[Eeglablist] About ERSP averaging and normalisation

[Scott Makeig]

Kazuhiro Shishida wrote:

>Dear Marco,
>
>Marco Buiatti <marco.buiatti at univ-paris5.fr> wrote:
>
>  
>
>>2) The non-linearity of the log causes averages over electrodes/subjects 
>>to be biased: don't you think this can be misleading?
>>    
>>
>
>The average of logarithmic value is the radical of products, for example,
>when A is a 1 x n vector,
>mean((log10(A)),2)=log10(prod(A,2)^(1/n)).
>Of course, it's different from log10(mean(A,2)).
>So, it is a problem which is physiologically better. I think intuitively that the former one is better, but also eager to know the answer about it.
>
>  
>
If the process creating the 'spectral perturbation' is multiplicative 
(acting as a gain control), then using log power measures the amount of 
'gain change'. If the process is additive, then log power may not be a 
useful concept - however, plotting linear power changes tends to 
discount decreases (here modeled as 'subtractions').

Scott Makeig