[Eeglablist] ICs with identical topographies

[Maximilien Chaumon]

Hi Guillaume,

Well, it makes sense to my intuitive understanding...
The two components don't cancel each other, but then sum up, more or less.
Does that mean that I could somehow treat them as "one" component?
I get your point with this superclean dataset example. And it also makes
sense with the other answers I got, with the rank of my data being not
exactly as high as I expect it to be. Now I need to understand why/how some
of my channels turn out to be linearly related...

Thanks,
Mx

2011/8/24 Guillaume Rousselet <Guillaume.Rousselet at psy.gla.ac.uk>

> Hey Max,
>
> your components don't cancel one another. In your example, the topographies
> and the time courses have opposite signs, so if you multiply one by the
> other, your two components are essentially the same.
> ICA returns maximally independent components, and therefore can still be
> correlated. The correlation can be quantified using the mutual information
> plugin, which I use to confirm my own judgement about component similarity
> when I look for eye blink ICs.
> Imagine you have an absolutely clean signal, with only highly reliable
> evoked activity and no noise at all. In that case you would expect to get n
> times your number of electrodes the same component. In my experience,
> cleaner datasets tend to have more correlated - almost identical - ICs.
> Makes sense?
>
> Best,
>
> Guillaume
>
>
> ************************************************************************************
> Guillaume A. Rousselet, Ph.D., senior lecturer & deputy post-graduate
> convenor
>
> Centre for Cognitive Neuroimaging (CCNi)
> Institute of Neuroscience and Psychology
> College of Medical, Veterinary and Life Sciences
> University of Glasgow
> 58 Hillhead Street
> G12 8QB
>
> http://www.psy.gla.ac.uk/staff/index.php?id=GAR01
>
> Email: Guillaume.Rousselet at glasgow.ac.uk
> Fax. +44 (0)141 330 4606
> Tel. +44 (0)141 330 6652
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>
> The University of Glasgow, charity number SC004401
>
> ************************************************************************************
>
> On 23 Aug 2011, at 15:24, Maximilien Chaumon wrote:
>
> Hi eeglabbers,
>
> I sometimes get ICs with extremely similar topographies and time courses,
> like on this slide<http://oszilla.hgs.hu-berlin.de/public/Similar_ICs.PNG>
> .
> I know that ICA returns independent components.
> Does that not mean that they should not look the same?
> I know the components are independent in a statistical sense, which is not
> the same as uncorrelated, but still. I'm a bit surprised. What do these two
> components mean if they cancel one another? well, do they?
>
> Sorry if my question is naive, but what is happening?
>
> The data is here <http://oszilla.hgs.hu-berlin.de/public/Similar_ICs.mat>.
>
> Best,
> Max
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