Occasionally I run across experimenters who express a desire to run a PPI, where the psych variable for the PPI is of the form "condition X vs fixation" or "condition X vs baseline."
In a thread from a few years ago, Darren G. claimed that to do this, one must model fixation explicitly in the conventional model that feeds into PPI. Donald (McLaren) has probably thought about this, but I haven't had time to read his code/poster.
Any thoughts? (Explicitly modeling fixation is laborious because it means all the subject-level models must be redesigned.)
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