Dear Afraim,
> Can someone tell me if it is possible to plot simple variance maps
> of voxel timeseries within SPM?
Yes, this is possible.
If you want to look at the estimated variance image, take a look at the
ResMS-image in your analysis directory.
This image contains the sum of squares of the residuals
divided by trace(RV), where R is the residual forming matrix and V =
KK', where K is the assumed temporal filter. (K Worsley and K Friston,
'Analysis of fMRI Time-Series Revisited -Again, Neuroimage 1995, p
173-181).
To compute something like a 'simple' variance map, by which I assume you
mean the variance around the mean, go for a t-test in 'Basic Models',
which implements a design matrix having one constant column. This takes
out the mean and leaves you with the variance in the ResMS-image. Note
that this variance estimation takes into account the potential temporal
correlation of the data.
Stefan
--
Stefan Kiebel
Functional Imaging Laboratory
Wellcome Dept. of Cognitive Neurology
12 Queen Square
WC1N 3BG London, UK
Tel.: +44-(0)20-7833-7478
FAX : -7813-1420
email: [log in to unmask]
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