Hi Kathleen
As you note the HRF + TD will account for some shifts in the timing of the
HRF. You could explicitly test for this by examining the hrf and TD
responses using F-tests at the 2nd level. You could also include the
dispersion derivative in the analyses to account for additional variance in
the canonical hrf.
Another way of analyzing the data would be by using an FIR basis set. This
would eschew any specific form for the HRF and allow you to assess for
changes in peri-stimulus BOLD responses based on your FIR time bins.
Darren
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping)
> [mailto:[log in to unmask]] On Behalf Of Kathleen W. Smith
> Sent: Friday, June 15, 2007 8:02 PM
> To: [log in to unmask]
> Subject: [SPM] hrf peak
>
> Dear Dr. Friston, Dr. Worsley, and other SPM experts,
>
> One of my committee members is setting me a very interesting
> challenge:
>
> "...my primary concern is confirming that your analysis is
> not compromised by using an incorrect hrf peak. We spent a
> lot of work deciding on the correct oxygenation peak delay to
> use in our analyses, and perhaps now (many years later) 6s is
> considered stable across studies. I've never seen that
> statement made, so my concern is that each study may need a
> different hrf peak.
> This is critical since all your analyses are based on the
> assumption that oxygenation will peak at 6s, and if it
> doesn't (or more troubling, if the peak differs by condition,
> which I sincerely hope it doesn't and I don't expect), your
> analyses are severely compromised. All that said, you can
> placate my concerns by choosing one set of predictions (your
> choice, but choose the most critical hypothesis), and showing
> that the observed hrf peak is the same for all conditions,
> and that the peak is close to 6s."
>
>
> Having read Kalina Christoff's procedures on identifying
> individual hrf's, I don't believe that I can establish the
> hrf for individual Ps retroactively from my existing data.
> Christoff indicates that such a study would be designed so
> that no stimulus would evoke a new hrf until the previous one
> had run its course.
>
> I know that the canonical hrf peaks at six seconds and that
> this is grounded in theory.
>
> I have been analysing my data using SPM2, using hrf+time derivative.
>
> I wonder whether there is an indirect way to address this
> question. Would any of you mind pointing me in the right direction?
>
> Many thanks!
>
> Kathleen Smith
>
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