Paul -
In a HBM1999 abstract, we certainly found that differences
of 3s made a big difference in the canonical HRF fit (ie
between synchronising the model with the first versus last
slice acquired with a TR of 3s). Synchronising with the middle
slice (ie a maximum difference of +/-1.5s) had less effect,
compared with either the first or last slice, but appreciable
differential variability remained.We also found that the inclusion
of the temporal derivative in F-tests captured most of this extra
variability (probably up to at least +/-1s).
Thus, I expect a timing error of 1s may well appear to have only
small effects, as you found. Note however that comparing the SPMs
of a Fixed Effects t-test may not be the best index of fit: the
overall fit may be poor, so differences less. Certainly shifting
a timeseries by +/-1s relative to a single HRF will produce
decreases in the canonical parameter estimate of 80-90%
(which could have implications for any Random Effects analysis
for example).
Rik
Paul Fletcher wrote:
> Rik and Richard,
>
> Many thanks for setting me straight re. SOA and TR.
>
> >Thus I would use slice-timing correction for any TR between 0-3s.
>
> My question here really is whether, although the slice timing works best at
> lower TRs, there is some length of TR below which there is no practical
> benefit of using it. In analysing some previous event-related data, I was
> struck by how a relatively sizeable error (about a second) in specifying
> when events occured, there seemed sufficient flexibility in the model to
> cope with the timing problems. That is, when I accidentally modelled all
> events as occuring (approximately) a second later than they actually did,
> the resulting activations (modeled solely with canonical HRF wave) differed
> little from the correctly modelled data. I assumed that this was a result
> of a very blurred temporal reposnse and some degree of flexibility in the
> way that SPM models the responses. This would suggest that the slice timing
> correction, though working best at low TRs, has its least practical value
> at these TRs. Does anyone out there have a feel for whether there is any
> lower limit below which correction for slice timing has lost its value?
>
> (I suppose this may be largely a hypothetical question as, even if there is
> no clear benefit at low TRs, it behoves one to model the data as accurately
> as possible).
>
> Any advice gratefully recieved.
>
> Paul Fletcher
> -----------------------------------------------------------------
> Paul Fletcher,
> Research Department of Psychiatry,
> University of Cambridge,
> Addenbrooke's Hospital,
> Hills Road,
> Cambridge,
> UK
> CB2 2QQ
>
> Tel 01223 336 988
> Fax 01223 336 581
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