[Eeglablist] artifacts in time freq plots

[ondrej lassak]

Hey guys

Arnaud is right (thx for the code analysis) its just a numerical
artifact, which is impossible to occur on a real life EEG, so Makoto
you can chill :D

This time I added to the ideal sinusoid pseudo white noise and since
it adds non 0 variance the numerical artifact cannot occure and one
get sweet line in the TF plot even when using base subtraction

try this code to see it for yourself:

fs=2048%samples/s
t=linspace(0,8,8*2048);%samples
f=6%Hz


x=.05*sin(2*pi*f*t);
x1=.05*sin(2*pi*f*t*2);
x=[x(1:8*2048/2) x1(1:8*2048/2)];
x=x+.01*rand(1,length(t))


figure; [ersp,itc,powbase,times,freqs,erspboot,itcboot] = newtimef(x,
8*2048, [-500 8000], 2048, 0,'baseline',[NaN],'basenorm','off',
'maxfreq' ,19,'nfreqs',150,'padratio', 32, 'scale', 'abs');
set(gcf,'Name',('original signal no baseline adjustment'))

figure; [ersp,itc,powbase,times,freqs,erspboot,itcboot] = newtimef(x,
8*2048, [-500 8000], 2048, 0,'baseline',[5000 6000],'basenorm','off',
'maxfreq' ,19,'nfreqs',150,'padratio', 32, 'scale', 'abs');
set(gcf,'Name','baseline adjusted [5000 6000] ms')



figure; [ersp,itc,powbase,times,freqs,erspboot,itcboot] = newtimef(x,
8*2048, [-500 8000], 2048, 0,'baseline',[2000 5000],'basenorm','off',
'maxfreq' ,19,'nfreqs',150,'padratio', 32, 'scale', 'abs');
set(gcf,'Name','baseline adjusted [2000 5000] ms')





On Wed, May 12, 2010 at 4:46 AM, Makoto Miyakoshi
<mataothefifth at yahoo.co.jp> wrote:
> Dear Ondrej,
>
> I replicated your results. This is shocking actually.
> I also found that the problem is probably in baseline subtraction because
> when I set 'baseline', NaN, the result seems right.
>
> Makoto
>
>
> --- ondrej lassak <tehuberpro at gmail.com> wrote:
>
>> I fed the TF analysis single sinusoid 6 and 12Hz and the TF plot
>> shows
>> multiple specral lines (more than two).
>> How can one rely on the TF when it introduces such massive artifacts
>> both in
>> pure FFT spectrogram and Wavelet scalogram?
>> Or am I doing something wrong? When only one freq during the whole
>> time span
>> is present the TF plots look like really bad moira and the presence
>> of the
>> freq is apparent only from the summation over time (left from the
>> main
>> plot).
>>
>>
>> The matlab report with function calls and resulting pictures is
>> attached
>> below (no scripts embedded in the html).
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