[FieldTrip] Problem with TFR calculation using multitaper

Andreas Sauer sauer.mpih at googlemail.com
Fri Nov 16 14:25:50 CET 2012


Dear all,

I have encountered a problem when trying to calculate the power spectrum of
my data and I was wondering if anyone could help me to determine the origin
of this problem.

For the analysis I used multitapers (dpss) with a 11 Hz freq-smoothing and
a time window with a length of 500 ms ending up with 10 tapers for the TFR
calculation (the freq range is 20 - 200 Hz):

cfg   = [];
cfg.output      = 'pow';
cfg.channel     = 'MEG';
cfg.method      = 'mtmconvol';
cfg.taper       = 'dpss';
cfg.keeptrials  = 'no';
cfg.foi         = [20:1:200];
cfg.toi         = [-0.5:0.05:1.0]; % the preprocessed data length is 2 sec,
from -0.8 to 1.2 s
cfg.tapsmofrq   = ones(length(cfg.foi),1).*11; % spectral smoothing = +/-
11 Hz
cfg.t_ftimwin   = ones(length(cfg.foi),1).*0.5; % length of time window =
500 ms
cfg.pad         = 'maxperlen';

TFR_Mult_DPSS_10tapers_fmin20Hz_fmax200Hz = ft_freqanalysis(cfg,DataOut);

After calculation I did a baseline normalization:

cfg = [];
cfg.baseline = [-0.4 -0.1];
cfg.baselinetype = 'relchange';

TFR_HighGamma{subject,condition} =
ft_freqbaseline(cfg,TFR_file.TFR_Mult_DPSS_10tapers_fmin20Hz_fmax200Hz);

averaged:

cfg         = [];
cfg.channel = 'ParOcc'; % parietal and occipital channels
GA_TFR_HighGamma{condition} =
ft_freqgrandaverage(cfg,TFR_HighGamma{:,condition});

and finally plotted the spectrum:

 time(1) = 3; % -400 ms
 time(2) = 27; % 800 ms
 indFreq = [20 200];
 freq = find(ismember(GA_TFR_HighGamma{i}.freq, indFreq) == 1);

 power = squeeze(mean(GA_TFR_HighGamma{i}.powspctrm,1));
 power = mean(GA_TFR_HighGamma{i}.powspctrm(:,:,time(1):time(2)),1);
 power = mean(power,1);

 fig = figure;
 set(fig,'PaperUnits','centimeters');
 set(fig,'Units','centimeters');
 set(fig,'PaperSize',[15 15]);
 set(fig,'PaperPosition',[0 0 14 14]);
 set(gcf,'Color',[1 1 1]);

 ha = gca;
 pcolor(GA_TFR_HighGamma{i}.time(time(1):time(2)),GA_TFR_HighGamma{i}.freq,squeeze(power));
 set(ha,'FontName','Arial','FontSize',10,'LineWidth',1.5);
 shading interp;
 xlim([-0.4 0.8]);
 ylim([40 120]);
 caxis([-.2 .2]);
 hold on;
 ha = gca;
 plot([0 0],[40 120],'k--','LineWidth',1);
 set(ha,'FontName','Arial','FontSize',10,'LineWidth',1.5);
 set(ha,'YTick',[40 60 80 100 120]);
 colorbar('SouthOutside');
 xlabel('Time [sec]');
 ylabel('Frequency [Hz]');

The resulting plot looks strange to me since there is this artifical "cut"
at 70 Hz and at 50 Hz. I checked the FAQs but my plot looks different from
the "strange" ones described there.
I also attached a screenshot of the MultiplotTFR and there you can see the
strange "effect" clearly in channels MLO22 or MRO22 for example.

Is something wrong with my TFR calculation? Or does the problem arise from
an earlier step in the preprocessing?

Any help or advice would be greatly appreciated!

Best,

Andreas

-- 
Andreas Sauer
Max Planck Institute for Brain Research
Deutschordenstra├če 46
60528 Frankfurt am Main
Germany

T: +49 69 96769 278
F: +49 69 96769 327
Email: sauer.mpih at gmail.com
www.brain.mpg.de
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