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

[David Soto]

Thanks Jo and Nick for the advise, the individual acc maps
which I registered to MNI prior to t-test look fine to me
(see example pic attached, spot with accuracy around 0.7)...there might be
something going on in the t-testing done  in FSL, though I cant see what-
as should work fine across imaging data types -

I will try your R code - thanks!

ds

On Thu, Sep 18, 2014 at 11:04 PM, J.A. Etzel <jetzel at artsci.wustl.edu>
wrote:

> I agree with Nick that something might have went wrong with the t-test.
> I've never tried one in fsl, either, but usually use R. Here's a little bit
> of R code to do a voxelwise t-test: http://mvpa.blogspot.com/2014/
> 09/demo-r-code-to-perform-voxelwise-t-test.html
>
> I assume you looked at the 19 subjects' accuracy maps before trying the
> t-test ...
>
> Jo
>
>
>
>
> On 9/18/2014 8:58 AM, Nick Oosterhof wrote:
>
>>
>> On Sep 18, 2014, at 3:36 PM, David Soto <d.soto.b at gmail.com> wrote:
>>
>>  I reiterate I did this separately for each of the 19 subjects.
>>>
>>> I then aimed to carry out a group analyses using the individual
>>> accuracy maps. to do this I merged the 19 nifti accuracy maps into
>>> a 4D file and run a one-sample t-test in FSL using randomise -i
>>> searchpred -o searchpredOneSampT -1 -v 5 -T.
>>>
>>> Weirdly the output gives a group map with all brain voxels over
>>> p<0.001 !? which cannot be right...
>>>
>>
>> I am not familiar with FSL's randomise tool (and a quick google
>> search only gave me limited information).
>>
>> One possibility: did you test against the null hypothesis of a mean
>> of zero? In your case chance level is .5 (=1/2), not 0. It could
>> explain why you are getting all highly significant voxels. If that is
>> the case, you should subtract .5 from the classification accuracy
>> maps (of individual subjects) before testing against a mean of zero.
>> _______________________________________________ Pkg-ExpPsy-PyMVPA
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>>
>>
>>  --
> Joset A. Etzel, Ph.D.
> Research Analyst
> Cognitive Control & Psychopathology Lab
> Washington University in St. Louis
> http://mvpa.blogspot.com/
>
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-- 
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