Abstract
This study used functional MRI (fMRI) to examine the neural effects of willfully allocating one's attention to one of two ongoing tasks. In a dual task paradigm, participants were instructed to focus either on auditory sentence comprehension, mental rotation, or both. One of the major findings is that the distribution of brain activation was amenable to strategic control, such that the amount of activation per task was systematically related to the attention‐dividing instructions. The activation in language processing regions was lower when attending to mental rotation than when attending to the sentences, and the activation in visuospatial processing regions was lower when attending to sentences than when attending to mental rotations. Additionally, the activation was found to be underadditive, with the dual‐task condition eliciting less activation than the sum of the attend sentence and attend rotation conditions. We also observed a laterality shift across conditions within language‐processing regions, with the attend sentence condition showing bilateral activation, while the dual task condition showed a left hemispheric dominance. This shift suggests multiple language‐processing modes and may explain the underadditivity in activation observed in the current and previous studies. Hum. Brain Mapp, 2007. © 2006 Wiley‐Liss, Inc.