[FieldTrip] Questions regarding cluster-based permutation tests on TFRs

[Gabriel Obregon-Henao]

Hello everyone,

I've been going over the tutorial on cluster-based permutation tests on
time-frequency data and I have a couple of questions I'd really appreciate
getting help with.

First, I was wondering if it's possible to compare the TFRs  in the
baseline with multiple non-overlapping segments in the activation period,
and if so, how would one go about doing so?
The lengths of my baseline periods vary between 1 - 1.5 s, but I can only
use the last 500 ms or so to avoid activity evoked by the onset of a
fixation cross within the ITI. On the other hand, the length of my
activation periods is fixed at 3 s. Since one has to use equal-length
intervals from the baseline and activation periods for the statistical
test, I can only select a 500 ms segment from my activation period. Is
there a workaround for testing the whole activation period against
baseline, such as breaking it into multiple 500 ms segments and correcting
for the MCP post-hoc? Alternatively, could one trick the test by upsampling
the baseline period so it contains the same number of samples as the
activation period?

Second, I was wondering why baseline normalization is not performed in the
between-trial and the within-subject experiment examples before running the
cluster-based permutation tests? This is in contrast to the example on
analyzing high-gamma activity from human ECoG recordings. Are there any
subtleties in the preprocessing stages that I'm overseeing (e.g., in the
cluster-based permutation tests tutorial the epochs are being re-centered
using the entire length of the epoch). Does baseline normalization depend
on the experimental question being asked  and/or does it affect the
sensitivity of the statistical  test (I know that it definitely changes the
interpretation of the results)?

Finally, I was wondering whether it matters whether one performs baseline
normalization (using a baseline other than 'absolute') on the trial or
average level, and how does it impact the results?

Thanks in advance!

--Gabriel

-- 
Gabriel Obregon-Henao
PhD Candidate in Neuroscience
University of Washington
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