Resumo

Objectives: The study used fMRI to investigate how the brain adapts when one is able to successfully perform two simultaneous tasks. A central issue in the understanding of multitasking is the automaticity of the tasks that make up a dual task. The reason that walking and chewing gum are so easy to perform concurrently is that both are automatic, by which we mean that they do not require appreciable executive control by the frontal-lobe systems. We investigated one dual task that combined two highly automatic tasks (listening comprehension) and one that combined two less-automatic, controlled tasks (math problems). The study investigated (1) the brain areas recruited for the dual task; (2) changes in cortical network synchronization (functional connectivity); and (3) brain markers in dual-tasking associated with individual differences in working memory capacity. Methods: Experiment 1. Event-related design with 3 experimental conditions: Left Ear Message, Right Ear Message, and Dual Message (both ears). Experiment 2. Event-related design with 3 experimental conditions: Mathematical multiplication problem presented Visually, Auditorilly, and Simultaneously Visually and Auditorilly. fMRI Procedures and Analyses: Sixteen oblique-axial slices; Siemens Allegra 3.0T scanner with a commercial birdcage, quadrature-drive radio-frequency head coil (EPI sequence, TR=1000 ms, TE=30 ms, flip angle=60°, 64x64 acquisition matrix, 3.125mm x 3.125mm x 5mm voxels, 1mm gap). Images were corrected for slice acquisition timing, motion-corrected, normalized to the MNI template, resampled to 2x2x2 mm voxels, and smoothed with an 8-mm Gaussian kernel to decrease spatial noise. Statistical analysis was performed on individual and group data by using the general linear model as implemented in SPM2. Results and Conclusions: The results show three characteristics of high-level dual-tasking: First, there was an increase in the functional connectivity among frontal and posterior areas of the brain in both experiments. The increase in synchronization of brain activation was brought about primarily by a change in the timing of left inferior frontal gyrus (LIFG) activation relative to posterior activation (temporal lobe for language; parietal lobe for math), bringing the LIFG activation into closer correspondence with the activation is posterior areas of the brain. Second, the results also show that the change in LIFG timing was greater in participants with lower working memory capacity. The shift in LIFG activation may be a brain marker of how the brain adapts to high-level dual-tasking. Third, dual-tasking did not recruit new activation in dorsolateral prefrontal cortex (DLPFC) in the language task; conversely, in the math task, there was recruitment of DLPFC in the dual task. The findings provide persuasive evidence that not all dual tasks require additional executive functioning and are suggestive of which task properties determine how the dual-tasking is controlled (automatic vs. controlled tasks). Understanding the core reasons why some brains can multitask while others cannot may make it possible to develop training programs aimed both at improving multitasking and at helping those who are initially unsuccessful at multitasking to improve their performance.