Dear All
I am helping someone analyse a set of experiments where the design
is always a simple ABABAB and there should be 12 scans per block.
Sometimes A is the active task and B the reference, sometimes vice
versa. My plan was to specify just the active condition in
each experiement (whether A or B) and model that. However we have a
bit of a problem in that, due to a cock-up over dummy scans, 2 scans
from the first A block are consistently missing and we therefore have
unequal epoch lengths (A is 10 12 12 rather than 12 12 12). As I
understand it, the spm 99 conceptualisation of epochs and how to
model them depend on each epoch of a condition being the same length
and we therefore can't specify A as things stand. So I'm considering
3 options.
1. Always specify B rather than A whichever is the active task, since
B DOES always have consistent length epochs. If B is the active task
then 1 0 gives us what we want, if A is the active task its -1 0.
2. Introduce an extra condition C, call the design ABCBCB, have A
with 10 second epoch, B and C with 12 and then look at the effects of
interest with contrasts [0.33 -1 0.67] or [-0.33 1 -0.67]
3. (risking excommunication from the spm fraternity here........)
make duplicate copies of the first 2 scans we do have in each
experiment thereby magically creating 12 scans per block.
My question: 1, 2 or 3 (or some fiendishly clever option 4 that has
not occurred to me)?
Thanks very much for your help
Rebecca
Dr Rebecca Elliott
Neuroscience and Psychiatry Unit
Room G907, Stopford Building
University of Manchester
Oxford Road
Manchester
M13 9PT
Phone: 0161 275 7433
Fax: 0161 275 7429
e-mail: [log in to unmask]
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