[Eeglablist] Are the results more significant on the scalp or inside brain?

Michal Vavrecka vavrecka at fel.cvut.cz
Tue May 13 14:26:08 PDT 2014

Hi Cyril,

On 13.5.2014 21:44, Dr Cyril Pernet wrote:
> Hi Makoto & Michal,
> I agree with Makoto about the ICA subspace which can be quite 
> different - there is however another thing to consider
> You said ' Should the effect be stronger (in terms of more 
> statistically significant electrodes (dipoles) and timeperiods) on 
> scalp electrodes or in DIPFIT clusters?'
> the problem here is that statistically significant is an estimate 
> under H0, so beside the hypothesis test, you cannot tell if the effect 
> is stronger or weaker in one case or the other because a p value tells 
> nothing about H1 -- to do that you need to look at the actual effect 
> size (like what is the mean uV difference between conditions) and not 
> base your judgment the (correted) p values.
It is correct from the statistical point of view. I was looking for some 
"common sense" interpretation, but I know that there are some 
limitations of direct comparison

> You could also test if the effects are different using a test for 
> apparied measures  (eg. a paired t-test between (condition A - 
> condition B) on one compoment vs (A -B) on one channel).
Thanks for advice


> Cyril
> --------------------------------
> Dear Michal,
> That's a simple but deep question.
> Theoretically the difference between condition can't be smaller in ICA 
> recults since canceling happens in the mixing process and not the 
> other way around (like the law of entropy?)
> However, I believe a major problem in comparing channels with ICs is 
> component selection. The question is how you guarantee that the ICs 
> you choose is a right representative (projecting source) to the 
> channel? What if some subject don't have such ICs? What if some 
> subjects have multiple of such ICs (subspace)?
> One way to investigate this problem is run pvaf analysis (you have 
> pvaftopo under EEGLAB plugin manager)
> I have an experience of computing the pvaf analysis across subjects 
> per cluster (unpublished data), and the result showed very large 
> standard deviations... it was like mean 30% and SD=30, range 5-80. 
> This means a cluster can explain a channel activity (in my result, of 
> course) only by 30%, and there are huge inter-subject variance.
> This being said, I think it is still ok to stay optimistic and take 
> the theoretical conclusion. You haven't observed horrendously 
> contradicting results, have you?
> Makoto
> 2014-05-12 14:02 GMT-07:00 Michal Vavrecka <vavrecka at fel.cvut.cz>:
> Hello,
> I do have few simple questions and I am curious about your intuitions 
> and arguments:
> I am finishing the paper where I did group analysis of two cognitive 
> states. I visualized both scalp maps and dipoles and their statistical 
> tests. Both visualization are based on fieldtrip monte carlo 
> permutation with cluster based statistics (correction for multiple 
> comparison). I would like to interpret the difference between results 
> on the scalp and inside the brain (DIPFIT). What are your intuitions:
> Should the effect be stronger (in terms of more statistically 
> significant electrodes (dipoles) and timeperiods) on scalp electrodes 
> or in DIPFIT clusters?
> How to interpret the stronger effect on the  scalp?
> Does the ICA and DIPFIT calculation somehow weaken the ERSP difference?
> My intuition is opposite as the source reconstruction has to clean the 
> noise and strengthen the effect that should result in more 
> statistically significant timeperiods in the spectrograms compared to 
> scalp data?
> Is there any paper that compares these two approaches?
> Thanks for your answers.
> Michal
> --
> Dr Cyril Pernet,
> Academic Fellow
> Brain Research Imaging Center
> Neuroimaging Sciences
> University of Edinburgh
> Western General Hospital
> Division of Clinical Neurosciences
> Crewe Road
> Edinburgh
> EH4 2XU
> Scotland, UK
> cyril.pernet at ed.ac.uk
> tel: +44(0)1315373661
> http://www.sinapse.ac.uk/
> http://www.sbirc.ed.ac.uk/cyril
> --
> The University of Edinburgh is a charitable body, registered in
> Scotland, with registration number SC005336.
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Michal Vavrecka
assistant professor
Biodat Research Group
Incognite Research Unit
Karlovo nam. 13
Prague 2
phone: +420224357609
cell: +420608661977
personal: http://bio.felk.cvut.cz/~vavrecka/
groups: http://incognite.felk.cvut.cz/

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