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Hi Catie, I hope this helps.... This 32 offset value comes from spm_get_ons.m. The stimulus function or box-car function has a length of nscans * microtime resolution (usually the number of slices) + 32. If you plot the field u in the U structure, you will realize that there is an offset of 32. just do figure, plot(U.u) and zoom This value of 32 is the length of the canonical hrf in seconds, which is computed in spm_get_bf. Also note that the length of bf is 32 * TR/microtime resolution, where TR/microtime resolution is your time between two matlab points. In the case you are not using the any of the canonical hrf choices (hrf or hrf + temporal derivative or hrf + temporal and dispersion derivative) then the default value of the window for your basis functions is 32 (see spm_get_bf.m), unless you input another value. In that case, I am not really sure whether this 32 offset is still appropiate or it must be changed to SPM.xBF.length. Then, your set of basis functions (bf) is convolved in spm_volterra with your stimulus function (U.u), and the output of convolution has a length of length(U.u) + length(bf) -1, so you must pad zeros before your stimulus function. According to matlab, there is no need to pad zeros to your signal since convolution is done in frequency domain. But i am not really sure if this padding is crucial for some some other spm files when plotting results. Any suggestions? Then downsampling is carried out and these signals in X are used to build the design matrix. So in my opinion, this is not a minor question since the downsampling is crucial for event related fmri and, especially when your TR is large. Note that there is another offset of T0 which is your microtime onset, and according to the SPM5 manual this must be set equal to the time point of your reference slice. So this is clearly related to the slice timing problem. Cesar Caballero Phd Student Computer Science School/SPMMRC University of Nottingham Catie Chang wrote: > Hi - > > This is a very very minor question, but I was just noticing that in > spm_fMRI_design.m (in SPM5), a 32-bin offset is used when resampling > the regressors at acquisition times. I was wondering where the 32 > comes from. > > Thanks :) > catie This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.