
Tue Oct 6 04:00:20 PDT 2015

```Dear all,

my question is somewhat related to this older post below:

I have precomputed power density in the STUDY function using the
parameters 'specmode',[psd] in order to use the spectopo function.

Now i am facing two problems:

1. The plot does not seem to be scaled in dB (as stated below), from
the script i gather that these must be the absolute power values in
µv²/Hz.. is that the case?

2. When I additionally use 'logtrials','on' as paramters the power
values seem to be scaled in dB however they still do not correspond to
the values that i calculated in the single subject mode using the
spectotopo function.

What could be the difference? And which one is the valid calculation?

Thanks a lot in advance for any hints and suggestions!

Best regards,

Roxanne

______________________________________________________________________________

Dear Marco,

if you use no option to compute spectrum, then the units are power
density (10*log(microvolt^2)/Hz) because the STUDY function uses the
spectopo function. If you use 'specmode', 'fft', when you computed the
spectrum then the unit is power values (10*log(microvolt^2)). These
two are proportional but I agree that they should be made consistent.
We have entered a bug report.

Best regards,

Arno

On Sep 19, 2011, at 6:33 AM, Marco Montalto wrote:

>* Dear all,
*> >* I have created a STUDY and pre-computed measures for power
spectra using the default spectopo parameters ("fft"). My question is
the following: the numerical values I extract using this code (example
is with channel Fpz):
*> >* [ STUDY specdata specfreqs] = std_specplot(STUDY, ALLEEG,
'channels', { 'Fpz' });
*> >* are power values or voltage or amplitude etc.?
*> >* If they are not power values can someone explain to me how I can
extract power values as I need these to calculate ERD/ERS?
*> >* Thanks and regards,
*>* Marco*
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