Friday, March 8, 2013

correlating: functional connectivity and RSA

It occurred to me that functional connectivity is sort of like RSA (representational similarity analysis): calculating a bunch of distances (Pearson correlation, often), but over timepoints (e.g. volumes) instead of voxels.
The sketches show how one cell of the dissimilarity matrix or one functional connection is calculated. I use "BOLD" in a loose sense; the signal has of course been scaled, etc., but the plotted values do derive from  "activation" as measured with BOLD.

I can make a similar sort of picture for linear SVM classification:

but it's quite a bit different. With classification we're analyzing (often) the test set accuracy which comes from testing the fitted classifier (represented by the purple dashed line) with new data. But with functional connectivity and RSA we're working with the correlation (or whatever distance metric) more directly, interpreting how related the BOLD is between two regions (over timepoints) or between two trials (over voxels).

1 comment:

  1. Hi Joset, I ran across your site again when I was looking for some other info and saw this post. You are totally right that RSA and FC matrices relate data across regions in similar ways. Maureen Ritchey in my lab also found that the two are significantly correlated with each other, in every participant that we've looked at (though the magnitude of the correlation as you might expect is modest). Here's the link to that paper:
    Cheers, CR