Wednesday, January 1, 2014

FIR Models Redux

As promised, here is a tutorial video on how to set up Finite Impulse Response (FIR) models in AFNI. The previous post contained the code for using FIR basis functions instead of traditional Gamma basis functions, which then spews estimates for activity at each time point you specify, which can be in as long or as short of a time window that you like. You can then extract these using a command like 3dbucket, e.g.

3dbucket -prefix VisCoeffs cstats.FT+orig'[12..22]'

You can then extract these timecourses by loading up a statistical dataset and overlaying blobs at certain thresholds, or just overlaying an ROI that you created and highlighting it through the "Rpt" button in the AFNI interface. Choose "Aux Dataset", load up the coefficients you extracted, and make a neat line, like this:

 Keep in mind that this was a block design, and that these were coefficients extracted from a visual ROI from a visual condition; so naturally the hemodynamic response will increase and plateau for the duration of the block. Often you will get much more complex shapes, which contains information about how timepoints along the hemodynamic response can differ across conditions, as well as hierophanies, if you look closely enough. In any case, don't get too carried away.

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