On Tue, 20 Dec 2011, Mike E. Klein wrote: > Does anyone know what could cause a negative shift in the searchlight > accuracy distribution for a 2-category Linear SVM classifier of single > subject? I'm seeing this in a couple of my subjects following a switch to > using zscore(dataset, chunks_attr='chunks', dtype='float32') …. so > zscoring against the mean and not against a baseline condition. (I'm > actually zscoring after removing the rest trials from the timeseries.) > While the vast majority of my subjects show a normal looking curve with a > peak right at 50%, a couple of subjects, while having a normal-esque > curve, show accuracy peaks at 40-45%.
I think I have observed that as well, but blindly attributed it just to a fact that peak might vary across subjects as well by chance... I should have compared against not z-scored data I guess... would you mind plotting/printing histograms of peaks across subjects (how many?) with and without z-scoring? > Generally speaking, I'm seeing group data accuracy peaks in more-or-less > our hypothesized areas of the brain, but I'm still a bit worried that I've > done something wrong somewhere along the line. I can't figure out why I'm > seeing the negative peak (in a couple of subjects) or why using zscore > against rest has such a detrimental effect on my accuracies… so it would be interesting to compare -- what if you make a scatter plot (pylab.scatter) of such "problematic" subject searchlight performances voxel by voxel (one axis performance with zscoring on the other -- without)? NB I have cooked a little helper function to easily create such plots and I haven't pulled it into PyMVPA yet -- I guess should do that some time soon -- =------------------------------------------------------------------= Keep in touch www.onerussian.com Yaroslav Halchenko www.ohloh.net/accounts/yarikoptic _______________________________________________ Pkg-ExpPsy-PyMVPA mailing list Pkg-ExpPsy-PyMVPA@lists.alioth.debian.org http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/pkg-exppsy-pymvpa