Although Positron Emission Tomography (PET) and functional magnetic resonance imaging (fMRI) studies commonly subtract data obtained during two or more experimental conditions to decompose a complex task, there have been few opportunities to evaluate this approach directly. In the present study, PET was used to study three motor speech tasks selected such that two were constituent components of the third, making possible a direct examination of decomposition by subtraction. In Experiment 1, a group of 13 right-handed normal volunteers participated in three activation studies: syllable repetition; phonation; and repetitive lip closure. A scanning session was devoted to a single task, repeated four times. In Experiment 2, six of the original subjects performed the same three activation studies during a single scanning session. Whether tasks were studied in separate scanning sessions or combined within a single session, the results of decomposition by compound subtraction differed significantly from the results obtained when individual tasks were compared to a simple baseline condition. These data failed to demonstrate task additivity, a necessary property if decomposition by subtraction is to provide an accurate characterization of the brain activity accompanying complex behavior.
Bibliographical noteFunding Information:
The results of Experiment 1 were presented at the Second International Conference on Functional Mapping of the Human Brain, Boston, MA, June, 1996. This work was supported by the Bob Allison Ataxia Research Center, by the Program in Hereditary Ataxia, and by the Program in Visualization of Functional Connectivity in the Brain Project. We thank D. Van Lancker and C. Iadecola for comments on this manuscript and D. Daley, C. Farmer, D. Hamm, and M. Kneer for assistance in data collection and processing. We also acknowledge the thoughtful comments of the reviewers, especially with respect to clarifying the presentation of the nonadditivity across tasks.