[FieldTrip] TF transform, data length + padding

[Pomper, Ulrich]

Dear community,
I have a question regarding  time-frequency transformation.
The data I want to analyse last from -800 to -200 ms before stimulus onset, I'm using a 400 ms taper.
Importantly, want to make sure that no stimulus-evoked activity leaks into my prestim analysis. 

I first tried to cut the segment down to  -1000 to 0 ms and used (zero) padding (see figure attached, top row). This would mean that the taper, at the last samplepoint of interest (-200ms), would include data up to 0ms
Suprisingly, I get quite different looking results when using a longer data segment (-1200 to 0 ms) (fig. bottom row) without padding.

Can someone explain to me what is going on here, and which approach would be more correct?
Either, the padding in the first variant introduces artificial low frequency activity, or the taper in the second variant uses more than 400ms, at least for the low frequencies.
Note that the difference between conditions (left coloumn) look almost identical, so whatever happens, it happens to both conditions very similarly.

I hope someone can educate me on that matter.
Cheers, Ulrich


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% TF transform %%

            cfg = [];
            cfg.continuous = 'no';
            cfg.channel      = 'all';
            cfg.output = 'pow';
            cfg.method       = 'mtmconvol';
            cfg.foi          =  2:0.5:35;
            cfg.toi          =  -1:0.01:0;
            cfg.taper   = 'hanning';
            cfg.tapsmofrq = 2;
            cfg.t_ftimwin    = 400
            % cfg.pad        = 5;
...
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