Hello Everyone,
 
I'm trying to understand how FSL creates its regresors from a 3 column stimulus file by recreating them myself. 

To do this, I'm comparing my own regressors to those created in FSL from a 3 column stimulus file, colvolved with the Double-Gamma HRF, at zero phase and without temporal filtering.

I've read elsewhere in the forums that the double gamma HRF used by fsl is the same one used by SPM, so I'm using that at 20 Hz as my convolution kernel.  I'm creating a 20Hz box car array from the 3 column stimulus file, and colvolving it with this HRF.  After correcting for some temporal offset, when I compare this to the regressor created in FSL, I'm still seeing subtle differences. (FSL's regressor in blue, mine is in green)
 
These differences may seem small, but it's important that I understand exactly what FSL is doing.

I'm wondering if anyone can shed some light onto what FSL is actually doing to create these regressors, as in, is there any filtering going on that we're not aware of, or is there some subtle difference between their HRF kernel and the one I'm using?  Is the fsl HRF actually saved anywhere that I could access directly? (I did try to look at this by creating a regressor with a single impulse, and the resulting regressors were the same to a very small margin of error, much less than above).  So, any help in this would be appreciated.  Thanks!