Age differences in the frontal lateralization of verbal and spatial working memory revealed by PET

Age-related decline in working memory figures prominently in theories of co gnitive aging. However, the effects of aging on the neural substrate of wor king memory are largely unknown. Positron emission tomography (PET) was use d to investigate verbal and spatial short-term storage (3 sec) in older and younger adults. Previous investigations with younger subjects performing t hese same tasks have revealed asymmetries in the lateral organization of ve rbal and spatial working memory. Using volume of interest (VOI) analyses th at specifically compared activation at sites identified with working memory to their homologous twin in the opposite hemisphere, we show pronounced ag e differences in this organization, particularly in the frontal lobes: In y ounger adults, activation is predominantly left lateralized for verbal work ing memory, and right lateralized for spatial working memory, whereas older adults show a global pattern of anterior bilateral activation for both typ es of memory. Analyses of frontal subregions indicate that several underlyi ng patterns contribute to global bilaterality in older adults; most notably , bilateral activation in areas associated with rehearsal, and paradoxical laterality in dorsolateral prefrontal sites (DLPFC; greater left activation for spatial and greater right activation for verbal). We consider several mechanisms that could account for these age differences including the possi bility that bilateral activation reflects recruitment to compensate for neu ral decline.