Dear List,
At the last FSL course, it was recommended that we use a fixed effects analysis at the mid level
(i.e., single-subject multiple-session) when fewer than 10 sessions had been collected for each
subject. I have questions regarding this recommendation.
Our data sets consist of 14 subjects with 2-4 runs each (done in a single session). (These data
were obtained from young children, and some of the runs have been excluded from the analysis
due to excessive head motion.) We are interested in making inferences about the population from
which 14 samples were drawn. Thus, my understanding is that you recommend using a fixed
effects analysis on the single-subject multiple-run level and a mixed effects analysis on the
multiple-subject/group level. (Please correct me if I’m wrong.)
When I compared the outputs of the fixed effects and mixed effects analyses for each subject
(combining multiple runs), I see greater extent of activations, and this conforms with what I have
reading (e.g., Friston et al., 1999), namely that a fixed effects analysis is generally more sensitive
to activations than a random effects analysis.
Subsequently, I performed two group-level analyses using a mixed effects analysis (FLAME 1); one
analysis had mid-level fixed effects stats as the input (FE-ME), and the other had mid-level mixed
effects stats as the input (ME-ME). Contrary to my expectation, some significant activations
present in the ME-ME analysis are less robust in the FE-ME analysis.
Why should using a mixed effects analysis at the mid level be more sensitive to activations (at
least in some areas) than using a mid-level fixed effects analysis? If we have a reason to suspect a
large run-to-run variability, would it be better to use a mixed effects analysis at the mid level, or
do you think that run-to-run variance cannot be estimated well with a small number of runs we
have in our data?
Also, I’d like to have better understanding of how these two different types of analyses are
performed in FEAT, and how the stats of these analyses are carried to the higher-level analysis.
Are there any documents or references on these you can point to me?
Thank you in advance for your advice!
-Yoshiko
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