Regionally specific sensitivity differences in fMRI and PET: Where do theycome from?

In this paper we report three neuroimaging studies of language that investi gate potential sources of inconsistency in measured hemodynamic responses: (1) between sessions for fMRI, including differences in hormonal status, (2 ) between sessions for PET, and (3) between scanning modalities (PET and fM RI). Differences in evoked responses between sessions of the same modality were small. In particular we did not find any effect of hormone levels when testing during the first and third weeks of the menstrual cycle (although we cannot exclude the possibility that activation in the temporoparietal re gions is sensitive to hormonal status). Comparing the two modalities showed that prefrontal regions were more activated in fMRI than in PET. This may relate to task switching between blocks in fMRI that is not induced by PET paradigms or increased error variance in these regions for PET. In contrast , temporal activations were found in PET more than in fMRI. We attribute th e lack of temporal activations, in fMRI, to a combination of factors, inclu ding susceptibility artifacts, anticipatory activity during the control con dition, discontinuous sampling of peristimulus time, and differences in the source, acquisition, and analysis of the measured signals. It is concluded that although there is sufficient reproducibility of results for these par adigms within each modality, the regionally specific differences in sensiti vity found between modalities warrant further investigation. These regional ly specific differences are important for a properly qualified interpretati on of activation profiles in fMRI. (C) 2000 Academic Press.