Hi AFNI gurus, I am new to fMRI and event-related design and would like to ask a design question. So, if I understand correctly an ideal design would randomize the stimuli (events) and also have an ideal ISI. My question is regarding a task in which each trial is composed of 3 stimuli, and there can be a total of 6 different trial types, so 18 separate events. Each of the 3 stimuli in a particular trial would follow each other in the same order (A, B, C), but the different trial types would be randomized. Thus, I would have something like this:

(A, B, C) ITI (D, E, F) ITI (G, H, I) ITI (J, K, L) ITI (M, N, O) ITI (P, Q, R) ITI (D, E, F) ITI (J, K, L) ITI (A, B, C)….etc.

I am interested in performing contrasts between A and J for example, but not between the 3 stimuli within a trial. Thus, when we speak of ISI and choosing the ideal time, is it between any stimuli within the entire task (e.g. A and B, M and N, etc.) or is it the ISI between the exact same stimulus (e.g. A and A) that will be deconvolved? Also, in this case, what would be the role of the ITI? I guess, I’m just getting confused between ISI and ITI, since I have heard them used interchangeably, but that doesn’t make sense to me.

Thanks for clarifying!

Anita