[Eeglablist] how to select channels for back-projection of ERP

[Elizabeth Milne]

Thanks Makoto, that is very helpful. However, I'm not sure which inputs are
required.
Can you clarify what I need to enter for fig, try_strings and catch_strings.
Best wishes,
Elizabeth

On 3 August 2015 at 23:09, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:

> Dear Elizabeth,
>
> A projection from ICs to channels is a 'forward projection'. We've been
> using the word 'backprojection' in the wrong way, so I corrected it (at
> least in my functions).
>
> In ICA, the polarity of map and time-series signal can be flipped by
> applying -1 to the both. This is a fundamental uncertainty.
>
> > If I want to use this, or any other, IC to measure the amplitude /
> latency of particular ERP deflections it is necessary to know which is the
> correct channel to choose for back-projection. However, I am unsure as to
> how to make this decision objectively.
>
> I wrote pvaftopo() plugin for this purpose
> http://sccn.ucsd.edu/wiki/Plugin_list_process
> pvaf is a new concept Scott proposed nearly 20 years ago which is
>
> percent variance accounted for (pvaf) = 100-100*var(all_data -
> selected_ICs)/var(all_data)
>
> We'll publish IEEE EMBC conference proceedings soon for this year about
> the study-level envtopo and pvaf, so you may want to refer to the paper
> once it's published (Lee et al., IEEE EMBC conf proc 2015)
>
> Pick up the channel that is closest to the peak in the scalp maps and you
> can choose the 'most accounted' channel objectively.
>
> I'm happy that there is finally a user asking for this plugin!
>
> Makoto
>
> On Fri, Jul 31, 2015 at 2:13 PM, Elizabeth Milne <e.milne at sheffield.ac.uk>
> wrote:
>
>> Dear EEGLAB team,
>> I have a question about resolving the "true" polarity of IC ERPs by
>> back-projecting to channel data. Using erpimage, I have obtained the ERP of
>> an IC, but the polarity is different (orthogonal) when back-projecting to
>> different channels. In another example from a different IC, the polarity of
>> the ERP does not change, but the amplitude of the signal is different
>> depending on which channel is selected for back-projection.
>>
>> If I want to use this, or any other, IC to measure the amplitude /
>> latency of particular ERP deflections it is necessary to know which is the
>> correct channel to choose for back-projection. However, I am unsure as to
>> how to make this decision objectively. Are there particular features that I
>> should look at to make this decision more systematic, i.e. dipole
>> projection? % power accounted for?
>>
>> If anyone can offer advice, or point out errors in my understanding I'd
>> be very grateful.
>>
>> Many thanks,
>> Elizabeth
>>
>> --
>> Elizabeth Milne
>> Reader in Cognitive Neuroscience
>> University of Sheffield
>> Western Bank
>> Sheffield
>> S10 2TN
>>
>> +44 (0) 114 222 6558
>>
>> http://www.autismresearchlab.group.shef.ac.uk/
>>
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>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>



-- 
Elizabeth Milne
Reader in Cognitive Neuroscience
University of Sheffield
Western Bank
Sheffield
S10 2TN

+44 (0) 114 222 6558

http://www.autismresearchlab.group.shef.ac.uk/
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