[FieldTrip] log transform of power values

Frederic Roux f.roux at bcbl.eu
Mon Dec 3 16:58:13 CET 2012

Dear all,

I am planning to run freq_statistics on single trial data,
with the power data computed in the following way:

cfg = [];
cfg.method = 'sincos';
etc ...
planar_data = ft_meplanar(cfg,raw_data);

cfg = [];
cfg.foi = 30:150;
cfg.output = 'pow';
cfg.taper = 'dpss';
etc ...

freq_planar = ft_freqanalysis(cfg,planar_data);

freq = ft_combineplanar([],freq_planar);

When I then look at the topographies 

cfg = [];
cfg.grad = freq.grad;
cfg.layout = ft_perpare_layout(cfg);
cfg.xlim = [.1 .6];
cfg.ylim = [40 60];
cfg.parameter = 'powspctrm';


I noticed that it makes a huge difference if the data are log transformed
before the plotting, ie 

freq.powspctrm = 20*log10(freq.powspctrm);

The data looks more 'clustered' when I do the log transform, whereas the 'raw' 
power values look scattered and like tiny islands.

Now I am wondering if this could help with the statistical analysis, meaning
that it would give me more power to detect spatio-temporal clusters.
(Baseline correction is not really a good idea, as there seems to be important
stuff going on in the baseline)

Any (quick) help and advice would be highly appreciated.


Frédéric Roux
Postdoctoral Researcher

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