Thank you very much Christopher! These information is very helpful and reassuring. I tested a couple of different minor modifications of the Lee et al., approach and found consistent results.
Best, Frances On Sun, May 10, 2015 at 9:17 AM, Christopher J Markiewicz <[email protected]> wrote: > On 05/09/2015 12:49 PM, Jingwen Jin wrote: > > > > Hi MVPA experts, > > > > I have a general question about conducting group-level analysis on the > > subjects' classification accuracy maps. Let's say I am doing a > > one-sample t-test to find the voxels that have high classification > > accuracy across subjects. Essentially, I am doing a t-test on percentage > > numbers (SVM classification accuracy measured as percentage correct). > > Since percentage is highly affected by the testing example numbers, and > > in general would probably not meet the normal distribution assumption > > for t-test. > > > > So my question is if people adjust for testing trial numbers or any sort > > of transformation? For example, I converted each voxel's percentage > > number to a z score at the individual subject's classification map > > level, and then do group-level t-test on these z score maps. I wonder if > > this is valid? > > What you describe sounds like the strategy used by Lee et al (2012) > <https://dx.doi.org/10.1523/JNEUROSCI.3814-11.2012> so there's > precedent. On the other hand, it's not clear to me how to do cluster > thresholding for multiple comparisons correction properly, using this > method. They use SPM8's random effects analysis, but if I recall > correctly that requires smoothness assumptions, while MVPA analyses > typically use unsmoothed volumes. > > There's also a much more intensive non-parametric test used by Stelzer > et al. (2012) <https://dx.doi.org/10.1016/j.neuroimage.2012.09.063>, but > it requires a lot of computing time and at least temporary storage space. > > -- > Christopher J Markiewicz > Ph.D. Candidate, Quantitative Neuroscience Laboratory > Boston University > > > _______________________________________________ > Pkg-ExpPsy-PyMVPA mailing list > [email protected] > http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/pkg-exppsy-pymvpa > -- Jingwen Frances Jin Department of Psychology Stony Brook University
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