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

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Mon Aug 3 15:09:02 PDT 2015


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
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