I'm designing an event-related fMRI study with 4 conditions: A,B,C,D,
where D is a null event (fixation). After reading Josephs and Henson
(1999) I still am not clear on 2 points:
1) If one wishes to have be sensitive to both main and differential
effects, is it critical to order the conditions such that each condition
(A,B,C,D) follows the other with equal frequency (i.e., a
pseudorandom, permuted design)?
Put another way, what is the disadvantage to a simple randomized design +
a null event for temporal jittering?
2) I have a TR= 2 seconds and a fixed ISI of 3.5 seconds. I haven't seen
any discussion of whether it is important to have each condition equally
represented at each peri-stimulus time point, but intuitively this would
seem to be important. For example, in this design there are 4
peri-stimulus positions: 0 (onset of stimulus coincides with scan), +.5
(stimulus presented at .5 sec after scan), -.5, and +1(same as -1).
Shouldn't the A condition (for example) be equally represented at each
peri-stimulus position, to avoid bias? Apologies if this has been covered
previously.
Stephan Hamann
Dept. of Psychology
Emory University
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