The cerebral hemispheres cooperate to perform complex but not simple tasks

Three experiments were designed to examine whether task complexity determin es the degree to which a division of processing across the hemispheres (i.e ., across-hemisphere processing) underlies performance when within- and acr oss-hemisphere processing are equally possible. When task complexity was re latively low, performance in a midline condition that allowed for either wi thin- or across-hemispheric processing resembled within-hemisphere performa nce (Experiments 1 and 2). However, when task complexity was high, performa nce in a midline condition (Experiments 1 and 2) and a lateralized conditio n, which also allowed for either within- or across-hemisphere processing (E xperiment 3), resembled across-hemisphere performance. Results complement a nd extend prior work (e.g., M. T. Banich & A. Belger, 1990) by indicating t hat the degree to which interhemispheric cooperation underlies performance changes with the complexity of the task being performed. This finding sugge sts that the hemispheres dynamically couple or uncouple their processing as a function of task complexity.