We are working on methods for analysis of fMRI data with very short ISI
and varying ISI. A rough draft of a paper is available on my web page
(www.sph.umich.edu/~jonraz/papers.html). As shown in this paper, even with
ISI=TR, if we are willing to assume additivity, then it is possible to
estimate the differential response. With a randomly varying ISI, it is
possible to estimate the hemodynamic responses to each stimulus type. I
am working with Bruce Turetsky and Dan Ragland on designing an experiment
with randomly varying ISI.
I do not believe that there is a minimum mean ISI that is necessary.
Clearly, if the maximum ISI is greater than the duration of the
hemodynamic response, then the responses can be estimated. The mean ISI
can be quite short, however, by including lots of short ISIs in the
experiment. This assumes additivity and that the response does not
vary according to ISI. For estimating differences, you do not need to vary
the ISI. Just use the shortest ISI that is not not shorter than the TR and
that you believe is reasonable from a biological and psychological
perspective.
Jonathan
On Thu, 10 Jun 1999, Andrew J. Saykin wrote:
> 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.
>
> Best wishes,
> Andy Saykin
>
>
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Jonathan Raz
On sabbatical from August 15, 1998 to June 30, 1999.
Email: [log in to unmask]
Home page: http://www.sph.umich.edu/~jonraz
Sabbatical address and phone numbers:
University of Pennsylvania Medical Center
Center for Clinical Epidemiology and Biostatistics
Division of Biostatistics
423 Guardian Drive
Philadelphia, PA 19104
Telephone: (215) 573-4467
Fax: (215) 573-4865
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