Multisubject fMRI studies and conjunction analyses

In this paper we present an approach to making inferences about generic act ivations in groups of subjects using fMRI. In particular we suggest that ac tivations common to all subjects reflect aspects of functional anatomy that may be "typical" of the population from which that group was sampled. Thes e commonalities can be identified by a conjunction analysis of the activati on effects in which the contrasts, testing for an activation, are specified separately for each subject. A conjunction is the joint refutation of mult iple null hypotheses, in this instance, of no activation in amy subject. Th e motivation behind this use of conjunctions is that fixed-effect analyses are generally more "sensitive" than equivalent random-effect analyses. This is because fixed-effect analyses can harness the large degrees of freedom and small scan-to-scan variability (relative to the variability in response s from subject to subject) when assessing the significance of an estimated response. The price one pays for the apparent sensitivity of fixed-effect a nalyses is that the ensuing inferences pertain to, and only to, the subject s studied. However, a conjunction analysis, using a fixed-effect model, all ows one to infer: (i) that every subject studied activated and (ii) that at least a certain proportion of the population would have shown this effect. The second inference depends upon a meta-analytic formulation in terms of a confidence region for this proportion. This approach retains the sensitiv ity of fixed-effect analyses when the inference that only a substantial pro portion of the population activates is sufficient. (C) 1999 Academic Press.