[Mne_analysis] About the units of the PSD

[Brunner, Clemens (clemens.brunner@uni-graz.at)]

Alejandro,

you are right that the unit of the PSD is V**2/Hz (or uV**2/Hz in the case of EEG). If dB=True, the unit should be dB without anything else in parentheses. Since MNE uses scipy.signal.spectrogram, my guess is that the labels are wrong. The ASD has units of V/sqrt(Hz) because the ASD is the square root of the PSD, so if MNE is really calculating the ASD this should be documented or changed.

I've created a new issue: https://github.com/mne-tools/mne-python/issues/4642

Clemens


> On 04.10.2017, at 02:31, Alejandro Weinstein <alejandro.weinstein at gmail.com> wrote:
> 
> Hi:
> 
> I have some questions regarding the units used in the PSD plot of raw signals.
> 
> As a reference for definitions related to the PSD I'm using this
> document: https://holometer.fnal.gov/GH_FFT.pdf . In particular, see
> Table 1 in page 7, and Eq. (14) in page 10.
> 
> Looking at the code, I see that all the calls to the spectrogram
> function are made with the scaling parameter set to "density" (that's
> the default value). According to the documentation, the description of
> the parameter is:
> 
> "Selects between computing the power spectral density (‘density’)
> where Sxx has units of V**2/Hz and computing the power spectrum
> (‘spectrum’) where Sxx has units of V**2, if x is measured in V and fs
> is measured in Hz. "
> 
> My understanding is that this implies that MNE always compute the
> power spectral density, with units uV**2/Hz (I'm focusing on EEG,
> which is scaled to uV).
> 
> On the other hand, the dB parameter of the raw.plot_psd function is
> documented as:
> 
> "If True, transform data to decibels. If False, plot amplitudes."
> 
> My questions is:
> 
> - When using dB=False, shouldn't the units used in the PSD plot be
> uV**2/Hz, instead of uV/sqrt{Hz} ?
> 
> Also related, I think that saying that dB=False corresponds to
> plotting the amplitudes is not correct, since that would require to
> take the square root of the PSD, which isn't the case.
> 
> Alejandro
> 
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