Dear SPMers
I'm still unable to resolve the issue below and am hoping to clarify,
as I cannot see the solution. Specifically, when running an efMRI
paradigm with a large interval between acquisitions, does 'Interscan
Interval' = TR in the level 1 model (as it notes in the manual, and in
the chapter 27 - face data - example), or does it include the silent
period?
If it is the TR, I assume to proceed you adjust your stimulus onset
times relative to each scan. It looks like this is what was done in
the face data example, as, the TR is listed as 2 secs, indicating the
experiment ran for at least 702 seconds (351 images), but the SOTS do
not go past 351 secs.
If so, is it the case that SPM cannot accommodate a design where
stimulus onsets occur more than a TR's length before scan
acquisition? Here, when you make the first SOT 0secs, the second SOT
occurs before the first.
A related query: If 'Interscan Interval' = TR, I am also wondering if
specifying the SOTS relative to the scan onset times and ignoring the
silent period between scans could create problems. Doesn't SPM need
the full time course of the experiment (inc. silent periods) to
correctly convolve the SOTs with the HRF? Without the full time
course wouldn't it treat each stimulus as activating a region from
resting state, and ignore the effects of repeated activation, thus
modeling the activation incorrectly?
Any advice much appreciated.
Christopher Benjamin
> Dear list,
>
> I am relatively new to SPM and have a quick query about model
> specification for a sparse event-
> related fMRI design.
>
> Specifically, in my design I have a TR of 4 seconds and the delay
> between one TR commencing and
> the next is 16.5 seconds (to allow for stimulus presentation and the
> HRF). In level one model
> specification would I
> – specify 16.5 for the Interscan Interval (not the 4s TR);
> – specify the SOTs in one long train (seconds) relative to the first
> SOT (specified as time = 0; Nb.
> have set Units of design = seconds, time bins = number of slices).
>
> Many thanks
>
> Christopher
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