[Eeglablist] Electrodes Values

[Arnaud Delorme]

Hi Stefan,

you are the one that wrote the original comment on GFP on the http://sccn.ucsd.edu/pipermail/eeglablist/2008/002132.html
Do you agree with Jeff that a N normalization is preferable to N-1?
Thanks,

Arno

On 21 May 2013, at 21:58, Barry Oken wrote:

> For Andreas, the rationale for N-1 in much of statistics is that it gives a more unbiased estimate of the parameter in the whole population. If one really just wants to know the parameter in one particular sample, be it a single EEG or a single group of subjects without thinking about the population that it was sampled  from, then, as Jeff commented, N is correct.
> Barry Oken
> 
> 
> -----Original Message-----
> From: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Eriksen, Jeff :LPH EEG
> Sent: Tuesday, May 21, 2013 12:44 PM
> To: Andreas Widmann; Arnaud Delorme
> Cc: eeglablist at sccn.ucsd.edu
> Subject: Re: [Eeglablist] Electrodes Values
> 
> All:
> 
> I believe we should use N, not N-1. This is a physical quantity, not a statistical estimate of some population parameter. If I want the RMS of a time series, I use every time sample and divide by the total number of time samples. If I want spatial global field power, I want to use all N spatial samples as well.
> 
> -Jeff Eriksen
> 
> -----Original Message-----
> From: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Andreas Widmann
> Sent: Tuesday, May 21, 2013 7:19 AM
> To: Arnaud Delorme
> Cc: eeglablist at sccn.ucsd.edu
> Subject: Re: [Eeglablist] Electrodes Values
> 
> Dear Arno,
> 
> your notion of "small normalization difference" made me curious where this definition of GFP might originate. Where is it defined with "N - 1 normalization"? Also in ERPLAB GFP appears to be implemented according to this definition.
> 
> However, in the original paper by Lehmann and Skrandies (1980) it is either defined as "root of the mean of the squared potential differences between all possible electrode pairs" (formula 1A) gfp = sqrt(1 / (2 * 32) * sum(sum((repmat(tmp(:, 1), [1 32]) - repmat(tmp(:, 1), [1 32])') .^ 2))) (similar to implementation in eeg_gfp.m) or as root mean squared voltage of the average referenced data (formula 1B), with gfp_1A = gfp_1B * sqrt(nChans).
> 
> Also some other papers which explicitly define the term use the latter definition (e.g., Brandeis et al., 1992; Roth et al. 1995; Murray et al., 2008; Koenig et al., 2009), that is, normalize by N not N - 1. Shouldn't we possibly better use gfp = std(tmp, 1) % GFP normalized by N to compute GFP?
> 
> What is the rationale behind N - 1 normalization? Any ideas?
> Thank you! Best,
> Andreas
> 
> Am 18.05.2013 um 19:21 schrieb Arnaud Delorme <arno at ucsd.edu>:
> 
>> Dear Kleber,
>> 
>> one way to get a global measure proportional to microvolt is to use the Global Field Power (GFP).
>> 
>> http://sccn.ucsd.edu/pipermail/eeglablist/2008/002132.html
>> 
>> The root mean square (RMS) value is also sometimes used.
>> The root mean square value on average reference data is similar to the GFP (with small normalization difference).
>> 
>> tmp = rand(32,10); % simulated data 32 channels 10 points
>> std(tmp) % GFP
>> sqrt(mean(tmp.^2)) % RMS
>> sqrt(sum((tmp-repmat(mean(tmp), [size(tmp,1) 1])).^2)/(size(tmp,1)-1)) 
>> % RMS on average reference with n-1 normalization (same as GFP)
>> 
>> I am not sure RMS on a common reference should be used. Any other comments on this appreciated.
>> Thanks,
>> 
>> Arno
>> 
>> On 16 May 2013, at 15:15, Kleber de Aguiar wrote:
>> 
>>> Hello Everyone in this list,
>>> 
>>> I would like to know if anyone of you can answer me a doubt about de 
>>> reading of the values of a group of electrodes in a EEG exam, as I"ll explain bellow:
>>> 
>>> 
>>> 1- Spliting the EEG recording in slots of time, that I will call them 
>>> as "samples";
>>> 
>>> 2- Assuming that "n" electrodes were used in this EEG Recording;
>>> 
>>> 3 - In every "sample" of the recording, there will have "n" 
>>> electrodes values corresponding for each electrodes;
>>> 
>>> MY QUESTIONS:
>>> 
>>> * How can I sum this electrodes values of the sample in a single 
>>> value that represents  the total of microvolts oh that "sample"?
>>> 
>>> * How can I describe the values of "y" samples in a single value that 
>>> represents these "y" sample values?
>>> 
>>> 
>>> Best regards,
>>> 
>>> Kleber, a fresh researcher in this field.
>>> 
>>> 
>>> 
>>> --
>>> Programa de Engenharia de Sistemas e Computação PESC/COPPE/UFRJ 
>>> Visite nossa página web (http://www.cos.ufrj.br)
>>>      ------***------
>>> Kleber de Aguiar
>>> Tecnólogo em Sistemas de Computação
>>> Mestrando em Engenharia de Sistemas e Computação - PESC/COPPE/UFRJ 
>>> Tutor Presencial CEDERJ/UAB - Curso de Computação UFF
>>> Twitter: @kleberIAguiar
>>> 
>>> 
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