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

Dr Cyril Pernet cyril.pernet at ed.ac.uk
Tue May 13 12:44:01 PDT 2014

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. 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).


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I have an experience of computing the pvaf analysis  across subjects per cluster (unpublished data), and the result showed  very large standard deviations...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? 

  2014-05-12 14:02 GMT-07:00 Michal Vavrecka <vavrecka at fel.cvut.cz>:

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.


Dr Cyril Pernet,
Academic Fellow
Brain Research Imaging Center
Neuroimaging Sciences
University of Edinburgh

Western General Hospital
Division of Clinical Neurosciences
Crewe Road
Scotland, UK

cyril.pernet at ed.ac.uk
tel: +44(0)1315373661

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
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