Interdependence of nonoverlapping cortical systems in dual cognitive tasks

One of the classic questions about human thinking concerns the limited abil ity to perform two cognitive tasks concurrently, such as a novice driver's difficulty in simultaneously driving and conversing. Limitations on the con current performance of two unrelated tasks challenge the tacitly assumed in dependence of. two brain systems that seemingly have little overlap. The cu rrent study used fMRI (functional magnetic resonance imaging) to measure co rtical activation during the concurrent performance of two high-level cogni tive tasks that involve different sensory modalities and activate largely n onoverlapping areas of sensory and association cortex. One task was auditor y sentence comprehension, and the other was the mental rotation of visually depicted 3-D objects. If the neural systems underlying the two tasks funct ioned independently, then in the dual task the brain activation in the main areas supporting the cognitive processing should be approximately the conj unction of the activation for each of the two tasks performed alone. We fou nd instead that in the dual task, the activation in association areas (prim arily temporal and parietal areas of cortex) was substantially less than th e sum of the activation when the two tasks were performed alone, suggesting some mutual constraint among association areas. A similar result was obtai ned for sensory areas as well. (C) 2001 Academic Press.