[Eeglablist] questions about PSD, FFT and baseline corrections for my epochs.

[Madhura Lotlikar]

Greetings,
I have questions about PSD, FFT and baseline corrections for my epochs.

I am looking at the overall PSD after the stimulus is presented to analyze the time-on-task effect later.

I ran the following code:
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[STUDY, ALLEEG]= std_precomp(STUDY, ALLEEG, 'channels' ,'interp','on','recompute','on','erp','on','erpparams',{'rmbase',[-300 -1] }, 'spec' , 'on', 'specparams', {'specmode','psd','logtrials','off', 'winsize', 650, 'overlap', 325, 'nfft', 650})

EEG = pop_select( EEG, 'time' ,[0 1]);    %to only extract post-stimulus spectra which is baseline-corrected.

[STUDY specdata_F specfreqs_F pgroup pcond pinter] = std_specplot( STUDY, ALLEEG, 'channels', {'AF3', 'FP1', 'FP2' , 'AFz', 'AF7', 'AF4', 'AF8', 'F5', 'F3', 'F7', 'Fz', 'F2', 'F4',' F6', 'F8'}, ...'averagechan' , 'on', 'plotsubjects', 'off', 'plotconditions', 'together', 'freqrange', [0 30])

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650 is the total number of data points in 1.3 seconds of epoch, where 0.3s is baseline and 1sec is post-stimulus. srate= 500Hz

My questions are:
1) I don't see the correct units on the Y-axis. If I am not wrong they should be microV sq./ Hz. But I  still see microvolts square.
2) I see a peak in the delta band which I hadn't seen before when I ran FFT only and followed the same pop_select method as mentioned above. Why would that be? Is this cause by the artifact because I sub-epoched the 1.3s to 1 second and given that I used window overlapping for PSD, it caused this ? If that is true should I instead remove the baseline, then sub epoch and then pre-compute the PSD? What's the best way to go about this?
3) Even after running PSD with the abovementioned parameters, the freq resolution is the same and the plots are not smoother than FFT only.


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Figures below: on the left is the frontal cluster after PSD 3 groups
On the right is the same frontal cluster 3 conditions on separate panel.

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