Andreas -
With such a short TR, there is no need for the temporal derivative
in order to capture slice-timing differences. However, different brain
regions may still have different HRF delays/shapes, which may require
more basis functions, such as the temporal derivative. I would therefore
recommend sticking with F-tests using both basis functions in your case
(within the context of a Fixed Effects analysis).
In other words, T-tests on the canonical HRF will be more powerful
for responses that match the canonical, but may miss noncanonical
responses. Furthermore, signifiant differences in the canonical
HRF parameter estimates cannot necessarily be interpreted as
magnitude differences unless other basis functions, such as the
temporal derivative, do not differ (eg look at the PSTH).
Rik
Andreas Bartsch wrote:
> Dear SPMers (Rik / JB / Christian?)-
> are t-testings on the hrf-regressor alone feasible when modelling the
> response by the canonical hrf and its temporal derivative? I know that
> usually you would recommend F-testing, but in my case the TR is rather short
> (~0.32s, at an SOA quite below 2s) when compared to the time constants (p in
> spm_hrf ) of the canonical hrf and I think this would give me some
> justification for t-testings here. I would prefer modelling the temporal
> derivative to account for variance due to different slice timings as well as
> to variable hrf-onsets in my particular case. Both the SPM{F} as well as the
> SPM{t} I have generated so far make some sense, but the latter are much
> better restricted to the areas expected of hosting the activation I am
> looking for.
> TIA-
> Andreas
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