[Mne_analysis] zero_mean parameter in induced_power

[Alexandre Gramfort]

hi Laetitia,

> I'm wondering, when calculating time-frequency data, what the parameter
> zero_mean means, and why the default parameter is False. To me, a zero mean

zero_mean means that the Gabor atom is corrected to be zero mean as a
proper wavelet.

it's the \kappa_\sigma in the first equation in:

http://en.wikipedia.org/wiki/Morlet_wavelet

it is False by default for historical default. When I implemented it I
copied the code from brainstorm and spm which at that time (maybe
still) does not correct with the \kappa_\sigma. It is not really a
problem in practice if you use more than 4 or 5 cycles but if you use
less that the zero mean correction starts to make a difference.

hope this helps,
Alex

PS : it will deserve a proper announcement but the new MNE v0.8 is
about to be released and the induced_power function is deprecated.
See:
http://martinos.org/mne/stable/auto_examples/time_frequency/plot_time_frequency_sensors.html
for an example of TFR in sensor space with the new tfr_morlet
function. Now zero_mean is set to true by default :)

> is a by-definition feature of a Morlet wavelet so I don't understand why we
> can choose to have non zero mean. Does this parameter change a lot the
> time-frequency pattern?
>
> Thanks a lot,
> Best
> Laetitia G.
>
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