[pymvpa] null classification performance in the presence of strong univariate signal??

David Soto d.soto.b at gmail.com
Thu Sep 11 15:15:04 UTC 2014

Hi Nick, sorry to come back to this point
but I am not clear why you mention that I have 2 samples
only for MVPA, I have condition A with 19 beta images (1 per subject)
and condition B with the same, merged and concatenated in one fule

If I run a basic nfold crosvalidation

clf = LinearNuSVMC()


cv=CrossValidation(clf, partitioner)

sl=sphere_searchlight(cv, radius=3, space='voxel_indices',

then I am training  SVM to distinguish condition A from B
in 18 subjects and then testing on the remaining 19th subject.....am i

the univariate stats are done as a A-B t-test on those images....

so why N=2? still did not figure out how the univariate t-test gives
strong signal in frontoparietal cortex but MVPA nothing


On Mon, Sep 8, 2014 at 11:50 AM, Nick Oosterhof <nikolaas.oosterhof at unitn.it
> wrote:

> On Sep 8, 2014, at 12:09 PM, David Soto <d.soto.b at gmail.com> wrote:
> > however I should say that I still do not get why using the very same
> input data, the univariate GLM picks the difference between the*cued and
> uncued* conditions but the MVPA seems not
> From what I understood, you were trying to use the beta estimates /from/
> the GLM as input for MVPA.
> The difference is that the GLM uses N samples (with N the number of
> acquired volumes; i.e. as a timeseries) as input, whereas your MVPA
> approach only uses 2 (which is much smaller than N). Two samples is just
> not enough to do MVPA, just as two samples is insufficient for any
> meaningful univariate statistics.
> You can use all N volumes as input for MVPA, for example see:
> http://www.pymvpa.org/examples/searchlight.html
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