Dear Chung-Haow Tu,
At 16:28 27/05/99 +0800, Chung-Haow Tu wrote:
| In the website, the information for the "enhanced PET models"
| including "time order entry of images in PET/SPECT & basic stats".
| Is that mean the model for PET in SPM99 now estimate the temporal
| effect? If that's true, is there any paper to read?
Nothing so sophisticated!
The SPM99 PET models now ask you for scans in time order, rather than by
condition as was the case with previous incarnations. You are then asked to
specify the conditions (for each subject) as a vector such as [1 2 1 2 ...]
or [1212...] or [abab...].
The main advantages of time order scan entry are that covariate entry is
straightforward (since the scans haven't been reordered), and that the
design matrix reflects the actual design of the experiment.
----------------
Since (with a good model) the residual errors from one PET scan to the next
are independent, there's no temporal autocorrelation to account for as with
fMRI time series.
You can of course model time effects using suitable functions of the scan
times, in the same way as with other covariates. See (for example)
Christian Buechels polynomial regression of PET rCBF (counts actually) on
word presentation rate:
Buchel C, Wise RJS, Mummery CJ, Poline J-B & Friston KJ (1996)
Nonlinear regression in parametric activation studies
NeuroImage 4, 60-66
----------------
Hope this helps,
-andrew
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