Age-Related Changes in Hemispheric Organization

What are the implications for cognition of the fact that we have two cerebral hemispheres? Although this is one of the most fundamental questions in cognitive neuroscience, no definite answer is presently available. For more than a century, researchers have wondered why we have two hemispheres, how they differ, and how they interact with each other. Anatomical and functional asymmetries have been identified, and different models of hemispheric interaction have been proposed. Recently, studies using positron emission tomography (PET) and functional MRI (fMRI) have revitalized this research domain by discovering new hemispheric asymmetries. Moreover, these studies have identified changes in the lateralization of brain activity as a function of population variables, such as aging. In fact, the most consistent finding in functional neuroimaging studies of cognitive aging is a reduction in the lateralization of prefrontal activations. This is not the first time that aging has been associated with changes in lateralization; for many years, cognitive aging psychologists discussed the possibility that the right hemisphere ages faster than the left. In general, however, these ideas have not been informed by theories regarding hemispheric specialization and interaction. The goal of this chapter is to address this disconnection and to link the topic of lateralization and aging to general issues regarding hemispheric organization.