On Mon, 2 Aug 1999, Andrew Holmes wrote:
Dear Andrew and Karl,
Thanks for your combined efforts. I have a few more queries in regard to
interactions.
In the example I gave, (two conditions (scans) plus one covariate measure
(verbal fluency) for each) the program requests the covariate as a single
matrix, which I've entered in the same order as the scans
[VF_cond1_s1, VF_cond2_s1...etc], but the program creates two covariate
matrices, one for each condition called VF@cond1, [log in to unmask] My questions
are:
1. What numeric value is the program deriving for these covariate matrices
where the measure has no equivalent scan? Is it simply the value in
the adjacent column with the sign changed? It's certainly non-zero
from the figure of the GLM, but I'd like to check the exact value.
2. Am I right with my interpretataion of the interactions [-1 1] and [1
-1]? Does this mean that the first [-1 1] is equivalent to a positive
correlation, where the t-map will display voxels which increase in
intensity between condition 1 and 2 in parallel with an increase in the
covariate (VF) value from condition 1 to condition 2, and vice versa for
the [1 -1] contrast?
3. > | other. Notice that this model implicitly also models the main
> | effect of the covariate (tested for with 1 1).
I'm not entirely clear how to interpret this (or it's mirror, [-1 -1]),
mainly because I'm not 100% sure of what the program is inserting in the
matrices. Presumably, this is not an interaction effect, but can you help
me out here?
Thank you for your continuing patience and assistance.
Regards - Mike
--
______________________________________________________________________
Mike Glabus PhD, Principal Physicist
MRC Brain Metabolism Unit Tel Intl: 44-(0)131 537 6818/6282
Royal Edinburgh Hospital FAX Intl: 44-(0)131 537 6110
Morningside Park Pager : 5236, via 0131 537 6000
Edinburgh, EH10 5HF, SCOTLAND http://www.pst.ed.ac.uk/staff_pages
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