> 1.) Why do you need a 2-way ANOVA? The order of the 1st level regressors is 1=pre hrf, 2=pre
> temporal
> derivative(TD), 2=post hrf, 4=post TD. So, [1 0 -1 0] tests hrf and [0 1 0 -1] tests TD. If so,
> you get two con
> images. Why would you need a 2-way ANOVA for this? Isn't it a oneway within-subjects ANOVA with 2
> levels?
You are right - trust me to be lazy - I meant a 1x2 oneway ANOVA.
(Though in fact there are no real "N-way" ANOVA options in SPM,
because it always assumes a pooled error).
> 2.) Is it true that the 1st level t-contrasts test only for greater activations before than after
> (ie pre minus post)?
> If one wanted to test both tails (ie pre>post or post>pre), would it be correct to run F contrasts
> on the first level
> and take these to the second level?
No - you only impose a sign (tail) on your test when you select a T-contrast
within a model - and this just affects the sign of the resulting statistic image
(spmT_*) and, more importantly, of the contrast image (con*img).
You do not need to test both tails at the 1st level in order to test both tails at
a higher level. All you need to do is remember the sign of your single, 1st level
contrast. So if your 1st level contrast was A-B, you can test for B>A in a
second-level one-sample t-test simply with the contrast [-1]. Or if you want
to test both tails, you simply evaluate the 2nd-level F-contrast [1].
NB: F-contrasts do not create contrast images - ie linear combinations of betas
(only the estimated sum of squares, ess*_imgs) - so are not used in this way.
Rik
----------------------------------------
Dr Richard Henson
MRC Cognition & Brain Sciences Unit
15 Chaucer Road
Cambridge
CB2 2EF, UK
Tel: +44 (0)1223 355 294 x522
Fax: +44 (0)1223 359 062
http://www.mrc-cbu.cam.ac.uk/~rik.henson
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