[Eeglablist] Extract absolute power using ERSP

[Thang Le]

Hi everyone,

I have a few questions regarding absolute power which, admittedly, is a
topic that has been discussed at length on here. Nevertheless, I'm still
not able to wrap my head around a number of things. I would great
appreciate your comments or redirection to an earlier post in case I missed
anything in my research.

In my STUDY design, there are three conditions. Due to the concerns of
using different baselines when comparing these conditions, I have opted to
analyze each condition with its own prestimulus baseline separately. In
order to do this, my intention is to first extract the absolute power (i.e.
set 'baseline', NaN). To give you an idea of what I'm doing, the following
is how I precompute my data:

%ERSP with no baseline
[STUDY, ALLEEG] = std_checkset(STUDY, ALLEEG);
[STUDY ALLEEG] = std_precomp(STUDY, ALLEEG,
{},'allcomps','on','recompute','on','interp','on','ersp','on',
'erspparams', {'cycles' [3 .5] 'baseline',NaN, 'freqscale' 'linear'
'timesout' 200 'freqs' [8 16] 'nfreqs' 18},'itc','on');

After this, I compute the stats for a cluster of electrodes of interest:

%Compute Statistics for Cluster, alpha frequencies only:
STUDY = pop_erspparams(STUDY, 'condstats', 'on','freqrange',[8 12]);
[STUDY erspdata ersptimes pgroup pcond pinter] =
std_erspplot(STUDY,ALLEEG,'timerange', [-5100 1800], 'baseline', NaN,
'channels',{ 'PZ', 'OZ', 'POZ', 'P1', 'P2', 'O1', 'O2','PO3','PO4'}); I
don't think the 'baseline' NaN is needed here since the data have been
precomputed without the baseline.

To extract the ersp data (for the first condition), I use the following
calculations

%Take average of all frequencies:
AllSsErspData_cond1 = mean(erspdata{1},1);
%Take Average ERSP data for all electrodes in cluster
ClustErspData_cond1 = mean(AllSsErspData_cond1,3);
%Compute ERS/ERD (percent change)
ClustERD_cond1 = (10.^(ClustErspData_cond1/10)-1)*100;

My questions are:

1. Is this the right way to obtain absolute power? Should I even look at
ERSP? As I understand ERSP measures the changes in amplitude of a frequency
spectrum at a given point of time relative to a prestimulus baseline.
Without the baseline being specified as in my case, I'm unsure how ERSP is
calculated. There was a post (
http://sccn.ucsd.edu/pipermail/eeglablist/2011/004229.html) which deals
with this but I'm still a bit hazy on the explanation.

1a. This is related to the question above: I don't think I can compute the
percentage change of Pfurtscheller's ERS/ERD as I have here in the last
step above since there is no baseline. Is this correct?


2. Would I be better off looking at power spectra instead?


Thank you so much for your help.

Thang Le
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