[Eeglablist] averaging

Campbell, Tom tom.campbell at helsinki.fi
Tue Feb 15 08:00:32 PST 2005


Hi,

Looking through the tutorials, I feel somewhat puzzled as to how EEGlab 
does its averaging for simple ERPs.

Consider a situation where I have 2 blocks. There is only one type of 
trigger event.

So, I can get an ERP average for each block with Plot->Channel ERPs-> In 
rect.array (launching the function pop_plotdata which outputs an ERP). 
This would correspond to something like TYPERNG 1 1; WAVEFORM in SCAN 3 
EDIT. 

The advice in the tutorial (I.7.2. Computing Grand Mean ERPs) would then 
seem to suggest to average these ERPs across blocks using  Plot -> 
Sum/Compare ERPs (that lauches the pop_comperp()). This would correspond 
to something like INDIVIDUAL in SCAN 3 STATS. In turn, when the individual 
average across all blocks for a each volunteer in this way, I would take 
all individual averages and use the same Plot -> Sum/Compare ERPs to get 
the grand average. This would correspond to something like GROUP in SCAN 3 
STATS.

My concern is that for a given participant, there may be less artefact-
free trials in Block 1 than Block 2 and this should be weighted 
accordingly so that Block 2 contributes more to the individual average. 
The grand average I would imagine to be a straightforward average of all 
the individual averages in the group and should be not weighted. 

I have a feeling of having missed the point, so please your advice on how 
to use EEGlab to compute individual and grand averages would be 
appreciated.
-- 
Parhain terveisin,

Tom Campbell, Ph.D.

Room 137
Helsinki Collegium for Advanced Studies
University of Helsinki, Finland
P.O.Box 4 (Fabianinkatu 24)
FIN-00014 University of Helsinki
Finland

Phone: +358-9 191 23256
Telefax: +358-9 191 22924
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E-mail: tom.campbell at helsinki.fi 

URL: http://www.mv.helsinki.fi/home/campbell/



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