[Mne_analysis] About the units of the PSD

[Alejandro Weinstein]

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