[FieldTrip] Oscillatory power normalization

Juan Kamienkowski jkamienk at gmail.com
Thu Jun 6 17:49:27 CEST 2013

Hi everybody,

More than one year later we come up with the same questions. Does anybody
have suggestions on this topic?

Thanks a lot!



On Fri, Mar 9, 2012 at 4:16 PM, Matt Mollison <matt.mollison at gmail.com>wrote:

> My questions essentially boil down to: what do people do for power
> normalization when assessing statistical differences?
> It gets more detailed below regarding examining event-related power
> changes relative to a baseline (within-subjects, comparing two conditions,
> stimulus onset = 0 ms). I didn't find much discussion of this on the list
> or the wiki. Any references for these issues would also be appreciated.
> (1) Does power data need to be baseline normalized for statistical tests
> comparing conditions? Normalization would put power on equal footing across
> all subjects, conditions, sensors, times, frequencies, etc., but it will
> surely affect power during the stimulus period in a particular way. If so,
> do the two (or more) conditions need to use the same baseline condition, or
> can each trial be normalized to its own pre-stim baseline period (a la
> ft_freqbaseline)? For either, it seems like you'd always need
> keeptrials='yes' in ft_freqanalysis. However, it does not seem to get
> normalized in the cluster_permutation_freq tutorial (within-subjects)---am
> I missing something?
> If we should normalize:
> (2) I've read a number of papers that Z-transform stimulus period power
> relative to pre-stim activity (subtract mean, divide by std) before doing
> statistics. I've also read a lot that don't mention baselines, or e.g. do a
> decibel [dB] transform. ft_freqbaseline does not have a Z-transform option.
> There is ft_preproc_standardize, but this seems to operate at a lower level
> than is usually recommended. Z-transforming seems like a good option, but
> how can I use it in the FT pipeline for within-subjects analyses
> (especially with keeptrials='no')? Alternatively, when should one use
> 'absolute', 'relative', or 'relchange'?
> Regarding choosing the baseline period:
> (3) It seems that the baseline period needs to precede stimulus onset by a
> sufficient amount of time so that the stimulus period doesn't bleed into
> the baseline; this time would be specific to both the frequency and either
> wavelet width or taper window length. For example, at 4 Hz with wavelet
> width=6 or a taper with 6 cycles per time window (t_ftimwin) the
> wavelet/window would be 1500 ms long, and the end of the baseline must
> precede stimulus onset by at least half this to keep them separate. At
> lower frequencies this could get quite unruly (e.g., 1 Hz would require
> ending 3000 ms before stimulus). Is this correct? Maybe that's why it's
> better to have a single separate baseline condition. Anyway, the
> timefrequencyanalysis tutorial seems to disregard this separation of
> baseline and stimulus activity (as have many papers I've read), so maybe
> I'm wrong about this being necessary.
> Thanks for your time,
> Matt Mollison
> --
> Univ. of Colorado at Boulder
> Dept. of Psychology and Neuroscience
> matthew.mollison at colorado.edu
> http://psych.colorado.edu/~mollison/
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Juan E Kamienkowski
Laboratorio de Neurociencia Integrativa
Departamento de Fisica, FCEN-UBA
Ciudad Universitaria, Pabellon I
(1428) Buenos Aires, Argentina
Phone: (54-11) 4576 3300 (282)
Fax: (54-11) 4576 3357
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