[Mne_analysis] Visualizing power spectrum density of sources on the brain

[Gladia Hotan]

Hi,

I have a couple of questions about the psd of sources:

1) I used

stc = mne.minimum_norm.compute_source_psd

to get an stc file containing the psd of the sources. It seems like in
this stc file,
frequency has replaced time, but is still called 'time':

<SourceEstimate  |  ... tmin : 0.0 (ms), tmax : 55.0 (ms), tstep :
0.25 (ms) ...>

(I set the function to compute from 0 to 55 Hz; the time duration of the data is
actually a lot longer). I can use stc.plot or
mne.plot_source_estimates to visualize this, but it treats
the frequencies like times. How can I plot the psd at a particular
frequency on the brain?

2) What are the units of the psd of the sources? According to the
tutorial <http://martinos.org/mne/stable/auto_examples/time_frequency/plot_source_power_spectrum.html#sphx-glr-auto-examples-time-frequency-plot-source-power-spectrum-py>
the psd is in dB,
but what is the reference quantity being used when converting to dB?
(Ie. what is Pref in P_db = 10*log10(P/Pref)?)
Am I right that the source amplitudes are in units of ampere-metres?

Thanks and Best,
Gladia
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