Are brain functions really additive?

Although Positron Emission Tomography (PET) and functional magnetic resonan ce imaging (fMRI) studies commonly subtract data obtained during two or mor e 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 con stituent components of the third, making possible a direct examination of d ecomposition by subtraction. In Experiment I, a group of 13 right-handed no rmal volunteers participated in three activation studies: syllable repetiti on; phonation; and repetitive lip closure. A scanning session was devoted t o a single task, repeated four times. In Experiment 2, six of the original subjects performed the same three activation studies during a single scanni ng session. Whether tasks were studied in separate scanning sessions or com bined within a single session, the results of decomposition by compound sub traction differed significantly from the results obtained when individual t asks were compared to a simple baseline condition. These data failed to dem onstrate task additivity, a necessary property if decomposition by subtract ion is to provide an accurate characterization of the brain activity accomp anying complex behavior. (C) 1999 Academic Press.