Dear all,

the suggested way to do a conjunction analysis in AFNI, in order to identify the brain areas that are commonly activated by two different conditions (or better, contrasts), is to compute the spatial intersection of the statistically thresholded maps corresponding to the two selected conditions/contrasts.

However, since those maps are typically thresholded using a combination of voxel-wise and cluster-extent thresholding, I am not so sure that the region of overlap of the thresholded maps can be considered statistically significant. As discussed at length in a recent paper (Woo, Krishnan & Wager, 2014, Neuroimage, 91:312-419), when using cluster-extent based thresholding we cannot make any inference about the activation of specific locations within a suprathreshold cluster: we can only say that "*some* signal must be present *somewhere* in the cluster" (quote from the above paper). Consequently, if the location of "true" activation within a given cluster is undetermined, it seems to me that the intersection area of two statistical maps that have been thresholded with the cluster-extent method cannot be claimed to be itself a spatially-precise locus of significant activation.

Is this a legitimate concern, and if so, what would be an alternative procedure for conjunction analysis?

Thanks in advance for any comment or clarification

best
giuseppe