[FieldTrip] Smoothing before permutation test

Ramirez Pasos, Ramirez_U at ukw.de
Wed Nov 2 18:02:00 CET 2016

Thank you David for your response! Just to be clear: by "smoothing" I'm not referring to the smoothing performed when using the ft_frequencyanalysis multitaper method, but some kind of spatial smoothing on the time-frequency matrix obtained via ft_frequencyanalysis - so that according to your suggestion, I would smooth each trial, then average trials for each subject/condition, and finally use these for statistical evaluation?  Is there a reason why smoothing each trial might be preferable to smoothing each subject's trial average?

Thank you so much in advance,
Von: fieldtrip-bounces at science.ru.nl [fieldtrip-bounces at science.ru.nl]" im Auftrag von "David Groppe [david.m.groppe at gmail.com]
Gesendet: Mittwoch, 2. November 2016 14:43
An: FieldTrip discussion list
Betreff: Re: [FieldTrip] Smoothing before permutation test

Since permutation tests exploit correlations between variables to increase sensitivity, smoothing each trial will increase your sensitivity.

On Wed, Nov 2, 2016 at 6:41 AM, Ramirez Pasos, <Ramirez_U at ukw.de<mailto:Ramirez_U at ukw.de>> wrote:
Dear Fieldtrippers,

I have a set of subcortical LFP signals from 8 patients, which I have analyzed with fieldtrip in order to obtain event-related time-frequency plots. Unfortunately, I have only three repetitions for each of my 4 conditions, so there's a lot of noise in the subject-averages of time-frequency data. I'm interested in testing baseline vs activation for each condition as well as comparisons between my 4 conditions (A1A2, A1B1, B1B2, A2B2).

Could anyone tell me their take on smoothing before permuting? Is that a valid procedure with such a small sample size? I've searched for literature discussing smoothing before permuting (mostly the Holmes papers), where much talk of smoothing refers to concepts such as "locally pooled variance" and "pseudo t-statistics," but I don't know how this fits with fieldtrip's cluster statistics functions.

Any thoughts would be greatly appreciated!

U. Ramirez
University of Wuerzburg

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