[Eeglablist] how to select channels for back-projection of ERP
e.milne at sheffield.ac.uk
Tue Aug 4 02:44:41 PDT 2015
Thanks Makoto, that is very helpful. However, I'm not sure which inputs are
Can you clarify what I need to enter for fig, try_strings and catch_strings.
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
> pvaf is a new concept Scott proposed nearly 20 years ago which is
> percent variance accounted for (pvaf) = 100-100*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!
> On Fri, Jul 31, 2015 at 2:13 PM, Elizabeth Milne <e.milne at sheffield.ac.uk>
>> 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 Milne
>> Reader in Cognitive Neuroscience
>> University of Sheffield
>> Western Bank
>> S10 2TN
>> +44 (0) 114 222 6558
>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
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> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
Reader in Cognitive Neuroscience
University of Sheffield
+44 (0) 114 222 6558
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