Dear Bill,
> regarding the limited replicability of activation between sessions
David McGonigle has undertaken a heroic examination of intersession
differences in activation pattern for simple visual, motor and cognitive
paradigms. Abstract below.
Best,
Geraint
--
Geraint Rees | Institute of Cognitive Neuroscience
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+44-(0)20-7679-5496 (work) | Alexandra House
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www.klab.caltech.edu/~geraint | London WC1N 3BG
Variability in fMRI: an examination of intersession differences.
McGonigle DJ, Howseman AM, Athwal BS, Friston KJ, Frackowiak RS, Holmes AP.
Wellcome Department of Cognitive Neurology, Institute of Neurology, London,
WC1N 3BG, United Kingdom.
The results from a single functional magnetic resonance imaging session are
typically reported as indicative of the subject's functional neuroanatomy.
Underlying this interpretation is the implicit assumption that there are no
responses specific to that particular session, i.e., that the potential
variability of response between sessions is negligible. The present study
sought to examine this assumption empirically. A total of 99 sessions,
comprising 33 repeats of simple motor, visual, and cognitive paradigms, were
collected over a period of 2 months on a single male subject. For each
paradigm, the inclusion of session-by-condition interactions explained a
significant amount of error variance (P < 0.05 corrected for multiple
comparisons) over a model assuming a common activation magnitude across all
sessions. However, many of those voxels displaying significant
session-by-condition interactions were not seen in a multisession
fixed-effects analysis of the same data set; i.e., they were not activated
on average across all sessions. Most voxels that were both significantly
variable and activated on average across all sessions did not survive a
random-effects analysis (modeling between-session variance). We interpret
our results as demonstrating that correct inference about subject responses
to activation tasks can be derived through the use of a statistical model
which accounts for both within- and between-session variance, combined with
an appropriately large session sample size. If researchers have access to
only a single session from a single subject, erroneous conclusions are a
possibility, in that responses specific to this single session may be
claimed to be typical responses for this subject.
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