[Eeglablist] Average re-reference

[Delorme, Arnaud]

Dear Chiara,

You do not need to re-reference the data before running ICA. Changing the reference will of course change the topographies you are observing. This is one limitation of working with channel data.

Best wishes,

Arno

> On Oct 20, 2020, at 10:35 PM, Chiara Terzo <Chiara.Terzo at iit.it> wrote:
> 
> Hi Joe,
> 
> 
> thank you for your reply. We are doing an ERP study on the Implicit Association Test (IAT), and specifically we look at N2 (cognitive conflict) and LPP (allocation of the attention towards motivationally relevant stimuli). Rereferencing does indeed affect ICA, but it does also affect the estimate of the evoked potentials (carried on the already cleaned data), especially affecting the late components. Indeed, when referencing to the right mastoid I get the LPP from 400 ms on. When referencing to the common average, the LPP disappears and I get instead a negative pattern from 300 ms on. The literature on ERPs and the IAT does not have a consistent reference, but on average they do not use the common average as a reference. Thus, what I am wondering is whether using the right mastoid as a reference would bias too much my ICA.
> 
> 
> Chiara
> 
> ________________________________
> From: Joseph Dien <jdien07 at me.com>
> Sent: 20 October 2020 22:40:33
> To: Chiara Terzo
> Cc: eeglablist at sccn.ucsd.edu
> Subject: Re: [Eeglablist] Average re-reference
> 
> Hi Chiara,
>   a bit more detail is needed to answer.  Rereferencing does indeed affect the output of ICA as it is reweighting things in the data but there isn’t a rule of thumb on this being good or bad.  I would say that it is best to use whatever reference you’re using in the rest of your analyses to keep consistent.  Could you provide more information on what you are doing and how you are interpreting the data?  I presume you are doing a spatial ICA on the averaged data?  Or not?
> 
> When you say the late potentials were negative, what are you looking at when you say this?  For example, the sign of the loadings (mixing matrix in ICA speak) is arbitrary and only relates to the sign of the original voltages once you multiply them by the activations to backproject the data.  This is because the same set of loadings account for both the time points where the voltages went negative and the ones where it went positive.  So it is only the product of the two that gives you the original sign for a given time point at a given channel.  So are you looking at the mixing matrix or are you looking at the back-projected data?
> 
> Anyway, more information needed before we can help you.
> 
> Cheers!
> 
> Joe
> 
> On Oct 20, 2020, at 09:23, Chiara Terzo <Chiara.Terzo at iit.it<mailto:Chiara.Terzo at iit.it>> wrote:
> 
> Dear all,
> 
> 
> I estimated evoked-potentials of an EEG data set collected with 32 channels. With the reference to the right mastoid I get a specific pattern of evoked that are consistent with the previous literature (i.e. positive late potentials around 400ms). However, if before doing ICA I re-reference my data to the common average (as it's highly recommended), I get a different pattern, such as late potentials  being negative. Which one of the two should then be reliable? Is it necessary to re-reference to average before doing ICA? And how much not doing so would bias my results?
> 
> 
> Thank you for your help,
> 
> 
> Chiara
> 
> 
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> --------------------------------------------------------------------------------
> 
> Joseph Dien, PhD
> Senior Research Scientist
> Department of Human Development and Quantitative Methodology
> University of Maryland, College Park
> E-mail: jdien07 at mac.com<mailto:jdien07 at mac.com>
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