[Eeglablist] averaging

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


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 

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 

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 
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

Phone: +358-9 191 23256
Telefax: +358-9 191 22924
Mobile: +358-4 154 09764 87
E-mail: tom.campbell at helsinki.fi 

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

More information about the eeglablist mailing list