[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
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