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
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
Tom Campbell, Ph.D.
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
More information about the eeglablist