Andy,
>
> Dear SPM group,
> In planning event related fMRI experiments with the expectation of
> analysis by SPM99 I am wondering what the minimum average ISI can be. We
> would like to optimize the number of stimuli we can present in a scan
> session. Assume the ISI is randomized within a defined window about the
> mean ISI and a TR of 3.3s for whole brain acquisition. I realize that with
> ISI's less than 16-20s we will limit our ability to characterize
> differential onset and temporal pattern of HRFs. So my question is, if we
> primarily want to detect relative HRF amplitude differences among classes of
> items, what is the smallest mean ISI that can be appropriately analyzed by
> SPM99. Has anyone tested this or run simulations? I'd greatly appreciate any
> thoughts.
If one assumes that neural activity change is linearly transformed to
fMRI signal change, then sensitivity to differences in amplitude
continues to increase as average ISI decreases for designs in which there
is some (non-unity) probability of a trial occurring every certain fixed
period of time ("minimal ISI)". This is described in submitted
work by Karl, Anders Dale, Oliver Josephs, and Rik Henson.
However, as current results (Friston, K., Josephs, O., Rees, G., Turner,
R. 1998. Nonlinear event-related responses in fMRI. Magnetic Resonance in
Medicine 39: 41-52.) suggest that the agreement of
the transform to linearity is quite poor at ISI's of around a second
or two, it would seem sensible to keep the minimal ISI no lower
than 2 seconds. This is simply a "guesstimate", though, and the correct
choice in general would be expected to depend on experiment and perhaps
regionally specific factors.
Sincerely,
Eric
Eric Zarahn
University of Pennsylvania
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