[FieldTrip] Calculating ERPs with tf_timelockbaseline

[Luca Moretti]

Dear Julian,

I have tried to follow your useful suggestion: I have plot ERP and Color_C
together. What I have found is that they generally don't overlap: however,
the waveforms are still identical. The only difference is that the baseline
corrected waveform starts (and of course ends) at more positive values.

Now I am confused: Is this what baseline correction should do?

Best,
Luca

Il giorno mar 9 apr 2019 alle ore 12:57 Julian Keil <julian.keil at gmail.com>
ha scritto:

> Dear Luca,
>
> have you plotted both datasets in one figure?
>
> e.g.
>
> figure
> cfg = [];
> cfg.layout = layout;
> cfg.interactive = 'yes';
> cfg.showoutline = 'yes';
> ft_multiplotER(cfg, ERP, Color_C);
>
> This way, you’ll see whether the two ERP traces actually overlap.
> However, if the average of your baseline is indeed „close to zero“ as you
> state, then the baselinecorrection will not have a huge impact. In other
> words, if there is nothing to subtract, the ERP will not change.
>
> Best,
>
> Julian
>
>
> Am 09.04.2019 um 11:55 schrieb Luca Moretti <moretti.l1991 at gmail.com>:
>
> Dear Julian,
>
> I mean the second one: the signal does not change from Color_C to ERP.
> Inspecting Color_C as you suggested, it looks like there is a difference
> with ERP. However, ft_multiplotER seems to fail in plotting the ERP data.
> At first I also thought that maybe I simply couldn't see the differences
> because the avarage was close to 0 in my baseline, but I tend to exclude
> this case as the figures are Always identical even when specifying
> different baseline periods, when plotting different electrodes, and when
> analyzing different subjects.
>
> Thank you again,
> Luca
>
> Il giorno mar 9 apr 2019 alle ore 10:28 Julian Keil <julian.keil at gmail.com>
> ha scritto:
>
>> Dear Luca,
>>
>> could you be a bit more precise in your description?
>> By saying that "figure Always turn out the same for each electrode“, do
>> you mean that the ERP is identical at each electrode?
>> In that case, I suggest going back a step and checking whether the signal
>> at each electrode is identical. That would indicate a problem earlier in
>> your pipeline, e.g. that you overwrite the single electrodes with a common
>> signal.
>>
>> Or do you mean, that the signal does not change from Color_C to ERP?
>> In that case, I suggest inspecting Color_C in close detail, and computing
>> the mean of the baseline interval by hand. If that value is close to zero,
>> you can’t expect drastic changes from the baseline correction.
>>
>> Hope that helps,
>>
>> Julian
>>
>> ________________
>> Prof. Dr. Julian Keil
>>
>> Biological Psychology
>> Olshausenstrasse 62 - R. 306
>> 24118 Kiel, Germany
>>
>> +49 - 0431 - 880 - 4872
>> http://www.biopsych.uni-kiel.de/en
>>
>>
>> Am 09.04.2019 um 10:11 schrieb Luca Moretti <moretti.l1991 at gmail.com>:
>>
>> Dear Fieldtrippers,
>>
>> I have a problem concerning the calculation of ERPs with
>> ft_timelockbaseline. After I have preprocessed the data (re-referenced,
>> filtered, cleaned from artifact), this is what I have done:
>>
>> inputfile = (strcat('Subject_2_clean.mat'));
>> load (inputfile);
>> layout = 'easycapM1';
>>
>> %% Calculating ERP
>>
>> cfg = [];
>> cfg.trials = find(data_clean_2.trialinfo==60);
>> Color_C = ft_timelockanalysis(cfg, data_clean_2);
>>
>> %% Plotting
>>
>> cfg = [];
>> cfg.baseline = [-.3 0];
>> cfg.channel = {'all'};
>> cfg.parameter = 'avg';
>> ERP = ft_timelockbaseline(cfg, Color_C);
>>
>> figure
>> cfg = [];
>> cfg.layout = layout;
>> cfg.interactive = 'yes';
>> cfg.showoutline = 'yes';
>> ft_multiplotER(cfg, ERP)
>>
>> %%
>>
>> My problem is that it doesn't matter what kind of baseline period I am
>> chosing, the figure Always turn out the same for each electrode. I have
>> even tried to set the baseline to the Whole trial period, but the figure I
>> get from plotting individual channels from the multiplot is Always the same.
>>
>> Can someone find something wrong with my code?
>>
>> Many thanks!
>> Luca
>>
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