Stephan -
> 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)?
It is not critical; it is, in theory, slightly better for a differential
effect
(eg [1 -1 0] or [1 1 -2] contrast) when you are in an intermediate SOA
(ISI)
regime of ~6-10s (and employ the usual bandpass filter). Note condition
D - the
fixation trials - are not modelled. Thus a "main effect" can be viewed
simply
as the differential effect between each condition and fixation; the
distinction
here applies simply to modelled versus nonmodelled event-types.
> Put another way, what is the disadvantage to a simple randomized design +
> a null event for temporal jittering?
No disadvantage - for short SOAs (as in your case) and null events, a
fully randomised design is best for sensitivity to differential and main
effects,
and is probably what you should use.
> 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.
In theory, it is best to have each condition equally represented at each
time point. In practice, we often assume this will be close to true with
enough randomised stimuli.
Note that in your case, I think there are 7 peristimulus sampling
points:
0 0.5 1.0 1.5 2.0 2.5 3.0
ie, you have an effective sampling rate of 2Hz (with a cycling period of
14s: 4 ISIs or 7 TRS).
Good luck!
Rik
---------------------------8-{)}-------------------------
DR R HENSON EMAIL [log in to unmask]
Wellcome Department of
Cognitive Neurology TEL (work1) +44 171 833 7483
12 Queen Square TEL (work2) +44 171 833 7472
London, WC1N 3BG FAX +44 171 813 1420
URL: http://www.psychol.ucl.ac.uk/rik.henson/index.html
---------------------------------------------------------
