[Eeglablist] Are the results more significant on the scalp or inside brain?
vavrecka at fel.cvut.cz
Tue May 13 14:26:08 PDT 2014
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
> 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 per cluster (unpublished data), and the result showed very large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
> EH4 2XU
> 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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