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Dear colleagues, I'm planning a block design fMRI experiment with at least 5 different conditions. The number of stimuli are different, and I'd like to avoid repetitions. I'd appreciate your comments on a few questions I have: 1) Would it be a problem to compare condition 1 (1 block, 7 stimuli with 3 sec ISI = 21 sec) to a condition 2 (3 blocks, 7 stimuli each with same ISI) ? 2) If we only plan to look at contrasts (cond 2 - cond 1) etc. and have all blocks randomized to avoid order effects, is there any reason to implement a rest between blocks ? 3) What is considered to be the minimum block duration (TR = 2sec) ? I'd assume at least 15-20 seconds in order to allow the hrf to peak and reach an optimum ? Is shorter than 15 sec feasible ? 4) is there a benefit in terms of statistical power to have one 30 sec block vs. 2 15 sec blocks ? Jiang