Dear spm-xperts,
we are analyzing a data set from 5 subjects that participated in 4 sessions,
each with 4 different tasks and would like to know whether there is
any task related effects, that is not confounded by session or subject.
We are interested in any difference between tasks (i.e. both task1>task2
and task2>task1, etc) and we thus supposed that F-tests is the preferred
method of analysis.
Our questions are:
Can the F-contrasts be created in analogy with a plain 3way ANOVA, with 3
main effects and 4 interaction terms?
In that case, are the contrasts proposed below correct for the main effect
of "task", and the interaction term (subject by session by task)?
Is it possible to pursue the analysis on the 2nd level, on the basis of
observations made using the F-tests? (i.e. after creating the relevant
T-contrasts) to assess differences in task, related to this particular
population of 5 subjects, assuming that the result from the 4 sessions are
representative of these 5 subjects?
Would it be preferable to use the approach in the paper by McGonigle
(Neuroimage, 11,708 2000), i.e. with one regressor describing a single
"task" and 4 different regressors that describing the sub-divisions of the
task (1-4), assessing the additional variance modeled by these 4 latter
regressors in an F-test. In this case, how are the additional confounds
(session and subject) handled?
Our data-set: The design matrix consists of 80 columns, 16 for each subject
(A-E),
subdivided in 4 tasks (1-4), each repeated in 4 sessions (1-4) such that
the columns
may be named:
A1a A1b A1c A1d A2a A2b A2c A2d A3a etc until E4d
Proposed F-contrasts:
1. Main effect of "task" (as average across subjects and sessions)
1 1 1 1 -1 -1 -1 -1 0 0 0 0 0 0 0 0 (repeated 4 more times in the same row)
1 1 1 1 0 0 0 0 -1 -1 1 -1 0 0 0 0 (repeated 4 more times in the same row)
1 1 1 1 0 0 0 0 0 0 0 0 -1 -1 -1 -1 (repeated 4 more times in the same row)
2. Interaction term, subject by session by task
1 -1 0 0
1 0 -1 0
1 0 0 -1
0 0 0 0 1 -1 0 0
0 0 0 0 1 0 -1 0
0 0 0 0 1 0 0 -1
etc until zeros(1,76) 1 0 0 -1
any input is appreciated!
Gisela
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Gisela E Hagberg, PhD
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Laboratory of Functional Neuroimaging
Fondazione Santa Lucia IRCCS
via Ardeatina 306
00179 Rome - Italy
voice: +390 6 51501349
fax: +390 6 51501213
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