Hi Malin, Yes, I agree with you that I also noted that sometimes exhaustive method gives worse results than partial. Just a notation point: in your chart "Averaged searchlight" line is exhaustive and "Searchlight" line is randomly picked? Those levels were somehow established separately?
Thanks On Tue, Feb 26, 2013 at 4:48 AM, Malin Björnsdotter < [email protected]> wrote: > Hi Vadim! > > I sequentially added more and more search volumes (lights), > approaching the number in the exhaustive map. At some point > (surprisingly early) in this process this random average map was > better (= had a larger area under the receiver operating > characteristic curve) than the non-average, exhaustive (searchlight) > map on simulated data. I have attached a hotted up version of a figure > from the Neuroimage paper, that may make it more clear: at less than > 5,000 classifiers (=searchlights) the Monte Carlo approach performed > at the same mapping level as the exhaustive (searchlight) algorithm > (which required over 25,000 classifiers, i.e. as many voxels as there > were in the brain). So, the number of search volumes you choose > depends on your definition of satisfactory performance. :) > > Also, there is a trick - in my approach, the search volume selection > is not quite random. I made sure that every voxel was included in the > same number of search volumes, i.e. I partitioned the entire brain > into search volumes (some much smaller than that specified by the > radius parameter) such that every voxel was included in one. > > ~Malin > > On Tue, Feb 26, 2013 at 8:09 PM, Vadim Axel <[email protected]> wrote: > > Indeed, very similar - I only make it not random, but rather sequentially > > iterating over all brain. In such a way each voxels participates in > roughly > > the same amount of lights. I could not figure out from paper, Malin, in > how > > many lights the voxel should participate in order to achieve satisfactory > > performance? > > > > On Tue, Feb 26, 2013 at 3:47 AM, Malin Björnsdotter > > <[email protected]> wrote: > >> > >> > In my method, the hitrate is assigned to all the > >> > voxels in the light and given that each voxel participates in many > >> > lights, > >> > the hitrates are averaged. So, using my method a voxel hitrate > reflects > >> > many > >> > possible environments. I try to compare the results of both and so far > >> > it > >> > seems that with your method the results are more patchy. > >> > >> That sounds pretty much exactly as what I've been doing. :-) Jo has an > >> excellent blog entry about this: > >> http://mvpa.blogspot.sg/2012/09/random-searchlight-averaging.html > >> > >> _______________________________________________ > >> Pkg-ExpPsy-PyMVPA mailing list > >> [email protected] > >> > http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/pkg-exppsy-pymvpa > > > > >
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