[Eeglablist] Problem with Envtopo
nicole strang
nicole_strang at hotmail.com
Mon Jun 13 09:55:28 PDT 2005
Hi Everyone,
I have an ERP dataset, collected using the GSN128 montage, from 16 subjects
in an experiment with two conditions. We initially analyzed the data by
comparing the grand averages for each condition using Netstation tools. We
found focal differences using the bootstapping method and wanted to further
elucidate the scalp distribution of the generator(s) that accounted for
theses differences. To do we concatenated the single subject averages for
each condition (32 total) and imported them into EEGLAB. We also reduced the
number of electrodes from 128 to 67 to provide sufficient data for ICA,
which we did using runica. Once we conducted the ICA we created a new
dataset (comprised only of the control condition, 16 epochs) from the parent
dataset. We used the EEGLAB option 'component ERPS with component maps
(compare)' to subtract the control condition from the parent dataset and
display the 7 largest components in the timeframe from 300ms to 400ms.
post-stimulus (this is the time period where we for ERP condition
differences). We did this with the thought that the components accounting
for the greatest variance would account for condition differences. My first
questions is whether this is an appropriate method of analysis? Secondly,
according to the latest eeglab manual there sould be a printout of the
variance accounted for by each component, however this is not the case. Is
there any way I can determine how much variance is accounted for by each
component? I realze that when you select each topoplot you get rv-Is this
relative variance or residual variance? Finally, I have a general ICA
question. In my analyses I get rvs and pvafs (I used a previous version of
EEGLAB to get the printout of pvaf for each component, however in this
version I recieve an error when I try to subtract on dataset from the other)
that are negative. What does this mean?
Thank you very much for your help,
Nicole Strang
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