Variability in fMRI: An examination of intersession differences

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 n o responses specific to that particular session, i.e., that the potential v ariability of response between sessions is negligible. The present study so ught to examine this assumption empirically. A total of 99 sessions, compri sing 33 repeats of simple motor, visual, and cognitive paradigms, were coll ected over a period of 2 months on a single male subject. For each paradigm , the inclusion of session-by-condition interactions explained a significan t amount of error variance (P < 0.05 corrected for multiple comparisons) ov er a model assuming a common activation magnitude across all sessions. Howe ver, many of those voxels displaying significant session-by-condition inter actions 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. Mos t voxels that were both significantly variable and activated on average acr oss all sessions did not survive a random-effects analysis (modeling betwee n-session variance). We interpret our results as demonstrating that correct inference about subject responses to activation tasks can be derived throu gh the use of a statistical model which accounts for both within- and betwe en-session variance, combined with an appropriately large session sample si ze. If researchers have access to only a single session from a single subje ct, erroneous conclusions are a possibility, in that responses specific to this single session may be claimed to be typical responses for this subject . (C) 2000 Academic Press.