[pymvpa] Searchlight and permutation tests.

[Roberto Guidotti]

Hi all,

I'm writing the second part of my story, begun with the thread "Balancing
with searchlight and statistical issue"!

I have a problem with searchlight results, I ran some whole brain
searchlight to decode betas of an fMRI dataset regarding a memory task
(this time the dataset is balanced).
I divided the dataset in 5 parts (11 betas per condition in each fold) and
run a searchlight using Linear SVM (C=1) with a leave-one out
cross-validation.
The across-subject average map has some suspicious results, the accuracy
histogram is not peaked at chance level (0.5) but is peaked at 0.55-0.56,
so the majority of voxels has that range of value. Do you think is it
reasonable? Or it depends on some cross-validation scheme, beta issue, or
who knows?

To validate that result I ran a exploratory permutation test (n=100) on a
single subject to look at accuracy distribution, in that subject, the
histogram after permutation test is correctly peaked at chance level (the
map with correct labeling is peaked at 0.57). I don't know if I'm correct
but this should validate the hypothesis that the map histogram peaked at
0.55-0.56 is reasonable!

Any comments is really appreciated!!

Thank you
Roberto
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